Is clonus (involuntary muscle contractions) normal in babies?

Clonus in Babies: Normal Phenomenon or Pathological Sign?

Some degree of ankle clonus can be normal in babies, but sustained clonus (more than ten beats) or clonus persisting beyond 8 months of age is abnormal and requires further evaluation. 1

Understanding Clonus in Infants

Clonus is characterized by involuntary and rhythmic muscle contractions that occur in response to sudden stretching of a muscle. In clinical practice, it is most commonly elicited at the ankle but can also be found at other locations including the patella, wrist, and jaw. 2

Key characteristics of clonus:

• Occurs with a frequency of 5-8 Hz 2
• Average period of oscillations for ankle clonus is approximately 160-200 ms 2
• The first beat is typically longer, with subsequent beats becoming shorter and stabilizing by the 4th or 5th beat 2

Normal vs. Pathological Clonus in Babies

When Clonus May Be Normal

• Brief clonus (fewer than ten beats) in infants under 8 months of age may be normal 1
• Transient clonus without other neurological signs often resolves spontaneously 1

When Clonus Is Concerning

• Clonus of more than ten beats at any age within the first year of life 1
• Clonus of ten or fewer beats that persists beyond 8 months of age 1
• Clonus accompanied by other neurological signs that appear within 4 months and persist beyond 5 months of age 1

Clinical Significance and Outcomes

Research has shown that among infants exhibiting ankle clonus within the first year of life:

• 57.4% had normal neurological outcomes 1
• 42.6% had pathological outcomes including: 1
  • Cerebral palsy (29%)
  • Mental retardation (7.1%)
  • Borderline intelligence (5.3%)
  • Motor delay (1.2%)

Differential Diagnosis

When evaluating clonus in infants, consider these conditions:

1. Neurological disorders:

   • Cerebral palsy - often associated with other signs of upper motor neuron dysfunction 3
   • Developmental delay with abnormal muscle tone 4

2. Non-epileptic motor phenomena:

   • Benign myoclonus of early infancy (BMEI) - characterized by nonepileptic spasms that may resemble epileptic spasms but with normal EEG and development 5
   • Startle-induced clonus - can mimic clonic seizures but is non-epileptic in origin 6

3. Normal developmental variation:

   • Transient clonus that resolves spontaneously 1

Evaluation Approach

When clonus is detected in an infant:

1. Document characteristics:

   • Number of beats (fewer than ten vs. more than ten) 1
   • Age of infant when clonus is observed 1
   • Presence of other neurological signs 1

2. Perform thorough neuromotor examination:

   • Assess postural tone (ventral suspension in younger infants, truncal positioning when sitting/standing in older infants) 4
   • Evaluate extremity tone (scarf sign in infants, popliteal angles after the first year) 4
   • Check for persistence of primitive reflexes or asymmetry/absence of protective reflexes 4
   • Observe quality and quantity of movement, including antigravity movement 4

3. Consider additional testing if concerning features are present:

   • EEG if seizures are suspected 6, 5
   • Measurement of creatine phosphokinase (CK) and thyroid-stimulating hormone (TSH) if low tone with weakness is identified 4

Management Recommendations

1. For normal/transient clonus:

   • Reassurance and routine developmental monitoring 1

2. For concerning clonus:

   • Close observation of clinical course for possible development of neurological abnormalities 1
   • Early referral to pediatric neurology for comprehensive evaluation 1
   • If clonus is causing functional impairment, treatment options may include baclofen, cold application, or botulinum toxin injections in selected cases 2

Important Considerations and Pitfalls

• Do not confuse benign myoclonus of early infancy with epileptic conditions like West's syndrome, as this may lead to unnecessary and potentially harmful antiepileptic therapy 5
• Clonus may be associated with abnormal general movements in infants at high risk for cerebral palsy 3
• Similar to clonus, tonic responses may indicate loss of supraspinal control 3
• Reflex irradiation (spread of reflex responses to other muscle groups) can occur in both typically developing infants and those at high risk for neurological disorders 3