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Prevalent involvement of thenar motor fibres in vineyard workers with carpal tunnel syndrome
Clinical Neurophysiology 121 (2010) 1251–1255
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Prevalent involvement of thenar motor fibres in vineyard workers with carpal tunnel syndrome M. Mondelli a,*, A. Baldasseroni b, A. Aretini a, F. Ginanneschi c, L. Padua d a
EMG Service, Local Health Unit 7, Siena, Italy Epidemiology Unit, Local Health Unit 10, Florence, Italy c Dept. Neurological, Neurosurgical and Behavioural Sciences, Siena University, Siena, Italy d Dept. Neurosciences, ‘‘Catholic” University and ‘‘don Grocchi” Foundation, Italy b
a r t i c l e
i n f o
Article history: Accepted 16 February 2010 Available online 15 March 2010 Keywords: Carpal tunnel syndrome Repetitive strain injury Thenar motor branch Vineyard workers
a b s t r a c t Objective: Carpal tunnel syndrome (CTS) has a high prevalence in agricultural workers, especially those engaged in vineyards. We postulated that vineyard CTS was electrophysiologically different from CTS of other subjects. We performed a retrospective cross-sectional electrophysiological study of two cohorts of consecutive patients with CTS, the first consisting of vineyard workers and the second, of other unselected types of workers, housewives and pensioners. Methods: Thirty-three vineyard workers (mean age 46.8 years, 42% women) and 205 patients with other occupations (mean age 53.7 years; 66% women) were enrolled. All patients underwent sensory and motor neurography of the median and ulnar nerves. Differences in demographic and electrophysiological findings between groups were calculated and multiple linear regression analysis was performed to eliminate the influence of potential confounding factors (age, sex, BMI, clinical severity of CTS) on the results of univariate difference analysis. Results: Univariate analysis showed that DML was longer and compound muscle action potential amplitude of the median nerve, recorded from the abductor pollicis brevis muscle, was smaller in vineyard workers than in the other CTS patients. These differences remained significant after adjusting the results for confounding factors. Conclusions: The vineyard workers showed a different pattern of CTS than the other patients: thenar motor fibres were more affected, presumably due to chronic compression on the thenar branch. This suggests an association between ‘‘common” CTS and thenar mononeuropathy. Significance: Occupational physiologists should clarify the mechanisms of neuromuscular engagement in particular jobs and ergonomists design suitable working tools, because many ‘‘individual” risk factors are difficult to change, but workplace-related risk factors can be modified. Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Carpal tunnel syndrome (CTS) is the most frequent focal neuropathy and the prevalence is particularly high in certain categories of manual workers (Rossignol et al., 1997; Palmer et al., 2007; Mattioli et al., 2009a). The risk factors associated with CTS include works that involve repeated forced movements of flexion–extension of the wrist and fingers with incongrue posture and use of vibrating instruments (Gell et al., 2005; Werner, 2006; Roquelaure et al., 2008a; Mattioli et al., 2009b).
* Corresponding author. Address: Servizio di EMG, ASL 7, Via Pian d’Ovile, 9, 53100 Siena, Italy. Tel.: +39 0577 535904; fax: +39 0577 535983. E-mail address: [email protected] (M. Mondelli).
Over the past 13 years, our outpatient EMG service has seen 28 consecutive cases of idiopathic mononeuropathy of thenar motor branch of the median nerve, eight of whom were agricultural workers engaged exclusively in the vineyards. We postulated that this particular type of work could be responsible for thenar motor neuropathy through direct compression on the motor branch of the median nerve (Mondelli et al., 2009). The aim of the present study was to determine whether CTS in vineyard workers was electrophysiologically different from that of other subjects, with particular attention to involvement of thenar motor fibres. We performed a retrospective cross-sectional study comparing electrophysiological findings of a cohort of consecutive vineyard workers with a cohort of consecutive unselected workers in other fields, pensioners and housewives, all with idiopathic CTS.
1388-2457/$36.00 Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2010.02.150
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2. Patients From 2 January 2006 to 31 December 2008, CTS was diagnosed for the first time in 1809 patients admitted consecutively to the EMG service of Local Health Unit No. 7 of Siena, Italy. This service is only open to unselected outpatients. The diagnosis of CTS was made on the basis of clinical findings and delayed distal conduction velocity of the median nerve (for details see Section 3). For clinical diagnosis, the inclusion criteria were those recommended by AAN (1993). All symptomatic hands were included in the ‘‘classic/probable” or ‘‘possible” categories according to the Katz hand diagram, as modified by the consensus document on the classification of CTS (Katz et al., 1990; Rempel et al., 1998). From 1809 patients with CTS, 33 consecutive cases were selected (1.8% of all patients with CTS: 14 women, 19 men, mean age 46.8 ± 11.7 years, range 29–70, median 46) on the basis of their work: all were agricultural workers engaged exclusively in vineyards and they did all the manual operations pertaining to the annual cycle of grape vines. By way of comparison, we enrolled 205 consecutive patients with idiopathic CTS presenting at the same service from 2 January to 31 May 2008; 136 were women and 69 men, mean age 53.7 ± 15.3 years (range 21–86, median 53). There were 81 blue-collar workers, 53 white-collar workers, 30 pensioners and 41 housewives. All subjects with diabetes, diseases of connective tissue or thyroid, renal failure, gout, onset of symptoms during pregnancy or lactation or <3 months after hand or wrist trauma with or without fracture were excluded in both groups. All patients with associated polyneuropathies and other mononeuropathies or radiculopathies of the upper limb were also excluded. Only patients with distal mononeuropathy of the median nerve diagnosed as ‘‘idiopathic” CTS were enrolled. Clinical severity of CTS was graded with a validated ordinal scale from 1 to 5, which considered symptoms and sensory changes and strength/trophism of the thenar muscles. The details of this scale, which is routinely used by Italian CTS study group, have already been reported (Giannini et al., 2002).
3. Electrophysiological methods The ‘‘standard” electrophysiological protocol was performed in the symptomatic hands of all CTS patients. In cases with bilateral CTS, only the results of the hand with the worst electrophysiological findings were included in the statistical analysis, because including a patient with bilateral CTS as two cases may be source of statistical bias and the results may be overstated if the correlation between the two hands is not taken into account (Padua et al., 2005). ‘‘Standard” electrophysiological study included motor conduction velocity (MCV) calculated in the elbow–wrist segment of the median nerve and the below elbow–wrist segment of the ulnar nerve, recording from the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles, respectively. Compound muscle action potential (CMAP) amplitude was measured from baseline to the next negative peak and distal motor latency (DML) was calculated at a fixed distance of 7 cm from the point of stimulation at the wrist to the muscle from which CMAP was recorded. Sensory conduction velocity (SCV) was calculated by orthodromic technique in the third and fourth finger–wrist tract for the median nerve (M3, M4) and in the fourth finger–wrist tract for the ulnar nerve (U4). Sensory action potential (SAP) amplitude was measured peak to peak. The second lumbrical-interosseous (2LI) DML test was also performed by the technique of Preston and Logigian (1992). The median and ulnar nerves were stimulated at the wrist 10 cm from surface recording electrodes placed over the motor point of the 2LI, lateral to the mid-point of the third metacarpal.
