This study of quantitative electromyography examines the influence of sample size on motor unit action potential (MUAP) tolerance limits, intertrial variability, and diagnostic sensitivity. We recorded 20 randomly selected MUAPs from the biceps muscle twice in 21 normal subjects, and once in 10 patients with myopathy. The 95% tolerance limits for mean total duration in normal subjects progressively narrowed from 6.6 to 14.2 rns for 5 MUAPs to 7.4 to 13.0 ms for 20 MUAPs. The 95% tolerance limits for intertrial variability were t22% for mean total duration of 20 MUAPs. Larger sample size had a greater effect on reducing intertrial variability than on narrowing 95% tolerance limits for amplitude and area. Quantitative EMG results for duration supported the presence of rnyopathy in 2 of 10 patients with analysis of 5 MUAPs, and 9 patients with analysis of 20 MUAPs. Although anatysis of 5 potentials may be adequate for diagnosis occasionally, quantitative analysis of 20 MUAPs narrows tolerance limits, reduces intertrial variability, and improves diagnostic sensitivity. Key words: quantitative electrornyography intertrial variability sample size diagnostic sensitivity myopathy MUSCLE & NERVE 15:277-281 1992
QUANTITATIVE MOTOR UNIT ANALYSIS: THE EFFECT OF SAMPLE SIZE JOHN W. ENGSTROM, MD, and RICHARD K. OLNEY, MD
Quantitative motor unit action potential analysis is a general technique of EMG recording in which MUAP features are quantified to objectively describe their characteristics. Some investigators have used this approach to create models of motor unit p h y s i ~ l o g y . ~ - 'Others ~ ~ ' ~ have used various methods to establish clinical techniques for the diagnosis of myopathic and neurogenic diseases.1-4,13,14 These techniques may be used in clinical studies to follow response to treatment or disease course because intertrial reproducibility can be defined quantitatively. Quantitative MUAP analysis was developed by Buchthal, who routinely studied 20 to 40 MUAPs
Department of Neurology, School of Medicine, University of California, San Francisco, California. Acknowledgments: The authors thank Dr. Robert Layzer for referring study patients, and the neurology housestaff and nursing staff for volunteering for the collection of normal data. Presented in part at the Annual Meeting of the American Association of Electromyography and Electrodiagnosis, Washington, DC, September 15, 1989. Address reprint requests to Dr. John W. Engstrom, Department of Neurology, Box 0114, University of California, San Francisco, CA 9414301 14. Accepted for publication March 19, 1991 CCC 0148-639W92/030277-05 $04.00 0 1992 John Wiley & Sons, Inc
Quantitative Electrornyography
per m ~ s c l e .The ~ ' ~effect of smaller MUAP sample sizes on quantitative motor unit analysis results has not been described. Thus, we performed a quantitative EMG study of the biceps brachii twice in normal subjects, to determine the influence of MUAP sample size on tolerance limits and intertrial reproducibility. We also examined the effect of sample size on diagnostic sensitivity in 10 patients with various types of myopathy. METHODS
Twenty-one volunteers (13 men, 8 women; aged 25 to 41 years) were studied after obtaining informed consent. None had a history, symptoms, or neurologic signs to suggest myopathic or neurogenic disease affecting the biceps brachii. In addition, we studied 10 patients with myopathies (aged 22 to 40 years). The selection criteria for inclusion in this study were: (1) referral for clinically suspected myopathy, and (2) early recruitment of motor unit action potentials. Nine of these myopathy patients had symmetric proximal weakness often associated with elevated CPK. The other patient had clinical features suggestive of the MELAS syndrome. Seven patients had muscle biopsies, with results supporting myopathy in each. The other three patients had an unequivocal Subjects.
