Benign Extra-Axial Fluid

Benign extra-axial fluid, previously known as idiopathic external hydrocephalus, is rapid head growth in neonates accompanied by increased fluid in the subarachnoid space.

It is detected typically between ages 3 and 8 months. The condition is self-limited with resolution of subarachnoid space enlargement by 2 years of age. Any developmental delay also resolves as the extra fluid resolves. The macrocephaly, however, usually persists. A family history of benign macrocephaly is also commonly seen.

Mild hydrocephalus is seen in 2/3 of patients. Grayscale ultrasound findings include sulcal dilatation with normal gyrus configuration. The cortical vein sign, initially described on MRI, can help differentiate subarachnoid fluid (seen in passive dilatation and benign extra-axial fluid) from subdural fluid (seen in trauma). Visualization of cortical veins within fluid collections localizes the collection to the subarachnoid space, making a subdural collection less likely.

Differential considerations include:

• Passive dilatation of the subarachnoid space due to brain atrophy
• Acquired extraventricular obstructive hydrocephalus
• Different stages of subdural hematoma, for example from nonaccidental trauma

References

• Chen CY, Chou TY, Zimmerman RA, Lee CC, Chen FH, Faro SH. Pericerebral fluid collection: differentiation of enlarged subarachnoid spaces from subdural collections with color Doppler US. Radiology. 1996 Nov;201(2):389-92.
• McCluney KW, Yeakley JW, Fenstermacher MJ, Baird SH, Bonmati CM. Subdural hygroma versus atrophy on MR brain scans: "the cortical vein sign". AJNR Am J Neuroradiol. 1992 Sep-Oct;13(5):1335-9.
• Seibert JJ, Avva R, Hronas TN, Mocharla R, Vanderzalm T, Cox K, Kinder D, Lidzy B, Knight KL. Use of power Doppler in pediatric neurosonography: a pictorial essay. Radiographics. 1998 Jul-Aug;18(4):879-90.