Innervation-zone width in disease and its changes with fatigue

Journal of Electromyography and Kinesiology Vol. 2, No. 4, pp 252-256 0 1993 Raven Press, Ltd., New York

Innervation-Zone

Width in Disease and its Changes with Fatigue Israel Yaar, M.D.

Neurology

Section,

VA Medical Center, Providence,

Rhode Island, U.S.A.

Summary: This study was designed to measure the average size of the motor unit functional innervation zone (FINZONE) in healthy and in diseased muscles, intramuscularly, in fatigue-inducing maximal voluntary contractions. The bicipital EMG interference patterns of 188 subjects (32 healthy, 83 neuropathic, 28 myasthenic, 13 myotonic, and 32 myopathic) were recorded with coaxial needle electrodes. From them, the FINZONE size criterion was computed, repeatedly, each 5.84 s to complete fatigue. The data display showed that the FINZONE size diminished with fatigue. Over all groups, this decrement was significant at p = 0.00001. Separately, only the normals and the neuropathic groups reached significance. Measured by the FINZONE size, the five groups divided into two significantly different clusters: (a) the {myopathic/ myasthenic} cluster with large FINZONE sizes and (b) the {normal/ neuropathic/myotonic} cluster with smaller ones. Several explanations for these findings were entertained. Also, as significant group differences were found, these results may prove helpful, complementing other variables, in the diagnosis of neuromuscular disorders. Both issues (FINZONE diagnostic value and FINZONE size decrement with fatigue) deserve additional study. Key Words: Innervation zone-EMG signal-Fatigue-Neuromuscular disorders.

conduction velocity (MFCV). This “zone width is expressed as standard deviation of the scattering of innervation points” (S), and will be referred to in this article as the functional innervation zone (FINZONE) for obvious reasons (&l&12). The present study was designed to verify INZONE width differences between healthy and diseased muscles and between the various neuromuscular disorders (NMDs) that were reported previously (5), this time with intramuscular recordings and in maximal voluntary contractions (MVCs). Also, this study was designed to follow the changes in FINZONE during fatigue, in healthy patients and in NMDs. This may help explain the changes observed in the EMG signal during fatigue, with special reference to differences between the various muscular disorders.

The innervation zone is that part of the muscle where neuromuscular joining takes place. This region or regions is usually located within short stretches of the muscle bulk (10-12). The spatial arrangement of the motor unit (MU) innervation zone (INZONE) is an important factor in the configuration of the motor unit action potential (MUAP) (2-5,13). The INZONE has been determined in healthy and diseased muscles by histological methods (1). The INZONE width can be determined from the electromyographic (EMG) signal power spectral moments (M,) and the muscle fiber

Accepted October 28, 1991. Address correspondence and reprint requests to Dr. I. Yaar at Neurology Section, VA Medical Center, Davis Park (127A), Providence, RI 02908, U.S.A.

252

INNERVATION

2.53

ZONE AND FATIGUE

METHODS AND MATERIALS

The MFCV in the equation above was replaced by

Subjects

MFCV = K3. M,/Mo

EMG interference patterns (EMGIPs) were recorded from bicep muscles of 188 subjects. There were 106 men and 82 women, 8-80 years old (average age of 43 + 15 years). Thirty-two of them were healthy subjects, 83 had peripheral polyneuropathy, 28 had myasthenia gravis, 13 had myotonia and 32 had myopathy. Recording The recordings were done according to our previously published technique (15) simultaneously from two locations in the biceps muscle, parallel, as much as possible, to the muscle fibers, and separated exactly 1 cm, in MVCs. The elbow was positioned at 90” and maintained that way throughout the study. The subjects were instructed to continue pulling in a competitive way for as long as they could and then were pleaded with to “go the extra mile.” The two EMGIP signals s,(t) and s,(t) were amplified and digitized at 5,000 samples/s. The frequency response of the system was set to bandpass 9-2,200 Hz (3 dB points), with attenuation of 30 dB/octave on both ends. Data Reduction and Variables Generation The two EMGIPs time series s,(t) and s,(t) each was segmented into three long overlapping segments, 16,384 data points (3.2768 s) long each, and into short overlapping segments of 256 data points (0.0512 s), each from within the long ones. The short segments were overlapped 50%, while the long segments were overlapped 60% of their lengths. s,(t) was subtracted from s,(t) for each epoch resulting in the differential signal s&). Each of the 256 data points of sAt> was Hanning windowed, at a time, fast Fourier transformed coefficients were squared, summed for each frequency, and averaged (to a total of) 381 times, generating the difference power spectrum {Gdd(f)}. The nominal frequency resolution was 19.5 Hz. The innervation zone width was calculated as follows (8): FINZONE

= K, * MFCV/BWE

The BWE was computed moments

from the power spectral

BWE = K4 . M$IX Gtd(f) M,, = K2 * IXj” * Gdd(f) Therefore, FINZONE = K * Ef.

