From the Guidelines
Clonus is indeed a test used to assess hyperreflexia, as it is characterized by rhythmic, involuntary muscle contractions and relaxations in response to sudden, maintained stretching of a muscle, often indicating upper motor neuron pathology and hyperreflexia 1.
Definition and Clinical Significance
Clonus refers to a series of rhythmic, involuntary muscle contractions and relaxations that occur in response to sudden, maintained stretching of a muscle. To test for clonus, a healthcare provider typically briskly dorsiflexes the patient's foot while the knee is held in a slightly flexed position, then maintains the stretch.
Diagnostic Criteria
In patients with hyperreflexia, this maneuver may produce rhythmic oscillations of the foot. The presence of sustained clonus (more than a few beats) generally indicates upper motor neuron pathology and hyperreflexia. This occurs because damage to descending inhibitory pathways from the brain leads to exaggerated stretch reflexes.
Clinical Evaluation
Clonus is often assessed alongside other tests of deep tendon reflexes, such as the patellar or Achilles reflex, to provide a comprehensive evaluation of the patient's neurological status. The severity of clonus is typically graded based on the number of beats observed, with more beats indicating more severe hyperreflexia. Some experts believe that clonus and hyperreflexia are “highly diagnostic for the serotonin syndrome and their occurrence in the setting of serotonergic drug use establishes the diagnosis” 1.
Key Points
- Clonus is a test used to assess hyperreflexia
- It is characterized by rhythmic, involuntary muscle contractions and relaxations in response to sudden, maintained stretching of a muscle
- The presence of sustained clonus indicates upper motor neuron pathology and hyperreflexia
- Clonus is often assessed alongside other tests of deep tendon reflexes to provide a comprehensive evaluation of the patient's neurological status
- The severity of clonus is typically graded based on the number of beats observed, with more beats indicating more severe hyperreflexia 1.
From the Research
Definition and Mechanism of Clonus
- Clonus is defined as involuntary and rhythmic muscle contractions caused by a permanent lesion in descending motor neurons 2.
- It is characterized by a frequency of 5-8 Hz and an average period of oscillations of approximately 160-200 ms 2.
- The exact mechanism of clonus remains unclear, but two hypotheses have been proposed: hyperactive stretch reflexes caused by self-excitation, and central generator activity that arises as a consequence of peripheral events 2.
Relationship between Clonus and Hyperreflexia
- Clonus is often associated with hyperreflexia, which is an increased muscle tone and augmented tendon reflex activity 3.
- Hyperreflexia is caused by disturbed inhibition of spinal reflexes following upper motor neuron dysfunction 3.
- Clonus can be considered a test of hyperreflexes, as it is a manifestation of the increased reflex gain and enlarged tissue viscoelasticity that occurs in upper motor neuron diseases 4.
Clinical Significance of Clonus
- Clonus is a common clinical symptom of spasticity, which is a major cause of motor disability worldwide 3.
- It can be treated with various therapies, including baclofen, cold application, botox, or phenol injections 2.
- Clonus can be used as an indicator of the severity of spasticity and the effectiveness of treatment 5.
Neurophysiological Mechanisms of Clonus
- Clonus is maintained by a reflex mechanism that involves the activation of Ia afferents and the inhibition of group II afferents 6.
- The duration and frequency of clonus are contingent upon joint angle and the amount of stretch placed on the triceps surae muscle group 6.
- Clonus can be abolished by blocking group I afferent fibers from the gastrocnemius-soleus muscle 6.