Journal of the Neurological Sciences, 1990,97:81-91

81

Elsevier JNS 03335

EMG power spectrum, turns-amplitude analysis and motor unit potential duration in neuromuscular disorders Anders Fuglsang-Frederiksen and Jesper Ronager Departments of Clinical Neurophysiology, Hvidovre Hospital and PJgshospitalet, University of Copenhagen, Copenhagen (Denmark)

(Received 17 November, 1989) (Revised, received29 January, 1990) (Accepted 30 January, 1990)

SUMMARY The diagnostic value of power spectrum analysis of the needle EMG pattern at a force of 30 ~o of maximum was compared to that of turns-amplitude analysis and to that of manual measurements of motor unit potential (MUP) duration in the brachial biceps muscle of 20 patients with myopathy and 11 patients with neurogenic disorders. In myopathy the power spectrum analysis had the same diagnostic value as the turns-amplitude analysis and MUP duration measurements and the 3 methods supplemented each other. In patients with neurogenic disorders the diagnostic value of the power spectrum analysis as well as that of the turns-amplitude analysis were lesser than that of MUP duration measurement. In diseased muscles the amount of high frequencies increased with increasing ratio of turns to mean amplitude while there was no relation between the power spectrum and the MUP changes. The results suggest that the power spectrum analysis of EMG can be used as a diagnostic tool in patients with neuromuscular disorders.

Key words: EMG pattern; Power spectrum; Turns-analysis; MUP duration; Neuromuscular disorders

Correspondenceto: A. Fuglsang-Frederiksen,M.D.,Ph.D.,DepartmentofClinicalNeurophysiology 145, Hvidovre Hospital, UniversityHospital of Copenhagen, Ketteg~d Alle 30, DK-2650 Hvidovre, Denmark.

0022-510X/90/$03.50 © 1990Elsevier SciencePublishers B.V.(BiomedicalDivision)

82 INTRODUCTION The frequency distribution of the EMG pattern is shifted towards higher frequencies in patients with myopathy and towards lower frequencies in patients with neurogenic disorders compared with controls (Walton 1952; Larsson 1975; FuglsangFrederiksen 1985; Ronager et al. 1989). The earlier studies of EMG frequency analysis made use of analogue filters (Walton 1952; Larsson 1975). Today, high resolution power spectrum analysis by fast Fourier transformation is available during the examination (Fuglsang-Frederiksen and Ronager 1988). The diagnostic yield may therefore be higher than indicated earlier. The aim of the present study was to compare the results of power spectrum analysis, turns-amplitude analysis and manual MUP analysis in muscles from patients with neuromuscular disorders. A preliminary report has been published elsewhere (Fuglsang-Frederiksen et al., 1986). A detailed description of the power spectrum analysis in controls and patients with neuromuscular disorders is described elsewhere (Ronager et al., 1989).

SUBJECTS AND METHODS The control material comprised 11 males and 9 females, aged 17-66 years. None had history or clinical signs of neuromuscular disorders. Informed consent was obtained from each subject. Twenty patients with myopathy and 11 patients with neurogenic disorders were examined. The patients were the same as in a previous study, except one patient who could not exert a force of 30~o (Ronager et al. 1989). Of 20 patients with myopathy (16 females, 4 males, age 19-74 years), 5 had progressive muscular dystrophy, 2 myotonic dystrophy, 3 congenital myopathy, 8 myopathy in collagen diseases, 1 steroid myopathy and 1 unclassified myopathy. The duration of symptoms was on average 7.8 years (SD 10.6). Of 12 patients with neurogenic disorders (5 females, 7 males, age 21-72 years) 1 had progressive spinal muscular atrophy, 4 motor neurone disease, 3 plexus brachialis lesion and 3 cervical root lesion. The duration of symptoms was on average 4.0 years (SD 6.7). The diagnosis was based on history, clinical findings, enzyme studies, conventional EMG of other muscles, muscle biopsies and nerve conduction studies. Informed consent was obtained from each patient. Force measurement

The force of the elbow flexion was measured with a strain-gauge dynamometer (Fuglsang-Frederiksen and M~msson 1975). A voltmeter connected to the dynamometer made it possible for the subject to monitor the exerted force. Although 75 ~o of the patients with myopathy and 82 Yo of the patients with neurogenic disorders had a decrease in maximum force of more than the limit of controls (30Yo), only half of the patients had more than a 50Yo decrease in force compared to controls.