Gd17 * ~G2,,Y)l~[~Gdf113

(1)

where FINZONE = the functional innervation zone width, MFCV = muscle fiber conduction velocity, BWE = equivalent noise bandwidth, M,, = spectral moment of order it, and K,-K4 and K = constants. The FINZONE values were calculated using Eq. (1) for each case, repeatedly each 5.84 s period, as study participants were exerting in MVCs to complete fatigue. The MFCV was not measured directly, but rather substituted by its linear correlate. Therefore, the constant “K,” is not known. The constant “K” in Eq. (1) is the product of several constants (some are known) and “K,.” Because “K3” was not computed, FINZONE will not be given in absolute units. Each period was normalized for the total duration of the MVC as a percent of the time it took to complete fatigue (PCF). These values were displayed for each diagnostic group in Fig. 1 after the best polynomial fit (up to order five) was applied (SPSSPC + , regression procedure, SPSS/ PC, Inc., Chicago, IL, U.S.A.). The coefficients and their statistics were evaluated in multivariate analysis of variance (MANOVA) analyses (SPSS/ PC, MANOVA procedure) and presented in Tables 1 and 2. The strength of FINZONE as a diagnostic criterion was evaluated in a jackknife discriminant analysis (SPSS, discriminant procedure aided by FORTRAN programming). RESULTS Figure 1 shows that the FINZONE decreased in size with fatigue in each diagnostic group. Table 1 shows the polynomial fit coefficients, best suited for each group, and shows that the overall group fatigue effect on the FINZONE was decremental at a significant level of p = 0.00001. However, calculated separately for each group, the fatigue trend

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I. YAAR

FIG. 1. Innervation-zone size: changes with fatigue by diagnostic groups. NOR, normal group; NEUR, neuropathic group; MG, myasthenic group; MYOT, myotonic group; MYOP, myopathy group; st. dev., standard deviations.

0

IO

20

30

40

E:\GDO\INZONEAI.ENG

50 X

60

reached significance only for the normal and neuropathic groups (Table 1). There was a significant group effect (p = 0.0001) in the all-groups MANOVA analysis, suggesting that some or all of the groups were significantly different in group-average FINZONE values. This finding made it permissible to perform separate repeated pairwise comparisons of the groups for differences in the FINZONE values. Figure 1 and Table 2 show that the normal, neuropathic, and myotonic groups kept significantly smaller FINZONE

TABLE 1. Polynomial jY to and multivariate analysis of the innervation-zone width changes with fatigue Group

No. of cases

C,b

C,

C,

Fatigue changes, P=

Nor Neur MG Myot Myop Total

32 83 28 13 32 188

10.6 10.5 12.0 11.0 12.0 11.1

-0.025 -0.006 -0.007 -0.011 -0.005 -0.02

0.018 0.01

0.006 0.01 NS NS NS 0.00001

C,-C:, = coefftcients of the polynomial fit. C, and C, were never significant. NS = not significant: p > 0.05. Nor = normal, Neur = neuropathy, MG = myasthenic, Myot = myotonic, and Myop = myopathic groups. a INZONE = C,, + C, * PCF + C, . PCF’ + CS * PCF’ + C, * PC? + Cs * PCF’, INZONE = innervation-zone width; PCF = % fatigue-see Fig. 1; and C, = first coefficient = innervation-zone width extrapolated to before MVC. b All C,, values are greater than 0 at p < 0.00001.

