What is a subacute infarct on a computed tomography (CT) head scan?

What is a Subacute Infarct on CT Head?

A subacute infarct on CT head refers to a stroke that occurred between approximately 8 days and 1 month prior, appearing as a well-defined hypodense (dark) area with more pronounced tissue loss and clearer borders than acute infarcts, often with associated mass effect that may worsen during this phase.

Temporal Definition and Pathophysiology

• The subacute phase of cerebral infarction is typically defined as occurring between 8 days and 1 month after the initial ischemic event 1
• During this period, secondary injury processes develop including worsening parenchymal edema, ischemia, hydrocephalus, and progressive or delayed hemorrhage 1
• Up to 30% of contusions may cause worsening mass effect during the subacute phase due to toxic metabolites released into surrounding tissue and cerebral autoregulation dysfunction 1

CT Imaging Characteristics

Noncontrast CT is more sensitive for evaluating the extent of ischemic changes on follow-up imaging than in the hyperacute setting 1

Key CT Findings in Subacute Infarcts:

• Well-demarcated hypodensity: The infarcted tissue appears darker (lower attenuation) than normal brain tissue with clearer borders than acute infarcts 2, 3
• Mass effect: Swelling and edema may be prominent, potentially causing midline shift or herniation 1
• Loss of gray-white matter differentiation: The normal distinction between cortex and white matter becomes obliterated 2, 4
• Sulcal effacement: Brain sulci become compressed or invisible due to swelling 2, 3

Critical Imaging Considerations

Contrast Administration Should Be Avoided:

There is no role for IV contrast in the CT evaluation of evolving or subacute infarct 1

• Contrast enhancement within previously undocumented subacute infarcts can paradoxically cause confusion with other more aggressive brain lesions or hemorrhagic conversion of prior infarcts 1
• This is a common pitfall that can lead to misdiagnosis

When to Image:

• Noncontrast CT is useful for early surveillance of ischemic strokes to evaluate for complications such as hemorrhagic conversion, mass effect, and herniation when clinically indicated 1
• CT is usually preferred for surveillance due to its quick repeatability and ease of comparison to prior examinations 1
• For patients who experience clinical deterioration, emergency CT scanning should be performed immediately 5

Clinical Implications

• Patients with significant changes in neurologic status during the subacute phase are at high risk for progressive intracranial injury and may require neurosurgical intervention 1
• CT can provide rapid, accurate assessment for progressive hemorrhage, herniation, and hydrocephalus 1
• The 24-hour follow-up CT scan after thrombolysis is required before starting anticoagulants or antiplatelet agents for secondary stroke prevention 5

Common Pitfalls

• Relying solely on clinical examination without follow-up imaging may miss asymptomatic hemorrhagic transformation 5
• Using contrast-enhanced CT can create diagnostic confusion by mimicking hemorrhagic transformation or tumor 1
• Delay in obtaining follow-up imaging may postpone initiation of secondary stroke prevention measures 5