[A case of possible multifocal motor neuropathy with suspected posterior interosseous nerve palsy].

We present the case of a 33-year-old man with progressive weakness in the left wrist and finger extensors. Initially, posterior interosseous nerve (PIN) palsy was suspected. However, nerve conduction studies detected conduction block and ultrasonography demonstrated swelling between the elbow and spiral groove in the left radial nerve. Anti-GM1 IgM antibodies were positive, leading to a diagnosis of possible multifocal motor neuropathy (MMN). Although MMN usually represents multifocal pure motor neuropathies, the involvement of a single nerve allows for diagnosis of possible MMN. PIN palsy of indeterminate etiology should prompt consideration, underscoring the utility of ultrasonography in the diagnostic process.