Acute-Onset Multifocal Motor Neuropathy (AMMN): How We Meet the Diagnosis

Multifocal Motor Neuropathy: A Narrative Review

Multifocal motor neuropathy (MMN) is an acquired autoimmune polyneuropathy that affects almost exclusively the motor nerve fibers. Typically seen in middle-aged adults, its predominant clinical feature is a chronically progressive asymmetric weakness that affects the distal upper extremities most significantly. Minor sensory symptoms, sensory examination findings or abnormal sensory nerve conduction studies can be seen in the lower extremities in a minority of patients. Electrodiagnostic studies reveal motor conduction blocks at noncompressible sites, and minor findings of other demyelinating features such as conduction slowing or temporal dispersion. Anti-GM1 antibody titers are elevated in less than half of MMN patients, and more recent studies suggest mechanisms including antibody-induced complement attack at the node of Ranvier with resulting ion channel dysfunction. Peripheral nerve magnetic resonance imaging and neuromuscular ultrasound often reveal non-uniform enlargement of the nerve roots, plexuses, or peripheral nerve segments, thus being useful in assisting diagnosis. The differential diagnosis of MMN mainly includes motor neuron disease or demyelinating sensorimotor polyneuropathies. Immunoglobulin therapy is the first-line and mainstay of treatment, being effective in maintaining or restoring muscle strength in the majority of patients. However, motor strength often slowly declines over the long term, even with maintenance immunoglobulin treatment. More effective immunotherapy is needed to halt the slow progression of MMN, and complement inhibition appears to be a promising option in the near future.

Finger drop sign as a new variant of acute motor axonal neuropathy

We propose the finger drop sign as a new clinical variant of acute motor axonal neuropathy (AMAN) defined by immunological and radiological evidence. We identified eight consecutive patients who had AMAN. All of them developed prominent involvement of the finger extensors. We performed magnetic resonance imaging (MRI) of the extremity muscles and serological assays for antiganglioside antibodies and Campylobacter jejuni. Patients with AMAN showed characteristic and a markedly sustained weakness of the finger extensors with a distinctive pattern of the finger drop sign. Limb MRI revealed unevenly distributed abnormal signals in the muscles mainly innervated by the posterior interosseous nerve. All tested patients showed positivity for immunoglobulin G antibody against ganglioside complex of GM1 and phosphatidic acid. A pathophysiological understanding of this unique syndrome can provide further insight into antiganglioside-antibody-mediated axonal injury in Guillain-Barré syndrome.