Hypertrophic branchial myopathy with uniform predominance of type 1 fibres. Case report.

A 17-year-old Japanese girl presented with left masseteric hypertrophy. She had a habit of chewing only on the affected side. Histochemical study of the masseter muscle showed myopathic changes characterised by uniform type 1 fibres with gross hypertrophy, increases of internal nuclei and split fibres, and endomysial fibrous tissue. This condition could be compatible with hypertrophic branchial myopathy, because it was confined to the masseter muscle and so cannot be classified into any known type of systemic myopathy.