Myokymia is caused by the hyperactivation of one or more motor units, which are the functional units that control muscle contraction. A motor unit consists of a single motor neuron and the group of skeletal muscle fibers it innervates. Since each motor unit innervates several hundred
muscle fibers, its activation causes a continuous rippling movement of the skin overlying the contracting muscle cells.
Eyelid myokymia, which may present as eyelid twitching, is common among healthy individuals and is usually triggered by stress, fatigue, lack of sleep, or caffeine. When myokymia occurs in the face, it most frequently affects the orbicularis oculi muscle, which is the muscle responsible for closing the eyelids. Less common causes of facial myokymia include damage to the facial nerve nucleus in the brainstem due to demyelinating disorders, such as multiple sclerosis or compression from brainstem tumors.
Limb or generalized myokymia, on the other hand, limb or generalized myokymia, are almost always a result of an underlying disorder affecting motor nerve function. Cervical or lumbar radiculopathies can cause limb myokymia; chronic entrapment of a nerve (e.g., median neuropathy at the carpal tunnel); or as a classic complication of radiation-induced nerve damage. Finally, generalized myokymia can be seen in acute inflammatory myopathies or neuropathies, such as Guillain-Barré syndrome or chronic inflammatory demyelinating polyneuropathy (e.g., multiple sclerosis), as well as in uremia, thyrotoxicosis, hereditary episodic ataxia, or due to timber rattlesnake venom. Although rare, generalized or limb myokymia may be seen after strenuous exercise in healthy individuals.