The reference electrode was placed on the palmar aspect of the proximal interphalangeal joint of the second finger. Differences between U4–M4 SCV, median–ulnar DML recording from APB and ADM, and 2LI-DML were also calculated. In view of the aim of the study, differences in median DML recording from second lumbrical (2L) and APB muscles and for comparison, in ulnar DML recording from second interosseous (2I) and ADM muscles, were also measured. The ‘‘standard” neurographic protocol has been integrated in our lab with other electrodiagnostic tests (SCV of median (M1) and radial (R1) nerves in the first finger–wrist tract and difference between R1–M1 SCV, conduction velocity of the median and ulnar nerves in 8 cm palm-to-writ segment, MCV of the ulnar nerve in the above–below segment, SCV of the ulnar nerve in the fifth finger–wrist tract) for certain patients with CTS symptoms or hand clumsiness, because thenar motor neuropathy or concomitant ulnar nerve entrapment syndromes at the wrist or elbow had been demonstrated in subjects engaged in particular manual jobs, including vineyard workers. Some of the above electrodiagnostic tests were also performed to increase the sensitivity of the ‘‘standard” electrophysiological protocol in the diagnosis of CTS. In other words, if only one electrodiagnostic test of the ‘‘standard” protocol was abnormal (for example absolute M4 SCV or U4–M4 SCV difference or 2LI-DML difference), we looked for at least one other abnormal value, because the abnormality of only one test could be random and two or more abnormal electrodiagnostic tests greatly reduced the probability of including false positive cases of CTS (Dorfman and Robinson, 1997; Werner, 2006). Standard needle EMG of the APB muscle was rarely performed in CTS patients. In addition a detailed ulnar nerve study was performed if paraesthesia, sensory loss or pain were outside the classical territory innervated by the median nerve, including the whole hand or the fifth finger. This was done to exclude ulnar nerve neuropathy or neurogenic thoracic outlet syndrome. Electrophysiological examination was also extended with other appropriate tests if symptoms suggested polyneuropathy (MCV of deep peroneal nerve, SCV of radial and sural nerves) or radiculopathy/plexopathy (standard needle electromyography of upper limb muscles). However, any of these diseases associated with CTS was an exclusion criterion for the study. For MCV/DML study, 9-mm Ag/AgCl electrodes were used as active and reference electrodes and electrical stimulus was delivered by a constant current stimulator through bipolar surface electrodes (2 cm interelectrode distance, cathode distal). For SCV study, ring electrodes were used to stimulate the fingers (cathode at first interphalangeal joint) and rubber suction electrodes on the wrist as recording electrodes. The skin temperature of the hand was maintained above 32 °C with an infrared lamp and measured with a digital thermometer. Neurographic values at least 2 SD above or below the means of the values belonging to a control group were considered abnormal. The control group included 66 healthy volunteers (38 women and 28 men, mean age 46.3, range 23–84, median 44.5 years) randomly selected among companions of the patients admitted to the EMG service between 2 January and 31 July 2008. They were 28 blue-collar workers, 24 white-collar workers, eight pensioners and six housewives. SAP and CMAP amplitudes were measured, but not used for electrophysiological confirmation of CTS diagnosis or included in the statistical analysis, except for APB CMAP and 2L CMAP. One neurophysiologist and one neurophysiological technician performed all examinations with the same equipment (Phasis I ESAOTE electromiograph, Florence, Italy) and our electrophysiological methods were in line with the steps of the AANEM guidelines for CTS and UNE (AAEM et al., 2002; Campbell et al., 1999).
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4. Statistical methods Univariate comparisons of quantitative variables were performed between the three groups (33 vineyard workers with CTS, 205 other CTS patients and 66 controls). The statistical significance of differences between groups was assessed by the F test for variance. a error was set at 5%. Multiple differences between groups were analyzed by the Bonferroni method to ensure against a error inflation. For categorical variables, the comparison was performed with the contingency table distribution and the v2 test. To check the influence of potential (‘‘a priori”) confounding variables on the electrophysiological findings, multiple linear regression was applied, entering the following independent variables: (1) Group (dummy, 0 = other subjects with CTS, 1 = vineyard workers with CTS); (2) Sex (dummy, 0 = Female, 1 = Male); (3) age of subjects (years); (4) BMI (kg/m2); (5) clinical severity scale (dummy, 0 = severity stages 1 and 2, i.e., CTS patients with only symptoms, 1 = severity stages 3–5, i.e., CTS patients with clinical signs of sensory/motor deficit). The dependent electrophysiological variables of the worst hand were: DML and CMAP amplitude of median nerve recording from APB and 2L muscles, DML of ulnar nerve recording from ADM and 2I muscles, M4, M3 and U4 SCV, and differences in U4–M4 SCV, 2LI-DML, median APB–ulnar ADM DML, median 2L-APB DML and ulnar 2I-ADM DML.
The parameter estimated by the regression model was b, the slope of the line tracing the relationship between independent variables and dependent variable. b can therefore be interpreted as the change in the dependent variable for a one-unit change in the independent variable. Entry of all variables in a single model made it possible to assess the weight of each, irrespective of the others. Significance was set at 95% (McNamee, 2005). 5. Results Sixteen out of 33 vineyard workers had bilateral CTS (48%), the other 17 had only one affected hand. The affected hand in monolateral cases and the worst hand in bilateral cases was always the dominant hand, except in one case. In the other 205 patients with CTS, 117 had bilateral CTS (57%). The affected hand in monolateral cases and the worst hand in bilateral cases was the dominant hand in 166 patients. Table 1 shows demographic findings, BMI, duration of symptoms, clinical severity, electrodiagnostic test results and statistical differences for the three groups. There were more dominant hands with CTS among vineyard workers (97%) than in the other CTS group (81%) (v2 test with Yates correction = 4.12, p = 0.042). There were more men among vineyard workers than in the other CTS group (v2 = 6.95, p = 0.008). There were significant differences in
Table 1 Demographic, clinical severity and electrophysiological findings in worst hand of 33 vineyard workers with CTS and 205 other CTS patients and in dominant hand of control group. Vineyard workers with CTS (dominant hand)
Other CTS patients
Controls (dominant hand)
Mean ± SD (range) or No. Mean ± SD (range) or No.
Mean ± SD (range) or No.
