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Predicting acute denervation in carpal tunnel syndrome
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Predicting Acute Denervation in Carpal Tunnel Syndrome Michael 21. Vennix, MD, David D. Hirsh, AID, Faye Y. Chiou-Tan, MD, C. Don Rossi, MS ABSTRACT. Vennix MJ, Hirsh DD, Chiou-Tan FY, Rossi CD. Predicting acute denervation in carpal tunnel syndrome. Arch Phys Med Rehabil 1998;78:306-312.
Objective: To determine which nerve conduction parameters can predict the presence of acute denervation in carpal tunnel syndrome. Setting: The electrodiagnostic laboratories of a hospital and a county hospital district. Design: A retrospective review. Patients: A total of 1,590 consecutive cases from January 1992 to June 1996, diagnosed as having median neuropathy at the wrist. Main Outcome Measures: Evidence of acute denervation on needle electromyography of the abductor pollicis brevis and its relationship to patient age, gender, and parameters obtained from nerve conduction studies, including median sensory latency and amplitude, and median motor latency and amplitude. Results: Logistic regression analysis identified gender, median motor latency, and median motor amplitude (all p --- .008) as contributing to the prediction of denervation. Needle examination of the cases with a median motor amplitude < 7 m V detected 95.3% (141/148) of all cases with denervation and could have spared 52% (708/1,362) of the population from a needle examination of the abductor pollicis brevis. Conclusion: The median motor amplitude can predict the presence of acute denervation in the thenar muscles in median neuropathy at the wrist and possibly eliminate a painful needle examination of the median-innervated thenar muscles in over 50% of the cases.
© 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation ARPAL TUNNEL SYNDROME (CTS) results from entrapment of the median nerve within the carpal canal and C frequently causes disability. Estimates of the prevalence of this disorder range from 55 to 125 cases per 100,000 people. 1,2 Recent analyses of the 1988 National Health Interview Survey estimate the prevalence of CTS among the working US adult population at 530 cases per 1 0 0 , 0 0 0 . 3,4 Release of the carpal canal is the most common surgical procedure of the hand, with
From the Department of Physical Medicine and Rehabilitation, Baylor College of Medicine (Drs, Vennix, Hirsh, Chiou-Tan, Mr. Rossi); The Methodist Hospital (Dr. Vennix); The Harris County Hospital District (Ben Taub General Hospital, Quentin Mease Community Hospital) (Drs. Vennix, Itirsh, Chiou-Tan); and the Office of Research, Division of Biostatistics (Mr. Rossi), Houston, TX. Mr. Rossi died April 7, 1997. Submitted for publication June 4, 1997. Accepted in revised form September 29, 1997. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Michael J. Vennix, MD, Baylor College of Medicine, Department of Physical Medicine and Rehabilitation, 6550 Fannin, Suite 1421, Houston, TX 77030. © 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/98/7903-453453.00/(3
Arch Phys Med Rehabil Vol 79, March 1998
estimates at more than 460,000 procedures performed each year and a direct medical cost of over $1.9 billion. 5,6 Electrodiagnostic testing with nerve conduction studies and needle electromyography is recognized as an objective tool which, combined with the clinical presentation, can substantially improve diagnostic reliability in C T S . 7 Refinements in nerve conduction studies over the past two decades have improved their sensitivity in diagnosis of median nerve compression. 8,9 These tests aid the physician in evaluating the physiologic integrity of the median nerve, determining prognosis, and planning treatment. In severe cases, abnormal spontaneous activity in the folrn of positive waves and fibrillation potentials can be observed on needle electromyography. However, needle examination of the thenar muscles is an insensitive means of detecting median neuropathy at the wrist. Previous studies reported denervation present in only 18% to 25% of cases that undergo electrodiagnostic testing. 8,1°,11 The usefulness of needle examination of the thenar muscles lies in the evaluation of the severity of median nerve injury and demonstration of axonal degeneration. The presence of clinical weakness in thumb abduction, visible evidence of atrophy in the thenar eminence, and findings of denervation on needle electrornyography of these muscles are all manifestations of a severe median neuropathy and may be indications for surgery in patients with C T S . 12"14 Experience has shown that patients with these findings can obtain relief from their symptoms of numbness and paresthesias after surgery, although they tend to have poor recovery of their motor deficits. 15,16 Despite technical advances, including teflon coating of needle electrodes and thinner needles (27 to 28 gauge), pain and discomfort during needle examination remain the most negative aspects of electrodiagnostic testing for the patient. 17q9 Studies have found the thenar eminence to be one of the most painful muscle groups to evaluate. 2° In the authors' experience, assessment of these muscles is poorly tolerated by many patients. In a recent survey of 110 electromyographers, 74% performed needle examination of the abductor pollicis brevis as a standard part of the electrodiagnostic testing in patients undergoing evaluation for presumed CTS. 2t If the findings of denervation could be predicted from the nerve conduction studies, it might be possible to spare a sizable percentage of patients from a painful needle examination of this muscle. We therefore undertook a retrospective review of patients examined during a 3.5-year period in multiple electrodiagnostic laboratories to evaluate the practicality of predicting denervation in CTS.
PATIENTS AND METHODS The electrodiagnostic laboratories of our hospital and local county hospital district evaluate more than 1,600 patients each year for lower motor neuron injuries. The most common request is evaluation for CTS. After approval from our institutional review board, all records between January 1992 and June 1996 were examined. Patients diagnosed with a nontraumatic median neuropathy at the wrist were included. Testing in all laboratories was performed with a Nomad LE EMG machine, a The information obtained for all cases included age, gender, median sensory latency, median sensory amplitude, median motor latency, median motor amplitude, and the
CTS DENERVATION, Vennix
presence or absence of denervation on needle examination of the abductor pollicis brevis. The electrical diagnosis of median neuropathy at the wrist was made if at least one of the following two criteria from the clinical practice recommendations of the American Association of Electrodiagnostic Medicine was met22: (1) relative prolongation of the standard median sensory peak latency by 0.5msec compared to the peak latency of the standard ulnar sensory response, and each examination was performed at a distance of 14cm23-25; or (2) relative prolongation of the short-segment, transcarpal median response by 0.5msec with palmar stimulation recording over the median nerve at the wrist with its ulnar comparison obtained by stimulation over the medial aspect of the palm recording over the ulnar nerve at the wrist, and each performed at a distance of 8cm.l°,26 A more stringent 0.5-msec median-ulnar difference on this transcarpal comparison was used, compared to the initially described 0.3-msec criterion to prevent inclusion of any potentially false-positive cases. 27 Median motor nerve conductions were recorded from the thenar muscles with stimulation performed at the wrist 8cm from the recording site. 28 The active electrode was placed halfway between the metacarpophalangeal joint of the thumb and the midpoint of the distal wrist crease. If the resultant motor response demonstrated an initial positive deflection, then the active electrode was relocated to adjacent sites on the thenar eminence until a response with an initial negative deflection and maximal motor amplitude was obtained. Repeat measurement was then performed to maintain an 8-cm distance between the recording site and the stimulation site at the wrist. The reference electrode was placed on the distal phalanx of the thumb. The absolute value for the upper limit of the median motor latency in our laboratories is 4.3msec. Comparative motor latency techniques between the median motor nerve with either the ipsilateral ulnar nerve or contralateral median nerve were not evaluated. For nerve conduction data, the amplitudes of all responses are measured from baseline-to-negative peak. The standard practice in our laboratories is to maintain skin temperatures above 30°C. Thermistor recordings are performed over the dorsum of the hand and hydrocollator packs used for warming the extremity when the skin temperature is found to be <30°C. In all cases, a needle examination was performed of the abductor pollicis brevis on the ipsilateral side using a tefloncoated, monopolar needle electrode made of 27- or 28-gauge stainless steel wire. The presence of denervation was defined as observation of sustained, abnormal spontaneous activity in the form of positive waves or fibrillations ranging from 0 to 4 + . Abnormalities on needle examination reflecting reinnervation, such as motor unit potentials with polyphasic appearance, or increased amplitude or duration, as well as abnormalities seen in motor unit recruitment indicative of axonal loss, were not considered for analytical purposes. Two types of analyses were applied to the data. The first type, logistic regression analysis, using STATA software, b produced odds ratios, Z scores, and 95% confidence intervals for the following independent variables: age, gender, median sensory latency, median sensory amplitude, median motor latency, and median motor amplitude. After this, a residual deviance analysis was applied and outliers that had extreme values for the independent variables were identified. A follow-up logistic regression analysis with the outliers omitted was performed for comparison with the initial group. In patients with increasingly severe median neuropathies, the median sensory and motor responses became progressively delayed. Median sensory and motor latencies can increase in
307
