To describe the clinical, electrodiagnostic (EDx), and imaging features of a chronic, immunoglobulin(IG)-responsive, focal posterior interosseous neuropathy (PIN) involving the posterior interosseous branch of the right radial nerve with 7 years of follow up.  

IG-responsive neuropathies include typical chronic inflammatory demyelinating polyneuropathy and variants such as multifocal motor neuropathy (MMN).  MMN typically presents with asymmetric and progressive weakness in multiple nerve distributions without sensory involvement. Rarely, a single nerve can be affected in an MMN variant known as monofocal motor neuropathy. Distinguishing MMN from other mononeuropathies is of importance because of immunomodulatory treatment responsiveness.

A descriptive case report and literature review.

The patient is a 59-year-old female with an 8-year history of right finger extension weakness (digits 3-5 > digit 2) and forearm cramping with finger extension. Repeated EDx studies demonstrated reduced recruitment, fibrillations, fasciculations/myokymia/cramp potentials in the right extensor digitorum communis and extensor indicis proprius indicating active denervation and peripheral nerve hyperexcitability in the posterior interosseous nerve distribution. Recent MR neurography of the right radial nerve and anti-GM1 serologies were normal. The patient has been treated with intravenous IG over the last three years, but only intermittently due to patient preference. Accordingly, she has had 4 stereotypical recurrences of weakness and cramping, but her clinical symptoms and examination have consistently and rapidly resolved with IVIG treatments.

While our case does not meet criteria for definite MMN, several features suggest a focal variant of multifocal motor neuropathy. These characteristics include IG-responsiveness, lack of overt reinnervation changes (enlarged motor units) on repeated EDx despite a duration of over 8 years, and evidence of peripheral nerve excitability. Whether this case represents PIN monofocal motor neuropathy or a novel immune-mediated syndrome is unclear, but this presentation suggests significant overlap with MMN.