MUSCLE & NERVE
March 1992
277
clinical diagnosis of myopathy (AZT myopathy, limb-girdle dystrophy, myotonic dystrophy). Electromyographic Technique. We performed the studies with standard electromyographic instrumentation (Dantec 1500 EMG System, San Jose, CA), filter settings of 2 Hz and 10 kHz, a sweep time of 10 msldivision, and a sensitivity of 100 pV1 division. We used concentric needle electrodes with an active surface area of 0.07 mm2. Skin temperature was maintained at 34°C with a heat lamp controlled by a thermistor placed outside the field of heat radiation. The average length of a study was 1 hour. The EMG needle was inserted into the medial head of the left biceps brachii, using 3 to 4 puncture sites at 2 to 3-cm intervals down the longitudinal axis of the muscle. Four to 7 recording locations were used in each puncture site by advancing- the recording electrode 3 mm or more between recordings. An amplitude trigger was utilized for selective recording. A rapid rise time and a minimum amplitude of 50 p V were required for inclusion. Needle position was not adjusted to maximize amp1itude. We units Only in the recruitment Oramong the first der. We recorded a second or third MUAP at a recording site only if stable triggering was possible without adiustment of needle Dosition. We re21 MUAPs per m u h e . corded 20 We averaged a minimum of 128 discharges, but averaged an additional 128 MUAPs when background contamination of MUAP morphology was present after averaging the first 128. The waveform of each recorded MUAP was transferred by a parallel digital interface from the Dantec 1500 EMG system to an Apple IIe computer and to a floppy disk for further analysis. Duration, amplitude, and area were measured automatically using a program adapted from Stalberg's previously described algorithm (method 2).' For MUAP onset, the computer scanned from the left to the right for the first 50 pV deviation from baseline. The prior 10 ms to the left was scanned from the 50 pV deviation point, to locate the point at which the slope was less than 8 pV1ms. Then, moving again toward the right, the onset was set at the first point, deviating more than 8 pV from the baseline. The end of the MUAP was defined in an analogous manner from the reverse direction. Amplitudes were measured peak-to-peak. The area was calculated by digitally integrating the absolute value of amplitude over the duration for each MUAP.
6
278
Quantitative Electromyography
The spike duration was determined from the onset to the end of the negative spike(s) greater than 50 FV in amplitude by manually setting computer cursors on the digitized waveforms. The main spike duration was determined from the onset to the end of the single phase of the highest amplitude spike, whereas the spike duration included all spikes greater than 50 FV in amplitude. In general, the onset of both spike duration and main spike duration was at the positive peak preceding the spike, while the end was at the positive peak following the spike. Spike duration and main spike duration differed only for MUAPs having 4 or more phases. We calculated the mean of each parameter for 5, 10, 15, and 20 MUAPs per study. For the mean total duration of all sampled MUAPs, we also compared the averages with and without inclusion of polyphasic MUAPs. ~h~ 95% tolerance limits from the mean were defined for our sample size with = o.90 for each MUAP parameter. We used tolerance limits, rather than confidence intervals, because the sample standard deviation in a small population is only an approximation of the true standard deviation. 1 1 Statistical
RESULTS
Frequency analysis of the data revealed a normal distribution for total duration, spike duration, main spike duration, and area, but amplitude measurements were skewed toward lower amplitudes. Determination of amplitude tolerance limits required logarithmic transformation of the data to obtain a normal distribution. The results of our quantitative MUAP studies in normal subjects are listed in Table 1. T h e mean of total duration was reduced by 0.2 ms by excluding polyphasic MUAPs. The mean values for duration, amplitude, and area fell within the 95% tolerance limits for all normal subjects in each trial after recording 20 MUAPs. As expected, on statistical grounds, the standard deviation for the means of all parameters was less for 20 MUAPs than for 5 MUAPs for the 21 normal subjects. As a result, the mean of total duration for normal subjects had 95% tolerance limits that progressively narrowed from 6.6 to 14.2 ms for the mean of 5 MUAPs to 7.4 to 13.0 ms for the mean of 20 MUAPs. Thus, these 95% tolerance limits were 36% broader for the mean of 5 MUAPs than the mean of 20 MUAPs (Fig. 1).
MUSCLE & NERVE
March 1992
Table 1. Motor unit parameters versus motor units sample size in normal subjects (n = 21). Number of motor units sampled Mean
?
1 SD
Total duration (ms)
Spike duration (ms)
Amplitude (in P V )
Area (I*V/ms)
4.2 f 0.6 4.3 2 0.5 4.2 2 0.5 4.2 f 0.4
285 f 57 293 f 53 299 f 53 302 ? 52
551 f 163 562 f 160 562 f 135 561 133
5 10 15 20
10.4 10.4 10.3 10.2
5 10 15 20
6.6- 14.2 6.9- 13.9 7.3- 13.3 7.4-13.0
2.8-6.4 3.0-6.0 3.2-5.8 3.5-5.5
2.7-5.7 3.0-5.6 2.9-5.5 3.2-5.2
166-469* 176-471* 185-468* 191-465*
142-960 162-962 224-900 228-894
5 10 15 20
2 29 f39
243 ? 45 2-34 529
f52
f70 257 f55 251
287 f76 f52 ? 46
2 1.5 f 1.4 2 1.2
f 1.1
4.6 4.5 4.5 4.5
f 0.7 2 0.6 2 0.5 f 0.4
Main spike duration (ms)
*
95% Tolerance limits
Intertrial variability-95% tolerance limitst
?26 f22
2 40 ? 29 527
*Range calculatd after logarithmic transformation (see text). fExpressed as a percent change from the mean of the first trial
The tolerance limits for spike duration were 90% broader for the mean of 5 MUAPs than for 20 MUAPs. Increasing sample size had a similar effect on the 95% tolerance limits for the means of other MUAP parameters. The upper limits of intertrial variability for all MUAP parameters decreased with increasing sample size also (Table 1). Intertrial variability was lowest for duration parameters and greatest for area and amplitude. MUAP parameters did not differ significantly as a function of sex. No significant effect of age on MUAP parameters could be observed in our patients, probably because the age range (27 to 43 years) of our normal subjects was narrow.
