What is Clonus
Clonus is a series of involuntary, rhythmic, and brisk muscle contractions and relaxations caused by upper motor neuron lesions, occurring at a frequency of 5-8 Hz with oscillation periods of approximately 160-200 milliseconds. 1
Clinical Definition and Characteristics
Clonus represents a pathological motor pattern that indicates corticospinal tract dysfunction. 1, 2 The phenomenon manifests as:
- Rhythmic muscular contractions triggered by peripheral stimuli that produce muscle stretching 1
- Common anatomical locations include the ankle (most frequent), patella, triceps surae, wrist, jaw, and biceps brachii 1
- Temporal pattern where the first beat is always longer, with subsequent beats shortening and stabilizing by the 4th or 5th oscillation 1
- Oscillation composition consisting of 45% plantar flexion and 55% dorsiflexion of the total period 1
Underlying Pathophysiology
The exact mechanism remains incompletely understood, with two competing hypotheses:
- Hyperactive stretch reflex theory (most widely accepted): Self-excitation of hyperactive stretch reflexes drives the rhythmic contractions 1
- Central generator theory: Central nervous system generator activity produces rhythmic stimulation of lower motor neurons in response to peripheral events 1
- Afferent involvement: Evidence suggests both group Ia and group II afferents participate in clonus generation, with sufficient time for spinal mechanisms beyond simple monosynaptic reflexes 1, 3
Etiologies
Clonus results from permanent lesions in descending motor neurons, including:
- Stroke (particularly middle cerebral artery territory) 4
- Multiple sclerosis 4, 5
- Metabolic derangements such as severe hepatic failure 4
- Serotonin syndrome 4
- Severe encephalopathy (can produce startle-induced clonus mimicking seizures) 2
Treatment Approaches
Pharmacological and interventional options include:
- Baclofen (myorelaxant) 1, 2
- Cold application 1
- Botulinum toxin injections 1
- Phenol injections 1
- Epidural spinal cord stimulation (emerging evidence shows immediate reduction in mechanically evoked clonus) 5
Clinical Pitfalls
Distinguish clonus from clonic seizures: Prolonged startle-induced clonus in encephalopathic patients can closely mimic epileptic seizures, requiring video-EEG confirmation of non-epileptic origin. 2 Antiepileptic drugs are ineffective for true clonus, while myorelaxants produce dramatic improvement. 2