Motor weakness can be due to dysfunction in the corticospinal tract, basal ganglia, spine, peripheral nerves, or muscle. A careful examination of the motor system enables the clinician to localize the lesion, build a differential diagnosis, and choose appropriate imaging and/or laboratory tests.
The limbs and shoulder girdle should be fully exposed, then inspected for the following:
Atrophy
Hypertrophy
Asymmetric development
Fasciculations
Myotonia
Other involuntary movements, including chorea (brief, jerky movements), athetosis (continuous, writhing movements), and myoclonus (shocklike contractions of a muscle)
Passive flexion and extension of the limbs in a relaxed patient provide information about muscle tone.
Atrophy is indicated by decreased muscle bulk, but bilateral atrophy or atrophy in large or concealed muscles, unless advanced, may not be obvious. In the elderly, loss of some muscle mass is common.
Hypertrophy occurs when one muscle must work harder to compensate for weakness in another; pseudohypertrophy occurs when muscle tissue is replaced by excessive connective tissue or nonfunctional material (eg, amyloid).
Fasciculations (brief, fine, irregular twitches of the muscle visible under the skin) are relatively common. Although they can occur in normal muscle, particularly in calf muscles of the elderly, fasciculations frequently indicate lesions of the lower motor neuron (eg, nerve degeneration or injury and regeneration).
Myotonia (slowed relaxation of muscle after a sustained contraction or direct percussion of the muscle) suggests myotonic dystrophy or another myotonic disorder and may be demonstrated by inability to quickly open a clenched hand.
Increased resistance followed by relaxation (clasp-knife phenomenon) and spasticity indicates upper motor neuron lesions.
Lead-pipe rigidity (uniform rigidity throughout the range of motion), often with cogwheeling, suggests a basal ganglia disorder.
(See also How to Assess Muscle Strength and Introduction to the Neurologic Examination.)