their value to a point at which the responses become unobtainable, resulting in latencies with a value of zero. Severe conduction block or complete axonal involvement can also result in latencies with a value of zero, To avoid this paradoxical trend in the data, an assumption was made to assign latency values for absent median sensory and motor responses that were beyond the most delayed median sensory and motor latencies found in the data. For the median sensory response, latencies of unobtainable recordings were designated a value of 10msec, whereas for the median motor response, the assigned value was determined to be 20msec. To evaluate the effect of these assumptions on the data, all values for median sensory and motor latencies were then given an ordinal classification and resubmitted to a logistic regression analysis. A second major type of analysis was then performed to determine the sensitivity of various parameters for the presence of acute denervation and to assess the number of people who could potentially be spared needle examination of the thenar muscles. Median sensory and motor conduction as well as needle electromyography data were analyzed to obtain the frequency of cases with acute denervation for all latencies and amplitudes observed. The number of cases with acute denervation was then summated for all cases that had a latency value up to any given latency. This number was expressed as a percentage of the total number of cases with denervation in the entire population. Analogously, the number of cases with denervation was also summated for all cases that had amplitude values up to any given amplitude. A percentage of the total number of cases with denervation in the entire population was then similarly calculated for all amplitudes. Additionally, the number of cases studied below any given latency or amplitude was subtracted from the total population. This was converted to a percentage of the total population and used to represent those cases that could potentially be spared a needle examination of the thenar muscles. RESULTS Of the initial 1,590 cases that met the electrical criteria for the diagnosis of median neuropathy at the wrist, 157 (9.9%) were excluded from the study for either a concomitant lower motor neuron injury (eg, radiculopathy, peripheral neuropathy, or ulnar neuropathy) or presence of anomalous innervation involving the ipsilateral median nerve (table 1). An additional 71 (4.5%) were also excluded because they refused needle examination of the thenar muscles. Of the 1,519 cases that underwent needle examination, 103 (6.6%) were found to have a concomitant lesion mimicking CTS (ie, cervical radiculopathy, peripheral neuropathy, or proximal median neuropathy). Exclusion of cases with a concomitant lower motor neuron injury as well as those refusing needle examination left a total of 1,362 cases for the purposes of data analysis. The mean age was 48.9 years (range 20 to 94 years) with a 4.9:1 ratio of women to men. On needle examination of the abductor pollieis brevis, 148 cases were found to have evidence of acute denervation (10.9% of the total). Six different factors (age, gender, median sensory latency, median sensory amplitude, median motor latency, and median motor amplitude) were entered as independent variables in a logistic regression analysis, and three were found to contribute to the prediction of denervation (table 2). From the 95% confidence intervals and Z scores, gender, median motor latency, and median motor amplitude were identified as significant independent variables. A residual deviance analysis identified 17 cases (1.2%) that were outliers with extremes in value for the independent variables measured. A repeat run of the logistic regression analysis with deletion of the 17 outliers Arch Phys Med Rehabil Vol 79, March 1998
308
CTS DENERVATION, Vennix
Table 1: Cases With Median Neuropathy at the Wrist (CTS) Excluded From Analysis for Reasons of Simultaneous Lower Motor Neuron Injury or Anomalous Innervation Anterior horn cell disease (poliomyelitis) Cervical radiculopathy C5 C5-6 C5-6-7 C6 C6-7 C6-7-8 C7 C8 Brachial plexus injury Proximal median neuropathy Anterior interosseous neuropathy Proximal ulnar neuropathy Distal ulnar neuropathy Posterior interosseous neuropathy Peripheral neuropathy Martin-Gruber anastamosis Total
1 39 10 11 2 4 5 2 2 3 1 4 1 36 2 3 57 13 157
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Fig 1. Percentage of total CTS population spared needle examination (D) and percentage of patients with denervation (11) versus median motor amplitude. For only those cases examined with a median motor amplitude less than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
motor latency of >4.5msec would need to have a needle examination of the abductor pollicis brevis and this would have spared only 26.7% (364/1,362) of cases from a needle examination. Changing the latency cutoff so that only those cases with a median motor latency of >6msec would undergo evaluation of the abductor pollicis brevis now identifies only 75% of the cases with denervation (111/148), but spares 66.5% (906/1,362) of the population. An evaluation of the median sensory amplitude in isolation (fig 3) demonstrates that a level of 95.3% sensitivity for all denervated cases is achieved only when all cases with an amplitude of <20jaV undergo needle examination. This cutoff would only have spared 26% (354/1,362) of the population. Decreasing the cutoff so that only those cases with a median sensory amplitude of < 151aV undergo evaluation of the abductor pollicis brevis would have found evidence of denervation in
Table 2: Factors Selected as Possible Predictors of Denervation in CTS as Analyzed With Logistic Regression CTS With Denewation 10,9% (n = 148) CTS Without Denervation 89,1% (n = 1,214) Noncontributing factors Age (yrs) Median sensory latency (msec) Median sensory amplitude (IJV) Contributing factors Women:men Median motor latency (msec) Median motor amplitude (mV)
55,0 _+ 14.2 (24-94) 8,0 ÷ 2.4 (3.5-10) 5 +_ 7 (0-31)
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2.3:1 8,7 + 4.4 (3.5-20) 3,0 _+ 2.5 (0-12)
5.6:1 5.4 -+ 1.6 (3.0-20) 7,7 ± 3.2 (0-20)
All values represented are mean _+ standard deviation w i t h exception of the w o m e n : m e n ratio. * p < .001; t p = ,008.
Arch Phys Med Rehabil Vol 79, March 1998
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resulted in the same three independent variables previously described now each possessing p values < .001. A pseudo R 2 value for the model was calculated to be .495, which indicated that approximately 50% of the variance in the likelihood ratio estimate of this model was accounted for by the independent variables that were introduced. A second analysis was then performed using each of the median conduction parameters in isolation, as well as combinations of median motor latency and amplitude, to maximize sensitivity and specificity. Optimization was observed when using median motor amplitude in isolation (fig 1). The graph illustrates that if only cases with a median motor amplitude of < 7 m V are evaluated with a needle examination of the abductor pollicis brevis, then 95.3 % (141/148) of all cases of denervation would have been found, and 52% (708/1,362) of patients could have been spared needle examination. Limiting the needle examination of the abductor pollicis brevis to those cases with a median motor amplitude of < 6 m V would have uncovered only 85.8% (127/148) of all cases with denervation, but would have increased the population spared to 62% (844/1,362). Alternatively, raising the cutoff so that all cases with a median motor amplitude of < 8 m V would undergo evaluation of the abductor pollicis brevis would have increased the sensitivity of denervation only to 97.3% (144/148), but the population spared would have also decreased to 39.9% (543/1,362). In comparison, when evaluating median motor latency in isolation (fig 2), in order to document the same 95.3% (141/148) of all possible denervation, any case with a median
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309
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CTS. Nowhere is this more evident than in our subset of patients from our county hospital district. These patients, who are predominantly referred from outlying clinics by primary care practitioners, now present more frequently for diagnostic testing in the early stages of CTS. They arrive more often with only intermittent, as opposed to constant, symptoms of numbness and paresthesias. The electrophysiologic abnormalities on nerve conduction studies are typically mild, and thenar atrophy is infrequent. In contrast to the decreasing frequency of denervation, some well-recognized features in CTS like age and gender distribution remain unchanged. In 1966, Phalen 3° found more than half the patients in his series to be between the ages of 40 and 60. From 1961 to 1980 the mean ages in the Rochester series 1 were found to be 5l years for women and 49.8 years for men. In our series, the mean age for 1,132 women was 49.1 years, whereas that for the 230 men was 47.9 years. This syndrome remains a very common disorder of individuals between the ages of 40 and 60, and this age group accounted for 66.5% of all cases that were diagnosed in our series. Similarly, of a total of 1,362 cases, 1,132 cases of median neuropathy occurred in women, resulting in a 4.9:1 ratio of women to men. When only the total number of patients was considered, this ratio was nearly identical (4.8:1) in a population of 1,025 patients that provided the 1,362 hands tested. Previous large studies have had women to men ratios that ranged from as low as 3:1 to as high as 10:1.1,7,31 Radecki 32 reported a larger ratio in wrist depth/width in women compared to men, as well as a larger ratio in women with a median
Fig 2. Percentage of total CTS population spared needle examination (D) and percentage of patients with denervation (11) versus median motor latency. For only those cases examined with a median motor latency more than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
