Benign infantile neurogenic muscle atrophy predominantly involving the upper extremities

An eight year-old girl was first noted to be hypotonic at 4 months of age. She had a delay in achieving developmental milestones and showed apparent weakness predominantly affecting muscles of the upper extremities. In the left biceps brachii muscle biopsy at the age of 5 months, there were groups of atrophic fibers and marked fiber type grouping, but intramuscular nerves were well myelinated and there were few type 2C fibers. She improved with age and learned to walk at 1 year and 4 months and had minimal weakness of the forearm muscles and mild atrophy of the muscles of the right upper extremity. She recently developed a mild scoliosis. Since the EMG was not neurogenic and no mutations were found in the survival motor neuron (SMN) gene, we believe that she does not have a degenerative neurogenic disorder but rather has an abnormal innervation of the skeletal muscles due either to anterior horn cell dysgenesis or anomalous peripheral nerve branching.