Differential diagnosis Other disorders that cause pure muscle weakness should be ruled out: Disorders of neuromuscular transmission Various myopathies (including noninflammatory and drug-induced) Spinal muscular atrophies (mostly in children) Polymyositis Dermatomyositis Thyroid disorders and adrenal disorders Electrolyte abnormalities (eg, hypokalemia, hypercalcemia, hypophosphatemia) Various infections (eg, syphilis, Lyme disease, hepatitis C) Autoimmune-mediated motor neuropathies Cervical spinal stenosis Behavioral variant of frontotemporal dementia (15 to 20% develop motor neuron disease) Heavy metal toxicity (eg, lead, mercury) When cranial nerves are affected, a treatable cause is less likely.