Ankle Clonus During Sleep in Children: Clinical Assessment
Ankle clonus during sleep is NOT a normal finding in healthy children and warrants careful neurological evaluation to distinguish benign from pathological presentations.
Key Distinguishing Features
When Ankle Clonus Suggests Pathology
The presence of sustained ankle clonus (>10 beats at any age in the first year, or any clonus >8 months of age) is always abnormal and requires close neurological follow-up. 1
Critical red flags that indicate pathological clonus include:
- Associated developmental or motor delays 2
- Abnormal muscle tone on examination 2
- Absent or delayed primitive reflexes 2
- Impaired antigravity movements 2
- Persistent neurological signs appearing before 4 months and continuing after 5 months of age 1
Among infants exhibiting ankle clonus in the first year of life, research demonstrates that 7.1% had intellectual disability and 1.2% had motor delay, emphasizing this is not a benign finding to dismiss 2.
Differential Diagnosis During Sleep
The movements observed during sleep must be distinguished from other sleep-related phenomena:
- Periodic limb movements during sleep (PLMS): Brief (0.5-10 seconds) recurrent movements occurring every 15-30 seconds, which are normal in children when <5 events/hour 3
- Benign neonatal sleep myoclonus: Occurs only in neonates (first 23 days of life), involves primarily distal upper limbs, lasts 10-20 seconds, and disappears by 7 months 4
- Cataplexy-related movements: In children with narcolepsy or Prader-Willi syndrome, characterized by prominent facial involvement and emotion-triggered episodes 2
Essential Clinical Evaluation
Perform a comprehensive neurological examination focusing on:
- Postural and extremity tone assessment 2
- Primitive reflex testing 2
- Quality and quantity of antigravity movement 2
- Presence of hyperreflexia 2
- Number of clonus beats (>10 beats is always pathological) 1
Medication history is critical: Clonus with serotonergic drug exposure (onset within 6-24 hours of starting/increasing medication) suggests serotonin syndrome, which presents with inducible or spontaneous clonus, hyperreflexia, mental status changes, and autonomic instability 2.
When to Pursue Further Testing
Polysomnography should be obtained if:
- A primary sleep disorder (narcolepsy, sleep apnea) is suspected based on excessive daytime sleepiness or other sleep symptoms 2
- The movements are rhythmic and occur every 15-30 seconds, suggesting PLMD (>5 events/hour is abnormal in children) 3
Clinical Significance and Prognosis
True clonus represents involuntary rhythmic muscle contractions (5-8 Hz frequency) caused by permanent lesions in descending motor neurons 5. In the pediatric population studied, 42.6% of infants with ankle clonus in the first year had abnormal outcomes: cerebral palsy (29%), mental retardation (7.1%), borderline intelligence (5.3%), or motor delay (1.2%) 1.
The mechanism involves hyperactive stretch reflexes from self-excitation, though central generator activity may also contribute 5, 6. This is fundamentally different from benign sleep movements, which lack the sustained rhythmic quality and associated neurological findings.
Management Approach
Close developmental monitoring is mandatory for any child with ankle clonus during sleep, with particular vigilance if:
- Clonus exceeds 10 beats 1
- Child is older than 8 months 1
- Any associated neurological signs are present 2, 1
The clinical course should be observed closely for possible development of cerebral palsy, developmental delays, or other neurological abnormalities, as these outcomes occurred in nearly half of affected infants in longitudinal studies 1.