Manual resetting of cursors for total duration measurements occurred in a mean of 3 MUAPs per trial. The most common reason for resetting the cursors was inability of the program to eliminate satellites. The mean number of polyphasic MUAPs among normal subjects per trial was one (range 0 to 4).Of the 3 normal subjects with 3 or 4 polyphasic MUAPs on one quantitative study, none had greater than 2 polyphasic MUAPs on the other quantitative study. Main spike duration differed from spike duration in a mean of two MUAPs (range 0 to 5 ) per trial. Results of quantitative MUAP analysis are presented for 10 patients with myopathy (Table 2).
16 14
I
12 10 8
21
11
6
I
4
5
10
15
20
Number of Motor Units
5
10
15
20
Number of Motor Units
FIGURE 1. Incremental narrowing of the 95% tolerance limits for normal subjects with larger sample size is illustrated for the mean Of total duration and spike duration in normal subjects.
Quantitative Electromyography
MUSCLE & NERVE
March 1992
279
Table 2. Motor unit parameters at 5 and 20 units among myopathic patients. Total duration (ms) Patient
Total spike duration (ms)
Amplitude (pV)
Area (pV.ms)
Age (years)
5
20
5
20
5
20
5
20
40 33 32 24 27 37 39 33 22 29
8.1 10.6 8.5 6.2* 7.7 7.5 6.2* 7.0 8.4 6.7
9.3 9.2 7.2* 6.V 6.9* 7.2* 6.V 7.1* 6.8* 7.1*
2.2* 5.4 4.0 2.9 3.9 3.6 3.4 3.7 4.0 3.8
2.5* 4.6 3.4* 3.2* 3.P 3.6 3.4* 3.7 3.3* 3.5
285 330 190 190 135* 293 102* 131* 233 120*
299 388 184* 220 175* 268 171* 185* 176* 198
340 480 260 270 230 310 170 21 0 380 200
410 500 22w 290 220* 310 230 270 250 250
6.6-14.2
7.4-13.0
2.8-6.4
3.5-5.5
166-469
191-465
142-960
228-894
1 2 3 4 5 6 7 8 9 10
95% tolerance limits 'Outside tolerance limits.
The mean for total duration was abnormal in 2 of 10 patients after sampling 5 MUAPs and abnormal in 8 of 10 patients after sampling 20 MUAPs. Exclusion of polyphasic MUAPs from the average reduced the mean of total duration for the patient group by 0.2 ms (range 0 to -0.5 ms) and did not increase the diagnostic sensitivity. One patient had normal results for the mean of all MUAP parameters, regardless of sample size. One patient had abnormal results for the mean of spike duration only, both at 5 and 20 MUAPs. DISCUSSION
The standard technique of sampling 20 to 40 motor units in each muscle was developed in the classic studies of Buchthal and Buchthal did not publish data regarding the advantages of using at least 20 motor units. Our results demonstrate that increasing MUAP sample size from 5 to 20 results in incremental narrowing of tolerance limits, reduced intertrial variability, and improved diagnostic sensitivity. Except for sample size, we used methodology similar to Buchthal, because he and his colleagues accumulated the most extensive data for different muscles across a wide age range. His techniques included meticulous attention to random sampling within each muscle, recording of intramuscular temperature, consideration of age-related effects, variation in motor unit parameters between muscles, consideration of electrode-related effects, and consistent sampling of units from the first several l 4- Our quantitative recruited motor ~ n i t s . ' - ~ , ' ~ recording technique also involved the use of an
280
Quantitative Electromyography
amplitude trigger, signal averaging, computer analysis of individual waveforms, a manual capability to override contaminated waveforms (i.e., satellites), and statistical analysis of 20 motor units per muscle for each parameter. Our mean total duration (10.2 ms) and its standard deviation (1.1 ms) for 20 motor units from the biceps bracchii are similar to Buchthal's data for the same muscle (mean total duration 10.6 ms; SD = 1 ms) among subjects of the same We used these techniques to examine the effects of motor unit sample size on tolerance limits, intertrial reproducibility, and diagnostic sensitivity. We found that duration measurements had better intertrial reproducibility than amplitude and area, although our use of a 10-kHz sampling frequency and random needle insertion may have affected the reproducibility of amplitude. T w o of our 10 patients with myopathy had an abnormally brief mean total duration at 5 MUAPs. Thus, quantitative analysis of five MUAPs may be ample to support a diagnosis of myopathy. We were able to support a diagnosis of myopathy with brief total duration in 6 additional patients by increasing the sample size to 20 MUAPs. One of 10 myopathy patients was identified with abnormally brief MUAPs, solely by use of the spike duration. Overall, results were abnormal for total duration in 8, spike duration in 7 and for either total or spike duration, in 9. Low mean amplitude of MUAPs supported the diagnosis of myopathy in 4 patients after analysis of 5 MUAPs and in 5 patients after analysis of 20 MUAPs. Measurement of MUAP area provided results supportive of myopathy in only 2 of 10 my-
