When does a subdural hematoma become chronic based on computed tomography (CT) criteria?

When Does a Subdural Hematoma Become Chronic Based on CT Criteria

A subdural hematoma is considered chronic when it appears hypodense (lower density than brain tissue) on CT scan, which typically occurs approximately 3 weeks after the initial bleeding event.

Radiological Evolution of Subdural Hematomas on CT

Subdural hematomas evolve through distinct phases that can be identified on CT imaging:

Acute Phase (0-3 days)

• Appears hyperdense (white/bright) compared to brain tissue
• Homogeneous high density that fills the subdural space
• Hounsfield units consistent with acute blood

Subacute Phase (3-21 days)

• Transitions from hyperdense to isodense (similar density to brain tissue)
• May be difficult to visualize directly during this phase
• Mixed density may be present as the hematoma begins to liquefy

Chronic Phase (>21 days/3 weeks)

• Appears hypodense (darker than brain tissue)
• Lens-shaped or crescent-shaped low-density area between skull and brain surface
• May show evidence of membranes forming within the hematoma

CT Classification of Chronic Subdural Hematomas

Based on CT appearance, chronic subdural hematomas can be further classified into three types 1:

1. Type 1 (37% of cases): Decreased attenuation (hypodense) compared to brain tissue, appearing as a lens-shaped low-density area

2. Type 2 (30.5% of cases): Mixed density with both low-density areas and zones of increased attenuation due to recent bleeding into a chronic hematoma (indicating rebleeding)

3. Type 3 (32.5% of cases): Isodense to brain tissue (difficult to visualize directly), diagnosed by midline shift, ventricular compression, and absence of other lesions

Advanced Imaging Considerations

While CT is the primary modality for initial evaluation, MRI offers superior sensitivity for detecting chronic subdural hematomas:

• MRI shows better visualization of membranes and internal architecture
• Gradient echo and susceptibility-weighted sequences are particularly helpful for identifying small hemorrhages 2
• Follow-up MRI at 2 months can help confirm resolution or evolution of the hematoma 2

Common Pitfalls in Diagnosis

1. Isodense chronic hematomas: During the isodense phase, subdural hematomas may be missed on non-contrast CT. Consider contrast-enhanced CT or MRI if clinical suspicion is high.

2. Bilateral hematomas: These can be difficult to diagnose due to lack of midline shift. Look for compression of ventricles and sulcal effacement on both sides.

3. Misinterpretation of density changes: The transition from hyperdense to hypodense is a continuum rather than an abrupt change. Serial imaging may be necessary to establish chronicity.

4. Rebleeding into chronic hematomas: This creates mixed density appearance (Type 2) that should not be confused with acute hemorrhage alone.

The time-based evolution of subdural hematomas on CT is critical for clinical decision-making regarding management approaches, with chronic subdural hematomas generally being less urgent for surgical intervention unless causing significant mass effect or neurological deterioration.