Tentorial hypoplasia refers to an underdevelopment or partial absence of the tentorium cerebelli, the dural fold that separates the cerebral hemispheres from the cerebellum. This uncommon anatomical variant can be isolated or associated with other intracranial abnormalities. When isolated, it often leads to localized brain herniation, typically the occipital lobes, through the defect into the posterior fossa 1.
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Epidemiology
Although few case reports exist in the literature, the prevalence of tentorial hypoplasia is estimated to be approximately 2.2% 1. There appears to be a female predilection 1.
Clinical presentation
Tentorial hypoplasia is usually an incidental finding, and in many cases, presenting symptoms (e.g. headache, vertigo, seizures) are difficult to ascribe to the brain herniation 1.
Visual field defects, particularly homonymous hemianopia, are caused by distortion of optic radiation in the herniated brain tissue has been confirmed using diffusion tensor imaging (DTI) in some cases 2.
Pathology
The cause of tentorial hypoplasia is unknown but most likely due to failure of normal tentorial development 3. Tentorial hypoplasia can be unilateral or bilateral, with one study reporting ~20% right-sided, ~30% left-sided, and ~55% bilateral cases ref.
Radiographic features
The features of tentorial hypoplasia and associated brain herniation are best assessed with MRI, although many will also be visible on CT.
MRI
Imaging findings include 1-4:
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focal defect in the tentorium cerebelli
unilateral (more common) or bilateral
best seen on coronal T2 weighted images
herniation of brain tissue (typically occipital lobe) through the defect into the posterior fossa
Diffusion tensor imaging (DTI) may demonstrate involvement of white matter tracts, particularly the optic radiation, in the herniated brain tissue 2.
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