Discussion
Arachnoid granulations appear very early in life, although they are uncommon before age 2 years.2 Classically, they have been understood to act as 1-way valves permitting the outflow of CSF from the subarachnoid space to the dural venous sinuses. However, increasing evidence shows they may only play a minor role in that process.12 The structure of arachnoid granulations is being reexamined. A recent microscopy study demonstrated structural heterogeneity with a fine, porous lining that permits flow.13 Additionally, associated immune components in the microenvironment suggests that arachnoid granulations may function similarly to lymph nodes as part of a central nervous system lymphatic network.13 Evidence is lacking for arachnoid granulations being the primary route of CSF outflow, and newer models include CSF exit pathways along the cranial nerves and drainage through lymphatics within the dura mater.12
New MRI systems have demonstrated that the prevalence of arachnoid granulations increases with age. One study found that all subjects in the aged 40 years cohort had detectable arachnoid granulations on images obtained with a 3T MRI system, with the main site being the superior sagittal sinus.2 The prevalence increased until age 40 years and then noticeably decreased. Not only did the prevalence increase in this pattern, but the total number of detectable arachnoid granulations followed a similar pattern.2 In addition, the detectable arachnoid granulations tend to be larger in older patients. Arachnoid granulations are very common in adults, but little is known about when and why brain tissue herniates through these structures.
This case series illustrates how a small amount of adult cerebral or cerebellar matter in large arachnoid granulations can herniate into the dural sinuses and diploic space. Although arachnoid granulations extending into the dural sinuses and diploic space are a relatively common finding on MRI, BHAGs are rare in these locations.1,2,8 Improved spatial resolution afforded by newer high-field scanners with thinner sections, such as very thin (1 mm) T1- and heavily T2-weighted 3 dimensional sequences may lead to increased detection of BHAG. Some of these herniations are small and may be easily missed or confused for normal arachnoid granulations on 3 to 5 mm thickness MRIs.
Despite increased recognition, it is still uncertain to what degree these herniations contribute to the clinical presentations. Associated neurologic symptoms may include seizures, headaches, tinnitus, syncope, and increased intracranial pressure.7-10
Three cases presented in this article demonstrated abnormal signals adjacent to the herniated brain, presumably due to dysplasia of gliotic tissue. In 1 study, parenchymal signal and structural changes occurred in about one-half of the reported BHAG, all of which were cerebellar herniations.7 In Case 1, the herniation and adjacent abnormal MRI signal corresponded to localization of the seizure semiology as obtained from patient history, strongly suggesting the BHAG played a role in the presentation. Signal abnormality accompanying an adjacent BHAG may suggest a higher likelihood that the BHAG has clinical relevance. However, the patient in Case 2 had a visual aura that corresponded to the BHAG location, so a signal abnormality may not be necessary for a patient to develop symptoms. Case 1 also included a history of documented traumatic brain injuries, suggesting that perhaps head trauma may facilitate BHAG development. Regardless, there is likely also a congenital component to their formation, as BHAG has been observed in the pediatric population.14
The patient's asymmetric left-sided hearing loss in Case 3 appeared unrelated to the BHAG as its location was in the contralateral cerebellar region and did not correspond to the patient’s clinical findings. The patient in Case 4 had a limited history regarding localization details of their prior presumed alcohol withdrawal seizure, such as head movements, eye deviation, or lateralized onset of convulsions. Given this limited data, it is unclear whether their prior seizure could have been related to BHAG or not. The patient in case 5 reported worsening headaches on the left side of his head, which corresponded to BHAG occurring on the left side. However, given that the increased T2 signal occurred in the left cerebellar hemisphere with BHAG in the left occipital bone, the occipital cortex was not involved. In this case, the BHAG would not explain the patient’s visual aura as such a lesion would have been expected in the right occipital cortex rather than its actual location in this patient’s left cerebellar hemisphere.