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only 90.5% of the denervated cases (134/148), but would have increased the population spared to 38.9% (530/1,362). A final evaluation was performed with several combinations of median motor amplitude and latency. The optimal combination of criteria is displayed in figure 4 when all cases with a median motor amplitude of < 6 m V underwent needle examination of the abductor pollicis brevis. Of those remaining with an amplitude of >6mV, if all cases with a median motor latency of >5.5msec were additionally examined, then cumulatively 95.3 % (141 ! 148) of all denervation would have been found, and 43.6% (594/1,362) of the cases would have been spared needle examination. If, instead, of all remaining median motor conductions with an amplitude of > 6 m V only those with a median motor latency of >7msec were additionally examined, then only 89.9% (133/148) of cases with spontaneous activity would have been recognized while 58.1% (778/1,362) would have been spared. DISCUSSION The percentage of patients with CTS who exhibit denervation has diminished over the past few decades. In 1974, Buchtal and associates 29 found 50% of their study patients had denervation on needle examination. During the past decade, this has dwindled to the range of 18% to 25%. 8,1°,u This study shows a continuation of this trend, with only 10.9% of patients having evidence of fibrillations or positive waves in the abductor pollicis brevis. We believe that this trend reflects earlier detection, resulting from education of primary care physicians and the population at large about the signs and symptoms of
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Fig 3. Percentage of total CTS population spared needle examination (El) and percentage of patients with denervation (11) versus median sensory amplitude. For only those cases examined with a median sensory amplitude less than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
Arch Phys Med Rehabi| Vol 79, March 1998
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Fig 4. Percentage of total CTS population spared needle examination (C]) and percentage of patients with denervation (m} versus median motor latency in those individuals with median motor amplitude of <6mV. Of 535 cases that had a median motor amplitude of <6mV, for only those examined with a median motor latency more than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
neuropathy versus those without one. He suggested that this factor best explains the greater prevalence of CTS in women. 33 Phalen 3° has also suggested that the increase in incidence of CTS in perimenopausal women may be related to hormonal factors, but the mechanism for this is unclear. The amplitude of the motor response reflects the total number of muscle fibers that are innervated. 34 Axonal injury to the motor component of the median nerve should ultimately result in diminution of this response. Many patients with small amplitude responses show no evidence of denervation on needle examination. Frequently, this is the result of substantial conduction block occurring at the wrist. Stimulation of the median motor nerve in the palm distal to the site of conduction block can give a larger amplitude response in these cases. 35 In chronic median nerve compressions, it is also possible for denervated muscle fibers to undergo a relatively rapid reinnervation with collateral sprouting from intact axons, thus minimizing the number of muscle fibers that receive no innervation. 34 These factors likely contribute to an inability of the logistic regression model to predict denervation when analyzing the median motor amplitude in isolation. Previous literature has attempted to establish a relationship between the median motor latency and the presence of denervation. In their series of 228 patients, Murga and associates36 found that when the distal median motor latency was prolonged >7msec the needle examination was always abnormal. Their data collection differed in two respects from ours. First, the median motor response was recorded using a distance of 6cm compared to 8cm in our laboratories. This probably results in a difference in latency values of only a few tenths of a millisecArch Phys Med Rehabil Vol 79, March 1998
ond, as evidenced by the upper limits of normal of the two median motor latencies (4.2msec in their series vs 4.3msec in ours). Secondly, their criteria for the presence of an abnormal needle examination included abnormalities in the recruitment pattern, and ours did not. Our criteria were restricted to the presence of abnormal spontaneous activity for two reasons. First, the process of quantitative analysis to accurately assess motor unit amplitude, duration, and recruitment is impractical in routine cases of CTS. In addition, the presence or absence of spontaneous activity is the most important information obtained from the needle examination that is used by the surgeon when considering operative treatment. Restricting our focus to the presence or absence of spontaneous activity left 158 cases of CTS that had a motor latency of >7msec and a normal needle examination. Using the median motor latency to predict denervation would have required setting a cutoff latency in which all cases with a latency of >4.5msec would have had to undergo needle examination of the abductor pollicis brevis in order to achieve >95% sensitivity of cases with denervation (fig 2). Unfortunately, this latency value is only slightly outside the normal range and would have spared only 26.7% of the population from a needle examination. This is further reflected in figure 5 from the logistic regression analysis, which shows a suboptimal relationship between the median motor latency and the probability of acute denervation on needle examination. A much broader range of median motor latencies having a high probability of denervation can be seen on the scatterplot when compared with the range of median motor amplitudes in figure 6 with a similar high probability of denervation. Werner and Albers 37 studied the relation between nerve conduction studies and the needle examination. They identified smaller median evoked amplitudes and more prolonged median latencies in patients with abnormalities on needle electromyography, but found their logistic regression model to be poorly predictive of these abnormalities on an individual basis. Our logistic regression model explained 50% of the variance in the likelihood ratio estimate of spontaneous activity for the population of cases examined. We agree that this model is not applicable in the clinical setting for predicting denervation in individual patients. The major finding of this study resulted not from the logistic regression model, but from the second analysis. Here it was found that the median motor amplitude in isolation can predict whether patients with CTS have axonal denervation on needle examination of the abductor pollicis brevis. Figure 1 illustrates this in a clinically relevant manner. If all persons with CTS with a median motor amplitude of < 7 m V underwent needle examination of the abductor pollicis brevis, 95.3% with denervation would have been identified. Raising this cutoff value to 8mV would have increased the sensitivity only a small amount, to 97.3%. Conversely, if only patients with a median motor amplitude of <6.0mV underwent needle sampling of the abductor pollicis brevis, the sensitivity decreases to 85.8%. Figure 6, derived from the logistic regression analysis, provides an additional perspective, showing a sharp increase in probability of denervation as the values for median motor amplitude diminish. It should be noted that evidence of chronic denervation and reinnervation were not assessed, and larger motor amplitudes could occur in cases with reinnervation. Analysis of figure 1 reveals that setting a cutoff for the median motor amplitude at 7mV would have spared 52% of the entire population from needle examination of the abductor pollicis brevis. Predictably, raising the cutoff value to 8mV would have diminished the percent of spared population to
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of the median nerve in the proximal forearm can all mimic the clinical presentation of CTS. In our study, 6.6% (103/1,519) of all patients undergoing a needle examination had one of the above lesions, in addition to meeting the electrical criteria for CTS. Previous studies have shown as high as 11% incidence of cervical radiculopathy in documented cases of C T S . 38 In conclusion, the median motor amplitude can be used to predict the presence of denervation for patients with CTS. We believe that for the first time guidelines based on a large, unselected population will aid the electromyographer in justifying when the abductor pollicis brevis should be sampled. Performing a needle examination of the abductor pollicis brevis only in those individuals with a median motor amplitude of < 7 m V identifies more than 95% of patients with denervation, sparing 52% of the patients from a painful needle examination of this muscle. For individuals with a median motor amplitude of >7mV, testing a median-innervated muscle distal to the site of entrapment rarely contributes to localization of pathology in CTS. In these cases, examination of intrinsic hand muscles provides more useful information about a possible C8-T1 radiculopathy
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39.9%. If this cutoff value were decreased to 6mV, then the percentage of population spared needle examination would increase to 62%. Selection of a median motor amplitude cutoff value of 7mV allows for a high degree of sensitivity and, if employed in this population, would have spared more than half the patients from undergoing the discomfort of a needle sampling the muscles of the thenar eminence. In many patients the clinical presentation and nerve conduction studies lead to a straightforward diagnosis of a median neuropathy at the wrist. In simple cases some practitioners limit the needle examination to a sampling of the thenar muscles. If the conduction studies predicted a very low probability of identifying denervation, then omission of the needle examination would not only avoid unnecessary pain, but potentially result in a cost savings. It is important to note that omission of sampling of the abductor pollicis brevis does not imply that patients will not require any type of needle examination. The value of the needle examination resides not only in its ability to assist in evaluating the severity of a median neuropathy at the wrist, but also in differentiating this entity from other lesions. Cervical radiculopathy, peripheral neuropathy, and entrapment 1.0
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Fig6. Probability of finding the presence of abnormal spontaneous activity (denervation) on needle examination of the abductor pollicis brevis vs median motor amplitude (in millivolts).
