Clonus is defined as involuntary and rhythmic muscle contractions caused by a permanent lesion in descending motor neurons, usually resulting from oscillations in the spinal stretch reflex. It most commonly occurs in the ankle when the foot is briskly dorsiflexed. Clonus is accompanied by spasticity and relates to lesions in upper motor neurons in the brain or spinal cord. Potential causes include central nervous system lesions or serotonin syndrome. Treatment options aim to suppress increased reflexes and muscle tone, such as with baclofen, cold application, or injections of botox or phenol.

1. CLONUS
Ade Wijaya, MD – April 2020

2. Definition
Involuntary and rhythmic muscle contractions caused by a permanent lesion in descending motor
neurons
It is usually considered to be a result of oscillations in the group Ia spinal stretch reflex
Hidler JM, Rymer WZ. A simulation study of reflex instability in spasticity: origins of clonus. IEEE Trans Rehabil Eng 1999; 7:327-40.

3. Introduction
◦ Clonus is accompanied by spasticity and other findings of reflex excitability
◦ It relates to lesions in upper motor neurons
◦ Clonus does not occur if the muscle is excessively hypertonic
◦ Any mechanism or pharmacological drug suppressing increased reflexes and muscle tone is also
prone to block the clonus
◦ Severe clonus can interrupt sleep and prevent the transfer capability and result in fatigue that
can decrease work performance
◦ Interfere with the posture and gait
◦ Clonus can also occur in normal individuals
◦ Most commonly seen in the ankle joint by briskly dorsiflexing the foot
Boyraz I, Uysal H, Koc B, Sarman H. Clonus: definition, mechanism, treatment. Med Glas (Zenica). 2015 Jan 5;12(1):19-26.
Zimmerman, B., & Hubbard, J. B. (2019). Clonus.

4. Clonus Reflexes
◦ Ankle/Achilles reflex (S1/S2 nerve routes)
◦ Jaw jerk/masseter: Trigeminal nerve, tested at the chin/mental protuberance
◦ Patellar/quadriceps/knee: L2 to L4 (mostly L4), tested just inferior to the patella (or by pushing
patella distally)
◦ Biceps: C5 to C6, just anterior to elbow
◦ Triceps: C7 to C8 (mostly C7), just posterior to the elbow
Boyraz I, Uysal H, Koc B, Sarman H. Clonus: definition, mechanism, treatment. Med Glas (Zenica). 2015 Feb;12(1):19-26.

5. Pathophysiology
◦ Self-perpetuating reactivation of peripheral muscle stretch circuits, with each beat producing
the next
◦ Initial appropriate external stimulus that leads to activation of the stretch reflex circuit followed
by a central signal which commands the muscles to continue to produce that motor response in
the absence of an appropriate stimulation of the stretch reflex
This can occur when there is a lesion to descending motor nerves, predominantly the dorsal
reticulospinal pathway, which can occur anywhere from the cortex to the spinal cord.
The inhibitory dampening effect of these descending nerves on alpha and gamma motor neurons
is removed, leading to a hyper excitatory state in the muscle stretch reflex circuit.
Boyraz I, Uysal H, Koc B, Sarman H. Clonus: definition, mechanism, treatment. Med Glas (Zenica). 2015 Feb;12(1):19-26.
Beres-Jones JA, Johnson TD, Harkema SJ. Clonus after human spinal cord injury cannot be attributed solely to recurrent muscle-tendon stretch. Exp Brain Res. 2003 Mar;149(2):222-36.

6. Etiology
CNS LESIONS (BRAIN / SPINAL CORD) ◦ SEROTONIN SYNDROME
Najari F, Alizadeh-Ghamsari A, Vahabzadeh M, Dadpour B, Reza Mousavi S, Baradaran Kayal I. A Study of the Importance of Clonus Symptoms in Patients with Tramadol Poisoning.J Toxicol. 2017;2017:2151536.

7. Treatment
◦ Baclofen
◦ Applying cold
◦ Botox or phenol injections
Boyraz I, Uysal H, Koc B, Sarman H. Clonus: definition, mechanism, treatment. Med Glas (Zenica). 2015 Feb;12(1):19-26.

8. Summary
◦ Involuntary and rhythmic muscle contractions caused by a permanent lesion in descending
motor neurons
◦ It is usually considered to be a result of oscillations in the group Ia spinal stretch reflex
◦ Most commonly seen in the ankle joint by briskly dorsiflexing the foot
◦ Clonus is accompanied by spasticity and other findings of reflex excitability
◦ It relates to lesions in upper motor neurons (CNS lesions / serotonin syndrome)
◦ Any mechanism or pharmacological drug suppressing increased reflexes and muscle tone is also
prone to block the clonus

9. THANK YOU