What is the characteristic appearance of subarachnoid hemorrhage on a plain Computed Tomography (CT) scan?

Characteristic Appearance of Subarachnoid Hemorrhage on Plain CT Scan

Subarachnoid hemorrhage appears on non-contrast CT as hyperdense (bright white) blood filling the basal cisterns, following the contour of sulci and gyri, and layering in the Sylvian fissures and convexity sulci. 1

Primary CT Characteristics

Acute Phase Appearance (First 3 Days)

• Blood appears as hyperdense material (brighter than normal brain tissue) distributed in the subarachnoid spaces, with CT sensitivity approaching 100% within the first 3 days after onset 1
• Classic distribution patterns include:
  • Blood in the basal cisterns (most common location) 1
  • Blood following the contour of sulci and gyri (distinguishes it from subdural hematoma) 1
  • Blood in the Sylvian fissures 1
  • Blood in the convexity sulci 1
  • Blood may layer in gravity-dependent locations such as the occipital horn of the lateral ventricle 1

Distribution Patterns by Etiology

Aneurysmal SAH typically shows:

• Diffuse blood distribution or blood concentrated around the circle of Willis 2
• May completely fill the lateral Sylvian or anterior interhemispheric fissures 3
• Can extend into the ventricular system 2

Perimesencephalic (non-aneurysmal) SAH shows a distinct pattern:

• Blood centered immediately anterior to the brainstem 3
• May extend to ambient cisterns or basal parts of Sylvian fissures 3
• Lateral Sylvian or anterior interhemispheric fissures are never completely filled 3
• No intraventricular extension 3
• This pattern has a 95% predictive value for negative angiography and excellent prognosis 3

Quantification Systems

Fisher Grading Scale

The Fisher classification quantifies SAH severity on non-contrast CT 2:

• Fisher Grade 1: No subarachnoid blood detected 2
• Fisher Grade 2: Diffuse or vertical layers <1 mm thick 2
• Fisher Grade 3: Localized clot or vertical layers ≥1 mm thick 2
• Fisher Grade 4: Intracerebral or intraventricular clot with diffuse or no subarachnoid blood 2

*Note: "Vertical" hemorrhage cisterns include interhemispheric, insular, and ambient cisterns 2

Temporal Evolution on CT

Subacute Phase (1-6 weeks)

• Blood may appear as layering of red blood cells in dependent locations 2
• Common sites include occipital horns of lateral ventricles, dorsal margins of Sylvian fissures, and sulci of cerebral convexities 2
• Blood density may become isodense, hypodense, or mixed density compared to adjacent brain tissue 2

Chronic Phase (>6 weeks)

• CT becomes less sensitive; hemoglobin degradation products are better visualized on MRI 2

Critical Diagnostic Pitfalls

Time-Dependent Sensitivity

• CT sensitivity decreases significantly after 6 hours from symptom onset 4
• Beyond 6 hours, lumbar puncture becomes mandatory when CT is negative 4
• Within 6 hours, high-quality third-generation CT interpreted by experienced neuroradiologists may obviate lumbar puncture in typical presentations 4

Pseudo-Subarachnoid Hemorrhage

• Hyperdensity in basal cisterns can occur without true SAH in conditions causing diffuse cerebral edema or increased intracranial pressure 5
• Absence of aneurysm on vascular imaging and autopsy confirmation of no hemorrhage distinguishes this entity 5
• Consider this diagnosis when vascular imaging is negative despite apparent SAH on CT 5

Distinguishing from Other Hemorrhage Types

Key differentiating features:

• Epidural hematoma: Occurs between skull and dura mater, does not follow sulci/gyri 1
• Subdural hematoma: Occurs between dura and arachnoid, does not follow sulci/gyri contour 1
• Intraparenchymal hemorrhage: Focal collection within brain parenchyma, not in subarachnoid space 1
• Intraventricular hemorrhage: Blood confined to ventricular system 1

Clinical Context and Next Steps

When CT Shows SAH

• Proceed immediately with CT angiography or catheter angiography to identify bleeding source, even with minimal visible blood 4
• CTA has 96.5% sensitivity and 88% specificity for aneurysms overall, but limited sensitivity for aneurysms <3 mm 4, 6
• If CTA is negative but SAH confirmed, perform digital subtraction angiography with 3D rotational angiography (>98% sensitivity and specificity) 4

When CT is Negative but Clinical Suspicion High

• Do not dismiss negative CT in high-risk presentations including thunderclap headache, photophobia, neck stiffness, or exertional onset 4
• Perform lumbar puncture looking for xanthochromia (100% sensitivity, 95.2% specificity when performed 12 hours to 2 weeks after onset) 1, 4
• Atypical presentations (primary neck pain without headache, syncope, new seizure, focal deficits) mandate full workup despite negative imaging 4

Associated Findings Suggesting Secondary Causes

• Radiologic red flags for underlying vascular lesions include: 2
  • Subarachnoid hemorrhage with unusual (non-circular) hematoma shape 2
  • Edema disproportionate to early timing of imaging 2
  • Unusual hemorrhage location 2
  • Presence of other abnormal structures like masses 2
  • Abnormal signal in cerebral sinuses (consider venous thrombosis) 2