What is a subdural hygroma?

What is a Subdural Hygroma?

A subdural hygroma is a collection of cerebrospinal fluid (CSF) in the subdural space, typically occurring after trauma or CSF leak, appearing as a hypodense fluid collection on imaging that lacks the enhancement and neomembrane characteristic of chronic subdural hematomas. 1, 2

Pathophysiology

The fundamental mechanism involves separation of the dura-arachnoid interface, which creates the subdural space where CSF can accumulate. 2 This occurs through several pathways:

• Traumatic tears in the arachnoid membrane allow CSF to leak from the subarachnoid space into the subdural compartment 1, 3
• Passive effusion develops when the brain shrinks due to atrophy, dehydration, or decreased intracranial pressure, creating space for fluid accumulation 2
• Spontaneous intracranial hypotension (SIH) from CSF leaks can produce subdural hygromas as a secondary finding 4

Clinical Presentation and Associated Conditions

Subdural hygromas are associated with:

• Trauma (most common cause, particularly after trivial head injury in patients with brain atrophy) 2, 3
• Iatrogenic causes including dural puncture or spinal procedures 5
• Spontaneous intracranial hypotension with CSF leaks 4, 6
• Meningitis (less common association) 5

When associated with SIH, patients typically present with postural headaches (83% of cases), along with nausea/vomiting (28%), and various neurological symptoms. 4

Diagnostic Imaging Features

MRI is the gold standard for diagnosis, particularly when evaluating for underlying causes like SIH. 4, 5

Key imaging characteristics on CT include:

• Hypodense subdural collection (CSF density) 3
• No enhancement or neomembrane formation (distinguishing it from chronic subdural hematoma) 3
• Minimum distance of 3 mm between skull and brain 3

When SIH is suspected, MRI of the brain with contrast and whole spine should be performed to identify imaging features including pachymeningeal enhancement, midbrain descent, subdural hygroma or hematoma, and to locate potential CSF leak sites. 4, 6

Clinical Significance and Natural History

Most subdural hygromas are benign epiphenomena that resolve spontaneously when the brain re-expands. 2 However, important considerations include:

• Outcome is closely related to the primary head injury, not the hygroma itself 2
• Early detection is possible within the first 24 hours after trauma, often appearing as small subdural effusion 3
• A minority can evolve into chronic subdural hematomas if conditions persist over several weeks 2
• Dramatic evolution can occur, particularly with dehydration as a triggering factor 7

Potential Complications

When associated with spontaneous intracranial hypotension, subdural hygromas can be part of a life-threatening clinical picture including:

• Cerebral venous thrombosis (reported in 86% of SIH cases with complications) 4
• Intraparenchymal hemorrhage (22% of complicated cases) 8
• Seizures (22% of complicated cases) 8
• Mortality risk of 3% in severe cases with CVT 8

Common Pitfalls

• Subdural hygromas lack specific symptoms and are often not diagnosed because they are overshadowed by other cranial or spinal pathologies 5
• Serial CT scanning is essential as hygromas can have dramatic evolution after prolonged periods of stability 7
• Do not confuse with chronic subdural hematoma—hygromas lack enhancement and neomembrane on imaging 3
• Always investigate for underlying CSF leak, particularly in cases associated with postural headaches 6, 5