83

Sampling of electrical activity EMGpattern. The electrical activity of the brachial biceps muscle sampled with a concentric needle electrode at 30~ of maximum force in ten sites of the muscle was amplified with a lower limiting frequency of 20 Hz and an upper limiting frequency of 10 kHz and recorded on an FM tape recorder with an upper limiting frequency of 12 kHz. Motor unit potentials. In each brachial biceps muscle of the patients at least 20 individual MUPs were sampled at weak effort and recorded on paper (Buchthal 1957). Analysis of electrical activity Power spectrum analysis at 30% of maximum. The electrical activity was analysed from tape by a spectrum analyser (Briiel & Kjaer 2033) with a sampling rate of 12.8 kHz using an anti-aliasing filter, fast Fourier transformation and a Hanning window. The individual spectrum was analysed over 80 msec from 0 Hz to 5 kHz using a linear scale with a resolution of 12.5 Hz; 100 dB corresponded to 0.01 V2. In order to reduce the noise 37 spectra were averaged over 800 msec of the EMG pattern at each site of the muscle (Fuglsang-Frederiksen and R~nager 1988). From each averaged spectrum mean power frequency (MPF) and the relative power (in parts per million of total power: ppm) at 140 Hz and 1400 Hz were obtained. The values were logarithmic transformed and the mean of 10 sites in each muscle was calculated and compared to mean+ 2 SD of controls (R~nager et al. 1989; Table 1). Regression analysis was used for testing correlations. In a previous study of EMG power spectrum in controls an analysis of the variation from subject to subject and the variation from site to site in the individual subject showed that analysis at 10 sites in each muscle is representative of the muscle (Christensen and Fuglsang-Frederiksen 1986). Turns-amplitude analysis at 30 % of maximum force. The number of turns per sec (i.e., number of successive potential reversals of more than 100 #V), the mean amplitude between turns and the ratio of turns to mean amplitude (automatic potential analyser APA6; Fitch 1967; Willison 1964; Fuglsang-Frederiksen 1981) was obtained from the same samples of electrical activity as used for power spectrum analysis. The mean of 10 sites in each muscle was compared to mean + 2 SD of the controls (Table 1). Individual motor unit potentials. In each muscle the mean duration and amplitude and incidence of polyphasic MUPs were obtained from manual measurement of at least 20 individual MUPs. The values were compared to age matched controls (Buchthal 1957; Rosenfalck and Rosenfalck 1975).

84 TABLE 1 CONTROL VALUES OF THE EMG PA'ITERN AT 30~o OF MAXIMAL EFFORT IN 20 SUBJECTS Mean Power spectrum analysis Mean power frequency (log Hz) Relative power 140 Hz (log ppm) Relative power 1400 Hz (log ppm) Turns-amplitude analysis Number of turns per sec (No/see) Mean amplitude (#V) Ratio turns/mean amplitude (No/#Vsec)

SD

2.280 4.580 1.790

0.045 0.065 0.185

505 501 1.080

66 104 0.215

RESULTS

Control subjects In the brachial biceps muscle of control subjects there was no relation between power spectrum and turns-amplitude parameters from subject to subject. Patients with myopathy In the brachial biceps muscle 10 of 20 patients with myopathy had increased relative power at 1400 Hz and 1 further patient had increased mean power spectrum of the EMG pattern (Fig. 1). Thus, 5 5 ~ of the patients had an increase in the amount of high frequencies of the EMG power spectrum. Increased turns, decreased mean 20 PATIENTSWITH MYOPATHY % 100

[] Power spectrum

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Fig. 1. Diagnostic yield of the power spectrum.analysis compared to that of the turns amplitude analysis (left) and to the measurement of MUP duration (middle) and the combined diagnostic yield of all 3 methods (right) in the brachial biceps muscle of 20 patients with myopathy. The percentage of patients with findings indicating a myopathic lesion is indicated on the scale to the left.

85 amplitude between turns or increased ratio of turns to mean amplitude were found in 55% of the patients with myopathy (Fig. 1). Decreased mean MUP duration was present in 45% (Fig. 1). 80% of the patients had increased incidence of polyphasic potentials (regarded as an unspecific finding) and all had normal mean amplitude of individual MUPs (regarded as an unreliable parameter). When power spectrum and turns-amplitude analysis were combined, 75 % of the patients had changes in the muscle indicating myopathy, in 20% due to changes in power spectrum alone and in another 20% due to changes in turns-amplitude analysis

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Fig. 2. Power at 1400 Hz in parts per million of total power (ppm, logarithmic values) as a function ofnumber of turns (above), mean amplitude (middle) and the ratio of turns to mean amplitude (below) in 20 patients with myopathy. Each point indicates the mean of 10 sites in the brachial biceps muscle of 1 pationt. 1400 Hz power (log ppm) = 2.925 - 0.002 amplitude (#V), r ffi - 0 . 6 8 7 , P < 0.001. 1400 Hz power (log ppm) = 0.166 + 0.660 ratio (No/#Vsec), r = 0.845, P < 0 . 0 0 1 .