MUSCLE & NERVE
March 1992
opathy patients, even after analysis of 20 potentials. All patients with abnormal amplitude or area after analysis of 20 MUAPs also had abnormally brief total duration. Thus, quantitative analysis of MUAP amplitude and area did not alone improve upon the diagnostic sensitivity of duration parameters among our patients. The only patient with a normal quantitative study had an advanced dystrophy accompanied by fiber type grouping on a biceps brachii muscle biopsy and some long duration motor units seen on the quantitative study. Significant chronic partial denervation with reinnervation is known to occur in advanced dystrophies, presumably due to degeneration and regeneration of intramuscular nerve twigs.6 The combined influences of chronic partial denervation with reinnervation and myopathy on MUAP parameters offer an explanation for normal quantitative results in occasional patients with definite myopathy.
Our results have implications for the performance of routine, nonquantitative EMG studies also. If a limited number of motor units are assessed subjectively, there is a low diagnostic sensitivity for supporting the diagnosis of myopathy. Under these circumstances, normal results have little power to exclude a myopathic diagnosis. Our results indicate that MUAP sample size is an important variable in the specification of recording conditions for quantitative MUAP analysis. The normal tolerance limits for quantitative MUAP studies are narrower and the results more reproducible with analysis of 20 MUAPs rather than smaller sample sizes. Furthermore, the diagnostic sensitivity of quantitative motor unit analysis in myopathy can be enhanced considerably by increasing the sample size to 20 MUAPs.
REFERENCES 1. Black JT, Bhatt GP, DeJesus PV, Schotland DL, Rowland LP: Diagnostic accuracy of clinical data, quantitative electromyography and histo-chemistry in neuromuscular disease. J Neurol Sci 1979;21:59-70. 2. Buchthal F, Guld C, Rosenfalck P: Action potential parameters in normal human nluscle and their dependence on physical variables. Acta Physiol Scand 1954;32:200-218. 3. Buchthal F, Pinelli P, Rosenfalck P: Action potential parameters in normal human muscle and their physiological determinants. Acta Physiol Scand 1954;32:219- 229. 4. Buchthal F, Rosenfalck P: Action potential parameters in different human muscles. Acta Psych et Neurol Scand 1955;30:25- 131. 5. Buchthal F, Kamieniecka Z: The diagnostic yield of quantified electromyography and quantified muscle biopsy in neuromuscular disorders. Muscle Nerve 1982;5:265- 280. 6. Hilton-Brown P, Stalberg E: T h e motor unit in muscular dystrophy, a single fiber EMG and scanning EMG study. J Neurol Neurosurg Psychiatry 1983;46:981-995. 7. Nandedkar SD, Sanders DB: Simulation of myopathic motor unit action potentials. Muscle Nerue 1989;12:197-202. 8. Nandedkar SD, Sanders DB: Principal component analysis of the features of concentric needle EMG motor unit action potentials. Muscle Nerve 1989; 12:288-293.
Quantitative Electrornyography
9. Nandedkar SD, Stalberg E: Simulation of single muscle fiber action potentials. Med Bzol Eng Comput 1983;21:158165. 10. Nandedkar SD, Stalberg E: Selectivity of electromyographic recording techniques: a simulation study. Med Biol Comput 1985;23:536- 540. 11. Recommended Standard for Normative Studies of Evoked Potentials, Statistical Anaylsis of Results, and Criteria for Clinically Significant Abnormality. J Clin Neurophysiol 1984; 1: 11- 14. 12. Rosenfalck A: Electromyography-Sensolyand Motor Conductaon Findings in Normal Subjects. Rigshospitalet-Copenhagen, Laboratory of Clinical Neurophysiology, 1975. 13. Sacco G, Buchthal F, Rosenfalck P: Motor unit potentials at different ages. Arch Neurol 1962;6:44-51. 14. Stalberg E, Andreassen S, Falck B, Lang H, Rosenfalck A, Trojaborg W: Quantitative analysis of individual motor unit potentials: a proposition for standardized terminology and criteria for measurement. J Clin Neurophysiol 1986;3:313-348. 15. Stewart CR, Nandedkar SD, Massey JM, Gilchrist JM, Barkhaus PE, Sanders DB: Evaluation of an automatic method of measuring features of motor unit action potentials. Muscle Nerve 1989;12:141- 148.
