Brain Herniation into Arachnoid Granulations: Clinical and Neuroimaging Features Greta B. Liebo, John (Jack) I. Lane, Jamie J. Van Gompel, Laurence J. Eckel, Kara M. Schwartz, Vance T. Lehman From the Department of Radiology (GBL, J(J)IL, LJE, KMS, VTL); and Department of Neurologic Surgery and Otolaryngology (JJVG), Mayo Clinic-Rochester, Rochester, MN. ABSTRACT BACKGROUND/PURPOSE: Brain herniation into presumed arachnoid granulations (BHAG) is an increasingly recognized entity. Though it has previously been described as isointense to brain matter on magnetic resonance imaging (MRI), we have encountered 21 cases in our clinical practice, many of which have signal abnormality in the herniated or adjacent parenchyma. We performed a systematic analysis on these cases to further understand the clinical significance. METHODS: This is a retrospective analysis of cases collected from our clinical MRI practice. Each case was evaluated for location, size, lobe of origin, associated signal abnormality, symptoms, and the presence or absence of intracranial hypertension. RESULTS: Twenty-one cases of BHAG identified in 16 patients. Seven cases (7/21, 33%) demonstrated signal abnormality in the herniated or underlying parenchyma. On serial imaging, the size of herniation either fluctuated (2/10, 20%) or stayed the same (8/10, 80%). The associated signal abnormalities increased (1/5, 20%), decreased (1/5, 20%), or remained stable (3/5, 60%). Four patients (4/16, 25%) had signs/symptoms that could be associated with the BHAG. Ten patients (10/16, 63%) had either imaging findings or clinical signs that could be associated with intracranial hypertension. CONCLUSION: In contrast to previously reported cases, our study demonstrates that BHAG can be associated with MRI signal abnormalities of the herniated and adjacent brain. Symptoms are typically absent, though may be present in a small subset of patients, regardless of the presence of signal change or encephalomalacia of the involved brain. Indirect evidence of intracranial hypertension is present in a majority of patients but completely lacking in others. Keywords: Brain herniation, arachnoid granulation, encephalocele. Acceptance: Received April 18, 2016. Accepted for publication April 28, 2016. Correspondence: Address correspondence to Greta B. Liebo, Department of Radiology, Mayo Clinic-Rochester, 200 First St SW, Rochester, MN 55905. E-mail: [email protected]. Presentation at Meetings: Portions presented in electronic poster exhibit at ASNR, 2015 Annual Meeting, Chicago, IL, April 25–30. “Brain Herniation into Arachnoid Granulations: an Uncommon Entity with Variable Clinical and Radiographic Manifestations.” Portions presented in electronic poster exhibit at ASHNR, 2015 Annual Meeting, Naples, Florida; September 9–13. “Arachnoid Granulations Involving the Underlying Brain: Imaging and Clinical Features.” Acknowledgements and Disclosure: None. J Neuroimaging 2016;00:1-7. DOI: 10.1111/jon.12366 Introduction condition.9–11,14,15 Symptoms attributed to these abnormalities Arachnoid granulations (AGs) are invaginations of the arach- are variable and largely anecdotal.2–4,9–11,13,15 noid membrane that protrude through gaps in the dura into In all previously reported cases, the MRI signal in the her- the dural venous sinuses and occasionally the calvarium. They niated brain parenchyma and surrounding CSF has been de- are functionally and histologically related to arachnoid villi, dif- scribed as isointense to the underlying brain and subarachnoid fering only in size and complexity, providing a route for cere- space. In our experience, these herniations are sometimes asso- bral spinal fluid (CSF) resorption into the bloodstream.1–4 Fre- ciated with parenchymal signal abnormality or focal encephalo- quently seen on imaging, AGs are considered normal anatomic malacia in the underlying brain. To further understand the structures, though rare cases have been reported that indirectly imaging and clinical significance of this increasingly recognized implicate AGs with symptoms such as headache, tinnitus, CSF entity, we performed a systematic analysis of all 21 examples of otorrhea, and intracranial venous hypertension.5–14 brain herniation into presumed AGs (BHAG) encountered in Recently, herniation of brain parenchyma into the dural our clinical practice, specifically correlating imaging and clini- venous sinuses and calvarium have been reported, sometimes cal findings to the presence or absence of parenchymal signal referred to as occult encephaloceles, as the herniation does abnormality in the herniation or underlying brain. not extend through the outer cortex of the calvarium or skull base.2–4,9–12,15–17 While it remains to be proven, the association Methods of the herniation with