Arch Phys Med Rehabil Vol 79, March 1998
312
CTS DENERVATION, Vennix
or peripheral neuropathy. Substitution of the ulnar-innervated first dorsal interosseous when the median motor amplitude is > 7 m V will provide comparable information to that obtained by a more painful assessment of the abductor pollicis brevis.
Acknowledgment: We dedicate this article to the memory of Charles Don Rossi, MS, who passed away on April 7, 1997. References 1. Stevens JC, Sun S, Beard CM, O'Fallon WM, Kurland LT. Carpal tunnel syndrome in Rochester, Minnesota, 1961 to 1980. Neurology 1988;38:134-8. 2. CDC morbidity and mortality weekly report, occupational disease surveillance: carpal tunnel syndrome, MMWR Morbid Mortal Wkly Rep 1989;38:485-9. 3. Tanaka S, Wild DK, Seligman PJ, Behrens V, Cameron L, Putz-Anderson V. The U.S. prevalence of self-reported carpal tunnel syndrome: 1988 National Health Interview Survey Data. Am J Public Health 1994;84:1846-8. 4. Tanaka S, Wild DK, Seligman PJ, Behrens V, Cameron L, Putz-Anderson V. Prevalence and work-relatedness of selfreported carpal tunnel syndrome among U.S. workers: analysis of the Occupational Health Supplement Data of 1988 Health Interview Survey. Am J Ind Med 1995;27:451-70. 5. Levine DW, Simmons BE Koris MJ, Daltroy LH, Hohl GG, Fossel AH, et al. A self-administered questionnaire for the assessment of severity of symptoms and functional status in carpal tunnel syndrome. J Bone Joint Surg Am 1993;75A: 1585-92. 6. Palmer DH, Hanrahan LR Social and economic costs of carpal tunnel surgery. Instr Course Lect 1995;44:167-72. 7. Phalen GS. Reflections on 21 years' experience with the carpal tunnel syndrome. JAMA 1970;212:1365-7. 8. Jackson D, Clifford JC. Electrodiagnosis of mild carpal tunnel syndrome. Arch Phys Med Rehabil 1989;70:199-204. 9. Uncini A, Di Musio A, Awad J, Manente G, Tafnro M, Gambi D. Sensitivity of three median-to-ulnar comparative tests in diagnosois of mild carpal tunnel syndrome. Muscle Nerve 1993;16: 1366-73, 10. Stevens JC. AAEE minimonograph #26: the electrodiagnosis of carpal tunnel syndrome. Muscle Nerve 1987;10:99-113. 11. Kimura I, Ayyar DR. The carpal tunnel syndrome: electrophysiological aspects of 639 symptomatic extremities. Electromyogr Clin Neurophysiol 1985 ;25:151-64. 12. Rhoades CE, Mowery CA, Gelberman RH. Results of internal neurolysis of the median nerve for severe carpal-tunnel syndrome. J Bone Joint Surg Am 1985;67A:253-6. 13. McFarland GB. Entrapment syndromes. In: McCollister EC, editor. Surgery of the musculoskeletal system. 2nd ed. New York: Churchill Livingstone; 1994. p. 961-81. 14. Tubiana R, Brockman R. General considerations in carpal tunnel syndrome. In: Tubiana R, editor. The hand--volume IV. Philadelphia (PA): WB Sannders Co.; 1993. p. 441-9. 15. Finestone HM, Woodbury GM, Collavini T, Marchuk Y, Maryniak O. Severe carpal tunnel syndrome: clinical and electrodiagnostic outcome of surgical and conservative treatment. Muscle Nerve 1996;19:237-9. 16. Nan HE, Lange B, Lange S. Prediction of outcome of decompression for carpal tunnel syndrome. J Bone Joint Surg Br 1988;13B: 391-4. 17. Meadows JC. Observations on muscle pain in man, with particular reference to pain during needle electromyography. J Neurol Neurosurg Psychiatry 1970;33:519-23. 18. Gans BM, Kraft GH. Pain perception in clinical electromyography. Arch Phys Med Rehabil 1977;58:13-6. 19. Khoshbin S, Hallett M, Lunbeck R. Predictors of patients' experience of pain in EMG. Muscle Nerve 1987; 10:629-32.
Arch Phys Med Rehabil Vol 79, March 1998
20. Spence WR, Guyton JD. Control of pain during electromyography. Arch Phys Med Rehabil 1966;47:771-4. 21. Mazur A, Wertsch JJ. Use of thenar electromyography in the evaluation of carpal tunnel syndrome. Muscle Nerve 1995;18: 1072. 22. Jablecki CK, Andary MT, Yuen TS, Wilkins DE, Williams FH. Literature review of nerve conduction studies and electromyography for the evaluation of patients with carpal tunnel syndrome. Muscle Nerve 1993;16:1392-1414. 23. Felsenthal G. Median and ulnar distal motor and sensory latencies in the same normal subject. Arch Phys Med Rehabil 1977;58:297302. 24. Felsenthal G. Carpal tunnel syndrome diagnosis. Arch Phys Med Rehabil 1979;60:90. 25. Felsenthal G, Spindler H. Palmar conduction time of median and ulnar nerves of normal subjects and patients with carpal tunnel syndrome. Am J Phys Med Rehabil 1979;58:131-8. 26. Daube JR. Percutaneous palmar median nerve stimulation for carpal tunnel syndrome. Electroencephalogr Clin Neurophysiol 1977;43:139-40. 27. Parks TJ, Wertsch JJ, Abu-Faraj ZU. Repeatability of transcarpal median and ulnar short segment latencies: considerations in delta determination [abstract]. Muscle Nerve 1994;17:1083. 28. Melvin JL, Schuchman JA, Lanese RR. Diagnostic specificity of motor and sensory nerve conduction variables in the carpal tunnel syndrome. Arch Phys Med Rehabil 1973;54:69-74. 29. Buchtal F, Rosenfalck A, Trojaborg W. Electrophysiological findings in entrapment of the median nerve at the wrist and elbow. J Neurol Neurosurg Psychiatry 1974;37:340-60. 30. Phalen GS. The carpal-tunnel syndrome: seventeen years' experience in diagnosis and treatment of six hundred fifty-four hands. J Bone Joint Surg Am 1966;48A:211-28. 31. De Krom MCTFM, Knipschild PG, Kester ADM, Thijs CJ, Boekkooi PF, Spaans E Carpal tunnel syndrome: prevalanee in the general population. J Clin Epidemiol 1992;45:373-6. 32. Radecki E A gender specific wrist ratio and the likelihood of a median abnormality at the carpal tunnel. Am J Phys Med Rehabil 1994;73:157-62. 33. Radecki R Personal factors and blood volume movement in causation of median neuropathy at the carpal tunnel. Am J Phys Med Rehabil 1996;75:235-8. 34, Dumitru D. Focal peripheral neuropathies. In: Dumitru D, editor. Electrodiagnostic medicine. Philadelphia (PA): Hanley and Belfus, Inc.; 1995. p. 851-927. 35. Pease WS, Cunningham ML, Walsh WE, Johnson EW. Determining neurapraxia in carpal tunnel syndrome. Am J Phys Med Rehabil 1988;66:117-9. 36. Murga L, Moreno JM, Men6ndez C, Castilla JM. The carpal tunnel syndrome, Relationship between median distal motor latency and graded results of needle electromyography. EIectromyogr Clin Neurophysiol 1994;34:377-84. 37. Werner RA, Albers JW. Relation between needle electromyography and nerve conduction studies in patients with carpal tunnel syndrome. Arch Phys Med Rehabil 1995 ;76:246-9. 38. Yu J, Bendler EM, Mentari A. Neurological disorders associated with carpal tunnel syndrome. Electromyogr Clin Neurophysiol 1979;19:27-32.
Suppliers a. Dynamic Engineering, 2575 West Beltline Highway, Middleton, WI 53562. b. STATA Corporation, 702 University Drive, College Station, TX 77840.
Predicting Acute Denervation in Carpal Tunnel Syndrome Michael 21. Vennix, MD, David D. Hirsh, AID, Faye Y. Chiou-Tan, MD, C. Don Rossi, MS ABSTRACT. Vennix MJ, Hirsh DD, Chiou-Tan FY, Rossi CD. Predicting acute denervation in carpal tunnel syndrome. Arch Phys Med Rehabil 1998;78:306-312.
