What are the features of hypoxemic ischemic encephalopathy on a non-contrast computed tomography (CT) scan of the brain?

CT Features of Hypoxic-Ischemic Encephalopathy

Non-contrast CT brain is relatively insensitive for detecting hypoxic-ischemic encephalopathy (HIE) in the acute phase, often appearing normal in the first 24-48 hours despite severe clinical presentation, but when positive, demonstrates loss of gray-white matter differentiation, diffuse cerebral edema with sulcal effacement, and hypodensity of deep gray structures (basal ganglia and thalami). 1, 2

Early CT Findings (First 24-48 Hours)

CT is frequently normal or shows minimal changes in acute HIE despite severe neurological impairment:

• Loss of gray-white matter differentiation is the earliest detectable sign, particularly in the cortical ribbon at the insular cortex and lentiform nucleus, indicating cytotoxic edema 1, 2
• Hypodensity of deep gray structures including the basal ganglia (caudate and lentiform nuclei) and thalami may be visible within hours 1
• Diffuse cerebral edema manifests as sulcal effacement and compression of CSF spaces, representing focal or diffuse brain swelling 2
• Hyperdense artery sign may occasionally be present, though this is more characteristic of acute ischemic stroke from arterial occlusion 2

Timing Considerations and Diagnostic Limitations

The major pitfall with CT in HIE is its poor sensitivity in the hyperacute phase:

• CT shows abnormalities in less than 50% of patients initially with standard sequences in acute ischemia 2
• Normal CT findings do not exclude HIE—clinical assessment and neurological examination remain paramount for diagnosis 2
• CT is particularly insensitive for detecting small cortical or subcortical lesions and posterior fossa involvement (brainstem and cerebellum) 1, 2
• In documented cases, CT performed on days 0-3 after cardiac arrest has shown no abnormalities despite severe clinical HIE and poor prognosis 3

Subacute CT Findings (Days to Weeks)

CT becomes more sensitive for detecting ischemic changes on follow-up imaging:

• Increased sensitivity for extent of ischemic changes compared to hyperacute imaging 2
• Progressive hypodensity in affected territories becomes more apparent
• Mass effect may develop or worsen with time
• Atypical findings may include slight subarachnoid hemorrhage at cortical surfaces, thought to result from transient blood-brain barrier damage caused by hypoxia and ischemia 3

Clinical-Radiological Correlation

Critical management principle: Prognosis and treatment decisions in HIE should NOT be based solely on CT findings:

• Variations in CT and MRI findings in HIE are substantial, with imaging potentially appearing normal despite severe clinical encephalopathy 3
• Neurological signs and symptoms must guide management in conjunction with imaging, not imaging alone 3
• The severity of clinical presentation (depth of coma, presence of myoclonus, reflexes) does not always correlate with early CT findings 3

Recommended Imaging Approach

For suspected HIE, MRI is the preferred imaging modality when clinically feasible:

• MRI with diffusion-weighted imaging (DWI) is the gold standard for ischemic core assessment, turning positive within minutes with high sensitivity and specificity 1
• MRI is superior to CT for detecting acute and small cortical or subcortical infarctions, especially in the posterior fossa 1
• CT remains the first-line emergency imaging to rule out hemorrhage and provide rapid assessment, but should not be relied upon to exclude HIE if clinical suspicion is high 1, 2

Practical Algorithm for HIE Imaging

1. Obtain non-contrast CT brain emergently to exclude hemorrhage and assess for gross abnormalities 1
2. Do not delay supportive care based on normal or equivocal CT findings if clinical presentation suggests HIE 2
3. Proceed to MRI brain (preferably with DWI sequences) when patient is stable enough for transport, ideally within 24-96 hours for optimal diagnostic and prognostic information 4
4. Repeat imaging if neurological status deteriorates or fails to improve as expected 2