What is Myokymia?
Myokymia is an involuntary movement disorder characterized by spontaneous, continuous, undulating muscle contractions that produce a distinctive worm-like or vermiform rippling appearance of the overlying skin, caused by repetitive firing of motor unit groups at the neuromuscular level. 1, 2
Clinical Characteristics
Distinctive Movement Pattern:
- The hallmark feature is slow, undulating, worm-like muscular contractions visible through the skin that distinguish it from simple fasciculations 1, 3
- Unlike fasciculations (single muscle twitches), myokymia produces tetanic-like contractions lasting several seconds 4
- The movements are continuous and spontaneous, occurring at rest 3
Associated Symptoms:
- Muscle stiffness is the most common presenting complaint (60% of cases) 5
- Muscle cramps occur in 12% of patients 5
- Muscle pain and itchy sensations may accompany the movements 1
- Excessive sweating (hyperhidrosis) can occur, particularly in Isaacs syndrome 1, 3
- Easy fatigability is frequently reported 3
Classification and Distribution
Anatomic Distribution:
- Facial myokymia: Affects facial muscles 1
- Limb myokymia: Involves extremity muscles 1
- Generalized myokymia: Affects multiple muscle groups throughout the body 3, 5
Clinical Syndromes:
- Idiopathic generalized myokymia (IGM): Affects men and women equally with mean age of onset 29 ± 19 years, with 30% having positive family history 5
- Isaacs syndrome: Myokymia associated with hyperhidrosis and dysautonomia 1
- Hereditary myokymia with episodic ataxia: Autosomal dominant condition caused by KCNA1 gene mutations 2
Electromyographic Features
Diagnostic EMG Findings:
- Continuous spontaneous repetitive discharges of motor units at rest are pathognomonic 3
- Grouped discharges fire irregularly at interburst frequencies of 2-300 Hz (typically 30-50 Hz) 3, 5
- Discharge duration ranges from 1-2 seconds with amplitude around 1 mV 3
- EMG is essential for diagnosis, as it can distinguish myokymia from fasciculations when clinical examination is ambiguous 4, 5
Underlying Etiologies
Common Associated Conditions:
- Multiple sclerosis 1
- Guillain-Barré syndrome 1, 4
- Hypothyroidism 1
- Gold salt therapy for rheumatoid arthritis 4
- Peripheral neuropathy 4
- Genetic mutations in voltage-gated potassium channels (KCNA1) 2
Important Caveat: Cases with completely normal neurological findings are rare, so thorough neurological examination is essential to identify underlying pathology 1
Pathophysiology
Site of Origin:
- The exact mechanism remains incompletely understood, with proposed sites including the neuromuscular junction, peripheral nerves, or spinal cord 1
- In hereditary forms, loss-of-function mutations in the Kv1.1 potassium channel subunit cause neuronal hyperexcitability 2
- Some cases show evidence of combined damage to peripheral nerves, neuromuscular junctions, and muscles 3
Treatment Approach
Effective Pharmacologic Options:
- Both phenytoin and carbamazepine are effective treatments for controlling symptoms 5
- Treatment selection should be guided by the underlying etiology when identified 1, 4
Clinical Pitfall: Do not dismiss muscle twitching as benign fasciculations without EMG confirmation, as myokymia requires specific evaluation for underlying neurological disease and responds to targeted anticonvulsant therapy 5