clonic

Clonic is an adjective used in medicine to describe phenomena related to clonus, a neuromuscular sign characterized by rhythmic, involuntary muscle contractions. Clonus arises from hyperexcitability of the stretch reflex circuitry, typically due to upper motor neuron lesions. Clinically, clonus most commonly presents as ankle clonus, a series of rhythmic plantar flexion movements triggered by a rapid stretch, though it can occur in other muscle groups. Clonic movements differ from myoclonus (brief, irregular jerks) and from tonic contractions (sustained stiffening). In epilepsy, the term clonic is used to describe the rhythmic jerking phase of a generalized tonic-clonic seizure, and in combination with tonic (tonic-clonic seizures). The clonic phase consists of repetitive, rhythmic contractions that may vary in amplitude and frequency and can last from seconds to minutes.

Sustained clonus, or continuous rhythmic jerking, is a sign of ongoing upper motor neuron dysfunction and may be associated with conditions such as stroke, spinal cord injury, multiple sclerosis, cerebral palsy, or other central nervous system insults. Diagnosis is clinical, often aided by elicitation of reflex-induced clonus during passive movement; imaging or electrophysiology may be used to identify an underlying lesion. Management focuses on addressing the underlying cause and, when seizures are present, applying standard antiepileptic therapy. Clonus is a sign rather than a disease, reflecting the state of the motor pathways in the central nervous system.