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Two sisters, 9 and 11 years old, with typical clinical symptoms of Schwartz-Jampel syndrome were investigated. Conventional electromyographic investigation with concentrical needle electrodes in the biceps brachii and tibialis anterior showed continuous muscle activity (myotonic burst, high frequency discharges of single motor units with “bizarre” rhythmic activity). The single motor unit action potential (MUAP) was studied in detail by monopolar surface selective electrode with a small leading off area. The pattern suggests that the muscle membrane alone is the not the only reason for abnormality.
Continuous muscle activity is a prominent symptom in patients with Schwartz-Jampel syndrome. Some authors maintain that this may originate in the nerve or end plate. Lehmann-Horn et al 1 showed two muscle membrane abnormalities by voltage clamp and patch clamp techniques and concluded that spontaneous activity in the Schwartz-Jampel syndrome originated in the muscle membrane itself. Arimura et al 2found a normal end plate function and assumed that the motor unit pattern influenced interdischarge interval changes. It is difficult to make a precise analysis of the MUAPs with concentric needle electrodes because of other interfering spontaneous activities. Thus a monopolar surface selective electrode with a small leading off area3was employed to obtain a more precise assessment of a single MUAP pattern.
The patients were two sisters, 9 and 11 years old, from consanguineous parents. They displayed short stature, bone deformities (kyphoscoliosis, pigeon breast, short neck, pes equinovarus), facial dysmorphism, muscle stiffness, and missing tendon reflexes in the lower limbs. Concentric needle EMG was performed when the patients were 7 and 9 years old and disclosed abnormality. The needle insertion, mechanical stimulation, and mild muscular contraction induced spontaneous activity. Myotonic discharges (fig 1 A and B) were found in all examined muscles (abductor digiti minimi, quadriceps femoris, tibialis anterior, biceps brachii).There were also spontaneous high frequency biphase potentials. Some of the high frequency discharges appeared as doublets or complex repetitive discharges. Routine nerve conduction studies (motor conduction velocity, distal latency, compound muscle action potentials, and sensory action potentials in upper and lower limbs) were normal.4 Electromyographic investigations of single MUAPs were performed in biceps brachii and tibialis anterior muscles. Involuntary motor unit activity was recorded by monopolar surface selective electrode with a small leading off area3for 30 minutes. A Mistro 5+electromyograph and a Teac type recorder were employed to register the action potentials. Distance between the negative peaks of MUAP was measured with a resolution of 0.1 ms. After applying these electrodes we found single MUAP trains between myotonic discharges. They showed without provoking a burst of activity, as usually happens during needle electromyography.
Motor unit firing began with doublet discharges (fig 2 trace 1). After a few seconds MUAP alternated between doublets and triplets (fig 2trace 2). and then the motor unit fired with stable triplets (trace 3). Similarly, triple discharges turned into quadruplets, and then multiplets (traces 4 to 11) and the number of firing impulses increased at the end of motor unit discharge. All multiplet impulses were similar in shape.
Amplitude of the second impulse in the doublet was lower or higher than the amplitude of the first impulse (fig 2 trace 1). Amplitude of the second impulse in the triplet was predominantly lower than that of the first impulse but could also be higher (traces 2 and 3), whereas amplitude of the third impulse varied depending on the interimpulse interval, and was higher after a larger interimpulse interval. Other multiple discharges followed the same pattern. At the end of motor unit firing, the amplitude of successive impulses smoothly decreased (traces 10 and 11).
Every successive change in the number of impulses in a train was marked by a longer interval between the latest impulse of the train and the “new” impulse. With the increase of the number of impulses in the train, the interval before the emergence of a “new” impulse decreased (fig 2 traces 12 to 18). Interimpulse interval in the multiplets had a latency of 2 to 10 ms. The phenomenon was found in both sisters.
Electrophysiological studies of patients with the Schwartz-Jampel syndrome (normal nerve conduction, spontaneous activity myotonic discharge) have implied that spontaneous activity originates in the muscle membrane. The single MUAP pattern of repetetive neuronal discharges, however, suggests that a defect in the muscle membrane is not the only reason of abnormality. The MUAP pattern found supposes an influence at a higher level than the muscle. Multiplet discharge with an interimpulse interval of 2 to 10 ms probably originated in the muscle membrane but the increasing number of impulses in multiplets and the long intermultiplet intervals cannot be explained simply by the known abnormalities of the muscle membrane. Moreover, doublet, triplet, and multiplet electromyographic phenomena are neuromyotonic. In our opinion the reasons for the abnormality are complex and not yet understood.
This research was supported by Grant L-420/1994-97 of the National Scientific Research Fund.
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