Rapid onset of coma and death can occur given the small space of the posterior fossa and rapid progression in an acute process such as large hypertensive hemorrhage. This patient demonstrates hydrocephalus with brain herniating through the quadrigeminal plate cistern and causing mass effect on the posterior midbrain. The ambient cisterns are also compressed. Alar brain herniations are often not discussed much due to the relative absence of clinical symptomatology. These can also be difficult to demonstrate on imaging studies if one is not attentive to the subtle findings and changes.
Clinical symptoms are generally minimal.
In general, this type of herniation is associated with other herniations such as subfalcine or transtentorial herniations which are more clinically apparent. Alar herniations can be divided into posterior and anterior herniations. These occur over the sphenoid wing itself. Frontal lobe masses will cause a posterior alar herniation as the frontal lobe extends posterior and inferiorly over the sphenoid ridge. Temporal lobe lesions or lesions of the insula can cause anterior alar herniations .