A 35-year-old Chinese male with no significant past medical history presented to our hospital with a 6 year history of bilateral hand weakness. The patient reported that he first noticed the weakness after a motor vehicle accident while riding his bicycle, but had little work-up because of his minor injuries. When presenting to the Emergency Department, he described the hand weakness as progressive, accompanied by pain and cramping in his upper extremities. The patient’s physical exam findings revealed poor strength in the wrist extensors and hand interossei. Power was 1/5 on wrist extension and finger abduction, and there was bilateral wrist drop (see image). He had absent reflexes in the brachioradialis, triceps, patellar and ankle innervations bilaterally. EMG nerve conduction studies of the upper extremities showed evidence of acquired poly-motor neuropathy with the presence of conduction blocks in multiple motor nerves of the upper extremities. Lab values including ESR, ANA, DNA (DS), SM and SM/SMP, SS-A, SS-B were all negative. His anti-GM1 IgM ratio was 1:1600, confirming our diagnosis of Multifocal Motor Neuropathy (MMN). He received 5 doses of IVIG, along with physical therapy, and responded well with improvement in his motor function.
Multifocal Motor Neuropathy (MMN) is an acquired autoimmune demyelinating neuropathy with slowly progressive weakness, fasciculation, and muscle cramping without loss of sensation. MMN presents similarly to ALS, but without the muscle wasting and rapid progression. The pathologic mechanism is believed to be secondary to autoantibodies against the GM-1 ganglioside within the nodes of Ranvier, causing nerve conduction to be blocked. The disease can be detected by looking for these antibodies in the blood, or by EMG studies and clinical criteria. It occurs twice as often in men, and most patients are diagnosed between the ages of 40-60 years. IVIG is the first choice in treatment with the hope of suppressing the over activity of the immune system, but other pharmacological therapies are available. Response to IVIG may occur fairly rapidly, but the dose and frequency may need to be individualized depending on the length and benefits.
MMN is a demyelinating neuropathy that is infrequently considered but can lead to a significant decrease in quality of life. It is important for hospitalists to consider MMN on the differential of demyelinating disorders as it causes significant disability but it does not shorten life, and the prognosis is generally good. It is a difficult disease to both diagnose and live with, but increased awareness will lead to better care and outcomes.