Uncommon Locations for Brain Herniations Into Arachnoid Granulations: 5 Cases and Literature Review
Background: Arachnoid granulations are extensions of the subarachnoid space, an important component of the complex circulation of brain cerebrospinal fluid. While these structures primarily transmit cerebrospinal fluid into the dural venous sinuses, they also may serve as a conduit for brain tissue herniation. Such occurrences have been referred to in the literature as brain herniations into arachnoid granulations (BHAGs), which are considered incidental and asymptomatic but can be associated with nonspecific neurologic symptoms such as headache, tinnitus, vertigo, and seizure. BHAGs can be visualized more readily due to improved cross-sectional magnetic resonance imaging (MRI) with increased spatial and contrast resolution.
Case Presentation: We present 5 cases where brain herniations were detected in patients undergoing MRI for various neurologic symptoms. All patients experienced chronic symptoms, including headaches and seizures. Two cases included BHAG in locations that were associated with the patients’ symptoms.
Conclusions: BHAGs are increasingly recognized due to improved spatial resolution in MRIs. While there is still no definitive evidence that these lesions are responsible for various neurologic symptoms, some of these abnormalities may hold clinical significance such as the visual symptoms seen in 2 of the cases described. BHAG can be associated with gliosis of adjacent brain tissue, which may be a mechanism for symptom development.
Case 1
A 30-year-old male with a history of multiple traumatic brain injuries presented for evaluation of seizures. The patient described the semiology of the seizures as a bright, colorful light in his right visual field, followed by loss of vision, then loss of awareness and full body convulsion. The semiology of this patient’s seizures was consistent with left temporo-occipital lobe seizure. The only abnormality seen in the brain MRI was the herniation of brain parenchyma originating from the occipital lobe into the transverse sinus, presumably through an arachnoid granulation (Figure 1). An electroencephalogram (EEG) was unremarkable, though the semiology of the seizure historically described by the patient was localized to the area of BHAG. The patient is currently taking antiseizure medications and has experienced no additional seizures.
Case 2
A male aged 53 years with a history of peripheral artery disease presented with a 6-month history of headaches and dizziness. The patient reported the onset of visual aura to his right visual field, starting as a fingernail-sized scintillating kaleidoscope light that would gradually increase in size to a round shape with fading kaleidoscope colors. This episode would last for a few minutes and was immediately followed by a headache. There was no alteration of consciousness during visual aura, although sometimes the patient would have right-sided scalp tingling. These episodes were often unprovoked, but occasionally triggered by bright lights. A single routine EEG was unremarkable. The patient reported headaches without aura, but not aura without headaches, which made occipital lobe seizure less likely. MRI demonstrated a small herniation of brain parenchyma into the inner table of the left occipital bone (Figure 2). The patient was diagnosed with migraine with aura, and the semiology of the visual aura corresponded to the location of the herniation in the left occipital region.
Case 3
A 77-year-old male with a history of left ear diving injury presented with left-sided asymmetric hearing loss and word recognition difficulty for several years. MRI obtained as part of his work-up to evaluate for possible schwannoma of the eighth left cranial nerve instead demonstrated an incidental right cerebellar herniation within an arachnoid granulation into the diploic space of the occipital bone (Figure 3). The BHAG for this patient appeared to be an incidental finding unrelated to his asymmetric hearing loss.
Case 4
A male aged 62 years with a history of metastatic esophageal cancer, substance abuse, and a prior presumed alcohol withdrawal seizure underwent an MRI for evaluation of brain metastasis after presenting to the hospital with confusion 1 day after starting chemotherapy (Figure 4). Nine years prior, the patient had an isolated generalized tonic-clonic seizure approximately 72 hours following a period of alcohol cessation. The MRI demonstrated an incidental left parasagittal herniation of left parietal lobe tissue through an arachnoid granulation into the superior sagittal sinus, in addition to metastatic brain lesions. An EEG showed mild encephalopathy without evidence of seizures. It was determined that the patient's confusion was most likely due to toxic-metabolic encephalopathy from chemotherapy.
Case 5
A 51-year-old male presented with worsening headache severity and frequency. He had a history of chronic headaches for about 20 years that occurred annually, but were now occurring twice weekly. The headaches often started with a left eye visual aura followed by pressure in the left eye, left frontal region, and left ear, with at times a cervicogenic component. No cervical spine imaging was available. An MRI revealed 2 small adjacent areas of cerebellar herniation into arachnoid granulations in the left occipital bone (Figure 5).
