Compressive neuropathy of the branch of the posterior interosseous nerve with isolated paralysis of the extensor digitorum longus muscle: A case report

Compressive neuropathies occur when a nerve is compressed in a closed osteofibrous tunnel. Here, we report a case of isolated paralysis of the extensor digitorum longus muscle due to compression of a branch of the posterior interosseous nerve (PIN) in the proximal forearm in a 56-year-old man presented with progressive weakness of the left hand over 18 months. During the first time of clinical consultation, we found a swelling in the extensor aspect of the left proximal forearm, and a firm, non-tender, mobile swelling located deep in the extensor muscles, without other muscle involvement. The sensation was intact. An ultrasound scan and magnetic resonance imaging scan confirmed the diagnosis of an intramuscular lipoma, which was further confirmed with histopathologic examination. The patient was treated with an excision of the lipoma and decompression of the nerve. Post-operatively, the patient was given regular physiotherapy, through which he achieved improvement in his finger extension within a year. This is an extremely rare case of isolated paralysis of the extensor digitorum longus due to compression of the branch of the PIN by an intermuscular lipoma in the forearms.