33 46.8 ± 11.7 (29–70)
205 53.7 ± 15.2 (21–86)
66 46.3 ± 13.8 (23–84)
14/19 26.6 ± 4 (19.5–37.5) 53.1 ± 36.3 (45.3–60.5)
152/53 27.3 ± 4.9 (16.7–46.2) 50.5 ± 38 (6–180)
38/28 24.6 ± 4 (17.2–36.2)
2.64 ± 1.1 (1–4)
2.48 ± 1.1 (1–4)
52.7 ± 4.1 (45.3–60.5) 6.65 ± 2.5 (3.24–12.3) 8.7 ± 4 (2.7–17.8) 4.69 ± 1 (3.16–6.66) 35.9 ± 15.4 (0–52.9) 31.9 ± 15.4 (0–49.3)
52.5 ± 4.1 (39.3–59.1) 5.46 ± 2.4 (3.05–27.9) 12.1 ± 5 (0.2–25.1) 5.09 ± 2 (3.2–23.4) 36.9 ± 13 (0–62.5) 31.7 ± 14.5 (0–61.3)
Ulnar nerve MCV m/s ADM DML ms 2I DML ms U4 SCV m/s
58.4 ± 4.5 3.12 ± 0.3 3.56 ± 0.3 54.7 ± 4.9
58.2 ± 3.6 3.03 ± 0.3 3.44 ± 0.4 55.6 ± 4.6
Differences 2LI-DML ms 2L-APB DML ms 2I-ADM DML ms APB–ADM DML ms U4–M4 SCV m/s
1.13 ± 0.9 (0.04–3.08) 1.96 ± 1.9 ( 6.06–0.05) 0.44 ± 0.3 ( 0.1–1) 3.53 ± 2.3 (0.44–8.64) 22.8 ± 14 (6.9–57)
Number of hands Age of subjects (years) Women/men (No.) BMI (kg/m2) Duration of CTS symptoms (months) Clinical severity scale Electrophysiological parameters Median nerve MCV m/s APB DML ms APB CMAP amplitude mV 2L DML ms M3 SCV m/s M4 SCV m/s
(50.9–65.7) (2.5–3.92) (3.02–4.12) (44–63)
(47.7–55.9) (2.4–4.55) (2.47–4.95) (36–64)
1.65 ± 2 ( 0.1–20.65) 0.37 ± 0.8 ( 4.5–3.85) 0.41 ± 0.4 ( 2.21–1.28) 2.43 ± 2.4 (0–25.15) 23.9 ± 14.1 (0–61)
ANOVA (F ratio/F prob.)/v2 Post hoc test: vineyard workers vs other CTS test between the three group p values groups
8.21/0.0000
0.035
7.5/0.02 8.65/0.0001 0.13/0.71
0.0008a n.s. n.s.
3.02/0.45
n.s.
56.2 ± 3.3 (50–61.3) 3.66 ± 0.31 (2.98–4.32) 15.6 ± 4.4 (6.4–27.3) 3.58 ± 0.3 (2.98–4.25) 55.3 ± 3.5 (46–62.3) 53.2 ± 3.5 (43.8–63.6)
23.3/0.0000 26.8/0.0000 24.7/0.0000 19.7/0.0000 61.8/0.0000 69.3/0.0000
n.s. 0.0008 0.001 n.s. n.s. n.s.
57.9 ± 3.4 (49–64.6) 3 ± 0.4 (2.24–3.98) 3.43 ± 0.3 (2.8–4) 56.4 ± 4.1 (47.3–64)
0.29/0.74 1.46/0.23 1.63/0.2 1.55/0.21
n.s. n.s. n.s. n.s.
0.15 ± 0.3 ( 0.07 ± 0.3 0.43 ± 0.3 ( 0.66 ± 0.4 ( 3.22 ± 3.2 (
19/0.0000 52.1/0.0000 0.12/0.89 25.8/0.0000 69.6/0.0000
n.s. <0.0001 n.s. 0.0016 n.s.
0.88–0.65) ( 0.6–0.6) 0.5–1.16) 0.22–1.44) 8.2–10)
The v2 test was only performed for sex and clinical severity scale; ANOVA test was performed for the other variables. The significance of the post hoc test for age, BMI and median and ulnar nerve electrophysiological findings between controls and vineyard workers with CTS and between controls and other CTS patients are not shown in this table. There were significant differences in age, BMI and all median nerve electrophysiological findings, but no significant differences in ulnar nerve electrophysiological findings. BMI = body mass index; 2L = 2nd lumbrical; 2I = 2nd interosseous; ADM = abductor digiti minimi muscle; APB = abductor pollicis brevis muscle; CMAP = compound muscle action potential; DML = distal motor latency; MCV = motor conduction velocity; SCV = sensory conduction velocity; M3 = median nerve, 3rd finger–wrist; M4 = median nerve, 4th finger–wrist; U4 = ulnar nerve, 4th finger–wrist. a v2 = 6.95.
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age, APB DML, APB CMAP amplitude of the median nerve, median APB-2L DML and median APB–ulnar ADM DML between vineyard workers and the other CTS patients. There were no other electrophysiological differences in median nerve conduction velocities, BMI, clinical severity or duration of CTS symptoms between the two groups of CTS patients. There were obviously significant differences in all electrophysiological sensory and motor conduction parameters of the median nerve between controls and the two groups of CTS patients, but not in those of the other nerves. There were also more cases with abnormal APB DML (>4.28 ms) among vineyard workers (90%) than in the other CTS group (60.5%) (v2 test with Yates correction = 4.54, p = 0.033), while there were no significant differences in the numbers of hands with abnormal values of M3 and M4 SCV, 2LI-DML or U4–M4 SCV. When linear regression analysis was performed, the electrophysiological differences in thenar motor conduction findings between vineyard workers and the other group of CTS patients, remained significant after adjusting for age, sex, BMI and CTS severity. Briefly, significant differences (b values with 95% confidence interval-CI) regarded only the following electrophysiological dependent variables: (a) APB DML (b = 1.09; CI 0.35–1.83), (b) APB CMAP (b = 3.96; CI 5.46 to 2.46), (c) median APB-2L DML (b = 1.55; CI 1.93 to 1.18), and (d) median APB–ulnar ADM DML (b = 1.04; CI 0.3–1.78). Thus thenar motor fibres were more involved in vineyard workers than the other CTS patients, while there were no differences in the severity of involvement of the sensory and 2L motor fibres between the two CTS groups, irrespective of age, BMI, sex and CTS clinical severity of patients.
6. Discussion Musculoskeletal disorders, including CTS, are frequent in the agricultural sector (Holmberg et al., 2002; Davis and Kotowski, 2007; Roquelaure et al., 2008b, 2009). This is a major problem in our province, where viticulture and wine production are the principal occupation. It is estimated that about 10,000 out of 269,000 of the population of Siena province work directly or indirectly in viticulture and wine production; about 3/5 are engaged in manual work. The results of our study adjusted for age, sex, BMI and CTS clinical severity demonstrated that CTS had different electrophysiological characteristics in vineyard workers than in our other CTS patients. In ‘‘classical” CTS the electrophysiological involvement of major diameter sensory fibres usually precedes that of motor fibres and there were generally more CTS cases with abnormal sensory electrodiagnostic tests than with abnormal motor involvement (Jablecki et al., 1993). Moreover isolated CTS with involvement of only thenar motor fibres is very rare (Repaci et al., 1999). In vineyard workers with CTS there was greater involvement of motor fibres of the median nerve supplying the thenar muscles (i.e., thenar motor branch), while involvement of sensory and motor fibres directed to the 2L muscle was similar to the other CTS group. Prevalent involvement of thenar motor fibres is unlikely to be due to anatomical variations in the course of the thenar motor branch (Lanz, 1977) or to variations in the intraneural funicular distribution of thenar motor fascicles (located on extreme radial side of nerve) (Mackinnon and Dellon, 1988). There are three main variations in course. The most frequent is ‘‘extraligamentous” where the thenar motor branch separates from the median nerve trunk distal to the transverse ligament. The branch may also arise within the carpal tunnel and wind around the distal edge of the transverse ligament outside the tunnel in order to supply the thenar muscles (‘‘subligamentous” course). The branch rarely pierces the flexor retinaculum (‘‘transligamentous” course) (Lanz, 1977).
