Clonus Reflex

Original Editor - Vidya Acharya
Top Contributors - Vidya Acharya, Amanda Ager, Naomi O'Reilly and Carin Hunter

Purpose[edit | edit source]

Clonus is a rhythmic, oscillating, stretch reflex. The cause of the oscillations are related to lesions in upper motor neurons and is generally accompanied by hyperreflexia. Therefore, clonus is used as part of the neurological physical exam to assess the status of a patient’s nervous system. It is also used in prognostic evaluation of seizures in certain drug overdoses, and therefore can be used in treatment. Clonus of the ankle results in repeated dorsiflexion of the ankle in response to brisk dorsiflexion of the foot. The test helps in differentiating between the involvement of the central nervous and peripheral nervous system.[1]

Technique[edit | edit source]

The technique is performed without any equipment[1]. This is a simple procedure. The patient lies in a relaxed supine position. The physiotherapist slightly flexes the leg to be examined at the knee and with the other hand gently moving the ankle a couple of times in dorsiflexion and plantarflexion of the ankle, before rapidly dorsiflexing the ankle. The therapist holds the ankle in dorsiflexion. The initial brisk dorsiflexion and sustained pressure can be done with slight eversion of the foot.

A positive Clonus sign is recorded when the examiner feels and sees the oscillations against this pressure.[2] 

  • Rhythm and number of beats can be appreciated.[1]
  • Each beat will be felt as a plantarflexion followed by a relaxation. The first beat is the longest, with decreasing duration of beats until the fourth beat, after which the beat frequency becomes equivalent from one to the next.[1]
  • Frequency of clonus: 5-8 Hz and the average period of oscillations of the ankle clonus is approximately 160–200 ms with plantar flexion comprising 45% of the period and dorsiflexion 55% of the period. Literature suggests that the duration of the dorsiflexion around 88.63±10.83 ms, and the duration of the plantarflexion 71.75±6.73 ms.[3]
  • According to the National Institute of Neurological Disorders and Stroke (NINDS)[4], the deep tendon reflexes are graded on a scale from 0 to 4. Clonus is graded as grade 4+. If clonus is greater than 10 beats, it is considered "sustained clonus," which can be denoted as a "5" or just documented as a rating of "4".

Evidence[edit | edit source]

Clonus can exist as a physical examination finding; it is a marker of hyperreflexia, which is part of an upper motor neuron syndrome and is generally accompanied by spasticity and manifests as a central nervous system insult.[1] Clonus appears several weeks after the presence or onset of a lesion and may be due to a number of causes like cerebral hemispheric or spinal cord damage. 

Spinal Cord Assessment Tool for Spastic Reflexes (SCATS) a clinical tool for the assessment of spasms and spastic hypertonia in spinal cord injury (SCI) assesses the severity of the 3 spasticity components: clonus, flexor spasm, and extensor spasm. Clonus is quantified in response to rapid dorsiflexion of the ankle. The duration of each clonic burst is timed and scored as follows:

  • 0 No reaction
  • 1 Mild: Clonus maintained <3s
  • 2 Moderate: Clonus persists 3-10s
  • 3 Severe: Clonus persists >10

One observational study showed a significant correlation between the SCATS clonus scores and the Modified Ashworth scores of the hip, knee, and ankle in patients with spinal cord injury, thus suggesting that SCATS is a reliable tool for assessing spasm activity and spastic hypertonia in patients with SCI[5]. The results demonstrated good interrater agreement (due to well-trained experienced testers and standardised SCATS) and test-retest agreement; however, a higher interrater agreement was noted than the test-retest agreement which could be due to change in spasticity with time (2 tests were performed 1 week apart).[5] However, there is no research examining the sensitivity of clonus in ruling out cord compressive myelopathy.[6]


References[edit | edit source]

  1. 1.0 1.1 1.2 1.3 1.4 Zimmerman B, Hubbard JB. Clonus.
  2. Fundamentals of Tests and Measures for the Physical Therapist Assistant. Chapter 9 Neuromuscular examination. Stacie J. Fruth, Carol Fawcett
  3. Boyraz I, Uysal H, Koc B, Sarman H. Clonus: definition, mechanism, treatment. Med Glas (Zenica). 2015 Jan 5;12(1):19-26.
  4. Rodriguez-Beato FY, De Jesus O. Physiology, Deep Tendon Reflexes. StatPearls [Internet]. 2020 Aug 27.
  5. 5.0 5.1 Pinar Akpinar Arzu AtıcıArzu AtıcıIlknur AktasReliability of the Spinal Cord Assessment Tool for Spastic Reflexes Archives of Physical Medicine and Rehabilitation 98(6)October 2016
  6. Cook CE, Hegedus E, Pietrobon R, Goode A. A pragmatic neurological screen for patients with suspected cord compressive myelopathy. Physical therapy. 2007 Sep 1;87(9):1233-42.
  7. Dr. jawad alhakeemSustained ankle clonus. Available from Accessed on 2/3/21