Tentorial hypoplasia

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 localised brain herniation, typically the occipital lobes, through the defect into the posterior fossa ¹.

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:

• 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.