The ‘‘transligamentous” and ‘‘subligamentous” variants when the branch forms an excessive angle around the distal edge of the transverse ligament may be risk factors for TMN. These variations could not be more frequent in certain types of workers than others. Moreover the transligamentous course is very rare in the general population (Alp et al., 2005). It seems more likely that certain vineyard tasks, performed at certain times of the year, may be responsible for prevalent involvement of thenar motor fibres. In our zone the vineyard production cycle is carried out manually and take approximately 800 h/hectare, of which about 50 h for pruning and at least 150 h for harvest. Some tasks, particularly pruning and harvest, expose workers to risk of CTS and of direct chronic compression of the thenar motor branch where it divides from the common trunk of the nerve outside the carpal canal. We doubt that prevalent involvement of thenar motor fibres in vineyard workers could be due to greater intracanal pressure, because the sensory fibres on the sides of the thenar motor funiculi in the common trunk of the median nerve are equally involved in vineyard workers and in other CTS patients. Direct pressure on the carpal tunnel may produce higher intracanal pressure and consequently direct nerve damage, but there is no epidemiological evidence that this could be an independent risk factor for CTS (Werner, 2006). In addition, CTS of vineyard workers was no more severe than that of the other CTS group, since the motor fibres innervating the second lumbrical muscle, that are spared in severe CTS because they are deeper, are also equally involved (Meena et al., 2008). Furthermore, since the side more often affected in vineyard workers was the dominant hand, exposed to greater stress than the non dominant hand, this type of manual activity may play a role in the pathogenesis of prevalent damage to the thenar motor fibres. Gender differences (CTS was more frequent in male than female vineyard workers) could be due simply to the fact that more men than women are engaged in this work. However gender risk is significantly different when evaluated in workplace (Giersiepen et al., 2000; Werner, 2006). Similar prevalent involvement of thenar motor than sensory fibres was observed in Brazilian hand-milkers with CTS (Kouyoumdjian and de Araújo, 2006). The type of movements of both hands of hand-milkers is very similar to that of the dominant hand of vineyard workers, although the strength applied and the frequency of the task are different. It therefore seems likely that in vineyard workers there is a combination of ‘‘usual” CTS with mononeuropathy of the thenar motor branch, the latter due to direct compression of the branch, rather than a simple variant of CTS with greater involvement of the motor fibres. To clarify whether CTS and thenar motor neuropathy are really independent and co-exist in these workers, it would be useful to calculate the prevalence of CTS, pure mononeuropathy of the thenar motor branch and association of the two in all employees (symptomatic and asymptomatic) of a number of vineyards, comparing these results with those obtained in a clerical group. This could be the subject of a specially designed future study. Being a retrospective study, our study is not without defects. In particular, EMG of APB muscle was rarely performed, and this could be important in thenar mononeuropathy. However, EMG of APB muscle is considered as ‘‘option” in the guidelines of AANEM and the real utility of EMG in CTS is still questioned (Balbierz et al., 1998; Gnatz, 1999; Conway, 1999; AAEM et al., 2002; Wee, 2002; Robinson, 2007). In addition, ultrasonographic study could provide additional information, but it was performed only in few patients and hand shape was not considered between confounding factors. Our observations should stimulate occupational physiologists to clarify the mechanisms of neuromuscular engagement in particular jobs like those of vineyard workers and ergonomists to pro-
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vide suitable tools. Though electric or pneumatic shears theoretically minimize the force applied and the time spent on pruning and harvest, in our area they are not widely used because of high cost, need for training and risk of cutting off the fingers. Pruning machines cannot be used because the rows of vines are too close for the machine to pass between them. In conclusion, it is important to obtain details on epidemiological and ergonomic findings and type of CTS damage, since many ‘‘individual” risk factors are difficult to change, but workplace-related risk factors can be modified. Conflict of interest The authors state that they had not financial support and have not conflicts of interest. Acknowledgment The authors thank Mr. Federico Marconi, director in chief of a winery producing ‘‘Brunello di Montalcino”, for providing information about viticulture in Siena province. References Alp M, Marur T, Akkin SM, Yalcin L, Demirci S. Ramification pattern of the thenar branch of the median nerve entering the thenar fascia and the distribution of the terminal branches in the thenar musculature: anatomic cadaver study in 144 hands. Clin Anat 2005;18:195–9. American Academy of Neurology. Practice parameter for carpal tunnel syndrome (summary statement): Report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology 1993;43:2406–9. American Association of Electrodiagnostic Medicine, American Academy of Neurology, American Academy of Physical Medicine and Rehabilitation. Practice parameter for electrodiagnostic studies in carpal tunnel syndrome: summary statement. Muscle Nerve 2002;25:918–22. Balbierz JM, Cottrell AC, Cottrell WD. Is needle examination always necessary in evaluation of carpal tunnel syndrome? Arch Phys Med Rehabil 1998;79:514–6. Campbell WW, Carroll DJ, Greenberg MK, Krendel DA, Pridgeon RM, Sitaram KP, et al. Practice parameter. Electrodiagnostic studies in ulnar neuropathy at the elbow. Neurology 1999;52:688–90. Conway RR. The role of needle electromyography in the evaluation of patients with carpal tunnel syndrome. Needle EMG is often unnecessary. Muscle Nerve 1999;22:284–5. Davis KG, Kotowski SE. Understanding the ergonomic risk for musculoskeletal disorders in the United States agricultural sector. Am J Ind Med 2007;50:501–11. Dorfman LJ, Robinson LR. AAEM minimonograph #47: normative data in electrodiagnostic medicine. Muscle Nerve 1997;20:4–14. Gell N, Werner RA, Franzblau A, Ulin SS, Armstrong TJ. A longitudinal study of industrial and clerical workers: incidence of carpal tunnel syndrome and assessment of risk factors. J Occup Rehabil 2005;15:47–55. Giannini F, Cioni R, Mondelli M, Padua R, Gregori B, D’Amico P, et al. A new clinical scale of carpal tunnel syndrome: validation of the measurement and clinical– neurophysiological assessment. Clin Neurophysiol 2002;113:71–7. Giersiepen K, Eberle A, Pohlabeln H. Gender differences in carpal tunnel syndrome? Occupational and non-occupational risk factors in a population-based case– control study. Ann Epidemiol 2000;10:481.