86 alone (Fig. 1). 75~o of the patients had changes in the muscle indicating myopathy when power spectrum analysis and duration of MUPs were combined. 30~o of the patients had an increase in the amount of high frequencies alone and 20~o had decreased duration of MUPs alone (Fig. 1). Combining power spectrum analysis, turns-amplitude analysis and duration of MUPs, 85 ~o of the 20 patients had changes indicating myopathy; 5 ~ of the patients with an increased amount of high frequencies alone, 10~o with abnormal turns-amplitude analysis alone and 10~o with decreased duration of MUPs alone (Fig. 1). The relative power at 1400 Hz increased with increasing ratio of turns to mean amplitude and decreased with increasing mean amplitude from subject to subject (Fig. 2). Similarly, the mean power frequency increased with increasing ratio of turns to mean amplitude (MPF(log Hz) = 2.033 + 0.222 ratio (No/#Vsec), r = 0.763, P < 0.001). There was no correlation between power spectrum parameters and number of turns or between power spectrum parameters and MUP measurements. There was no correlation between the power spectrum parameters and duration of symptoms. Patients with neurogenic disorders In the brachial biceps muscle of 11 patients with neurogenic disorders 6 had decreased relative power at 1400 Hz of the EMG pattern and one further had increased relative power at 140 Hz (Fig. 3). Thus 6 4 ~ of the patients had a decrease in the amount of high frequencies or an increase in the amount of low frequencies of the EMG power spectrum or both. Decreased number of turns, increased mean amplitude between turns or decreased ratio of turns to mean amplitude were found in 73 ~o of the patients (Fig. 3). Increased duration of individual MUPs in the brachial biceps muscle was found in 91 ~o of the patients (Fig. 3), an increased incidence of

11 PATIENTS WITH NEUROGENIC DISORDERS °/01 O0

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S Fig. 3. Diagnostic yield of the power spectrum analysis compared to that of the turns-amplitude analysis (left) and the measurement of MUP duration (middle) and the combined diagnostic yield of all 3 methods (fight) in the brachial biceps muscle of 11 patients with neurogenic disorders. The percentage of patients with findings indicating a neurogenic lesion is indicated on the scale to the left.

87 p o l y p h a s i c M U P s was found in 72~o o f the p a t i e n t s ; in all a s s o c i a t e d with i n c r e a s e d duration. I n c r e a s e d m e a n a m p l i t u d e o f individual M U P s was found in all the patients with i n c r e a s e d duration. The p o w e r s p e c t r u m analysis d i d n o t a d d to the diagnostic yield o f the t u r n s - a m p l i t u d e analysis. P o w e r s p e c t r u m a n d t u r n s - a m p l i t u d e analysis d i d n o t s u p p l e m e n t the diagnostic yield o f m a n u a l M U P m e a s u r e m e n t s . The relative p o w e r at 1400 H z d e c r e a s e d with d e c r e a s i n g ratio o f turns to m e a n a m p l i t u d e as well as with d e c r e a s i n g turns from subject to subject (Fig. 4). The p o w e r

I00 ppm

11 PATIENTS WITH NEUROGENIC DISORDERS

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Fig. 4. Power at 1400 Hz in ppm of total power (logarithmic values) as a function of number of turns (above), mean amplitude (middle) and the ratio of turns to mean amplitude (below) in 11 patients with neurogenic disorders. Each point indicates the mean of 10 sites in the brachial biceps muscle of I patient. 1400 Hz power (log ppm) = 0~646 + 0.002 turns (No/sec), r = 0.609, P< 0.05. 1400 Hz power (log ppm) = 1.076 + 0.734 ratio (No/#Vsec), r = 0.746, P < 0.01.

88 No 800.

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Fig. 5. Number of turns analysed at a force of 30 % of maximum as a function of percent change in duration of individual M U P s sampled at weak effort in 11 patients with neurogenic disorders. Each point indicates the mean value of a brachial biceps muscle. Turns (No/sec) = 511 - 2 . 2 duration (%change), r = - 0 . 6 5 3 ,

P.~q).05.

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Fig. 6. Ratio of turns to mean amplitude at a force of 30% of maximum as a function of amplitude of individual M U P s sampled at weak effort in 11 patients with neurogenic disorders. Each point indicates the mean value of a bracbial biceps muscle. Ratio (No/gVsec) = 1.010- 0.0004 amplitude (pV), r = 0.680,

P