In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan.


Clonus is a rhythmic, oscillating, stretch reflex, the cause of which is not totally known; however, it relates to lesions in upper motor neurons and therefore is generally accompanied by hyperreflexia. Therefore, clonus is used as part of the neurological physical exam to evaluate the status of a patient’s nervous system. It can be evaluated in many joints but is most commonly seen in the ankle joint by briskly dorsiflexing the foot. An illustrative example of what clonus might look like is a clinician tapping the patellar tendon once lightly and a subsequent series of obvious repeated knee extensions and relaxations going on for a dozen beats. The test is free of cost, generally requires no equipment and can be performed rapidly and easily in a hospitalized patient. Like other signs of upper motor neuron syndrome clonus indicates some insult to the central rather than peripheral nervous system, so part of its utility as a clinical examination skill is in differentiating the two. It has other roles, for instance, it is prognostic of seizures in certain drug overdoses and therefore can be used to decide when to treat this condition prophylactically.

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