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Gnatz SM. The role of needle electromyography in the evaluation of patients with carpal tunnel syndrome. Needle EMG is important. Muscle Nerve 1999;22:282–3. Holmberg S, Stiernström EL, Thelin A, Svärdsudd K. Musculoskeletal symptoms among farmers and non-farmers: a population-based study. Int J Occup Environ Health 2002;8:339–45. Jablecki CK, Andary MT, So YT, Wilkins DEWilliams FH for AAEM Quality Assurance Committee. Literature review of the usefulness of nerve conduction studies and electromyography for the evaluation of patients with carpal tunnel syndrome. Muscle Nerve 1993;16:1392–414. Katz JN, Stirrat CR, Larson MG, Fossel AH, Eaton HM, Liang MH. A self-administered hand symptom diagram for the diagnosis and epidemiologic study of carpal tunnel syndrome. J Rheumatol 1990;17:1495–8. Kouyoumdjian JA, de Araújo RG. Carpal tunnel syndrome and manual milking: nerve conduction studies in 43 cases. Arq Neuropsiquiatr 2006;64(3-B):747–9. Lanz U. Anatomical variations of the median nerve in the carpal tunnel. J Hand Surg 1977;2A:44–53. Mackinnon SE, Dellon AL. Anatomic investigations of nerves at the wrist: I. Orientation of the motor fascicle of the median nerve in the carpal tunnel. Ann Plast Surg 1988;21:32–5. Mattioli S, Baldasseroni A, Curti S, Cooke RM, Mandes A, Zanardi F, et al. Incidence rates of surgically treated idiopathic carpal tunnel syndrome in blue- and white-collar workers and housewives in Tuscany, Italy. Occup Environ Med 2009a;66:299–304. Mattioli S, Baldasseroni A, Bovenzi M, Curti S, Cooke RM, Campo G, et al. Risk factors for operated carpal tunnel syndrome: a multicenter population-based case– control study. BMC Public Health 2009b;9:343. McNamee R. Regression modelling and other methods to control confounding. Occup Environ Med 2005;62:500–6. Meena AK, Srinivasa Rao B, Sailaja S, Mallikarjuna M, Borgohain R. Second lumbrical and interossei latency difference in Carpal Tunnel Syndrome. Clin Neurophysiol 2008;119:2789–94. Mondelli M, Aretini A, Ginanneschi F, Padua L. Electrophysiological study of 28 cases of thenar motor mononeuropathy. Eur J Neurol 2009;6(Suppl. 3):423. Padua L, Pasqualetti P, Rosenbaum R. One patient, two carpal tunnels: statistical and clinical analysis – by hand or by patient? Clin Neurophysiol 2005;116:241–3. Palmer KT, Harris EC, Coggon D. Carpal tunnel syndrome and its relation to occupation: a systematic literature review. Occup Med (Lond) 2007;57:57–66. Preston DC, Logigian EL. Lumbrical and interossei recording in carpal tunnel syndrome. Muscle Nerve 1992;15:1253–7. Rempel D, Evanoff B, Amadio PC, de Krom M, Franklin G, Franzblau A, et al. Consensus criteria for the classification of carpal tunnel syndrome in epidemiologic studies. Am J Public Health 1998;88:1447–51. Repaci M, Torrieri F, Di Blasio F, Uncini A. Exclusive electrophysiological motor involvement in carpal tunnel syndrome. Clin Neurophysiol 1999;110:1471–4. Robinson LR. Electrodiagnosis of carpal tunnel syndrome. Phys Med Rehabil Clin N Am 2007;18:733–46. Roquelaure Y, Ha C, Fouquet N, Descatha A, Leclerc A, Goldberg M, et al. Attributable risk of carpal tunnel syndrome in the general population: implications for intervention programs in the workplace. Scand J Work Environ Health 2009;35:342–8. Roquelaure Y, Ha C, Pélier-Cady MC, Nicolas G, Descatha A, Leclerc A, et al. Work increases the incidence of carpal tunnel syndrome in the general population. Muscle Nerve 2008a;37:477–82. Roquelaure Y, Ha C, Nicolas G, Pélier-Cady MC, Mariot C, Descatha A, et al. Attributable risk of carpal tunnel syndrome according to industry and occupation in a general population. Arthritis Rheum 2008b;59:341–8. Rossignol M, Stock S, Patry L, Armstrong B. Carpal tunnel syndrome: what is attributable to work? The Montreal study. Occup Environ Med 1997;54:519–23. Wee AS. Needle electromyography in carpal tunnel syndrome. Electromyogr Clin Neurophysiol 2002;42:253–6. Werner RA. Evaluation of work-related carpal tunnel syndrome. J Occup Rehabil 2006;16:207–22.
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Prevalent involvement of thenar motor fibres in vineyard workers with carpal tunnel syndrome M. Mondelli a,*, A. Baldasseroni b, A. Aretini a, F. Ginanneschi c, L. Padua d a
EMG Service, Local Health Unit 7, Siena, Italy Epidemiology Unit, Local Health Unit 10, Florence, Italy c Dept. Neurological, Neurosurgical and Behavioural Sciences, Siena University, Siena, Italy d Dept. Neurosciences, ‘‘Catholic” University and ‘‘don Grocchi” Foundation, Italy b
a r t i c l e
i n f o
Article history: Accepted 16 February 2010 Available online 15 March 2010 Keywords: Carpal tunnel syndrome Repetitive strain injury Thenar motor branch Vineyard workers
a b s t r a c t Objective: Carpal tunnel syndrome (CTS) has a high prevalence in agricultural workers, especially those engaged in vineyards. We postulated that vineyard CTS was electrophysiologically different from CTS of other subjects. We performed a retrospective cross-sectional electrophysiological study of two cohorts of consecutive patients with CTS, the first consisting of vineyard workers and the second, of other unselected types of workers, housewives and pensioners. Methods: Thirty-three vineyard workers (mean age 46.8 years, 42% women) and 205 patients with other occupations (mean age 53.7 years; 66% women) were enrolled. All patients underwent sensory and motor neurography of the median and ulnar nerves. Differences in demographic and electrophysiological findings between groups were calculated and multiple linear regression analysis was performed to eliminate the influence of potential confounding factors (age, sex, BMI, clinical severity of CTS) on the results of univariate difference analysis. Results: Univariate analysis showed that DML was longer and compound muscle action potential amplitude of the median nerve, recorded from the abductor pollicis brevis muscle, was smaller in vineyard workers than in the other CTS patients. These differences remained significant after adjusting the results for confounding factors. Conclusions: The vineyard workers showed a different pattern of CTS than the other patients: thenar motor fibres were more affected, presumably due to chronic compression on the thenar branch. This suggests an association between ‘‘common” CTS and thenar mononeuropathy. Significance: Occupational physiologists should clarify the mechanisms of neuromuscular engagement in particular jobs and ergonomists design suitable working tools, because many ‘‘individual” risk factors are difficult to change, but workplace-related risk factors can be modified. Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Carpal tunnel syndrome (CTS) is the most frequent focal neuropathy and the prevalence is particularly high in certain categories of manual workers (Rossignol et al., 1997; Palmer et al., 2007; Mattioli et al., 2009a). The risk factors associated with CTS include works that involve repeated forced movements of flexion–extension of the wrist and fingers with incongrue posture and use of vibrating instruments (Gell et al., 2005; Werner, 2006; Roquelaure et al., 2008a; Mattioli et al., 2009b).
* Corresponding author. Address: Servizio di EMG, ASL 7, Via Pian d’Ovile, 9, 53100 Siena, Italy. Tel.: +39 0577 535904; fax: +39 0577 535983. E-mail address: [email protected] (M. Mondelli).
