Benign enlargement of the subarachnoid spaces

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General

• AKA
• Benign external hydrocephalus
• Benign enlargement of the subarachnoid spaces
• External hydrocephalus
• Benign external hydrocephalus

• A form of communicating HCP

• There are often enlarged basal cisterns and widening of the anterior interhemispheric fissure. No other symptoms or signs should be present (although there may be slight delay only in motor milestones due to the large head).

Definition

• A rapid increase in head circumference, combined with enlarged subarachnoid spaces as seen on neuroimaging—especially overlying the frontal lobes—and normal or only moderately enlarged ventricles

• Ventriculomegaly without periventricular lucency, and non-tense fontanels

Number

• 0.6% of children in a tertiary paediatric neurology centre have it

• 2/3 boys

• In young children until 2 yrs

• M>F

Mechanism

• Caused by immature arachnoid villi not able to absorb the CSF that is produced continuously → accumulated CSF then expands the ventricles and the subarachnoid space inside the compliant and growing skull of an infant → avoiding a marked increase in intracranial pressure

• A transient communicating hydrocephalus

• After 2 years of age, the capacity for cerebrospinal fluid drainage can attain 2-4 times the normal rate of production.

Risk factors

• Mainly none

• Can be associated with
• Some craniosynostoses (especially plagiocephaly),
• Intraventricular haemorrhage
• Superior vena cava obstruction.

Natural history

• Delayed in motor or language development at 5 months of age, but by 15–18 months of age all but one were found to be normal

Evaluation

• Clinical
• Tense anterior fontanel
• Dilated scalp veins
• Frontal bossing
• Head circumference that crosses centiles
• Head circumference usually stabilizes before the age of 18 months

• Radiology

• Imaging shows
• Traversing bridging veins (vs subdural collection which does not have bridging veins)

Differential diagnosis

• Benign subdural collections (or extra-axial fluid) of infancy.

• Symptomatic chronic extra-axial fluid collections (or chronic subdural hematoma), which may be accompanied by seizures, vomiting, headache… and may be the result of child abuse.
• EH, MRI or CT may demonstrate cortical veins extending from the surface of the brain to the inner table of the skull coursing through the fluid collection (“cortical vein sign”),
• Subdural hematomas compress the subarachnoid space, which apposes the veins to the surface to the brain.

• Genetic conditions that should be considered when a patient manifests enlargement of the subarachnoid spaces which include
• Several of the mucopolysaccharidoses,
• Achondroplasia,
• Sotos syndrome,
• The inborn error of metabolism glutaric aciduria type I

Treatment

• EH usually compensates by 12–18 mos age without shunting.

• Recommended: follow serial ultrasound and/or CT to rule out abnormal ventricular enlargement.
• Emphasize to parents that this does not represent cortical atrophy.
• Due to increased risk for positional molding, parents may need to periodically reposition the head while the child is sleeping.

• A shunt may rarely be indicated when the collections are bloody (consider the possibility of child abuse) or for cosmetic reasons for severe macrocrania or frontal bossing.