What is the difference between acute and chronic subdural hematoma on computed tomography (CT) scan?

Differences Between Acute and Chronic Subdural Hematoma on CT Scan

The key difference between acute and chronic subdural hematoma on CT is that acute subdural hematomas appear hyperdense (white), while chronic subdural hematomas appear hypodense (dark) relative to brain parenchyma.

Acute Subdural Hematoma on CT

• Appearance: Hyperdense (white) crescent-shaped collection along the inner table of the skull 1
• Density: 50-100 Hounsfield units (HU)
• Time frame: Within 72 hours of injury
• Additional features:
  • Sharp inner margin
  • May demonstrate mass effect with midline shift
  • May be associated with other traumatic findings (contusions, subarachnoid hemorrhage)
  • Typically homogeneous in density, but can have mixed density in 39% of cases 2

Chronic Subdural Hematoma on CT

• Appearance: Hypodense (dark) crescent-shaped collection
• Density: Similar to cerebrospinal fluid (CSF)
• Time frame: Generally >3 weeks after injury
• Additional features:
  • Often bilateral
  • May have internal septations
  • Can have calcifications in very chronic cases
  • May have encapsulating membranes that enhance with contrast

Subacute Subdural Hematoma on CT

• Appearance: Isodense (same density as brain)
• Time frame: Approximately 1-3 weeks after injury
• Diagnostic challenge: May be difficult to detect without contrast enhancement 3
• Key finding: Contrast enhancement reveals displaced cortical vessels and enhancing membranes 3

Atypical Presentations

1. Isodense acute subdural hematoma:

   • Can occur in severely anemic patients (hemoglobin <10 g/dL) 4
   • May be missed without contrast enhancement
   • Critical diagnostic trap that can compromise patient care 4

2. Mixed-density acute subdural hematoma:

   • Present in 39% of acute cases 2
   • Associated with larger hematoma size (average 18.1mm vs 8.0mm)
   • Higher mortality rate (50% vs 26%) compared to homogeneous hematomas 2
   • May mimic epidural hematoma when it has a convex inner margin 2

Predictors of Spontaneous Resolution

• Use of antiplatelet agents before injury
• Presence of a low-density band between the hematoma and inner skull bone (indicating CSF infusion into subdural space) 5

Risk Factors for Progression of Conservatively Managed Acute Subdural Hematomas

• Increased maximal hematoma thickness
• Low hemoglobin level
• High leukocyte count 6

Clinical Implications

1. Diagnostic approach:

   • Non-contrast head CT is the first-line imaging test for suspected acute head trauma 1
   • Consider contrast-enhanced CT when isodense subdural hematoma is suspected, especially in anemic patients 3
   • MRI is more sensitive for detecting subacute and chronic subdural collections 1

2. Management considerations:

   • Patients with thicker acute subdural hematomas, lower hemoglobin levels, and higher leukocyte counts require closer observation due to higher risk of requiring delayed surgical intervention 6
   • Patients on antiplatelet therapy may have higher likelihood of spontaneous resolution 5

Common Pitfalls

1. Missing isodense subdural hematomas - particularly in anemic patients or those in the subacute phase; contrast enhancement is essential for diagnosis 4, 3

2. Misinterpreting mixed-density acute subdural hematomas as chronic or subacute when they represent unclotted blood, serum extrusion during clot retraction, or CSF within the subdural space 2

3. Overlooking bilateral subdural hematomas - always examine both sides carefully, as bilateral collections are common, especially in chronic cases

4. Underestimating the significance of small acute subdural hematomas - even small collections can expand rapidly and require surgical intervention

By understanding these differences in CT appearance and being aware of atypical presentations, clinicians can more accurately diagnose subdural hematomas and provide appropriate management to reduce morbidity and mortality.