Over the past 13 years, our outpatient EMG service has seen 28 consecutive cases of idiopathic mononeuropathy of thenar motor branch of the median nerve, eight of whom were agricultural workers engaged exclusively in the vineyards. We postulated that this particular type of work could be responsible for thenar motor neuropathy through direct compression on the motor branch of the median nerve (Mondelli et al., 2009). The aim of the present study was to determine whether CTS in vineyard workers was electrophysiologically different from that of other subjects, with particular attention to involvement of thenar motor fibres. We performed a retrospective cross-sectional study comparing electrophysiological findings of a cohort of consecutive vineyard workers with a cohort of consecutive unselected workers in other fields, pensioners and housewives, all with idiopathic CTS.
1388-2457/$36.00 Ó 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.clinph.2010.02.150
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2. Patients From 2 January 2006 to 31 December 2008, CTS was diagnosed for the first time in 1809 patients admitted consecutively to the EMG service of Local Health Unit No. 7 of Siena, Italy. This service is only open to unselected outpatients. The diagnosis of CTS was made on the basis of clinical findings and delayed distal conduction velocity of the median nerve (for details see Section 3). For clinical diagnosis, the inclusion criteria were those recommended by AAN (1993). All symptomatic hands were included in the ‘‘classic/probable” or ‘‘possible” categories according to the Katz hand diagram, as modified by the consensus document on the classification of CTS (Katz et al., 1990; Rempel et al., 1998). From 1809 patients with CTS, 33 consecutive cases were selected (1.8% of all patients with CTS: 14 women, 19 men, mean age 46.8 ± 11.7 years, range 29–70, median 46) on the basis of their work: all were agricultural workers engaged exclusively in vineyards and they did all the manual operations pertaining to the annual cycle of grape vines. By way of comparison, we enrolled 205 consecutive patients with idiopathic CTS presenting at the same service from 2 January to 31 May 2008; 136 were women and 69 men, mean age 53.7 ± 15.3 years (range 21–86, median 53). There were 81 blue-collar workers, 53 white-collar workers, 30 pensioners and 41 housewives. All subjects with diabetes, diseases of connective tissue or thyroid, renal failure, gout, onset of symptoms during pregnancy or lactation or <3 months after hand or wrist trauma with or without fracture were excluded in both groups. All patients with associated polyneuropathies and other mononeuropathies or radiculopathies of the upper limb were also excluded. Only patients with distal mononeuropathy of the median nerve diagnosed as ‘‘idiopathic” CTS were enrolled. Clinical severity of CTS was graded with a validated ordinal scale from 1 to 5, which considered symptoms and sensory changes and strength/trophism of the thenar muscles. The details of this scale, which is routinely used by Italian CTS study group, have already been reported (Giannini et al., 2002).
3. Electrophysiological methods The ‘‘standard” electrophysiological protocol was performed in the symptomatic hands of all CTS patients. In cases with bilateral CTS, only the results of the hand with the worst electrophysiological findings were included in the statistical analysis, because including a patient with bilateral CTS as two cases may be source of statistical bias and the results may be overstated if the correlation between the two hands is not taken into account (Padua et al., 2005). ‘‘Standard” electrophysiological study included motor conduction velocity (MCV) calculated in the elbow–wrist segment of the median nerve and the below elbow–wrist segment of the ulnar nerve, recording from the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles, respectively. Compound muscle action potential (CMAP) amplitude was measured from baseline to the next negative peak and distal motor latency (DML) was calculated at a fixed distance of 7 cm from the point of stimulation at the wrist to the muscle from which CMAP was recorded. Sensory conduction velocity (SCV) was calculated by orthodromic technique in the third and fourth finger–wrist tract for the median nerve (M3, M4) and in the fourth finger–wrist tract for the ulnar nerve (U4). Sensory action potential (SAP) amplitude was measured peak to peak. The second lumbrical-interosseous (2LI) DML test was also performed by the technique of Preston and Logigian (1992). The median and ulnar nerves were stimulated at the wrist 10 cm from surface recording electrodes placed over the motor point of the 2LI, lateral to the mid-point of the third metacarpal.
The reference electrode was placed on the palmar aspect of the proximal interphalangeal joint of the second finger. Differences between U4–M4 SCV, median–ulnar DML recording from APB and ADM, and 2LI-DML were also calculated. In view of the aim of the study, differences in median DML recording from second lumbrical (2L) and APB muscles and for comparison, in ulnar DML recording from second interosseous (2I) and ADM muscles, were also measured. The ‘‘standard” neurographic protocol has been integrated in our lab with other electrodiagnostic tests (SCV of median (M1) and radial (R1) nerves in the first finger–wrist tract and difference between R1–M1 SCV, conduction velocity of the median and ulnar nerves in 8 cm palm-to-writ segment, MCV of the ulnar nerve in the above–below segment, SCV of the ulnar nerve in the fifth finger–wrist tract) for certain patients with CTS symptoms or hand clumsiness, because thenar motor neuropathy or concomitant ulnar nerve entrapment syndromes at the wrist or elbow had been demonstrated in subjects engaged in particular manual jobs, including vineyard workers. Some of the above electrodiagnostic tests were also performed to increase the sensitivity of the ‘‘standard” electrophysiological protocol in the diagnosis of CTS. In other words, if only one electrodiagnostic test of the ‘‘standard” protocol was abnormal (for example absolute M4 SCV or U4–M4 SCV difference or 2LI-DML difference), we looked for at least one other abnormal value, because the abnormality of only one test could be random and two or more abnormal electrodiagnostic tests greatly reduced the probability of including false positive cases of CTS (Dorfman and Robinson, 1997; Werner, 2006). Standard needle EMG of the APB muscle was rarely performed in CTS patients. In addition a detailed ulnar nerve study was performed if paraesthesia, sensory loss or pain were outside the classical territory innervated by the median nerve, including the whole hand or the fifth finger. This was done to exclude ulnar nerve neuropathy or neurogenic thoracic outlet syndrome. Electrophysiological examination was also extended with other appropriate tests if symptoms suggested polyneuropathy (MCV of deep peroneal nerve, SCV of radial and sural nerves) or radiculopathy/plexopathy (standard needle electromyography of upper limb muscles). However, any of these diseases associated with CTS was an exclusion criterion for the study. For MCV/DML study, 9-mm Ag/AgCl electrodes were used as active and reference electrodes and electrical stimulus was delivered by a constant current stimulator through bipolar surface electrodes (2 cm interelectrode distance, cathode distal). For SCV study, ring electrodes were used to stimulate the fingers (cathode at first interphalangeal joint) and rubber suction electrodes on the wrist as recording electrodes. The skin temperature of the hand was maintained above 32 °C with an infrared lamp and measured with a digital thermometer. Neurographic values at least 2 SD above or below the means of the values belonging to a control group were considered abnormal. The control group included 66 healthy volunteers (38 women and 28 men, mean age 46.3, range 23–84, median 44.5 years) randomly selected among companions of the patients admitted to the EMG service between 2 January and 31 July 2008. They were 28 blue-collar workers, 24 white-collar workers, eight pensioners and six housewives. SAP and CMAP amplitudes were measured, but not used for electrophysiological confirmation of CTS diagnosis or included in the statistical analysis, except for APB CMAP and 2L CMAP. One neurophysiologist and one neurophysiological technician performed all examinations with the same equipment (Phasis I ESAOTE electromiograph, Florence, Italy) and our electrophysiological methods were in line with the steps of the AANEM guidelines for CTS and UNE (AAEM et al., 2002; Campbell et al., 1999).
