Clonus

Clonus is a rhythmic, involuntary series of muscle contractions triggered when a tendon is placed under sudden, sustained stretch. It reflects an overactive spinal reflex arc — the same loop responsible for the ordinary knee-jerk, amplified to the point of self-sustaining oscillation. The ankle is the most commonly tested site, though clonus can also be elicited at the knee or wrist.

In most clinical contexts clonus signals upper motor neuron damage. In the context of psychoactive substances, it surfaces as one of the cardinal neuromuscular signs of serotonin syndrome — alongside hyperreflexia and muscle rigidity — and its presence is part of what separates serotonin toxicity from similar-looking presentations such as neuroleptic malignant syndrome or stimulant agitation.

How it is done

Clonus is assessed by applying a brisk, sustained passive stretch to a muscle group and observing whether the muscle responds with repeated, rhythmic contractions rather than a single reflex jerk. At the ankle — the standard site — the clinician rapidly dorsiflexes the foot and holds it, counting how many oscillations occur before movement stops.

Sustained clonus — generally defined as contractions continuing beyond a few cycles — carries more diagnostic weight than one or two beats, which can appear in otherwise healthy people. Inducible ankle clonus is one of the specific findings in the Hunter Criteria, the clinical framework most widely used to diagnose serotonin toxicity.

When it matters

Clonus becomes a clinical priority when someone who has recently taken serotonergic drugs presents with agitation, elevated heart rate, sweating, or raised body temperature. In that setting, inducible or spontaneous clonus — especially at the ankle — shifts the working diagnosis toward serotonin toxicity and changes what the response needs to be.

The urgency matters because serotonin toxicity can progress to severe hyperthermia, and hyperthermia at the extreme end is the mechanism of life-threatening harm. Early identification of the neuromuscular triad — hyperreflexia, clonus, agitation — is what allows the clinical response to stay ahead of that progression. This is a medical emergency requiring emergency medical care, not observation.

What it cannot tell you

A positive finding does not establish serotonin toxicity on its own. Clonus occurs in other conditions — spinal cord injury, stroke, and certain metabolic disturbances among them — and its significance depends on the full clinical picture: drug history, onset pattern, temperature, and the presence of other neuromuscular signs such as hyperreflexia or muscle rigidity.

Conversely, the absence of clonus does not rule out serotonin toxicity. Milder presentations may show only hyperreflexia and agitation without reaching the threshold for observable clonus. Serotonin toxicity exists on a spectrum, and clonus marks a particular severity band, not the condition as a whole.

Clonus also says nothing about which substance caused the reaction, how much was taken, or how quickly the situation will progress. It is a snapshot of neuromuscular excitability at the moment of examination, not a measure of trajectory.