Multicore disease in twins.

Multicore disease in identical twin boys presented in infancy as generalized weakness and torticollis. Motor milestones such as sitting, standing, and running were delayed, although by the age of 6 years marked improvement in muscle strength had occurred. Serum enzymes were normal. Muscle biopsy revealed multifocal areas of decreased oxidative enzyme activity. Ultrastructurally, these areas were characterized by myofilament disruption and Z-band streaming.