Type II fiber predominance with motor neuron dysfunction.

Several variations in the muscle fiber distribution and differentiation have been described. Different hypotheses, including maturational dysfunction of motor neurons have been postulated. We present a 28 year-old male with proximal weakness, and negative family history. Motor strength was 4-/5 proximally and 4+/5 distally in the extremities. He was areflexic with trace triceps jerks. Electrophysiological studies showed motor neuron dysfunction. Marked type II fiber predominance 97% was noted without group atrophy on a vastus-lateralis muscle biopsy. Type 2A were the largest with type 2B and 2C smaller than type 1. A deltoid muscle biopsy and electrophysiological studies performed nine years earlier depicted the same changes. With electrophysiological studies consistent with static motor neuron dysfunction and clinical and pathological presentation of myopathy, we propose a functional abnormality of motor neuron in-utero leading to abnormal muscle fiber differentiation and growth.