Clonus is a neurological sign characterized by involuntary, rhythmic muscle contractions and relaxations. This phenomenon is an abnormal reflex response that occurs when a muscle is suddenly stretched. The presence of clonus signals an underlying issue within the central nervous system, indicating a disruption in the communication pathways that control muscle movement. It is not a disease in itself but rather a physical manifestation doctors look for during a neurological examination.
Defining Clonus and its Neurological Basis
Clonus represents a state of hyperreflexia, meaning the body’s normal stretch reflex has become overactive and exaggerated. The root cause lies in damage to the Upper Motor Neuron (UMN) pathways, which are the descending nerve fibers that originate in the brain and travel down the spinal cord. These pathways are responsible for sending inhibitory signals to the motor neurons in the spinal cord, essentially dampening the reflex response and preventing over-reaction to a stretch.
When the UMN pathways are damaged, this inhibitory control is lost, leading to a hyperexcitable state in the muscle stretch reflex circuit. A sudden stretch of the muscle then triggers a positive feedback loop: the initial contraction stretches the opposing muscle, which triggers its own reflex, and the cycle repeats rhythmically. Clinicians distinguish between unsustained clonus, which involves a few beats that quickly fade, and sustained clonus, which continues for ten beats or more as long as the stretch is maintained. Sustained clonus is a definitive sign of underlying central nervous system pathology.
Where and How Clonus Presents
Although clonus can appear in several muscle groups, it is most frequently observed in the lower limbs, particularly the ankle. This is often called ankle or foot clonus, and it involves the foot rapidly jerking up and down (dorsiflexion and plantarflexion).
A healthcare provider typically elicits ankle clonus manually as part of a physical exam. The patient’s knee is supported in a slightly bent position, and the examiner then briskly flexes the foot upward toward the shin, maintaining the pressure. If clonus is present, the foot will begin the characteristic rhythmic beating against the examiner’s hand. Less common sites where this reflex can be tested include the knee, where patellar clonus is elicited by a sharp downward push on the kneecap, and sometimes the wrist or the jaw.
Medical Conditions Associated with Clonus
Clonus is a sign of an Upper Motor Neuron Syndrome, and its presence points toward damage anywhere along the central nervous system pathways from the brain to the spinal cord. A wide range of medical conditions can cause this damage.
Common Causes of Clonus
- Stroke, where damage to brain tissue can affect the motor control areas.
- Spinal cord injury, as the trauma directly damages the descending nerve tracts.
- Chronic neurological disorders, such as Multiple Sclerosis (MS) and Amyotrophic Lateral Sclerosis (ALS).
- Cerebral palsy, a group of disorders affecting movement and posture in children due to brain damage occurring before or during birth.
- Acute conditions like severe traumatic brain injury.
- Brain infections, such as meningitis.
- Serotonin syndrome, caused by a toxic buildup from certain drug overdoses.
Diagnosis and Treatment Approaches
The diagnosis of clonus begins with the physical examination, where the healthcare provider attempts to elicit the rhythmic contractions. The severity is assessed by counting the number of beats, with sustained clonus indicating more significant neurological involvement. Once clonus is confirmed, the next step involves identifying the underlying cause through a thorough patient history and specialized testing. This may include imaging, such as MRI scans of the brain and spinal cord, and laboratory tests like blood work or spinal fluid analysis.
Treatment for clonus focuses on managing the underlying condition and providing symptomatic relief to improve function and comfort. Medications are a mainstay of symptomatic treatment, particularly muscle relaxants that decrease muscle spasticity and excitability. Commonly prescribed oral medications include baclofen and tizanidine, which help calm overactive nerve signals. For localized or severe clonus, targeted injections of botulinum toxin (Botox) can temporarily weaken the specific muscle, reducing involuntary contractions. Physical therapy is also a component of management, utilizing stretching and specialized bracing to maintain range of motion.