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4. Statistical methods Univariate comparisons of quantitative variables were performed between the three groups (33 vineyard workers with CTS, 205 other CTS patients and 66 controls). The statistical significance of differences between groups was assessed by the F test for variance. a error was set at 5%. Multiple differences between groups were analyzed by the Bonferroni method to ensure against a error inflation. For categorical variables, the comparison was performed with the contingency table distribution and the v2 test. To check the influence of potential (‘‘a priori”) confounding variables on the electrophysiological findings, multiple linear regression was applied, entering the following independent variables: (1) Group (dummy, 0 = other subjects with CTS, 1 = vineyard workers with CTS); (2) Sex (dummy, 0 = Female, 1 = Male); (3) age of subjects (years); (4) BMI (kg/m2); (5) clinical severity scale (dummy, 0 = severity stages 1 and 2, i.e., CTS patients with only symptoms, 1 = severity stages 3–5, i.e., CTS patients with clinical signs of sensory/motor deficit). The dependent electrophysiological variables of the worst hand were: DML and CMAP amplitude of median nerve recording from APB and 2L muscles, DML of ulnar nerve recording from ADM and 2I muscles, M4, M3 and U4 SCV, and differences in U4–M4 SCV, 2LI-DML, median APB–ulnar ADM DML, median 2L-APB DML and ulnar 2I-ADM DML.
The parameter estimated by the regression model was b, the slope of the line tracing the relationship between independent variables and dependent variable. b can therefore be interpreted as the change in the dependent variable for a one-unit change in the independent variable. Entry of all variables in a single model made it possible to assess the weight of each, irrespective of the others. Significance was set at 95% (McNamee, 2005). 5. Results Sixteen out of 33 vineyard workers had bilateral CTS (48%), the other 17 had only one affected hand. The affected hand in monolateral cases and the worst hand in bilateral cases was always the dominant hand, except in one case. In the other 205 patients with CTS, 117 had bilateral CTS (57%). The affected hand in monolateral cases and the worst hand in bilateral cases was the dominant hand in 166 patients. Table 1 shows demographic findings, BMI, duration of symptoms, clinical severity, electrodiagnostic test results and statistical differences for the three groups. There were more dominant hands with CTS among vineyard workers (97%) than in the other CTS group (81%) (v2 test with Yates correction = 4.12, p = 0.042). There were more men among vineyard workers than in the other CTS group (v2 = 6.95, p = 0.008). There were significant differences in
Table 1 Demographic, clinical severity and electrophysiological findings in worst hand of 33 vineyard workers with CTS and 205 other CTS patients and in dominant hand of control group. Vineyard workers with CTS (dominant hand)
Other CTS patients
Controls (dominant hand)
Mean ± SD (range) or No. Mean ± SD (range) or No.
Mean ± SD (range) or No.
33 46.8 ± 11.7 (29–70)
205 53.7 ± 15.2 (21–86)
66 46.3 ± 13.8 (23–84)
14/19 26.6 ± 4 (19.5–37.5) 53.1 ± 36.3 (45.3–60.5)
152/53 27.3 ± 4.9 (16.7–46.2) 50.5 ± 38 (6–180)
38/28 24.6 ± 4 (17.2–36.2)
2.64 ± 1.1 (1–4)
2.48 ± 1.1 (1–4)
52.7 ± 4.1 (45.3–60.5) 6.65 ± 2.5 (3.24–12.3) 8.7 ± 4 (2.7–17.8) 4.69 ± 1 (3.16–6.66) 35.9 ± 15.4 (0–52.9) 31.9 ± 15.4 (0–49.3)
52.5 ± 4.1 (39.3–59.1) 5.46 ± 2.4 (3.05–27.9) 12.1 ± 5 (0.2–25.1) 5.09 ± 2 (3.2–23.4) 36.9 ± 13 (0–62.5) 31.7 ± 14.5 (0–61.3)
Ulnar nerve MCV m/s ADM DML ms 2I DML ms U4 SCV m/s
58.4 ± 4.5 3.12 ± 0.3 3.56 ± 0.3 54.7 ± 4.9
58.2 ± 3.6 3.03 ± 0.3 3.44 ± 0.4 55.6 ± 4.6
Differences 2LI-DML ms 2L-APB DML ms 2I-ADM DML ms APB–ADM DML ms U4–M4 SCV m/s
1.13 ± 0.9 (0.04–3.08) 1.96 ± 1.9 ( 6.06–0.05) 0.44 ± 0.3 ( 0.1–1) 3.53 ± 2.3 (0.44–8.64) 22.8 ± 14 (6.9–57)
Number of hands Age of subjects (years) Women/men (No.) BMI (kg/m2) Duration of CTS symptoms (months) Clinical severity scale Electrophysiological parameters Median nerve MCV m/s APB DML ms APB CMAP amplitude mV 2L DML ms M3 SCV m/s M4 SCV m/s
(50.9–65.7) (2.5–3.92) (3.02–4.12) (44–63)
(47.7–55.9) (2.4–4.55) (2.47–4.95) (36–64)
1.65 ± 2 ( 0.1–20.65) 0.37 ± 0.8 ( 4.5–3.85) 0.41 ± 0.4 ( 2.21–1.28) 2.43 ± 2.4 (0–25.15) 23.9 ± 14.1 (0–61)
ANOVA (F ratio/F prob.)/v2 Post hoc test: vineyard workers vs other CTS test between the three group p values groups
8.21/0.0000
0.035
7.5/0.02 8.65/0.0001 0.13/0.71
0.0008a n.s. n.s.
3.02/0.45
n.s.
56.2 ± 3.3 (50–61.3) 3.66 ± 0.31 (2.98–4.32) 15.6 ± 4.4 (6.4–27.3) 3.58 ± 0.3 (2.98–4.25) 55.3 ± 3.5 (46–62.3) 53.2 ± 3.5 (43.8–63.6)
23.3/0.0000 26.8/0.0000 24.7/0.0000 19.7/0.0000 61.8/0.0000 69.3/0.0000
n.s. 0.0008 0.001 n.s. n.s. n.s.
57.9 ± 3.4 (49–64.6) 3 ± 0.4 (2.24–3.98) 3.43 ± 0.3 (2.8–4) 56.4 ± 4.1 (47.3–64)
0.29/0.74 1.46/0.23 1.63/0.2 1.55/0.21
n.s. n.s. n.s. n.s.
0.15 ± 0.3 ( 0.07 ± 0.3 0.43 ± 0.3 ( 0.66 ± 0.4 ( 3.22 ± 3.2 (
19/0.0000 52.1/0.0000 0.12/0.89 25.8/0.0000 69.6/0.0000
n.s. <0.0001 n.s. 0.0016 n.s.
0.88–0.65) ( 0.6–0.6) 0.5–1.16) 0.22–1.44) 8.2–10)
The v2 test was only performed for sex and clinical severity scale; ANOVA test was performed for the other variables. The significance of the post hoc test for age, BMI and median and ulnar nerve electrophysiological findings between controls and vineyard workers with CTS and between controls and other CTS patients are not shown in this table. There were significant differences in age, BMI and all median nerve electrophysiological findings, but no significant differences in ulnar nerve electrophysiological findings. BMI = body mass index; 2L = 2nd lumbrical; 2I = 2nd interosseous; ADM = abductor digiti minimi muscle; APB = abductor pollicis brevis muscle; CMAP = compound muscle action potential; DML = distal motor latency; MCV = motor conduction velocity; SCV = sensory conduction velocity; M3 = median nerve, 3rd finger–wrist; M4 = median nerve, 4th finger–wrist; U4 = ulnar nerve, 4th finger–wrist. a v2 = 6.95.