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70

80

90

100

FATIGUE

average size than the myopathic and myasthenic ones throughout fatigue. Analysis of FINZONE values of the different groups at no fatigue (Fig. 1, % fatigue = 0) could not separate the myotonic group from the others. Practically, as judged by the FINZONE size, the data reflect two distinct clusters: (a) the {myopathic/myasthenic} group with large FINZONE sizes and (b) the {normal/neuropathic/myotonic} group with smaller FINZONE sizes. The MANOVA procedure showed these two clusters to be different at p = 0.0001 significance level. Also, repeated jackknife discriminant analysis showed that if the question to answer is whether a subject belongs to either one of the clusters above, using FINZONE alone, a 71% correct classification can be reached. TABLE 2. Pairwise repeated-measures MANOVA comparisons for coefficients C, to C,; p values table Group

NOR

NEUR

MG

MYOT

NEUR

n = 115; p = 0.6 n = 59; p = 0.0001 n = 45; p = 0.2 n=64 p = 0.005

-

-

-

MG MYOT MYOP

n = 110; p=O.OOOl n = 96; p = 0.3 n = 115; p = 0.0001

n p n p

= = = =

40; 0.03 59; 0.9

n =45; p = 0.15

-

Nor = normal, Neur = neuropathy, MG = myasthenic, Myot = myotonic, and Myop = myopathic groups.

INNERVATION

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ZONE AND FATIGUE

The MANOVA procedure showed no significant effect of sex or age, and there was no significant differential effect of fatigue on the various groupsthey all showed decrement in the FINZONE size and no group’s decrement was (statistically) significantly higher than any other’s (a nonsignificant group x fatigue interaction factor).

DISCUSSION Correlation Between Findings and Known Pathology This study emphasizes relationships and changes, the relative FINZONE size in the various NMDs and healthy groups, and its relative changes with fatigue. Intentionally, there was no attempt to quantify the FINZONE size in exact units as it involves exact computations of the MFCV and several constants. It is interesting that the results seem to aggregate the five groups into two clinically viable and sound clusters: the {normal/neuropathic/myotonic} group with “normal” FINZONE sizes, and the {myopathic/myasthenic} group with significantly larger FINZONE sizes. When neuropathic denervation/ reinnervation processes are in action, it is known [refs. 5 and 6 (pp. 380-4)1 that the reinnervating sprout has preference to grow back into the old neuromuscular junction (NMJ). These processes are likely to increase the size of the MU but are unlikely to increase the size of the INZONE-overlapping and nearby MUs are known to have similar sizes and locations for their INZONEs (4). This agrees well with the above clustering of the normals with the neuropathics. When there is random damage to muscle fibers, those parts of the fibers that are denervated at times are reinnervated from their original MUs or from nearby MUs. Obviously, these new NMJs are not inside the original INZONE, resulting in the unavoidable expansion of the INZONE size. This process is common in myopathies (5,7) and to a lesser extent affects myasthenic muscles too (ref. 6, p. 245). It is also possible that another reason for the large INZONE of myasthenic muscle is the complex and ongoing remodulation of terminal innervating branches and the NMJs themselves (ref. 6, p. 245). This agrees with the above clustering of myasthenics with myopathits.

FINZONE Size Decrements with Fatigue One of the reasons this study was undertaken was to measure synchronization of MUs during fatigue. The logic was that, for a “function detector” like the FINZONE criterion, several synchronized MUs should look like one big MU. As it is known that synchronization of MUs increases in fatigue (9,14), the latter was expected to translate into the detection of increasing FINZONE size with fatigue. Obviously, the inverse resulted-FINZONE decreased significantly with fatigue. At this time and with these data, there is no good and decisive explanation for this phenomenon. It is possible that during fatigue, mainly in MVC-induced fatigue (which most likely was accompanied by ischemia of the muscles), there might have been a decrease in the muscle volume conductance that resulted in the EMG electrodes’ increasing failure to record from faraway muscle fibers, causing (functional) restriction of the MUs. Those muscle fibers still recorded from are near the needle electrode and near one another. As nearby muscle fibers also have similar and closely positioned NMJs (4,5), this may explain the decrement in the FINZONE. This process, if it exists, may actually be a stronger process than is evident from Fig. l-and what one sees is the result of this process being attenuated by synchronization. The key factor (the changes in muscle volume conductance with fatigue and its effect on the FINZONE) seems to be an important issue and in need of additional research.

Diagnostic Value The size of the INZONE is a known and expected parameter differentiating NMDs from one another and from healthy muscles (5). The INZONE size is one of the few important factors in determining the shape of the MUAP in health and in disease (3,4). It seems logical, therefore, that the FINZONE size will serve well as a diagnostic tool. When used alone, FINZONE showed some promise in discriminating among those two clusters (up to 71% correctly classified subjects). As it is likely that some of the biceps muscles that were recorded from subjects with diagnosed NMDs were actually normal, this discriminating power is quite good. This should encourage future studies using FINZONE in combinations with other variables.

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I. YAAR Acknowledgment: I thank Mrs. Sheryl Chicoine for her help in manuscript preparation. This work was supported by a VA Merit Review.

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