A new congenital neuromuscular disease with trilaminar muscle fibers.

An infant was born with marked rigidity, a paucity of spontaneous movements, and increased serum creatine phosphokinase activity. Electromyogram was normal. A muscle biopsy, taken at the age of 7 weeks, contained numerous fibers composed of three concentric zones, warranting the designation "trilaminar fibers." Electronmicroscopy showed the innermost zone contained a densely packed collection of mitochondria, glycogen, electron opaque material, and single filaments. The middle zone consisted of myofibrils with Z-band smearing. The outer zone resembled a sarcoplasmic mass. Extrajunctional acetylcholine receptor (AChR) was present in the trilaminar fibers between the middle and outer zones. The increased muscle tone and extrajunctional AChR suggest and altered neural influence, but the markedly increased creatine phosphokinase activity is more characteristic of muscle damage. Precise definition of the nature of the defect awaits further study.