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age, APB DML, APB CMAP amplitude of the median nerve, median APB-2L DML and median APB–ulnar ADM DML between vineyard workers and the other CTS patients. There were no other electrophysiological differences in median nerve conduction velocities, BMI, clinical severity or duration of CTS symptoms between the two groups of CTS patients. There were obviously significant differences in all electrophysiological sensory and motor conduction parameters of the median nerve between controls and the two groups of CTS patients, but not in those of the other nerves. There were also more cases with abnormal APB DML (>4.28 ms) among vineyard workers (90%) than in the other CTS group (60.5%) (v2 test with Yates correction = 4.54, p = 0.033), while there were no significant differences in the numbers of hands with abnormal values of M3 and M4 SCV, 2LI-DML or U4–M4 SCV. When linear regression analysis was performed, the electrophysiological differences in thenar motor conduction findings between vineyard workers and the other group of CTS patients, remained significant after adjusting for age, sex, BMI and CTS severity. Briefly, significant differences (b values with 95% confidence interval-CI) regarded only the following electrophysiological dependent variables: (a) APB DML (b = 1.09; CI 0.35–1.83), (b) APB CMAP (b = 3.96; CI 5.46 to 2.46), (c) median APB-2L DML (b = 1.55; CI 1.93 to 1.18), and (d) median APB–ulnar ADM DML (b = 1.04; CI 0.3–1.78). Thus thenar motor fibres were more involved in vineyard workers than the other CTS patients, while there were no differences in the severity of involvement of the sensory and 2L motor fibres between the two CTS groups, irrespective of age, BMI, sex and CTS clinical severity of patients.
6. Discussion Musculoskeletal disorders, including CTS, are frequent in the agricultural sector (Holmberg et al., 2002; Davis and Kotowski, 2007; Roquelaure et al., 2008b, 2009). This is a major problem in our province, where viticulture and wine production are the principal occupation. It is estimated that about 10,000 out of 269,000 of the population of Siena province work directly or indirectly in viticulture and wine production; about 3/5 are engaged in manual work. The results of our study adjusted for age, sex, BMI and CTS clinical severity demonstrated that CTS had different electrophysiological characteristics in vineyard workers than in our other CTS patients. In ‘‘classical” CTS the electrophysiological involvement of major diameter sensory fibres usually precedes that of motor fibres and there were generally more CTS cases with abnormal sensory electrodiagnostic tests than with abnormal motor involvement (Jablecki et al., 1993). Moreover isolated CTS with involvement of only thenar motor fibres is very rare (Repaci et al., 1999). In vineyard workers with CTS there was greater involvement of motor fibres of the median nerve supplying the thenar muscles (i.e., thenar motor branch), while involvement of sensory and motor fibres directed to the 2L muscle was similar to the other CTS group. Prevalent involvement of thenar motor fibres is unlikely to be due to anatomical variations in the course of the thenar motor branch (Lanz, 1977) or to variations in the intraneural funicular distribution of thenar motor fascicles (located on extreme radial side of nerve) (Mackinnon and Dellon, 1988). There are three main variations in course. The most frequent is ‘‘extraligamentous” where the thenar motor branch separates from the median nerve trunk distal to the transverse ligament. The branch may also arise within the carpal tunnel and wind around the distal edge of the transverse ligament outside the tunnel in order to supply the thenar muscles (‘‘subligamentous” course). The branch rarely pierces the flexor retinaculum (‘‘transligamentous” course) (Lanz, 1977).
The ‘‘transligamentous” and ‘‘subligamentous” variants when the branch forms an excessive angle around the distal edge of the transverse ligament may be risk factors for TMN. These variations could not be more frequent in certain types of workers than others. Moreover the transligamentous course is very rare in the general population (Alp et al., 2005). It seems more likely that certain vineyard tasks, performed at certain times of the year, may be responsible for prevalent involvement of thenar motor fibres. In our zone the vineyard production cycle is carried out manually and take approximately 800 h/hectare, of which about 50 h for pruning and at least 150 h for harvest. Some tasks, particularly pruning and harvest, expose workers to risk of CTS and of direct chronic compression of the thenar motor branch where it divides from the common trunk of the nerve outside the carpal canal. We doubt that prevalent involvement of thenar motor fibres in vineyard workers could be due to greater intracanal pressure, because the sensory fibres on the sides of the thenar motor funiculi in the common trunk of the median nerve are equally involved in vineyard workers and in other CTS patients. Direct pressure on the carpal tunnel may produce higher intracanal pressure and consequently direct nerve damage, but there is no epidemiological evidence that this could be an independent risk factor for CTS (Werner, 2006). In addition, CTS of vineyard workers was no more severe than that of the other CTS group, since the motor fibres innervating the second lumbrical muscle, that are spared in severe CTS because they are deeper, are also equally involved (Meena et al., 2008). Furthermore, since the side more often affected in vineyard workers was the dominant hand, exposed to greater stress than the non dominant hand, this type of manual activity may play a role in the pathogenesis of prevalent damage to the thenar motor fibres. Gender differences (CTS was more frequent in male than female vineyard workers) could be due simply to the fact that more men than women are engaged in this work. However gender risk is significantly different when evaluated in workplace (Giersiepen et al., 2000; Werner, 2006). Similar prevalent involvement of thenar motor than sensory fibres was observed in Brazilian hand-milkers with CTS (Kouyoumdjian and de Araújo, 2006). The type of movements of both hands of hand-milkers is very similar to that of the dominant hand of vineyard workers, although the strength applied and the frequency of the task are different. It therefore seems likely that in vineyard workers there is a combination of ‘‘usual” CTS with mononeuropathy of the thenar motor branch, the latter due to direct compression of the branch, rather than a simple variant of CTS with greater involvement of the motor fibres. To clarify whether CTS and thenar motor neuropathy are really independent and co-exist in these workers, it would be useful to calculate the prevalence of CTS, pure mononeuropathy of the thenar motor branch and association of the two in all employees (symptomatic and asymptomatic) of a number of vineyards, comparing these results with those obtained in a clerical group. This could be the subject of a specially designed future study. Being a retrospective study, our study is not without defects. In particular, EMG of APB muscle was rarely performed, and this could be important in thenar mononeuropathy. However, EMG of APB muscle is considered as ‘‘option” in the guidelines of AANEM and the real utility of EMG in CTS is still questioned (Balbierz et al., 1998; Gnatz, 1999; Conway, 1999; AAEM et al., 2002; Wee, 2002; Robinson, 2007). In addition, ultrasonographic study could provide additional information, but it was performed only in few patients and hand shape was not considered between confounding factors. Our observations should stimulate occupational physiologists to clarify the mechanisms of neuromuscular engagement in particular jobs like those of vineyard workers and ergonomists to pro-
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