General

• Disuse of muscles
• Anemia
• Malnutrition
• Electrolyte abnormality, eg, hypercalcemia, hypokalemia, hyponatremia
• Endocrine disease, eg, hypothyroidism, diabetes mellitus, Cushing’s syndrome, Addison’s disease, hyperaldosteronism
• Chronic lung or heart disease
• Malingering

Upper motor neuron disease

• Stroke
• Space-occupying lesion, eg, brain tumor
• Spinal cord lesion
• Multiple sclerosis

Lower motor neuron disease

• Anterior horn cell, eg, poliomyelitis, amyotrophic lateral sclerosis (ALS), West Nile virus
• Motor nerve root, eg, disk protrusion
• Nerve plexus, eg, brachial plexus neuropathy
• Peripheral nerve, eg, diabetes mellitus, alcoholism, heavy metals, Guillain-Barré syndrome
• Neuromuscular junction, eg, myasthenia gravis, Eaton-Lambert syndrome, botulism, organophosphate poisoning, tick paralysis, drug-induced, eg, aminoglycoside
• Myopathy, eg, polymyositis, corticosteroids, hypothyroidism, alcohol
• Other: aortic dissection, periodic paralysis, eg, hypokalemic, hyperkalemic, thyrotoxic

• Myopathy
• Peripheral neuropathy (polyneuropathy)
• Fatigue
• Immobility (especially in elderly)
• Catatonia or stiffness
• Falls & gait disorders