Fasciculations are rapid, flickering, twitching, involuntary movements within a muscle belly resulting from spontaneous activation of a bundle, or fasciculus, of muscle fibers (i.e., a motor unit), insufficient to move the joint. Fasciculations may also be induced by lightly tapping over a partially denervated muscle belly. The term was formerly used synonymously with fibrillation, but the latter term is now reserved for contraction of a single muscle fibre, or a group of fibers smaller than a motor unit.
Brief and localized fasciculations can be a normal finding (e.g., in the intrinsic foot muscles, especially abductor hallucis, and gastrocnemius, but not tibialis anterior), particularly if unaccompanied by other neurological symptoms and signs (wasting, weakness, sensory disturbance, sphincter dysfunction). Persistent fasciculations most usually reflect a pathological process involving the lower motor neurones in the anterior (ventral) horn of the spinal cord and/or in brainstem motor nuclei, typically motor neurone disease (in which cramps are an early associated symptom). Facial and perioral fasciculations are highly characteristic of Kennedy’s disease (X-linked bulbospinal neuronopathy). However, fasciculations are not pathognomonic of lower motor neurone pathology since they can on rare occasions be seen with upper motor neurone pathology.
The pathophysiological mechanism of fasciculations is thought to be spontaneous discharge from motor nerves, but the site of origin of this discharge is uncertain. Although ectopic neural discharge from anywhere along the lower motor neurone from cell body to nerve terminal could produce fasciculation, the commonly encountered assumption that it originates in the anterior horn cell body is not supported by the available evidence, which points to a more distal origin in the intramuscular nerve terminals. In addition, denervation of muscle fibers may lead to nerve fibre sprouting (axonal and collateral) and enlargement of motor units which makes fasciculations more obvious clinically. Fasciculations may be seen in:
Motor neurone disease with lower motor neurone involvement (i.e., progressive muscular atrophy, progressive bulbar atrophy variants)
Spinal muscular atrophy
Cervical radiculopathy (restricted to myotomal distribution) Multifocal motor neuropathy with conduction block
Benign fasciculation syndrome: typically seen only after exercise and without associated muscle atrophy or weakness
Cramp fasciculation syndrome
Kennedy’s disease (X-linked bulbospinal neuronopathy; especially perioral)
Almost any lower motor neurone disease, especially compression Metabolic causes: thyrotoxicosis, tetany, after acetylcholinesterase
inhibitors, anesthetic muscle relaxants.
Fasciculations may need to be distinguish from myokymia or neuromyotonia.
Blexrud MD, Windebank AJ, Daube JR. Long-term follow-up of 121 patients with benign fasciculations. Annals of Neurology 1993; 34: 622625
Desai J, Swash M. Fasciculations: what do we know of their significance? Journal of the Neurological Sciences 1997; 152 (suppl1): S43-S48 Layzer RB. The origin of muscle fasciculations and cramps. Muscle Nerve 1994; 17: 1243-1249