Type 2B muscle fibre deficiency in myotonia and paramyotonia congenita. A genetically determined histochemical fibre type pattern?

The histochemical ATPase fibre type pattern was examined in muscle biopsy samples obtained from patients with recessive myotonia, paramyotonia and from one patient with dominant myotonia. Absence (less than or equal to 5%) of 2B fibres was a genuine finding in the minority of the cases. In additional cases of recessive myotonia, a deficiency (less than or equal to 15%) of 2B fibres was observed. Absence or deficiency of 2B fibres was not related to the minor myopathic alterations or to (para-)myotonic activity. It is hypothesised that absence of 2B fibres is a dominant or a recessive autosomal trait, and deficiency of 2B fibres is a recessive trait. Reported findings and our own observations suggest the possibility of a genetic combination of myotonia and absence/deficiency of 2B fibres. Implications of these hypotheses are proposed.