What is a hygroma in the brain?

What is a Hygroma in the Brain?

A hygroma in the brain is a collection of cerebrospinal fluid (CSF) in the subdural space—the area between the dura mater and the arachnoid membrane—that appears as a hypodense (low-density), non-enhancing fluid collection on imaging. 1

Pathophysiology and Mechanism

Subdural hygromas develop when CSF accumulates in the subdural space due to separation of the dura-arachnoid interface. 2 This can occur through several mechanisms:

• CSF leakage through a dural defect, leaking meningeal diverticulum, or CSF-venous fistula, particularly in spontaneous intracranial hypotension (SIH) 3
• Downward traction on meninges from brain sagging due to decreased CSF volume, which creates space for fluid accumulation 3
• Traumatic separation of the dura-arachnoid interface, even from trivial trauma, especially when brain shrinkage occurs from atrophy, dehydration, or decreased intracranial pressure 2, 4

Clinical Presentation

The presenting symptoms depend on the underlying cause and size of the hygroma:

• Orthostatic headache (pain worsening when upright, improving when lying down) is characteristic of SIH-related hygromas 3
• Most subdural hygromas are asymptomatic and discovered incidentally, as they typically do not cause significant mass effect 2, 5
• Symptomatic hygromas with mass effect may present with neurological deficits related to compression 6
• Clinical outcome is more closely related to the primary injury (trauma or underlying pathology) rather than the hygroma itself 2, 5

Imaging Characteristics

MRI with and without contrast is the optimal imaging modality for diagnosing subdural hygromas and identifying associated features. 1

Key imaging findings include:

• Hypodense/hypointense subdural fluid collection on CT/MRI without enhancement or neomembrane formation 4
• Associated findings in SIH-related hygromas: engorgement of venous sinuses, pachymeningeal enhancement, midbrain descent, and convex superior surface of the pituitary 1
• Minimum distance of 3 mm between skull and brain is used as a diagnostic criterion 4

Important Clinical Distinctions

Do not confuse subdural hygromas with cystic hygromas (lymphatic malformations), which are completely different entities:

• Cystic hygromas are lymphatic malformations caused by lymphatic dysplasia, typically occurring in the neck region, not intracranially 3, 7
• Cystic hygromas are associated with chromosomal abnormalities (Turner syndrome in 50-80% of cases) and present prenatally or in early childhood 3

Natural History and Evolution

Most subdural hygromas resolve spontaneously when the brain re-expands and underlying conditions are addressed. 2, 5

• Early detection is common, with hygromas often appearing within the first 24 hours post-trauma as small subdural effusions that may evolve into larger collections 4
• Chronic evolution is possible but uncommon, with a few hygromas potentially transforming into chronic subdural hematomas if conditions persist for several weeks 2
• Serial imaging is essential to monitor evolution, as some hygromas can demonstrate rapid volume increase after prolonged slow evolution 8

Management Approach

Treatment should be guided by symptoms and underlying etiology, with most hygromas managed conservatively. 6, 5

Conservative Management (Majority of Cases)

• Treat the underlying CSF leak as the primary strategy for SIH-related hygromas 6
• Epidural blood patch should be prioritized as initial treatment for SIH-associated hygromas 6
• Bed rest in supine position as much as possible 6
• Serial CT or MRI monitoring to follow evolution 5

Surgical Intervention (Minority of Cases)

• Burr hole drainage is indicated only for symptomatic hygromas with significant mass effect causing neurological compromise 6, 5
• Surgery should be combined with treatment of any underlying CSF leak 6
• Surgical treatment rarely results in significant clinical improvement because outcomes are primarily determined by associated brain injuries rather than the hygroma itself 5

Critical Pitfalls to Avoid

• Do not rush to surgical intervention—the majority of subdural hygromas resolve spontaneously and surgery is rarely beneficial 2, 5
• Do not attribute poor outcomes to the hygroma alone—associated brain injuries (contusions, hematomas) are the primary determinants of prognosis 2, 5
• Do not confuse terminology—"cystic hygroma" refers to lymphatic malformations, not subdural fluid collections 3, 7
• Do not miss the underlying cause—always investigate for SIH with complete brain and spine MRI when subdural hygromas are identified 1, 6