Saturday, May 30, 2009

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

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