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281
QUANTITATIVE MOTOR UNIT ANALYSIS: THE EFFECT OF SAMPLE SIZE JOHN W. ENGSTROM, MD, and RICHARD K. OLNEY, MD
Quantitative motor unit action potential analysis is a general technique of EMG recording in which MUAP features are quantified to objectively describe their characteristics. Some investigators have used this approach to create models of motor unit p h y s i ~ l o g y . ~ - 'Others ~ ~ ' ~ have used various methods to establish clinical techniques for the diagnosis of myopathic and neurogenic diseases.1-4,13,14 These techniques may be used in clinical studies to follow response to treatment or disease course because intertrial reproducibility can be defined quantitatively. Quantitative MUAP analysis was developed by Buchthal, who routinely studied 20 to 40 MUAPs
Department of Neurology, School of Medicine, University of California, San Francisco, California. Acknowledgments: The authors thank Dr. Robert Layzer for referring study patients, and the neurology housestaff and nursing staff for volunteering for the collection of normal data. Presented in part at the Annual Meeting of the American Association of Electromyography and Electrodiagnosis, Washington, DC, September 15, 1989. Address reprint requests to Dr. John W. Engstrom, Department of Neurology, Box 0114, University of California, San Francisco, CA 9414301 14. Accepted for publication March 19, 1991 CCC 0148-639W92/030277-05 $04.00 0 1992 John Wiley & Sons, Inc
Quantitative Electrornyography
per m ~ s c l e .The ~ ' ~effect of smaller MUAP sample sizes on quantitative motor unit analysis results has not been described. Thus, we performed a quantitative EMG study of the biceps brachii twice in normal subjects, to determine the influence of MUAP sample size on tolerance limits and intertrial reproducibility. We also examined the effect of sample size on diagnostic sensitivity in 10 patients with various types of myopathy. METHODS
Twenty-one volunteers (13 men, 8 women; aged 25 to 41 years) were studied after obtaining informed consent. None had a history, symptoms, or neurologic signs to suggest myopathic or neurogenic disease affecting the biceps brachii. In addition, we studied 10 patients with myopathies (aged 22 to 40 years). The selection criteria for inclusion in this study were: (1) referral for clinically suspected myopathy, and (2) early recruitment of motor unit action potentials. Nine of these myopathy patients had symmetric proximal weakness often associated with elevated CPK. The other patient had clinical features suggestive of the MELAS syndrome. Seven patients had muscle biopsies, with results supporting myopathy in each. The other three patients had an unequivocal Subjects.
MUSCLE & NERVE
March 1992
277
clinical diagnosis of myopathy (AZT myopathy, limb-girdle dystrophy, myotonic dystrophy). Electromyographic Technique. We performed the studies with standard electromyographic instrumentation (Dantec 1500 EMG System, San Jose, CA), filter settings of 2 Hz and 10 kHz, a sweep time of 10 msldivision, and a sensitivity of 100 pV1 division. We used concentric needle electrodes with an active surface area of 0.07 mm2. Skin temperature was maintained at 34°C with a heat lamp controlled by a thermistor placed outside the field of heat radiation. The average length of a study was 1 hour. The EMG needle was inserted into the medial head of the left biceps brachii, using 3 to 4 puncture sites at 2 to 3-cm intervals down the longitudinal axis of the muscle. Four to 7 recording locations were used in each puncture site by advancing- the recording electrode 3 mm or more between recordings. An amplitude trigger was utilized for selective recording. A rapid rise time and a minimum amplitude of 50 p V were required for inclusion. Needle position was not adjusted to maximize amp1itude. We units Only in the recruitment Oramong the first der. We recorded a second or third MUAP at a recording site only if stable triggering was possible without adiustment of needle Dosition. We re21 MUAPs per m u h e . corded 20 We averaged a minimum of 128 discharges, but averaged an additional 128 MUAPs when background contamination of MUAP morphology was present after averaging the first 128. The waveform of each recorded MUAP was transferred by a parallel digital interface from the Dantec 1500 EMG system to an Apple IIe computer and to a floppy disk for further analysis. Duration, amplitude, and area were measured automatically using a program adapted from Stalberg's previously described algorithm (method 2).' For MUAP onset, the computer scanned from the left to the right for the first 50 pV deviation from baseline. The prior 10 ms to the left was scanned from the 50 pV deviation point, to locate the point at which the slope was less than 8 pV1ms. Then, moving again toward the right, the onset was set at the first point, deviating more than 8 pV from the baseline. The end of the MUAP was defined in an analogous manner from the reverse direction. Amplitudes were measured peak-to-peak. The area was calculated by digitally integrating the absolute value of amplitude over the duration for each MUAP.
