Motor unit size in muscular dystrophy, a macro EMG and scanning EMG study.

Abstract

SUMMARY Patients with muscular dystrophy were investigated with Macro EMG to study activity from whole individual motor units, and with Scanning EMG to study the distribution of activity within the motor unit. Macro motor unit potentials were normal or only slightly reduced in amplitude. In Scanning EMG the units had unchanged mean length compared with normal, but an uneven distribution of the activity. This was also seen in severely weak muscles. The findings are interpreted to be the result of degenerative and regenerative processes, giving rise to remodelling of the motor unit.The local fibre density (FD) measured with single fibre electromyography (SFEMG) is increased in muscular dystrophies.'-3 An increased FD is commonly seen in neurogenic disorders where it is interpreted as a sign of reinnervation, giving dense motor units with increased number of fibres.4 In these cases the concentric needle EMG (CNEMG) also indicates an increase in number of muscle fibres in the motor unit within the pick-up area of the electrode. In muscular dystrophies, however, CNEMG does not indicate dense motor units. Instead the typical findings of reduced duration and amplitude of the motor unit potentials are considered to reflect smaller than normal motor units.5 Thus in muscular dystrophies the two techniques give apparently contradictory findings. This may be due to the difference in pick up area of the two electrodes, although the concentric electrode is also relatively selective, recording from only a small part of the entire motor unit.

Estimation of the number of motor units based on macro-EMG.

“…The MU uptake area of the macro-electrode therefore has a radius of about D + R. On the analogy of a muscle fibre, the contribution of a MU to the macro-SEMAP decreases rapidly to zero when the distance from the macro-electrode to the centre of a MU increases from D-R to D + R. The radius of the MU uptake area of the macro-electrode will therefore be by approximation D. The mean cross-section of MU territories in the tibialis anterior muscle is about 8 mm. 15 16 The MU uptake area of the macro-electrode in the tibialis anterior muscle will therefore be a cylinder with a radius of about 4 mm and a length of about 23 mm. In line with this suggestion is the fact that the MUD ranges between 82 and 10, whereas estimates of the total number of MUs within the tibialis anterior muscle based on axon counts give a number of 370 MUs7 and 455.…”

Macroelectromyography: A review of the technique and its value in the investigation of neuromuscular disorders

“…Macro‐EMG also shows a tendency for slightly reduced amplitudes in the primary necrotizing myopathies, such as the muscular dystrophies and inflammatory myopathies (Fig. 7), supporting the notion of muscle‐fiber loss 10. However, in many patients, the median values lie within the normal range, and in some, the macro‐MUP amplitudes may be increased 4, 6.…”

Scanning Electromyography

“…The number of silent areas has also been measured in the biceps brachii. A mean of 0.5 silent areas (Hilton-Brown & Stålberg, 1983b) and 0.15 ± 0.36 silent areas with a range of 0 to 1 (Stålberg & Diószeghy, 1991) have been reported. In the tibialis anterior, a mean of 0.2 silent areas (Hilton-Brown & Stålberg, 1983b) and 0.19 ± 0.47 silent areas with a range of 0 to 2 (Stålberg & Diószeghy, 1991) have been reported.…”

“…A mean of 0.5 silent areas (Hilton-Brown & Stålberg, 1983b) and 0.15 ± 0.36 silent areas with a range of 0 to 1 (Stålberg & Diószeghy, 1991) have been reported. In the tibialis anterior, a mean of 0.2 silent areas (Hilton-Brown & Stålberg, 1983b) and 0.19 ± 0.47 silent areas with a range of 0 to 2 (Stålberg & Diószeghy, 1991) have been reported. The length of the individual motor unit fractions in the biceps brachii has been reported as 1.56 ± 1.07 mm with a range of 0.35 to 5.10 mm (Navallas & Stålberg, 2009).…”