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Neuroradiology Case of the Week

Case 193

Guruprasad A Srinath, MD and Per-Lennart Westesson, MD, PhD, DDS

Clinical Presentation: Enlarging head

Radiological Findings:

Diagnosis: External hydrocephalus

Discussion: External hydrocephalus (EH) is a condition in which infants with rapidly enlarging heads are found to have a CT scan that shows widening of the subarachnoid space with mild or no ventricular dilation. The characteristic CT findings in EH are bifrontal widening of the subarachnoid space and widened interhemispheric fissure frontally with only mild enlargement of the rest of the subarachnoid space. Occasional mild to moderate ventriculomegaly is also present. In most cases the radiologic findings can be helpful to distinguish this from cerebral atrophy, even without the clinical history of an enlarging head. The cases of cerebral atrophy show prominent sulci throughout without disproportionate bifrontal widening of the subarachnoid space. The radiologic abnormalities in these infants usually disappear by 2 years of age. EH is an age-related self-limited condition occurring in infants with open cranial sutures and that usually resolves without intervention by 2-3 years of age. Color Doppler US depiction of the cortical vein sign appears to be as effective as MR imaging in differentiating enlargement of the subarachnoid fluid space rooms effusion. Positive cortical vein sign is visualization of color-coded cortical veins that cross fluid collection sat cerebral convexities in subarachnoid fluid space. The veins were displaced and embedded within the echogenic pia-arachnoid that surrounds the brain or were trapped in the subarachnoid spaces between the neo-membrane and cortical surface in subdural effusion. Differentiation between subdural effusion and enlargement of the subarachnoid spaces can be established by focusing on two aspects of MR imaging findings: 1) the intensity of the fluid, which is either iso- or hyperintense relative to CSF, and 2) the presence or absence of vascular flow-void areas in the fluid spaces. In benign enlargement of the subarachnoid spaces, the fluid was isointense in relation to CSF, and vascular flow-void areas were seen in at least one of the MR imaging sequences.

References:

1. Maytal J, Alvarez LA, Elkin CM, Shinnar S: External hydrocephalus: radiologic spectrum and differentiation from cerebral atrophy. AJR Am J Roentgenol. 1987 Jun;148(6):1223-30. [Medline]
2. 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. Radiology. 1996 Nov;201(2):389-92. [Medline]
3. Aoki N: Extracerebral fluid collections in infancy: role of magnetic resonance imaging in differentiation between subdural effusion and subarachnoid space enlargement. J Neurosurg. 1994 Jul;81(1):20-3. [Medline]