Objective: To determine which nerve conduction parameters can predict the presence of acute denervation in carpal tunnel syndrome. Setting: The electrodiagnostic laboratories of a hospital and a county hospital district. Design: A retrospective review. Patients: A total of 1,590 consecutive cases from January 1992 to June 1996, diagnosed as having median neuropathy at the wrist. Main Outcome Measures: Evidence of acute denervation on needle electromyography of the abductor pollicis brevis and its relationship to patient age, gender, and parameters obtained from nerve conduction studies, including median sensory latency and amplitude, and median motor latency and amplitude. Results: Logistic regression analysis identified gender, median motor latency, and median motor amplitude (all p --- .008) as contributing to the prediction of denervation. Needle examination of the cases with a median motor amplitude < 7 m V detected 95.3% (141/148) of all cases with denervation and could have spared 52% (708/1,362) of the population from a needle examination of the abductor pollicis brevis. Conclusion: The median motor amplitude can predict the presence of acute denervation in the thenar muscles in median neuropathy at the wrist and possibly eliminate a painful needle examination of the median-innervated thenar muscles in over 50% of the cases.
© 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation ARPAL TUNNEL SYNDROME (CTS) results from entrapment of the median nerve within the carpal canal and C frequently causes disability. Estimates of the prevalence of this disorder range from 55 to 125 cases per 100,000 people. 1,2 Recent analyses of the 1988 National Health Interview Survey estimate the prevalence of CTS among the working US adult population at 530 cases per 1 0 0 , 0 0 0 . 3,4 Release of the carpal canal is the most common surgical procedure of the hand, with
From the Department of Physical Medicine and Rehabilitation, Baylor College of Medicine (Drs, Vennix, Hirsh, Chiou-Tan, Mr. Rossi); The Methodist Hospital (Dr. Vennix); The Harris County Hospital District (Ben Taub General Hospital, Quentin Mease Community Hospital) (Drs. Vennix, Itirsh, Chiou-Tan); and the Office of Research, Division of Biostatistics (Mr. Rossi), Houston, TX. Mr. Rossi died April 7, 1997. Submitted for publication June 4, 1997. Accepted in revised form September 29, 1997. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the authors or upon any organization with which the authors are associated. Reprint requests to Michael J. Vennix, MD, Baylor College of Medicine, Department of Physical Medicine and Rehabilitation, 6550 Fannin, Suite 1421, Houston, TX 77030. © 1998 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation 0003-9993/98/7903-453453.00/(3
Arch Phys Med Rehabil Vol 79, March 1998
estimates at more than 460,000 procedures performed each year and a direct medical cost of over $1.9 billion. 5,6 Electrodiagnostic testing with nerve conduction studies and needle electromyography is recognized as an objective tool which, combined with the clinical presentation, can substantially improve diagnostic reliability in C T S . 7 Refinements in nerve conduction studies over the past two decades have improved their sensitivity in diagnosis of median nerve compression. 8,9 These tests aid the physician in evaluating the physiologic integrity of the median nerve, determining prognosis, and planning treatment. In severe cases, abnormal spontaneous activity in the folrn of positive waves and fibrillation potentials can be observed on needle electromyography. However, needle examination of the thenar muscles is an insensitive means of detecting median neuropathy at the wrist. Previous studies reported denervation present in only 18% to 25% of cases that undergo electrodiagnostic testing. 8,1°,11 The usefulness of needle examination of the thenar muscles lies in the evaluation of the severity of median nerve injury and demonstration of axonal degeneration. The presence of clinical weakness in thumb abduction, visible evidence of atrophy in the thenar eminence, and findings of denervation on needle electrornyography of these muscles are all manifestations of a severe median neuropathy and may be indications for surgery in patients with C T S . 12"14 Experience has shown that patients with these findings can obtain relief from their symptoms of numbness and paresthesias after surgery, although they tend to have poor recovery of their motor deficits. 15,16 Despite technical advances, including teflon coating of needle electrodes and thinner needles (27 to 28 gauge), pain and discomfort during needle examination remain the most negative aspects of electrodiagnostic testing for the patient. 17q9 Studies have found the thenar eminence to be one of the most painful muscle groups to evaluate. 2° In the authors' experience, assessment of these muscles is poorly tolerated by many patients. In a recent survey of 110 electromyographers, 74% performed needle examination of the abductor pollicis brevis as a standard part of the electrodiagnostic testing in patients undergoing evaluation for presumed CTS. 2t If the findings of denervation could be predicted from the nerve conduction studies, it might be possible to spare a sizable percentage of patients from a painful needle examination of this muscle. We therefore undertook a retrospective review of patients examined during a 3.5-year period in multiple electrodiagnostic laboratories to evaluate the practicality of predicting denervation in CTS.
PATIENTS AND METHODS The electrodiagnostic laboratories of our hospital and local county hospital district evaluate more than 1,600 patients each year for lower motor neuron injuries. The most common request is evaluation for CTS. After approval from our institutional review board, all records between January 1992 and June 1996 were examined. Patients diagnosed with a nontraumatic median neuropathy at the wrist were included. Testing in all laboratories was performed with a Nomad LE EMG machine, a The information obtained for all cases included age, gender, median sensory latency, median sensory amplitude, median motor latency, median motor amplitude, and the
CTS DENERVATION, Vennix
presence or absence of denervation on needle examination of the abductor pollicis brevis. The electrical diagnosis of median neuropathy at the wrist was made if at least one of the following two criteria from the clinical practice recommendations of the American Association of Electrodiagnostic Medicine was met22: (1) relative prolongation of the standard median sensory peak latency by 0.5msec compared to the peak latency of the standard ulnar sensory response, and each examination was performed at a distance of 14cm23-25; or (2) relative prolongation of the short-segment, transcarpal median response by 0.5msec with palmar stimulation recording over the median nerve at the wrist with its ulnar comparison obtained by stimulation over the medial aspect of the palm recording over the ulnar nerve at the wrist, and each performed at a distance of 8cm.l°,26 A more stringent 0.5-msec median-ulnar difference on this transcarpal comparison was used, compared to the initially described 0.3-msec criterion to prevent inclusion of any potentially false-positive cases. 27 Median motor nerve conductions were recorded from the thenar muscles with stimulation performed at the wrist 8cm from the recording site. 28 The active electrode was placed halfway between the metacarpophalangeal joint of the thumb and the midpoint of the distal wrist crease. If the resultant motor response demonstrated an initial positive deflection, then the active electrode was relocated to adjacent sites on the thenar eminence until a response with an initial negative deflection and maximal motor amplitude was obtained. Repeat measurement was then performed to maintain an 8-cm distance between the recording site and the stimulation site at the wrist. The reference electrode was placed on the distal phalanx of the thumb. The absolute value for the upper limit of the median motor latency in our laboratories is 4.3msec. Comparative motor latency techniques between the median motor nerve with either the ipsilateral ulnar nerve or contralateral median nerve were not evaluated. For nerve conduction data, the amplitudes of all responses are measured from baseline-to-negative peak. The standard practice in our laboratories is to maintain skin temperatures above 30°C. Thermistor recordings are performed over the dorsum of the hand and hydrocollator packs used for warming the extremity when the skin temperature is found to be <30°C. In all cases, a needle examination was performed of the abductor pollicis brevis on the ipsilateral side using a tefloncoated, monopolar needle electrode made of 27- or 28-gauge stainless steel wire. The presence of denervation was defined as observation of sustained, abnormal spontaneous activity in the form of positive waves or fibrillations ranging from 0 to 4 + . Abnormalities on needle examination reflecting reinnervation, such as motor unit potentials with polyphasic appearance, or increased amplitude or duration, as well as abnormalities seen in motor unit recruitment indicative of axonal loss, were not considered for analytical purposes. Two types of analyses were applied to the data. The first type, logistic regression analysis, using STATA software, b produced odds ratios, Z scores, and 95% confidence intervals for the following independent variables: age, gender, median sensory latency, median sensory amplitude, median motor latency, and median motor amplitude. After this, a residual deviance analysis was applied and outliers that had extreme values for the independent variables were identified. A follow-up logistic regression analysis with the outliers omitted was performed for comparison with the initial group. In patients with increasingly severe median neuropathies, the median sensory and motor responses became progressively delayed. Median sensory and motor latencies can increase in
307
