A 61-year-old woman was followed for multifocal motor neuropathy (MMN) with conduction block since young adulthood. She had bilateral clinical and neurophysiologic involvement of both median and ulnar nerves, as also the right peroneal nerve. In a recent reevaluation, we decided to complete her investigation with a brachial plexus magnetic resonance imaging (MRI) that, besides diffuse thickening of the right brachial plexus trunks, showed a round hyperintense cyst that compressed several branches distally (Figure). Brachial plexus MRI may be important [...]