6
278
Quantitative Electromyography
The spike duration was determined from the onset to the end of the negative spike(s) greater than 50 FV in amplitude by manually setting computer cursors on the digitized waveforms. The main spike duration was determined from the onset to the end of the single phase of the highest amplitude spike, whereas the spike duration included all spikes greater than 50 FV in amplitude. In general, the onset of both spike duration and main spike duration was at the positive peak preceding the spike, while the end was at the positive peak following the spike. Spike duration and main spike duration differed only for MUAPs having 4 or more phases. We calculated the mean of each parameter for 5, 10, 15, and 20 MUAPs per study. For the mean total duration of all sampled MUAPs, we also compared the averages with and without inclusion of polyphasic MUAPs. ~h~ 95% tolerance limits from the mean were defined for our sample size with = o.90 for each MUAP parameter. We used tolerance limits, rather than confidence intervals, because the sample standard deviation in a small population is only an approximation of the true standard deviation. 1 1 Statistical
RESULTS
Frequency analysis of the data revealed a normal distribution for total duration, spike duration, main spike duration, and area, but amplitude measurements were skewed toward lower amplitudes. Determination of amplitude tolerance limits required logarithmic transformation of the data to obtain a normal distribution. The results of our quantitative MUAP studies in normal subjects are listed in Table 1. T h e mean of total duration was reduced by 0.2 ms by excluding polyphasic MUAPs. The mean values for duration, amplitude, and area fell within the 95% tolerance limits for all normal subjects in each trial after recording 20 MUAPs. As expected, on statistical grounds, the standard deviation for the means of all parameters was less for 20 MUAPs than for 5 MUAPs for the 21 normal subjects. As a result, the mean of total duration for normal subjects had 95% tolerance limits that progressively narrowed from 6.6 to 14.2 ms for the mean of 5 MUAPs to 7.4 to 13.0 ms for the mean of 20 MUAPs. Thus, these 95% tolerance limits were 36% broader for the mean of 5 MUAPs than the mean of 20 MUAPs (Fig. 1).
MUSCLE & NERVE
March 1992
Table 1. Motor unit parameters versus motor units sample size in normal subjects (n = 21). Number of motor units sampled Mean
?
1 SD
Total duration (ms)
Spike duration (ms)
Amplitude (in P V )
Area (I*V/ms)
4.2 f 0.6 4.3 2 0.5 4.2 2 0.5 4.2 f 0.4
285 f 57 293 f 53 299 f 53 302 ? 52
551 f 163 562 f 160 562 f 135 561 133
5 10 15 20
10.4 10.4 10.3 10.2
5 10 15 20
6.6- 14.2 6.9- 13.9 7.3- 13.3 7.4-13.0
2.8-6.4 3.0-6.0 3.2-5.8 3.5-5.5
2.7-5.7 3.0-5.6 2.9-5.5 3.2-5.2
166-469* 176-471* 185-468* 191-465*
142-960 162-962 224-900 228-894
5 10 15 20
2 29 f39
243 ? 45 2-34 529
f52
f70 257 f55 251
287 f76 f52 ? 46
2 1.5 f 1.4 2 1.2
f 1.1
4.6 4.5 4.5 4.5
f 0.7 2 0.6 2 0.5 f 0.4
Main spike duration (ms)
*
95% Tolerance limits
Intertrial variability-95% tolerance limitst
?26 f22
2 40 ? 29 527
*Range calculatd after logarithmic transformation (see text). fExpressed as a percent change from the mean of the first trial
The tolerance limits for spike duration were 90% broader for the mean of 5 MUAPs than for 20 MUAPs. Increasing sample size had a similar effect on the 95% tolerance limits for the means of other MUAP parameters. The upper limits of intertrial variability for all MUAP parameters decreased with increasing sample size also (Table 1). Intertrial variability was lowest for duration parameters and greatest for area and amplitude. MUAP parameters did not differ significantly as a function of sex. No significant effect of age on MUAP parameters could be observed in our patients, probably because the age range (27 to 43 years) of our normal subjects was narrow.
