What does clonus (involuntary muscle contractions) during sleep in pediatric patients indicate?

Clonus During Sleep in Children: Clinical Significance

Clonus-like movements during sleep in children are most commonly benign sleep myoclonus, not true clonus, and require differentiation from pathological conditions through careful clinical evaluation.

Understanding the Distinction

True clonus is involuntary rhythmic muscle contractions (5-8 Hz frequency) caused by permanent lesions in descending motor neurons and indicates upper motor neuron dysfunction 1. However, what appears as "clonus" during sleep in children is typically benign neonatal sleep myoclonus (BNSM) or other non-pathological sleep-related movements 2, 3, 4.

Benign Sleep Myoclonus in Infants and Young Children

Key Diagnostic Features

Benign neonatal sleep myoclonus presents with the following characteristics:

• Onset: Occurs within the first days to weeks of life (median 3 days, range 1-16 days) 3
• Sleep-specific: Myoclonic jerks occur only during sleep and stop abruptly when the child is aroused 2, 3, 4
• Distribution: Predominantly bilateral, synchronous, and symmetrical; facial muscles are typically not involved 3
• Duration: Episodes last from seconds to 30 minutes 2, 3
• EEG findings: No epileptiform activity during events—this is the critical distinguishing feature 2, 3, 4
• Resolution: Spontaneously resolves within 2-10 months (median 2 months) of life 3, 4
• Outcome: Consistently favorable with normal neurological development 2, 4

Critical Clinical Pitfall

BNSM is frequently misdiagnosed as epileptic seizures, and infants should not be treated with antiepileptic drugs 2. This misdiagnosis occurs because primary care physicians often do not consider sleep myoclonus in the differential 2.

Pathological Clonus: When to Be Concerned

Red Flags Indicating True Pathological Clonus

True clonus in children warrants neurological evaluation when accompanied by:

• Persistence beyond infancy (after 10-12 months) 3, 4
• Presence during wakefulness 2, 1
• Associated developmental delay or motor delay 5
• Abnormal muscle tone (hypotonia or hypertonia) 5
• Absent or delayed primitive reflexes 5
• Impaired antigravity movements 5

Specific Conditions to Consider

In children with cataplexy (particularly those with Prader-Willi syndrome or narcolepsy), clonus-like movements may resemble clonic, atonic, or myoclonic seizures but occur without loss of consciousness 6. These children often have:

• Prominent facial involvement with tongue and perioral muscle movements 6
• Episodes triggered by emotion (though not always in children) 6
• Associated excessive daytime sleepiness 6

In serotonin syndrome, clonus is a highly diagnostic feature when occurring with serotonergic drug exposure 6, 7, 8. Key distinguishing features include:

• Inducible or spontaneous clonus with hyperreflexia 6, 7
• Onset within 6-24 hours of starting/increasing serotonergic medications 7, 8
• Associated mental status changes and autonomic instability 6, 7
• This is a medical emergency requiring immediate intervention 8

Evaluation Algorithm

Step 1: Determine Sleep-Wake Pattern

• If movements occur ONLY during sleep and stop with arousal: Consider benign sleep myoclonus 2, 3, 4
• If movements occur during wakefulness or persist after arousal: Evaluate for pathological clonus 2, 1

Step 2: Assess Age and Onset

• Onset in first weeks of life, resolving by 10 months: Consistent with BNSM 3, 4
• Persistent beyond 12 months or new onset after infancy: Requires neurological evaluation 5, 3

Step 3: Neurological Examination

Perform thorough assessment including:

• Postural and extremity tone 5
• Primitive reflexes 5
• Quality and quantity of antigravity movement 5
• Presence of hyperreflexia (suggests upper motor neuron pathology) 6, 5, 1

Step 4: Medication and Exposure History

• Recent serotonergic medication use: Consider serotonin syndrome 6, 7, 8
• Antipsychotic use: Consider neuroleptic malignant syndrome 6

Step 5: Diagnostic Testing When Indicated

• Video-EEG during events: Essential to differentiate benign myoclonus (normal EEG) from epileptic seizures 2, 3, 4
• Consider creatine kinase and TSH if low tone with weakness is present 5
• Polysomnography: May be indicated if sleep disorder suspected (narcolepsy, sleep apnea) 6

Clinical Outcomes and Prognosis

Benign Sleep Myoclonus

• 100% favorable outcome with normal development 2, 4
• No treatment required; parental reassurance is key 2, 4

Pathological Clonus

Research indicates that among infants exhibiting ankle clonus in the first year:

• 7.1% had intellectual disability 5
• 1.2% had motor delay 5

These statistics emphasize the importance of distinguishing benign from pathological presentations and ensuring appropriate developmental monitoring.

Management Approach

For suspected benign sleep myoclonus:

• Provide parental education and reassurance 2, 4
• Avoid antiepileptic drugs 2
• Document resolution by 10 months of age 3, 4
• No further intervention needed if typical presentation 4

For pathological clonus or atypical presentations:

• Refer to pediatric neurology 5
• Obtain video-EEG to exclude epilepsy 2, 3
• Monitor developmental milestones closely 5
• Consider underlying neurological conditions requiring specific treatment 6