NursingCenter Blog

How To Assess for Clonus

Oct 10 2022 by Myrna Buiser Schnur, MSN, RN

As you complete your neurologic assessment and test your patient’s deep tendon reflexes, if the reflexes appear to be hyperactive, take a moment to check for clonus. Clonus is an abonormal movement marked by a rhythmic alternating muscle contraction and relaxation occurring in rapid succession brought on by stretching the tendon (Gelb, 2022; Hinkle, 2021).

Clonus is a marker of hyperreflexia, and may be associated with lesions in the upper motor neurons (Zimmerman & Hubbard, 2022). It is generally accompanied by other upper motor neuron signs including spasticity and weakness.

Test for Clonus (Bickley et al., 2021)

To test for ankle or Achilles clonus, begin by supporting the knee in a partially flexed position. Using your other hand, dorsiflex and plantar flex the foot a few times and ask your patient to relax. Next, sharply dorsiflex the foot and maintain it in dorsiflexion. Observe for rhythmic oscillations between dorsiflexion and plantar flexion. The ankle should not react to this motion and should remain in dorsiflexion. If the patient is tense or has just exercised, there may be a few clonic beats. However, sustained clonus indicates central nervous system (CNS) disease affecting the corticospinal tract. Clonus must be present for a reflex to be graded 4. Some clinicians will use a grade of 5 to designate sustained clonus, and grade 4 for unsustained clonus that eventually subsides after 2 to 10 beats (Gelb, 2022). Other joints can be tested for clonus. For example, a sharp downward movement of the patella may elicit patellar clonus in the extended knee. The jaw, wrist joint, biceps, and triceps may be utilized as well.

Clonus – A Sign of Several Disorders

Clonus can signal disorders such as stroke, encephalopathy, cerebral palsy, multiple sclerosis, and spinal cord insult. It is often used as a test in suspected serotonin syndrome, which can be brought on by psychiatric drugs including monoamine oxidase inhibitors, selective serotonin reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors, tricyclic antidepressants, or street drugs such as cocaine, ecstasy, and amphetamines. Clonus may also predict seizures in patients who have overdosed on tramadol, a serotonin and norepinephrine reuptake inhibitor (SNRI) (Zimmerman & Hubbard, 2022).

Testing for clonus is effective and efficient and can be performed without special equipment. If clonus is observed, refer the patient to a neurologist for further evaluation.