28 Atypical Motor Neuron Disorders
There are a heterogeneous group of motor neuron disorders that are rare but nonetheless important to recognize, because they often can mimic the presentation of amyotrophic lateral sclerosis (ALS). These often are referred to as atypical motor neuron disorders. Although many of the atypical motor neuron disorders share some features with ALS, they often can be distinguished by their clinical and electrophysiologic characteristics (Boxes 28–1 and 28–2).
Box 28–1
Clinical Clues of An Atypical Motor Neuron Disorder
Non-myotomal pattern of weakness
Absence of significant muscle wasting in chronically weak limbs
Predominantly lower motor neuron signs
Presence of sensory symptoms and/or signs
Cerebellar, extrapyramidal, cognitive, and/or psychiatric dysfunction
Duration of illness longer than 5 years
Onset of illness before age 40 years
History of human immunodeficiency virus infection
Box 28–2
Electrodiagnostic Clues of An Atypical Motor Neuron Disorder
Conduction block on motor nerve conduction studies (not at entrapment sites)
Markedly slowed conduction velocities or prolonged distal latencies (not at entrapment sites)
Sensory nerve conduction abnormalities
Prominent complex repetitive discharges
Facial fasciculations/grouped repetitive motor unit discharges with activation
Acute or subacute neuropathic pattern on needle electromyogram