By Michael Weiss, MD, University of Washington Medical Center
Most peripheral neuropathies are associated with sensory disturbances such as numbness and tingling. However, there are also forms of neuropathies that only involve motor nerves and produce such symptoms as weakness, twitching and cramping. Some neuropathies may be autoimmune and others are not, and this will dictate the treatment.
When people with these symptoms seek medical attention, a neurologist will perform a full physical and neurological examination, as well as an electrodiagnositic study, including needle EMG (electromyography) and nerve conduction studies. Blood work may also be done to look for the presence of autoantibodies.
Motor neuropathies must be distinguished from motor neuron disorders, diseases in which the nerve cells from which motor nerves arise start to degenerate. The most common form of motor neuron disease is amyotrophic lateral sclerosis (ALS, aka Lou Gehrig’s disease), a relatively uncommon disorder that has limited treatment and is terminal. There have been numerous unsuccessful studies looking at ways to slow progression of ALS, and hope for a cure continues. This is an uncommon disease, though not rare, with an incidence of five to eight in 1000,000 new cases a year. People with ALS typically experience upper motor neuron symptoms such as weakness, incoordination and stiffness, and demonstrate increased reflexes and muscle tone on examination. They also have lower motor neuron symptoms such as muscles cramping and twitching (fasciculations) and muscle wasting (atrophy).
There are many diagnoses that mimic ALS, and several are treatable. Some people are diagnosed with these other syndromes initially only to be told they have ALS later as the condition progresses. Others may be told they have ALS, and then further workup, leads to another diagnosis. This can be frightening and frustrating for patients, caregivers and physicians.
Disorders that can mimic ALS include cervical myelopathy, multiple sclerosis, certain tumors and even hyperthyroidism. In certain cases, a definitive diagnosis is never made. Other motor phenotypes include spinal-bulbar muscular atrophy, an inherited motor neuron disorder, and motor CIDP. Spinal-bulbar muscular atrophy affects one in 500,000, and purely motor CIDP is even rarer. CIDP is also treatable with intravenous immune globulin (IVIG).
One of the more common diseases mimicking motor neuron disease is multifocal motor neuropathy (MMN), a neuropathy that is autoimmune and treatable. The incidence of MMN is less than ALS, occurring in about one to two per 100,000 people. Onset is generally between 30 years and 60 years of age. Motor neuropathies are not seen with upper-motor neuron signs, but their symptoms and signs overlap with the lower-motor neuron presentation of ALS. Anti-GM1 autoantibodies may be found in the blood of some patients. Additionally, conduction block is typically demonstrated on a motor nerve conduction study. MMN is also treated with IVIG.