Symptomatic distal myopathy with cardiomyopathy due to a MYH7 mutation

Mutations in the myosin heavy chain gene (MYH7) can cause several distinct phenotypes depending on the location of the mutation: hypertrophic cardiomyopathy (several exons), myosin storage myopathy (exon 37/39) or Laing distal myopathy (exons 32-36). Here, we describe a unique combination of hypertrophic cardiomyopathy and hypertrophic distal myopathy in a family with a MYH7 Val606Met mutation (exon 16).

Relation between muscle fiber conduction velocity and fiber size in neuromuscular disorders

To determine the relation between muscle fiber conduction velocity (MFCV) and muscle fiber diameter (MFD) in pathological conditions, we correlated invasively measured MFCV values with MFD data obtained from muscle needle biopsies in 96 patients with various neuromuscular disorders. MFCV was significantly correlated with MFD and independent of the underlying disorder. Pathological diameter changes were fiber-type dependent, with corresponding MFCVs. A linear equation expresses the relation well: MFCV (m/s) = 0.043·MFD (μm) + 0.83. We conclude that fiber diameter determines MFCV largely independent of the underlying neuromuscular disorders studied.

Recognising F-response interference as a source of increased jitter in stimulated single fibre EMG

OBJECTIVE

To determine the source of an abnormal pattern of latency shifts leading to falsely high jitters in single fibre electromyography (SFEMG).

METHODS

We observed a sudden shortening of the latency to an individual single fibre spike component followed by a gradual return to baseline values during stimulation single fibre electromyography (SFEMG) of the facial muscle. The pattern could be reproduced in healthy controls.

RESULTS

The sudden decrease in latency proved to follow an additional discharge of the muscle fibre, not due to the external stimulus. This additional discharge was identified as an F-response.

CONCLUSIONS

The mechanism is thought to be a higher muscle fibre conduction velocity resulting from a temporary increase in stimulus frequency, in the form of an extra impulse along the muscle fibre represented by the F-response.

SIGNIFICANCE

The typical abnormal pattern should be recognised because it can falsely increase the mean jitter. We advice to increase the time base to 50 ms if this pattern is observed and to exclude the affected potentials from jitter measurements.