their value to a point at which the responses become unobtainable, resulting in latencies with a value of zero. Severe conduction block or complete axonal involvement can also result in latencies with a value of zero, To avoid this paradoxical trend in the data, an assumption was made to assign latency values for absent median sensory and motor responses that were beyond the most delayed median sensory and motor latencies found in the data. For the median sensory response, latencies of unobtainable recordings were designated a value of 10msec, whereas for the median motor response, the assigned value was determined to be 20msec. To evaluate the effect of these assumptions on the data, all values for median sensory and motor latencies were then given an ordinal classification and resubmitted to a logistic regression analysis. A second major type of analysis was then performed to determine the sensitivity of various parameters for the presence of acute denervation and to assess the number of people who could potentially be spared needle examination of the thenar muscles. Median sensory and motor conduction as well as needle electromyography data were analyzed to obtain the frequency of cases with acute denervation for all latencies and amplitudes observed. The number of cases with acute denervation was then summated for all cases that had a latency value up to any given latency. This number was expressed as a percentage of the total number of cases with denervation in the entire population. Analogously, the number of cases with denervation was also summated for all cases that had amplitude values up to any given amplitude. A percentage of the total number of cases with denervation in the entire population was then similarly calculated for all amplitudes. Additionally, the number of cases studied below any given latency or amplitude was subtracted from the total population. This was converted to a percentage of the total population and used to represent those cases that could potentially be spared a needle examination of the thenar muscles. RESULTS Of the initial 1,590 cases that met the electrical criteria for the diagnosis of median neuropathy at the wrist, 157 (9.9%) were excluded from the study for either a concomitant lower motor neuron injury (eg, radiculopathy, peripheral neuropathy, or ulnar neuropathy) or presence of anomalous innervation involving the ipsilateral median nerve (table 1). An additional 71 (4.5%) were also excluded because they refused needle examination of the thenar muscles. Of the 1,519 cases that underwent needle examination, 103 (6.6%) were found to have a concomitant lesion mimicking CTS (ie, cervical radiculopathy, peripheral neuropathy, or proximal median neuropathy). Exclusion of cases with a concomitant lower motor neuron injury as well as those refusing needle examination left a total of 1,362 cases for the purposes of data analysis. The mean age was 48.9 years (range 20 to 94 years) with a 4.9:1 ratio of women to men. On needle examination of the abductor pollieis brevis, 148 cases were found to have evidence of acute denervation (10.9% of the total). Six different factors (age, gender, median sensory latency, median sensory amplitude, median motor latency, and median motor amplitude) were entered as independent variables in a logistic regression analysis, and three were found to contribute to the prediction of denervation (table 2). From the 95% confidence intervals and Z scores, gender, median motor latency, and median motor amplitude were identified as significant independent variables. A residual deviance analysis identified 17 cases (1.2%) that were outliers with extremes in value for the independent variables measured. A repeat run of the logistic regression analysis with deletion of the 17 outliers Arch Phys Med Rehabil Vol 79, March 1998
308
CTS DENERVATION, Vennix
Table 1: Cases With Median Neuropathy at the Wrist (CTS) Excluded From Analysis for Reasons of Simultaneous Lower Motor Neuron Injury or Anomalous Innervation Anterior horn cell disease (poliomyelitis) Cervical radiculopathy C5 C5-6 C5-6-7 C6 C6-7 C6-7-8 C7 C8 Brachial plexus injury Proximal median neuropathy Anterior interosseous neuropathy Proximal ulnar neuropathy Distal ulnar neuropathy Posterior interosseous neuropathy Peripheral neuropathy Martin-Gruber anastamosis Total
1 39 10 11 2 4 5 2 2 3 1 4 1 36 2 3 57 13 157
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Fig 1. Percentage of total CTS population spared needle examination (D) and percentage of patients with denervation (11) versus median motor amplitude. For only those cases examined with a median motor amplitude less than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
motor latency of >4.5msec would need to have a needle examination of the abductor pollicis brevis and this would have spared only 26.7% (364/1,362) of cases from a needle examination. Changing the latency cutoff so that only those cases with a median motor latency of >6msec would undergo evaluation of the abductor pollicis brevis now identifies only 75% of the cases with denervation (111/148), but spares 66.5% (906/1,362) of the population. An evaluation of the median sensory amplitude in isolation (fig 3) demonstrates that a level of 95.3% sensitivity for all denervated cases is achieved only when all cases with an amplitude of <20jaV undergo needle examination. This cutoff would only have spared 26% (354/1,362) of the population. Decreasing the cutoff so that only those cases with a median sensory amplitude of < 151aV undergo evaluation of the abductor pollicis brevis would have found evidence of denervation in
Table 2: Factors Selected as Possible Predictors of Denervation in CTS as Analyzed With Logistic Regression CTS With Denewation 10,9% (n = 148) CTS Without Denervation 89,1% (n = 1,214) Noncontributing factors Age (yrs) Median sensory latency (msec) Median sensory amplitude (IJV) Contributing factors Women:men Median motor latency (msec) Median motor amplitude (mV)
55,0 _+ 14.2 (24-94) 8,0 ÷ 2.4 (3.5-10) 5 +_ 7 (0-31)
48.2 -+ 11.5 (20-94) 5.5 -- 2.2 (3.0-10) 16 +_ 13 (0-78)
2.3:1 8,7 + 4.4 (3.5-20) 3,0 _+ 2.5 (0-12)
5.6:1 5.4 -+ 1.6 (3.0-20) 7,7 ± 3.2 (0-20)
All values represented are mean _+ standard deviation w i t h exception of the w o m e n : m e n ratio. * p < .001; t p = ,008.
Arch Phys Med Rehabil Vol 79, March 1998
~ .--.
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resulted in the same three independent variables previously described now each possessing p values < .001. A pseudo R 2 value for the model was calculated to be .495, which indicated that approximately 50% of the variance in the likelihood ratio estimate of this model was accounted for by the independent variables that were introduced. A second analysis was then performed using each of the median conduction parameters in isolation, as well as combinations of median motor latency and amplitude, to maximize sensitivity and specificity. Optimization was observed when using median motor amplitude in isolation (fig 1). The graph illustrates that if only cases with a median motor amplitude of < 7 m V are evaluated with a needle examination of the abductor pollicis brevis, then 95.3 % (141/148) of all cases of denervation would have been found, and 52% (708/1,362) of patients could have been spared needle examination. Limiting the needle examination of the abductor pollicis brevis to those cases with a median motor amplitude of < 6 m V would have uncovered only 85.8% (127/148) of all cases with denervation, but would have increased the population spared to 62% (844/1,362). Alternatively, raising the cutoff so that all cases with a median motor amplitude of < 8 m V would undergo evaluation of the abductor pollicis brevis would have increased the sensitivity of denervation only to 97.3% (144/148), but the population spared would have also decreased to 39.9% (543/1,362). In comparison, when evaluating median motor latency in isolation (fig 2), in order to document the same 95.3% (141/148) of all possible denervation, any case with a median
95.0%
All Cases of CTS 100% (n = 1,362)
48.9 _+ 12 (20-94) 5.8 _+ 2.3 (3-10) 14 ~ 13 (0-78) 4.9:1" 5.8 _+ 2.3 (3-20) t 7.2 ÷ 3.5 (0-20)*
309
CTS DENERVATION, Vennix 100.0%
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CTS. Nowhere is this more evident than in our subset of patients from our county hospital district. These patients, who are predominantly referred from outlying clinics by primary care practitioners, now present more frequently for diagnostic testing in the early stages of CTS. They arrive more often with only intermittent, as opposed to constant, symptoms of numbness and paresthesias. The electrophysiologic abnormalities on nerve conduction studies are typically mild, and thenar atrophy is infrequent. In contrast to the decreasing frequency of denervation, some well-recognized features in CTS like age and gender distribution remain unchanged. In 1966, Phalen 3° found more than half the patients in his series to be between the ages of 40 and 60. From 1961 to 1980 the mean ages in the Rochester series 1 were found to be 5l years for women and 49.8 years for men. In our series, the mean age for 1,132 women was 49.1 years, whereas that for the 230 men was 47.9 years. This syndrome remains a very common disorder of individuals between the ages of 40 and 60, and this age group accounted for 66.5% of all cases that were diagnosed in our series. Similarly, of a total of 1,362 cases, 1,132 cases of median neuropathy occurred in women, resulting in a 4.9:1 ratio of women to men. When only the total number of patients was considered, this ratio was nearly identical (4.8:1) in a population of 1,025 patients that provided the 1,362 hands tested. Previous large studies have had women to men ratios that ranged from as low as 3:1 to as high as 10:1.1,7,31 Radecki 32 reported a larger ratio in wrist depth/width in women compared to men, as well as a larger ratio in women with a median
Fig 2. Percentage of total CTS population spared needle examination (D) and percentage of patients with denervation (11) versus median motor latency. For only those cases examined with a median motor latency more than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
80.0% -r~ .
