Definition of Clonus
Clonus is a series of involuntary, rhythmic muscle contractions and relaxations that occur at a frequency of 5-8 Hz (approximately 160-200 ms per oscillation cycle), typically triggered by sudden muscle stretch and sustained by hyperactive stretch reflexes. 1, 2
Clinical Characteristics
Clonus manifests as rhythmic oscillations at specific anatomical sites including the ankle (most common), patella, wrist, jaw, and biceps brachii 1
Each oscillation cycle consists of two phases: plantar flexion (45% of the period) and dorsiflexion (55% of the period), with the first beat always longer than subsequent beats, which stabilize by the 4th or 5th contraction 1
The rhythmic pattern is synchronous across muscle groups when present, with clonic EMG activity in the soleus, gastrocnemius, and tibialis anterior occurring simultaneously 3
Underlying Pathophysiology
Clonus results from permanent lesions in descending motor pathways (upper motor neuron injury), leading to loss of supraspinal inhibition and hyperexcitable spinal reflexes 1, 2
Two mechanisms contribute to clonus generation: (1) self-excitation through hyperactive stretch reflexes with long reflex pathway delays, and (2) central generator activity producing rhythmic stimulation of lower motor neurons 1, 4
The condition requires both increased motoneuron excitability (reduced firing threshold) and long reflex delays typical of distal limb muscles to produce sustained oscillations 4
Diagnostic Context
Clonus is a key neuromuscular sign in serotonin syndrome, where it appears as part of the diagnostic triad alongside mental status changes and autonomic hyperactivity 5
Inducible clonus (elicited by rapid dorsiflexion) and spontaneous clonus are distinguished clinically, with spontaneous clonus being more specific for severe pathology 5
Hyperreflexia and clonus together are highly diagnostic for upper motor neuron dysfunction and are specifically sought in conditions like serotonin syndrome and spinal cord injury 5, 3
Common Etiologies
Neurological lesions including stroke, spinal cord injury, multiple sclerosis, and other conditions affecting descending motor pathways 1, 2
Metabolic disturbances such as severe hepatic failure 2
Drug-induced states particularly serotonin syndrome from excessive serotonergic activity 6, 5
Electrical burn injury as an unusual delayed neurological complication appearing approximately 3 weeks post-injury 7