Fatigability

Undue fatigability is common in neuromuscular clinics but non-neuromuscular causes are much more common than neuromuscular causes. Generalized fatigue is commonly caused by anemia, hypothyroidism, obstructive sleep apnea, depression, chronic fatigue syndrome, uremia, COPD, etc. Physiological fatigue is accentuated by neuromuscular disorders. Most strikingly, myasthenia gravis causes undue fatigue of the ocular, chewing, swallowing, and breathing muscles. However, ALS, myopathies, and motor neuropathies are also associated with abnormal fatigue. Myasthenia rarely causes isolated fatigue. Examination for fatigability should be part of neuromuscular evaluation and is conducted by inducing repetitive or sustained contraction of the suspected muscles (typically extraocular muscles) for a minute and reevaluation after 2 minutes of rest of the tested muscles.