Section 122.4: Hypoxic-Ischemic Encephalopathy (HIE)

Etiology · Pathophysiology · Sarnat staging · MRI patterns · Therapeutic hypothermia · aEEG · Prognosis
❄️ Key therapy: Therapeutic hypothermia (33.5°C for 72h) started within 6h reduces death/disability

🧠 HIE: Core Concepts & Therapeutic Hypothermia

📊 Sarnat Staging (Term HIE)
Stage I (mild): hyperalert, normal tone, no seizures. Stage II (moderate): lethargy, hypotonia, seizures. Stage III (severe): stupor/coma, flaccid, decerebrate posturing.
❄️ Therapeutic Hypothermia
Start within 6h of birth, target core 33.5°C for 72h, then slow rewarming. Reduces death and major neurodisability at 18-22 months.
🧬 MRI Patterns
Basal ganglia/thalamus injury (worst prognosis), watershed (parasagittal) injury, selective neuronal necrosis.
⚡ aEEG/EEG
Background pattern correlates with outcome. Burst suppression or flat tracing → poor prognosis. Normal sleep-wake cycling → good.
📈 Poor prognostic signs
pH <6.7, base deficit >25, absent spontaneous respirations at 20 min, severe BG/T lesions on MRI, status epilepticus.
💊 Seizure management
Phenobarbital 20 mg/kg IV (up to 40-50). Levetiracetam as second-line. Treat subclinical seizures (continuous EEG).
🔬 Pathophysiology: Primary energy failure (first 6h) → secondary energy failure (12-72h) due to excitotoxicity, inflammation, oxidative stress. Hypothermia interrupts secondary phase.

🔍 Approach to neonatal encephalopathy: possible HIE

1
Identify at-risk infant – Sentinel event (cord prolapse, abruption, uterine rupture), abnormal FHR tracing, Apgar ≤5 at 10 min, cord pH <7.0 or base deficit ≥12.
2
Neurologic exam (Sarnat staging) – Assess level of consciousness, tone, reflexes, seizures. Moderate-severe HIE qualifies for therapeutic hypothermia.
3
Exclude other causes – Sepsis, metabolic disorders (ammonia, lactate), intracranial hemorrhage (US), genetic epilepsies, drug exposure.
4
Neurophysiologic monitoring – aEEG or conventional EEG within 6h to assess background and detect seizures.
5
Neuroimaging – MRI with DWI between day 3-5 optimal for injury pattern; MRI at term (or day 5-7) for prognosis.

📋 Stepwise management of HIE and therapeutic hypothermia

1
Stabilization & cooling initiation – Maintain normothermia (36.5-37°C) during transport; start servo-controlled cooling (whole body or head) by 6h of age. Target esophageal/rectal 33.5°C.
2
Cardiorespiratory support – Avoid hyperoxia (SpO2 90-95%), normocarbia (PaCO2 45-55), maintain MAP >40 mmHg. Inotropes if hypotension.
3
Seizure management – Continuous EEG or aEEG. Phenobarbital 20 mg/kg IV; add levetiracetam or fosphenytoin for refractory seizures. Avoid long-acting sedatives.
4
Metabolic & organ support – Monitor glucose (avoid hypoglycemia), calcium, electrolytes. Renal function, liver enzymes, coagulopathy (common).
5
Rewarming – After 72h, rewarm slowly (0.5°C/h) to 36.5°C. Monitor for rebound hyperthermia, seizures, hypotension.
6
MRI and follow-up – MRI at day 5-7 (or term equivalent). Neurodevelopmental follow-up at 18-24 months (Bayley scales).
📌 Inclusion criteria for cooling (typical): GA ≥36 wk, birthweight ≥1800 g, pH ≤7.0 or base deficit ≥16 within 1h, or Apgar ≤5 at 10 min + moderate/severe encephalopathy.

🧠 Rapid reflex prompts – HIE & cooling

📌 First-line anticonvulsant in HIE?
Phenobarbital 20 mg/kg IV (up to 40-50 mg/kg).
📌 MRI finding with worst prognosis?
Severe basal ganglia/thalamus (BG/T) injury with absent PLIC signal.
📌 What aEEG background predicts poor outcome?
Burst suppression, continuous low voltage, or flat tracing.
📌 Optimal timing of MRI after HIE?
Day 3-7 (DWI most sensitive early; conventional T1/T2 at day 5-7).
📌 Contraindication to therapeutic hypothermia?
Major congenital anomaly, severe IUGR, active bleeding, <36 wk GA, decision for comfort care only.
📌 Which organ systems are commonly injured in HIE?
Brain, heart (myocardial stunning), kidney (ATN), liver, adrenal, GI (NEC/perforation), bone marrow (DIC).
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