Clonus Assessment in Hypertensive Disorders of Pregnancy
Clonus should be assessed as part of routine maternal monitoring in preeclampsia by briskly dorsiflexing the foot with the knee flexed and observing for rhythmic, involuntary muscle contractions; sustained clonus (≥3 beats) indicates hyperreflexia and signals increased risk of eclamptic seizures, warranting immediate consideration for magnesium sulfate prophylaxis. 1
What is Clonus?
Clonus is a series of rhythmic, involuntary muscular contractions that occur in response to sudden stretching of a muscle. In the context of preeclampsia, it represents hyperreflexia—an exaggerated deep tendon reflex response that reflects central nervous system irritability and increased risk of progression to eclampsia (seizures).
How to Measure Clonus
Technique:
- Position the patient with the knee flexed at approximately 90 degrees
- Grasp the foot firmly around the metatarsals
- Briskly dorsiflex the foot (push the foot upward toward the shin) and maintain gentle sustained pressure
- Observe for rhythmic beats: Normal response shows no beats or 1-2 transient beats; sustained clonus (≥3 beats) is abnormal
- Test both ankles for comparison
Clinical Significance:
The presence of sustained clonus indicates:
- Heightened neuromuscular excitability
- Increased risk of eclamptic seizures
- Need for closer monitoring and potential magnesium sulfate administration
Integration into Preeclampsia Monitoring
Maternal monitoring in preeclampsia must include clinical assessment of clonus as part of the neurological examination 1. This assessment should be performed:
- At initial diagnosis of preeclampsia
- During ongoing inpatient monitoring
- When evaluating for severe features
- Before and during magnesium sulfate therapy
When Clonus Triggers Action:
Women with preeclampsia who have hypertension with neurological signs or symptoms (including sustained clonus) should receive magnesium sulfate for convulsion prophylaxis 1.
Additional abnormal neurological features warranting delivery include:
- Severe intractable headache
- Repeated visual scotomata
- Convulsions
- Sustained clonus with other severe features
Common Pitfalls
- Confusing transient beats with sustained clonus: 1-2 beats can be normal, especially in anxious patients; only sustained rhythmic contractions (≥3 beats) are pathological
- Inadequate dorsiflexion force: Too gentle a stimulus may miss hyperreflexia
- Testing with the knee extended: This can produce false results; the knee must be flexed
- Ignoring unilateral findings: Even unilateral sustained clonus warrants concern
- Failing to document: Serial assessments help track disease progression
The assessment of clonus is a simple bedside test that provides critical information about central nervous system involvement in preeclampsia and helps guide decisions about seizure prophylaxis and timing of delivery.