Posterior interosseous neuropathy: distinguishing from a proximal radial neuropathy

The posterior interosseous nerve is the terminal motor branch of the radial nerve that innervates the extensor carpi ulnaris and the extensors of the thumb and fingers. We describe a case of a posterior interosseous neuropathy presenting with the typical 'finger drop' and partial 'wrist drop'. We focus on the clinical signs that distinguish it from a more proximal radial neuropathy, clarified by nerve conduction studies and needle electromyography. Multimodal imaging of the forearm did not identify a compressive lesion. Persistent symptoms prompted surgical exploration 5 years after initial onset. It identified compression of the posterior interosseous nerve in the region of the arcade of Frohse and leash of Henry. The sites were decompressed and concurrent salvage secondary reconstructive tendon transfers were required in view of the severe axonal loss with minimal chance of functional reinnervation.