Severe distal muscle involvement and mild sensory neuropathy in a boy with infantile onset Pompe disease treated with enzyme replacement therapy for 6 years.

Enzyme replacement therapy in infantile onset Pompe disease has led to a new phenotype with features not known in the pre-enzyme replacement therapy era. We investigated the origin of a rapidly emerging and severe weakness of the foot dorsiflexors in a 7-year-old boy after 6.5 years of enzyme replacement therapy. Electroneurography yielded normal findings except low compound muscle action potentials of the extensor digitorum brevis muscles after stimulation of the peroneal nerves. Electromyography of the tibial muscle demonstrated a myopathic pattern. Tibial muscle, sural nerve, and skin biopsy showed a myopathy with empty and glycogen containing vacuoles, a mild loss of myelinated and unmyelinated axons, and a moderately reduced intraepidermal nerve fiber density. These findings provide evidence for a severe distal muscle involvement and a mild sensory neuropathy evolving during the course of disease after long-term enzyme replacement therapy, thereby expanding the new emerging phenotype of infantile onset Pompe disease.