.
.
.
.
.
"r" 100.0%
95.0% 70.0% 90.0%
only 90.5% of the denervated cases (134/148), but would have increased the population spared to 38.9% (530/1,362). A final evaluation was performed with several combinations of median motor amplitude and latency. The optimal combination of criteria is displayed in figure 4 when all cases with a median motor amplitude of < 6 m V underwent needle examination of the abductor pollicis brevis. Of those remaining with an amplitude of >6mV, if all cases with a median motor latency of >5.5msec were additionally examined, then cumulatively 95.3 % (141 ! 148) of all denervation would have been found, and 43.6% (594/1,362) of the cases would have been spared needle examination. If, instead, of all remaining median motor conductions with an amplitude of > 6 m V only those with a median motor latency of >7msec were additionally examined, then only 89.9% (133/148) of cases with spontaneous activity would have been recognized while 58.1% (778/1,362) would have been spared. DISCUSSION The percentage of patients with CTS who exhibit denervation has diminished over the past few decades. In 1974, Buchtal and associates 29 found 50% of their study patients had denervation on needle examination. During the past decade, this has dwindled to the range of 18% to 25%. 8,1°,u This study shows a continuation of this trend, with only 10.9% of patients having evidence of fibrillations or positive waves in the abductor pollicis brevis. We believe that this trend reflects earlier detection, resulting from education of primary care physicians and the population at large about the signs and symptoms of
E LU 60.0%
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Fig 3. Percentage of total CTS population spared needle examination (El) and percentage of patients with denervation (11) versus median sensory amplitude. For only those cases examined with a median sensory amplitude less than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
Arch Phys Med Rehabi| Vol 79, March 1998
310
CTS DENERVATION, Vennix
70,0%
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60.0% 98.0%
g E 50.0% 96.0%
._g
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Fig 4. Percentage of total CTS population spared needle examination (C]) and percentage of patients with denervation (m} versus median motor latency in those individuals with median motor amplitude of <6mV. Of 535 cases that had a median motor amplitude of <6mV, for only those examined with a median motor latency more than the given value on the x-axis, the corresponding curves provide the percentage of the population that was found to have evidence of denervation (148 cases) and the percentage of total cases (1,362) that could have been spared a needle examination of the abductor pollicis brevis.
neuropathy versus those without one. He suggested that this factor best explains the greater prevalence of CTS in women. 33 Phalen 3° has also suggested that the increase in incidence of CTS in perimenopausal women may be related to hormonal factors, but the mechanism for this is unclear. The amplitude of the motor response reflects the total number of muscle fibers that are innervated. 34 Axonal injury to the motor component of the median nerve should ultimately result in diminution of this response. Many patients with small amplitude responses show no evidence of denervation on needle examination. Frequently, this is the result of substantial conduction block occurring at the wrist. Stimulation of the median motor nerve in the palm distal to the site of conduction block can give a larger amplitude response in these cases. 35 In chronic median nerve compressions, it is also possible for denervated muscle fibers to undergo a relatively rapid reinnervation with collateral sprouting from intact axons, thus minimizing the number of muscle fibers that receive no innervation. 34 These factors likely contribute to an inability of the logistic regression model to predict denervation when analyzing the median motor amplitude in isolation. Previous literature has attempted to establish a relationship between the median motor latency and the presence of denervation. In their series of 228 patients, Murga and associates36 found that when the distal median motor latency was prolonged >7msec the needle examination was always abnormal. Their data collection differed in two respects from ours. First, the median motor response was recorded using a distance of 6cm compared to 8cm in our laboratories. This probably results in a difference in latency values of only a few tenths of a millisecArch Phys Med Rehabil Vol 79, March 1998
ond, as evidenced by the upper limits of normal of the two median motor latencies (4.2msec in their series vs 4.3msec in ours). Secondly, their criteria for the presence of an abnormal needle examination included abnormalities in the recruitment pattern, and ours did not. Our criteria were restricted to the presence of abnormal spontaneous activity for two reasons. First, the process of quantitative analysis to accurately assess motor unit amplitude, duration, and recruitment is impractical in routine cases of CTS. In addition, the presence or absence of spontaneous activity is the most important information obtained from the needle examination that is used by the surgeon when considering operative treatment. Restricting our focus to the presence or absence of spontaneous activity left 158 cases of CTS that had a motor latency of >7msec and a normal needle examination. Using the median motor latency to predict denervation would have required setting a cutoff latency in which all cases with a latency of >4.5msec would have had to undergo needle examination of the abductor pollicis brevis in order to achieve >95% sensitivity of cases with denervation (fig 2). Unfortunately, this latency value is only slightly outside the normal range and would have spared only 26.7% of the population from a needle examination. This is further reflected in figure 5 from the logistic regression analysis, which shows a suboptimal relationship between the median motor latency and the probability of acute denervation on needle examination. A much broader range of median motor latencies having a high probability of denervation can be seen on the scatterplot when compared with the range of median motor amplitudes in figure 6 with a similar high probability of denervation. Werner and Albers 37 studied the relation between nerve conduction studies and the needle examination. They identified smaller median evoked amplitudes and more prolonged median latencies in patients with abnormalities on needle electromyography, but found their logistic regression model to be poorly predictive of these abnormalities on an individual basis. Our logistic regression model explained 50% of the variance in the likelihood ratio estimate of spontaneous activity for the population of cases examined. We agree that this model is not applicable in the clinical setting for predicting denervation in individual patients. The major finding of this study resulted not from the logistic regression model, but from the second analysis. Here it was found that the median motor amplitude in isolation can predict whether patients with CTS have axonal denervation on needle examination of the abductor pollicis brevis. Figure 1 illustrates this in a clinically relevant manner. If all persons with CTS with a median motor amplitude of < 7 m V underwent needle examination of the abductor pollicis brevis, 95.3% with denervation would have been identified. Raising this cutoff value to 8mV would have increased the sensitivity only a small amount, to 97.3%. Conversely, if only patients with a median motor amplitude of <6.0mV underwent needle sampling of the abductor pollicis brevis, the sensitivity decreases to 85.8%. Figure 6, derived from the logistic regression analysis, provides an additional perspective, showing a sharp increase in probability of denervation as the values for median motor amplitude diminish. It should be noted that evidence of chronic denervation and reinnervation were not assessed, and larger motor amplitudes could occur in cases with reinnervation. Analysis of figure 1 reveals that setting a cutoff for the median motor amplitude at 7mV would have spared 52% of the entire population from needle examination of the abductor pollicis brevis. Predictably, raising the cutoff value to 8mV would have diminished the percent of spared population to
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of the median nerve in the proximal forearm can all mimic the clinical presentation of CTS. In our study, 6.6% (103/1,519) of all patients undergoing a needle examination had one of the above lesions, in addition to meeting the electrical criteria for CTS. Previous studies have shown as high as 11% incidence of cervical radiculopathy in documented cases of C T S . 38 In conclusion, the median motor amplitude can be used to predict the presence of denervation for patients with CTS. We believe that for the first time guidelines based on a large, unselected population will aid the electromyographer in justifying when the abductor pollicis brevis should be sampled. Performing a needle examination of the abductor pollicis brevis only in those individuals with a median motor amplitude of < 7 m V identifies more than 95% of patients with denervation, sparing 52% of the patients from a painful needle examination of this muscle. For individuals with a median motor amplitude of >7mV, testing a median-innervated muscle distal to the site of entrapment rarely contributes to localization of pathology in CTS. In these cases, examination of intrinsic hand muscles provides more useful information about a possible C8-T1 radiculopathy
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39.9%. If this cutoff value were decreased to 6mV, then the percentage of population spared needle examination would increase to 62%. Selection of a median motor amplitude cutoff value of 7mV allows for a high degree of sensitivity and, if employed in this population, would have spared more than half the patients from undergoing the discomfort of a needle sampling the muscles of the thenar eminence. In many patients the clinical presentation and nerve conduction studies lead to a straightforward diagnosis of a median neuropathy at the wrist. In simple cases some practitioners limit the needle examination to a sampling of the thenar muscles. If the conduction studies predicted a very low probability of identifying denervation, then omission of the needle examination would not only avoid unnecessary pain, but potentially result in a cost savings. It is important to note that omission of sampling of the abductor pollicis brevis does not imply that patients will not require any type of needle examination. The value of the needle examination resides not only in its ability to assist in evaluating the severity of a median neuropathy at the wrist, but also in differentiating this entity from other lesions. Cervical radiculopathy, peripheral neuropathy, and entrapment 1.0
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Arch Phys Med Rehabil Vol 79, March 1998
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or peripheral neuropathy. Substitution of the ulnar-innervated first dorsal interosseous when the median motor amplitude is > 7 m V will provide comparable information to that obtained by a more painful assessment of the abductor pollicis brevis.
