In 2000, a 30-year-old man presented with 1-year history of a left-sided extensor weakness of his upper limb. Pre-existing illness, trauma and hereditary neuromuscular diseases were denied.

On physical examination, his left triceps muscle appeared atrophic. The muscle strength of the elbow extensors and the left triceps reflex were diminished. He had no pain and no sensory disturbances. MRT of the cervical spine and the NLG (left median, radial nerve) were inconspicuous. EMG revealed neuropathic changes in triceps brachii muscle. No diagnosis was established, and follow-ups demonstrated a stable course.

In 2002, the patient was reassessed without any further findings. Several differential diagnoses were …