Agitation, tachycardia, and fever occur in many toxicologic emergencies. Clonus is different: in the right exposure context, it is one of the most useful bedside clues pointing toward serotonin toxicity.

Bottom Line

Serotonin toxicity produces a characteristic pattern of neuromuscular excitation, and clonus is one of its highest-value bedside findings. Spontaneous, inducible, or ocular clonus carries more diagnostic weight than nonspecific findings such as agitation, tachycardia, hypertension, or diaphoresis. [1] [2]

That is why the Hunter Serotonin Toxicity Criteria are built heavily around clonus and hyperreflexia.

The key point is not that clonus proves serotonin toxicity by itself.

It is that clonus sharply strengthens the diagnosis when the exposure history and clinical context are compatible.

What Exactly Is Clonus?

Clonus is a series of rhythmic, involuntary muscle contractions triggered by sudden stretch.

At the bedside, ankle clonus is commonly tested by rapidly dorsiflexing the foot and maintaining the stretch. Repetitive oscillation of the foot represents clonus.

Clonus should not be confused with tremor. Tremor is typically a more continuous oscillatory movement, while inducible clonus is provoked by sustained stretch.

Serotonin toxicity may produce:

• Inducible clonus

• Spontaneous clonus

• Ocular clonus

These findings reflect the increased neuromuscular excitability produced by excessive serotonergic activity. [1]

Why Is Clonus More Useful Than Tachycardia or Agitation?

Many poisoned patients are tachycardic, anxious, agitated, sweaty, hypertensive, or febrile.

Those findings can occur with sympathomimetic poisoning, anticholinergic toxicity, withdrawal syndromes, infection, heat illness, and many other conditions.

Clonus and hyperreflexia are substantially more characteristic of serotonin toxicity.

This distinction becomes especially useful when competing toxidromes look similar. A patient with agitation, mydriasis, and tachycardia could have several different toxic syndromes. Adding inducible clonus and hyperreflexia changes the diagnostic picture considerably. [2]

Why Does Hunter Put So Much Weight on Clonus?

The original Hunter Serotonin Toxicity Criteria were derived from patients exposed to serotonergic drugs.

Among numerous clinical findings, the features most useful for the decision rules included clonus, agitation, diaphoresis, tremor, and hyperreflexia. In the original study, the resulting criteria were reported to be more sensitive and slightly more specific than the older Sternbach criteria when compared with diagnosis by a clinical toxicologist. [1]

In a patient with a plausible serotonergic exposure, the Hunter rules use:

spontaneous clonus, or combinations such as inducible or ocular clonus with agitation or diaphoresis, as key diagnostic patterns.

This is why simply checking the temperature and heart rate is not enough.

The neuromuscular examination matters.

Does the Absence of Clonus Rule Out Serotonin Toxicity?

No.

Hunter is a useful clinical decision rule, not a definitive laboratory test, and its original validation was principally in an overdose population. Contemporary reviews caution against assuming identical diagnostic performance in every therapeutic, perioperative, inpatient, or mixed-exposure setting. [2]

A 2026 systematic review of 764 published adult cases labeled as serotonin syndrome found that 65% fulfilled the Hunter criteria when the reports were reassessed. That number should not be interpreted as the sensitivity of Hunter in routine practice because published case reports vary in diagnostic quality and completeness. It does reinforce the need to use the criteria within a full clinical assessment rather than as an infallible test. [3]

In severe toxicity, marked muscle rigidity may also make clonus harder to demonstrate.

Don’t Miss

Clonus is most meaningful when paired with a biologically plausible serotonergic exposure.

Finding a brisk reflex or a few beats of ankle oscillation should not automatically label every patient as having serotonin toxicity.

Likewise, do not let nonspecific findings such as tachycardia, agitation, or fever outweigh a careful neurologic examination.

When serotonin toxicity is being considered, examine the ankles, reflexes, muscle tone, and eye movements rather than relying only on vital signs.

Clinical Pearl

Agitation and tachycardia tell you the patient is activated. Clonus helps tell you why.

When serotonin toxicity is on the differential:

Check the ankles and the eyes—not just the monitor.

Related MedicalToxic

Serotonin Syndrome (Serotonin Toxicity): Symptoms, Diagnosis, Drug Interactions, and Treatment

References

1. Dunkley EJC, Isbister GK, Sibbritt D, Dawson AH, Whyte IM. The Hunter Serotonin Toxicity Criteria: simple and accurate diagnostic decision rules for serotonin toxicity. QJM. 2003;96(9):635–642. doi:10.1093/qjmed/hcg109.

2. Chiew AL, Isbister GK. Management of serotonin syndrome (toxicity). British Journal of Clinical Pharmacology. 2025;91(3):654–661. doi:10.1111/bcp.16152.

3. Blyzniuk B, Danukalo M, Gastaldon C, et al. Precipitants and clinical features of serotonin syndrome: a systematic review with patient-level analysis of published case reports and series. European Journal of Clinical Pharmacology. 2026;82(8):210. doi:10.1007/s00228-026-04118-3.