Myotonic discharges are a frequent symptom in myofibrillar myopathies

Aims: In patients with myofibrillary myopathies (MFM), myotonic discharges have occasionally been detected by needle electromyography (EMG). Nevertheless, this peculiar type of spontaneous serial discharges has not attracted special interest in the genetically heterogeneous MFMs.

Patients: Therefore, the EMG and histopathological features were analysed in 6 patients with genetically confirmed MFM (n=1 MYOT, n=1 DES, n=2 ZASP, n=2 FLNC).

Results: Fibrillations, positive sharp waves and myotonic discharges were found in all 6 patients, pseudomyotonic discharges in 5. Myotonic discharges were detected in about 50% of the analysed muscles independent of the site, namely in distal (3/6) and proximal limb muscles (4/6) as well as in paravertebral muscles (3/6). Clinical signs of myotonia could not be elicited in any patient. The degree of myotonic discharges does not seem to be related to the number of muscle fibres affected by vacuoles and myofibrillary accumulations.

Conclusion: According to our data, the spectrum of disorders, in which myotonic discharges can be regularly observed, has to be extended by the group of MFMs, irrespective of the underlying genotype. Beside the classic myotonias and myotonic dystrophies myofibrillary myopathies should be considered in cases with myotonic discharges.