presumed preexisting AGs has been sug- Case Identification gested in prior reports. Some authors have suggested that the abnormality occurs spontaneously,2,10,11 while others have pos- Institution review board approval was obtained for this Health tulated that intracranial hypertension could be a predisposing Insurance Portability and Accountability Act compliant study. Copyright ◦C 2016 by the American Society of Neuroimaging 1 Cases were identified by the authors during routine clinical scanned, though all had a slice thickness of <1.5 mm. A tabu- practice between 2012 and 2015. The diagnosis was based on lated overview of the cases can be found in Table 1. criteria established from previously reported cases of AGs on magnetic resonance (MR) imaging3,8,18 in conjunction with (i) Imaging Analysis direct visualization of brain herniation into the AG or (ii) teth- The majority of the herniations occurred within presumed AGs ering of brain to the AG with underlying parenchymal signal along inner table of the calvaria or skull base (16/21, 76%). abnormality and architectural distortion. All cases meeting di- Fourteen of 21 (67%) involved the occipital crests and 2/21 agnostic imaging criteria by group consensus were included. (10%) involved the posterior petrous bones. The remaining five Two cases were confirmed pathologically. (24%) herniations occurred into presumed AGs protruding into the dural venous sinuses, including four (19%) in the transverse Imaging Analysis sinus and one (5%) in the torcula herophili. Four of 16 (25%) pa- All images were reviewed separately by two neuroradiologists tients had multiple herniations: three cases with two herniations (GBL and VTL), the latter of whom holds a certificate of added each and one case with three separate herniations. All hernia- qualification in neuroradiology and 3 years’ experience. Al- tions in these 4 patients involved herniation of the cerebellar though only one MRI was requisite for inclusion, all available hemisphere into the occipital crests. cross sectional imaging studies of the brain were analyzed in All herniations arose either from the posterior fossa (cerebel- each patient. The utilization of high-resolution 3-dimensional lum) or the cerebrum immediately adjacent to it. Herniations of gradient echo or spinecho imaging was also recorded, defined the cerebellar hemispheres (n = 19, 90%) exceeded that of the by a maximum slice thickness of 1.5 mm. cerebral hemispheres (n = 2, 10%). Cerebellar hemisphere her- Each case was evaluated carefully to assess (1) AG loca- niations were on the right in 9 cases and on the left in 10 cases; tion; (2) mean AG diameter; (3) lobe of origin; (4) presence no cases of herniation of the cerebellar vermis were identified. or absence of signal abnormality in the herniated brain; or (5) Herniations arising from the cerebral hemispheres included one encephalomalacia in the underlying brain parenchyma. When case (4.5%) originating from the left inferior temporal gyrus and serial examinations were available, the stability or evolution of one case (4.5%) originating from the lateral left inferior occipital these features was documented. lobe. For patients with multiple presumed AGs containing herni- Overall, MR signal abnormality in the herniated and/or un- ated brain parenchyma, each area was analyzed separately. All derlying brain was identified in 7/21 (33%) cases of BHAG, present and prior examinations were then analyzed for recog- involving 7/16 (44%) of patients. This subset of patients had nized neuroimaging signs of possible intracranial hypertension a mean age of 58 years (range 45–70) and 5/7 (71%) were fe- including intraparenchymal mass effect, thinning or erosions of males. All cases involved the cerebellum. Two cases (2/7, 29%) the calvarium and skull base, empty sella, optic sheath pseu- demonstrated T2 prolongation in the herniated tissue sugges- domeningoceles, and narrowing of the dural venous sinuses. tive of simple vasogenic edema (cases 2 and 11; Fig 1). One case (1/7, 14%) demonstrated T2 prolongation and enhance- Clinical Chart Analysis ment in the tissue subjacent to cerebellar herniation suggestive of more severe vasogenic edema related to ischemic injury (case The electronic medical record was systematically reviewed by 8; Fig 2). Four cases (4/7, 57%) demonstrated T2 prolongation, a board-certified neurologic surgeon (JJVG) with 5 years of ex- fluid-attenuated inversion recovery (FLAIR) suppression, and perience and clinical interest in skull base CSF leaks. Clinical T1 shortening in the parenchyma subjacent to the presumed histories were reviewed for any past or present neurologic sign AG suggestive of focal cerebellar encephalomalacia. Three of or symptom that could conceivably correlate with the MRI