Clonus is an involuntary, rhythmic muscle contraction and relaxation. This abnormal reflex response often signals an underlying neurological issue, typically involving damage to the upper motor neurons in the brain or spinal cord. Understanding clonus is crucial for addressing its underlying cause.
Understanding Clonus
Clonus is often described as a “beating” or “jerking” in a limb. It commonly occurs in the ankle, where the foot will bounce up and down, but can also affect the knees, wrists, fingers, or jaw.
Unlike brief, irregular muscle twitches or tremors, clonus is characterized by its sustained, repetitive nature, typically triggered by a sudden stretch of the muscle.
During a neurological examination, a healthcare provider might elicit clonus by rapidly stretching a muscle, such as quickly flexing the foot upward toward the shin and maintaining that pressure. If clonus is present, the foot will then involuntarily bounce rhythmically. This rhythmic movement can continue for as long as the pressure is applied.
The Underlying Neurological Mechanism
Clonus arises from an exaggerated stretch reflex, a phenomenon also known as hyperreflexia. Normally, upper motor neurons, which are nerve cells connecting the brain and spinal cord, send inhibitory signals that regulate muscle activity and reflexes. Damage to these upper motor neurons disrupts these inhibitory signals.
When this damage occurs, the normal control over the reflex arc is lost, leading to an overactive response. A sudden stretch of the muscle then triggers a repetitive cycle of contraction and relaxation, resulting in the characteristic rhythmic oscillations of clonus. This disinhibition of the stretch reflex causes clonus.
Conditions Indicated by Clonus
Clonus is not a standalone diagnosis but a sign that points to an issue within the central nervous system.
- Multiple Sclerosis (MS): An autoimmune disorder where the protective sheath around nerves in the brain and spinal cord is attacked, disrupting nerve signals and often leading to clonus.
- Stroke: Involves interrupted blood supply to the brain, causing clonus if the affected area controls movement. This damage leads to miscommunication between brain regions, resulting in unregulated muscle tone and contractions.
- Spinal Cord Injuries: Frequently result in clonus due to upper motor neuron damage, causing involuntary rhythmic shaking in the limbs.
- Cerebral Palsy: A group of disorders affecting movement and muscle tone due to brain damage during development, also commonly presents with clonus.
- Brain or Spinal Cord Tumors: Can cause clonus by compressing or damaging neural pathways.
- Severe Head Trauma: Can also lead to upper motor neuron damage, manifesting as clonus.
- Medications or Drug Withdrawal: Particularly those affecting serotonin levels like some antidepressants, can induce clonus as a side effect. Serotonin syndrome, a potentially life-threatening reaction from too much serotonin, is a notable example.
When to Consult a Healthcare Professional
Any new onset of clonus warrants a medical evaluation. It is particularly important to seek professional attention if the clonus is persistent, severe, or interferes with daily activities like walking or sleeping. Consulting a healthcare provider is also advised if clonus is accompanied by other neurological symptoms. These may include muscle weakness, numbness, changes in vision, balance issues, or pain. A doctor can accurately assess the underlying cause through a physical examination and further diagnostic tests.