What is the cause of jerky, shaky movements in a 3-month-old infant upon waking?

Jerky, Shaky Movements in a 3-Month-Old Upon Waking

The most likely cause is benign jitteriness, a normal maturational phenomenon that resolves spontaneously by 7-8 months of age with excellent prognosis, but you must immediately exclude metabolic derangements (hypoglycemia, hypocalcemia, hypomagnesemia) and distinguish this from seizures using the passive flexion test. 1, 2

Immediate Bedside Assessment

First, perform the passive flexion test: Gently hold and flex the affected limb during an episode. 1, 3

• If movements stop with passive flexion → This indicates benign jitteriness, not seizures 1, 3
• If movements continue despite passive flexion → This suggests seizures and requires immediate EEG 1, 4

Second, assess consciousness and associated features: 1, 5

• Consciousness preserved, no eye deviation, no autonomic changes → Benign jitteriness or Benign Myoclonus of Early Infancy (BMEI) 1, 5
• Altered consciousness, abnormal eye movements, or autonomic changes → Seizures requiring urgent neuroimaging and EEG 1, 4

Urgent Laboratory Workup

Check immediately (before pursuing extensive neurological evaluation): 1

• Serum glucose - Hypoglycemia is the most common provoked cause of jitteriness 1
• Serum calcium - Hypocalcemia causes both jitteriness and provoked seizures 1
• Serum magnesium - Hypomagnesemia presents similarly 1

Correct any metabolic abnormalities immediately - This is the primary management priority and may resolve symptoms entirely. 1

Maternal Substance Exposure History

Obtain comprehensive maternal drug history, though most withdrawal syndromes would have resolved by 3 months: 1, 5

• SSRIs (escitalopram) - Cause tremors/jitteriness within hours to days, typically lasting 1-4 weeks 1
• Benzodiazepines (clonazepam) - Cause tremors with onset hours to weeks, potentially lasting 1.5-9 months 1
• Opioids - Withdrawal in 55-94% of exposed neonates with tremors 1
• Barbiturates - Severe tremors with onset in first 24 hours or delayed up to 10-14 days 1

Note: While most drug withdrawal presents earlier, benzodiazepine withdrawal can persist to 3 months. 1

Benign Conditions at 3 Months

Benign Jitteriness (Most Likely)

This is the most common diagnosis in a 3-month-old with isolated jerky movements upon waking: 2

• Presents as stimulus-sensitive tremor predominantly affecting limbs 1, 3
• Resolves spontaneously at mean age of 7.2 months 2
• Associated with excellent prognosis - 92% have normal neurodevelopment at 3 years 2
• May be accompanied by mildly increased tone or hyperactive reflexes that are transient 2

Benign Myoclonus of Early Infancy (BMEI)

Consider if movements are brief myoclonic jerks in clusters: 1, 5

• Typically presents between 4-7 months (slightly older than this patient) 5
• Myoclonic jerks of head and/or upper limbs occurring in clusters 1, 5
• Each attack lasts only seconds but multiple episodes per day are common 5
• Triggered by excitement, frustration, postural changes, or sensory stimuli 5
• Consciousness preserved, usually during wakefulness 1, 5
• Requires normal ictal EEG to confirm diagnosis 1, 5
• Resolves spontaneously by age 2 years without developmental abnormalities 1, 5

When to Pursue Advanced Neurological Evaluation

Reserve neuroimaging and EEG for: 1

• Movements that do not stop with passive flexion 1
• Abnormal eye movements or autonomic changes present 1
• Focal neurologic findings on examination 1
• Abnormal tone or dysmorphic features 1
• Head lag, inability to grasp, or asymmetric hand use (suggests cerebral palsy) 5

If seizures are suspected, obtain: 4

• Video-EEG during episodes to capture ictal activity 4
• Brain MRI to identify etiology (hypoxic-ischemic injury is most common cause at 46-65%) 4

Management Algorithm

1. Test passive flexion during episode - Does it stop the movements? 1, 3
2. Check glucose, calcium, magnesium immediately - Correct any abnormalities 1
3. Assess consciousness and eye movements - Are they normal? 1
4. Obtain maternal substance exposure history - Any relevant exposures? 1
5. Perform focused neurological exam - Any focal findings, abnormal tone, or asymmetries? 1, 5

If all metabolic tests normal, movements stop with passive flexion, consciousness preserved, and no focal findings:

• Reassure parents this is benign jitteriness with excellent prognosis 1, 2
• Provide regular developmental monitoring to ensure normal progression 1, 5
• Expect spontaneous resolution by 7-8 months of age 2

If any concerning features present:

• Proceed immediately with neuroimaging and EEG 1, 4
• Refer to pediatric neurology urgently 4

Critical Pitfalls to Avoid

• Do not delay metabolic correction while pursuing extensive neurological workup - Hypoglycemia, hypocalcemia, and hypomagnesemia require immediate treatment 1
• Do not confuse myoclonic jerks with epileptic seizures based solely on jerking quality - Use the passive flexion test to differentiate 1, 3
• Do not overlook maternal substance exposure - Benzodiazepine withdrawal can have delayed onset up to weeks and persist for months 1
• Do not miss respiratory distress - Head nodding synchronized with breathing is a medical emergency, not a benign movement disorder 5, 4
• Do not assume all tremor is pathologic - Jitteriness beyond the neonatal period as a sole finding is benign in 92% of cases 2