What are the differences between chronic subdural hematoma and chronic subdural hygroma?

Differentiating Chronic Subdural Hematoma from Chronic Subdural Hygroma on CT

Chronic subdural hematoma (CSDH) and chronic subdural hygroma (SDH) differ primarily in their CT density (Hounsfield Units): CSDH typically appears hypodense to isodense (0-30 HU) with heterogeneous density or layering, while subdural hygroma appears uniformly hypodense (0-15 HU), matching CSF density.

Key CT Imaging Differences

Hounsfield Unit Characteristics

Chronic Subdural Hematoma:

• Density range: 0-30 HU, often isodense to brain parenchyma in the chronic phase 1
• Heterogeneity: Frequently shows mixed density with sedimentation levels or internal septations 1
• Evolution pattern: Progresses from hyperdense (acute) → isodense (subacute) → hypodense (chronic) over 1-3 weeks 2
• Higher CT values within the collection correlate with increased risk of evolution and recurrence 3

Subdural Hygroma:

• Density range: 0-15 HU, uniformly hypodense matching CSF 4
• Homogeneity: Typically uniform, water-like density without internal complexity 4
• No blood products: Pure CSF accumulation without hemorrhagic components initially 5

Morphological Distinctions

Shape and Border Characteristics

Both collections share:

• Convex outer border (against skull)
• Concave inner border (against brain) 1
• Crescent-shaped configuration along convexity

Critical differentiating features:

CSDH:

• Membrane formation: Develops neo-membranes (outer and inner capsules) with neovascularization from middle meningeal artery branches 2
• Thickness: Often thicker collections (>10mm associated with higher evolution risk) 3
• Internal architecture: May show septations, loculations, or fluid-fluid levels 1

Hygroma:

• No organized membranes initially (though can develop over time) 4
• Thinner collections typically
• Smooth internal architecture without septations 4

Pathophysiologic Context

Understanding the Continuum

The unified transdural-pressure gradient model 5 explains both entities:

• Hygroma formation: Results from CSF dissection into the dural border cell layer when transdural pressure gradients increase and venous/lymphatic drainage cannot accommodate elevated fluid flux
• Evolution to CSDH: Sustained pressure gradients trigger mechanotransductive angiogenesis, forming fragile neovessels that leak plasma proteins and erythrocytes, transforming hygroma into hematoma

Approximately 25% of traumatic subdural hygromas evolve into CSDH 3, particularly in:

• Patients ≥60 years old
• Thicker initial hygromas
• Higher initial CT density values (suggesting early blood contamination)

Diagnostic Algorithm

Step 1: Assess CT Density

• 0-15 HU, homogeneous → Likely hygroma
• 15-30 HU or mixed density → Likely CSDH
• Isodense to brain → CSDH (diagnostic challenge requiring MRI)

Step 2: Evaluate Internal Architecture

• Uniform, water-like → Hygroma
• Layering, septations, heterogeneity → CSDH

Step 3: Look for Membrane Enhancement

• If available, contrast-enhanced CT showing membrane enhancement strongly suggests CSDH with organized capsule 1

Step 4: Consider Clinical Context

• Recent trauma with orthostatic headache → Consider hygroma, especially if spontaneous intracranial hypotension suspected 6
• Elderly patient, anticoagulation, remote trauma → More likely CSDH
• Progressive symptoms over weeks → CSDH more likely

Common Diagnostic Pitfalls

Bilateral Isodense CSDH

The most challenging diagnostic scenario occurs with bilateral isodense CSDH 1, which can mimic:

• Cerebral atrophy with enlarged subarachnoid spaces
• Bilateral hygromas

Key distinguishing features:

• CSDH causes mass effect with sulcal effacement and ventricular compression
• Atrophy shows prominent sulci and enlarged ventricles
• Look for midline shift (suggests CSDH even if bilateral)

When CT is Insufficient

MRI is nearly 100% sensitive for distinguishing these entities 1:

• T1-weighted: CSDH shows variable signal intensity (often hyperintense); hygroma matches CSF (hypointense)
• T2-weighted: Both hyperintense, but CSDH may show heterogeneity
• FLAIR: Hygroma suppresses like CSF; CSDH shows incomplete suppression
• Gradient echo/SWI: Detects microbleeds and hemosiderin in CSDH, absent in pure hygroma

Clinical Implications

Management differs significantly:

• Hygroma: Often managed conservatively unless symptomatic mass effect; address underlying cause (CSF leak, intracranial hypotension) 6
• CSDH: Higher threshold for surgical intervention (burr hole drainage, middle meningeal artery embolization) due to neovascular membranes and recurrence risk 2

For patients with subdural collections and suspected spontaneous intracranial hypotension: MRI brain with contrast and whole spine imaging should be performed to investigate CSF leak 6. Small or asymptomatic collections should be managed conservatively while treating the underlying leak.