Manual resetting of cursors for total duration measurements occurred in a mean of 3 MUAPs per trial. The most common reason for resetting the cursors was inability of the program to eliminate satellites. The mean number of polyphasic MUAPs among normal subjects per trial was one (range 0 to 4).Of the 3 normal subjects with 3 or 4 polyphasic MUAPs on one quantitative study, none had greater than 2 polyphasic MUAPs on the other quantitative study. Main spike duration differed from spike duration in a mean of two MUAPs (range 0 to 5 ) per trial. Results of quantitative MUAP analysis are presented for 10 patients with myopathy (Table 2).
16 14
I
12 10 8
21
11
6
I
4
5
10
15
20
Number of Motor Units
5
10
15
20
Number of Motor Units
FIGURE 1. Incremental narrowing of the 95% tolerance limits for normal subjects with larger sample size is illustrated for the mean Of total duration and spike duration in normal subjects.
Quantitative Electromyography
MUSCLE & NERVE
March 1992
279
Table 2. Motor unit parameters at 5 and 20 units among myopathic patients. Total duration (ms) Patient
Total spike duration (ms)
Amplitude (pV)
Area (pV.ms)
Age (years)
5
20
5
20
5
20
5
20
40 33 32 24 27 37 39 33 22 29
8.1 10.6 8.5 6.2* 7.7 7.5 6.2* 7.0 8.4 6.7
9.3 9.2 7.2* 6.V 6.9* 7.2* 6.V 7.1* 6.8* 7.1*
2.2* 5.4 4.0 2.9 3.9 3.6 3.4 3.7 4.0 3.8
2.5* 4.6 3.4* 3.2* 3.P 3.6 3.4* 3.7 3.3* 3.5
285 330 190 190 135* 293 102* 131* 233 120*
299 388 184* 220 175* 268 171* 185* 176* 198
340 480 260 270 230 310 170 21 0 380 200
410 500 22w 290 220* 310 230 270 250 250
6.6-14.2
7.4-13.0
2.8-6.4
3.5-5.5
166-469
191-465
142-960
228-894
1 2 3 4 5 6 7 8 9 10
95% tolerance limits 'Outside tolerance limits.
The mean for total duration was abnormal in 2 of 10 patients after sampling 5 MUAPs and abnormal in 8 of 10 patients after sampling 20 MUAPs. Exclusion of polyphasic MUAPs from the average reduced the mean of total duration for the patient group by 0.2 ms (range 0 to -0.5 ms) and did not increase the diagnostic sensitivity. One patient had normal results for the mean of all MUAP parameters, regardless of sample size. One patient had abnormal results for the mean of spike duration only, both at 5 and 20 MUAPs. DISCUSSION
The standard technique of sampling 20 to 40 motor units in each muscle was developed in the classic studies of Buchthal and Buchthal did not publish data regarding the advantages of using at least 20 motor units. Our results demonstrate that increasing MUAP sample size from 5 to 20 results in incremental narrowing of tolerance limits, reduced intertrial variability, and improved diagnostic sensitivity. Except for sample size, we used methodology similar to Buchthal, because he and his colleagues accumulated the most extensive data for different muscles across a wide age range. His techniques included meticulous attention to random sampling within each muscle, recording of intramuscular temperature, consideration of age-related effects, variation in motor unit parameters between muscles, consideration of electrode-related effects, and consistent sampling of units from the first several l 4- Our quantitative recruited motor ~ n i t s . ' - ~ , ' ~ recording technique also involved the use of an
280
Quantitative Electromyography
amplitude trigger, signal averaging, computer analysis of individual waveforms, a manual capability to override contaminated waveforms (i.e., satellites), and statistical analysis of 20 motor units per muscle for each parameter. Our mean total duration (10.2 ms) and its standard deviation (1.1 ms) for 20 motor units from the biceps bracchii are similar to Buchthal's data for the same muscle (mean total duration 10.6 ms; SD = 1 ms) among subjects of the same We used these techniques to examine the effects of motor unit sample size on tolerance limits, intertrial reproducibility, and diagnostic sensitivity. We found that duration measurements had better intertrial reproducibility than amplitude and area, although our use of a 10-kHz sampling frequency and random needle insertion may have affected the reproducibility of amplitude. T w o of our 10 patients with myopathy had an abnormally brief mean total duration at 5 MUAPs. Thus, quantitative analysis of five MUAPs may be ample to support a diagnosis of myopathy. We were able to support a diagnosis of myopathy with brief total duration in 6 additional patients by increasing the sample size to 20 MUAPs. One of 10 myopathy patients was identified with abnormally brief MUAPs, solely by use of the spike duration. Overall, results were abnormal for total duration in 8, spike duration in 7 and for either total or spike duration, in 9. Low mean amplitude of MUAPs supported the diagnosis of myopathy in 4 patients after analysis of 5 MUAPs and in 5 patients after analysis of 20 MUAPs. Measurement of MUAP area provided results supportive of myopathy in only 2 of 10 my-
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opathy patients, even after analysis of 20 potentials. All patients with abnormal amplitude or area after analysis of 20 MUAPs also had abnormally brief total duration. Thus, quantitative analysis of MUAP amplitude and area did not alone improve upon the diagnostic sensitivity of duration parameters among our patients. The only patient with a normal quantitative study had an advanced dystrophy accompanied by fiber type grouping on a biceps brachii muscle biopsy and some long duration motor units seen on the quantitative study. Significant chronic partial denervation with reinnervation is known to occur in advanced dystrophies, presumably due to degeneration and regeneration of intramuscular nerve twigs.6 The combined influences of chronic partial denervation with reinnervation and myopathy on MUAP parameters offer an explanation for normal quantitative results in occasional patients with definite myopathy.