Acknowledgment: We dedicate this article to the memory of Charles Don Rossi, MS, who passed away on April 7, 1997. References 1. Stevens JC, Sun S, Beard CM, O'Fallon WM, Kurland LT. Carpal tunnel syndrome in Rochester, Minnesota, 1961 to 1980. Neurology 1988;38:134-8. 2. CDC morbidity and mortality weekly report, occupational disease surveillance: carpal tunnel syndrome, MMWR Morbid Mortal Wkly Rep 1989;38:485-9. 3. Tanaka S, Wild DK, Seligman PJ, Behrens V, Cameron L, Putz-Anderson V. The U.S. prevalence of self-reported carpal tunnel syndrome: 1988 National Health Interview Survey Data. Am J Public Health 1994;84:1846-8. 4. Tanaka S, Wild DK, Seligman PJ, Behrens V, Cameron L, Putz-Anderson V. Prevalence and work-relatedness of selfreported carpal tunnel syndrome among U.S. workers: analysis of the Occupational Health Supplement Data of 1988 Health Interview Survey. Am J Ind Med 1995;27:451-70. 5. Levine DW, Simmons BE Koris MJ, Daltroy LH, Hohl GG, Fossel AH, et al. A self-administered questionnaire for the assessment of severity of symptoms and functional status in carpal tunnel syndrome. J Bone Joint Surg Am 1993;75A: 1585-92. 6. Palmer DH, Hanrahan LR Social and economic costs of carpal tunnel surgery. Instr Course Lect 1995;44:167-72. 7. Phalen GS. Reflections on 21 years' experience with the carpal tunnel syndrome. JAMA 1970;212:1365-7. 8. Jackson D, Clifford JC. Electrodiagnosis of mild carpal tunnel syndrome. Arch Phys Med Rehabil 1989;70:199-204. 9. Uncini A, Di Musio A, Awad J, Manente G, Tafnro M, Gambi D. Sensitivity of three median-to-ulnar comparative tests in diagnosois of mild carpal tunnel syndrome. Muscle Nerve 1993;16: 1366-73, 10. Stevens JC. AAEE minimonograph #26: the electrodiagnosis of carpal tunnel syndrome. Muscle Nerve 1987;10:99-113. 11. Kimura I, Ayyar DR. The carpal tunnel syndrome: electrophysiological aspects of 639 symptomatic extremities. Electromyogr Clin Neurophysiol 1985 ;25:151-64. 12. Rhoades CE, Mowery CA, Gelberman RH. Results of internal neurolysis of the median nerve for severe carpal-tunnel syndrome. J Bone Joint Surg Am 1985;67A:253-6. 13. McFarland GB. Entrapment syndromes. In: McCollister EC, editor. Surgery of the musculoskeletal system. 2nd ed. New York: Churchill Livingstone; 1994. p. 961-81. 14. Tubiana R, Brockman R. General considerations in carpal tunnel syndrome. In: Tubiana R, editor. The hand--volume IV. Philadelphia (PA): WB Sannders Co.; 1993. p. 441-9. 15. Finestone HM, Woodbury GM, Collavini T, Marchuk Y, Maryniak O. Severe carpal tunnel syndrome: clinical and electrodiagnostic outcome of surgical and conservative treatment. Muscle Nerve 1996;19:237-9. 16. Nan HE, Lange B, Lange S. Prediction of outcome of decompression for carpal tunnel syndrome. J Bone Joint Surg Br 1988;13B: 391-4. 17. Meadows JC. Observations on muscle pain in man, with particular reference to pain during needle electromyography. J Neurol Neurosurg Psychiatry 1970;33:519-23. 18. Gans BM, Kraft GH. Pain perception in clinical electromyography. Arch Phys Med Rehabil 1977;58:13-6. 19. Khoshbin S, Hallett M, Lunbeck R. Predictors of patients' experience of pain in EMG. Muscle Nerve 1987; 10:629-32.
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20. Spence WR, Guyton JD. Control of pain during electromyography. Arch Phys Med Rehabil 1966;47:771-4. 21. Mazur A, Wertsch JJ. Use of thenar electromyography in the evaluation of carpal tunnel syndrome. Muscle Nerve 1995;18: 1072. 22. Jablecki CK, Andary MT, Yuen TS, Wilkins DE, Williams FH. Literature review of nerve conduction studies and electromyography for the evaluation of patients with carpal tunnel syndrome. Muscle Nerve 1993;16:1392-1414. 23. Felsenthal G. Median and ulnar distal motor and sensory latencies in the same normal subject. Arch Phys Med Rehabil 1977;58:297302. 24. Felsenthal G. Carpal tunnel syndrome diagnosis. Arch Phys Med Rehabil 1979;60:90. 25. Felsenthal G, Spindler H. Palmar conduction time of median and ulnar nerves of normal subjects and patients with carpal tunnel syndrome. Am J Phys Med Rehabil 1979;58:131-8. 26. Daube JR. Percutaneous palmar median nerve stimulation for carpal tunnel syndrome. Electroencephalogr Clin Neurophysiol 1977;43:139-40. 27. Parks TJ, Wertsch JJ, Abu-Faraj ZU. Repeatability of transcarpal median and ulnar short segment latencies: considerations in delta determination [abstract]. Muscle Nerve 1994;17:1083. 28. Melvin JL, Schuchman JA, Lanese RR. Diagnostic specificity of motor and sensory nerve conduction variables in the carpal tunnel syndrome. Arch Phys Med Rehabil 1973;54:69-74. 29. Buchtal F, Rosenfalck A, Trojaborg W. Electrophysiological findings in entrapment of the median nerve at the wrist and elbow. J Neurol Neurosurg Psychiatry 1974;37:340-60. 30. Phalen GS. The carpal-tunnel syndrome: seventeen years' experience in diagnosis and treatment of six hundred fifty-four hands. J Bone Joint Surg Am 1966;48A:211-28. 31. De Krom MCTFM, Knipschild PG, Kester ADM, Thijs CJ, Boekkooi PF, Spaans E Carpal tunnel syndrome: prevalanee in the general population. J Clin Epidemiol 1992;45:373-6. 32. Radecki E A gender specific wrist ratio and the likelihood of a median abnormality at the carpal tunnel. Am J Phys Med Rehabil 1994;73:157-62. 33. Radecki R Personal factors and blood volume movement in causation of median neuropathy at the carpal tunnel. Am J Phys Med Rehabil 1996;75:235-8. 34, Dumitru D. Focal peripheral neuropathies. In: Dumitru D, editor. Electrodiagnostic medicine. Philadelphia (PA): Hanley and Belfus, Inc.; 1995. p. 851-927. 35. Pease WS, Cunningham ML, Walsh WE, Johnson EW. Determining neurapraxia in carpal tunnel syndrome. Am J Phys Med Rehabil 1988;66:117-9. 36. Murga L, Moreno JM, Men6ndez C, Castilla JM. The carpal tunnel syndrome, Relationship between median distal motor latency and graded results of needle electromyography. EIectromyogr Clin Neurophysiol 1994;34:377-84. 37. Werner RA, Albers JW. Relation between needle electromyography and nerve conduction studies in patients with carpal tunnel syndrome. Arch Phys Med Rehabil 1995 ;76:246-9. 38. Yu J, Bendler EM, Mentari A. Neurological disorders associated with carpal tunnel syndrome. Electromyogr Clin Neurophysiol 1979;19:27-32.
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