Extracerebral fluid collections in infants

Justine Ker BA (

Justine Ker of Tulane University School of Medicine has no relevant financial relationships to disclose.

Stephen L Nelson Jr MD PhD (

Dr. Nelson of Tulane University School of Medicine received honorariums from BioMarin and LivaNova for speaking engagements.

Michael V Johnston MD, editor. (

Dr. Johnston of Johns Hopkins University School of Medicine has no relevant financial relationships to disclose.

Originally released February 13, 2002; last updated January 19, 2021; expires January 19, 2024


Extracerebral fluid collections in infants are common and of multiple etiologies. Most acute subdural hematomas occur non-accidentally. Infections can lead to subdural effusions and empyemas. Arachnoid cysts are the most common intracranial cysts. In otherwise normal infants with a large head, a common finding is benign enlargement of the subarachnoid space or subarachnomegaly, which has been called a variety of names in the literature, making interpretation of treatment and outcome data difficult.

Key points


• Extracerebral fluid collections are common in infants.


• Infants with benign enlargement of the subarachnoid space have large heads, often a family history of large heads, and imaging findings of frontal predominance of extraaxial fluid with no or mild ventriculomegaly.


• These infants have normal development other than sometimes minor delays that resolve; they have otherwise normal neurologic examinations and favorable outcomes.


MRI can readily distinguish between subarachnoid and subdural fluid collections and is the study of choice.


• Preferred terms for this condition are “benign enlargement of the subarachnoid spaces of infancy” or “subarachnomegaly,” and other terms for this condition should be avoided.

Historical note and terminology

Intracranial fluid collections in infants were described initially by Galen and Oribasius in the 1850s. Dandy and Blackfan first introduced the term external hydrocephalus to describe infants with increased intracranial pressure and enlarged subarachnoid spaces (Dandy and Blackfan 1913). During the 20th century, it was recognized that bloody and clear fluid could arise from a number of causes other than hydrocephalus and that this fluid could accumulate in the subdural as well as subarachnoid spaces. Subdural hygromas were described (Naffziger 1924), and chronic subdural hematoma was recognized as a frequent cause of head enlargement following trauma (Ingraham and Matson 1944). Subdural effusions were reported to be a common complication of meningitis in infants (Bloor et al 1950; McKay et al 1950). Subdural hematomas and effusions were noted as complications of ventricular shunt procedures (Anderson 1952; Davidoff and Feiring 1953). However, the distinction between the subarachnoid and subdural spaces was not always clearly determined; the literature is, therefore, often confusing regarding causes, treatment, and outcomes of these collections.

The more common extracerebral fluid collections encountered in infants include:


• dilated subarachnoid spaces (subarachnomegaly)/external hydrocephalus/multiple other names (see below)
• post-shunt subdural and subarachnoid effusions
• postinfectious subdural effusions
subdural empyemas
• malignant subdural effusions
chronic subdural hematoma
• subdural hygromas
• arachnoid and other extracerebral cysts

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