Our results have implications for the performance of routine, nonquantitative EMG studies also. If a limited number of motor units are assessed subjectively, there is a low diagnostic sensitivity for supporting the diagnosis of myopathy. Under these circumstances, normal results have little power to exclude a myopathic diagnosis. Our results indicate that MUAP sample size is an important variable in the specification of recording conditions for quantitative MUAP analysis. The normal tolerance limits for quantitative MUAP studies are narrower and the results more reproducible with analysis of 20 MUAPs rather than smaller sample sizes. Furthermore, the diagnostic sensitivity of quantitative motor unit analysis in myopathy can be enhanced considerably by increasing the sample size to 20 MUAPs.
REFERENCES 1. Black JT, Bhatt GP, DeJesus PV, Schotland DL, Rowland LP: Diagnostic accuracy of clinical data, quantitative electromyography and histo-chemistry in neuromuscular disease. J Neurol Sci 1979;21:59-70. 2. Buchthal F, Guld C, Rosenfalck P: Action potential parameters in normal human nluscle and their dependence on physical variables. Acta Physiol Scand 1954;32:200-218. 3. Buchthal F, Pinelli P, Rosenfalck P: Action potential parameters in normal human muscle and their physiological determinants. Acta Physiol Scand 1954;32:219- 229. 4. Buchthal F, Rosenfalck P: Action potential parameters in different human muscles. Acta Psych et Neurol Scand 1955;30:25- 131. 5. Buchthal F, Kamieniecka Z: The diagnostic yield of quantified electromyography and quantified muscle biopsy in neuromuscular disorders. Muscle Nerve 1982;5:265- 280. 6. Hilton-Brown P, Stalberg E: T h e motor unit in muscular dystrophy, a single fiber EMG and scanning EMG study. J Neurol Neurosurg Psychiatry 1983;46:981-995. 7. Nandedkar SD, Sanders DB: Simulation of myopathic motor unit action potentials. Muscle Nerue 1989;12:197-202. 8. Nandedkar SD, Sanders DB: Principal component analysis of the features of concentric needle EMG motor unit action potentials. Muscle Nerve 1989; 12:288-293.
Quantitative Electrornyography
9. Nandedkar SD, Stalberg E: Simulation of single muscle fiber action potentials. Med Bzol Eng Comput 1983;21:158165. 10. Nandedkar SD, Stalberg E: Selectivity of electromyographic recording techniques: a simulation study. Med Biol Comput 1985;23:536- 540. 11. Recommended Standard for Normative Studies of Evoked Potentials, Statistical Anaylsis of Results, and Criteria for Clinically Significant Abnormality. J Clin Neurophysiol 1984; 1: 11- 14. 12. Rosenfalck A: Electromyography-Sensolyand Motor Conductaon Findings in Normal Subjects. Rigshospitalet-Copenhagen, Laboratory of Clinical Neurophysiology, 1975. 13. Sacco G, Buchthal F, Rosenfalck P: Motor unit potentials at different ages. Arch Neurol 1962;6:44-51. 14. Stalberg E, Andreassen S, Falck B, Lang H, Rosenfalck A, Trojaborg W: Quantitative analysis of individual motor unit potentials: a proposition for standardized terminology and criteria for measurement. J Clin Neurophysiol 1986;3:313-348. 15. Stewart CR, Nandedkar SD, Massey JM, Gilchrist JM, Barkhaus PE, Sanders DB: Evaluation of an automatic method of measuring features of motor unit action potentials. Muscle Nerve 1989;12:141- 148.
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