“Giant” Arachnoid Granulations Just Like CSF?: NOT!!

Discussion

AGs are functionally and histologically related to AV, which are universally present and function in the filtration and resorption of CSF into the venous circulation. AV are formed by microscopic protrusions of arachnoid tissues into the venous sinuses via openings in the dura (Fig 4). It is thought that some AV hypertrophy is in response to increasing CSF volume and pressure, forming macroscopic lobulated AGs.^1,8

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Fig 4.

Cross-sectional graphic of a giant venous sinus AG projecting into a dural venous sinus. A core of CSF-filled collagenous trabeculation (open arrows) extends from the subarachnoid space into the granulation and is covered by an apical cap of arachnoid cells. CSF channels (arrows) extend through the cap to the sinus endothelium and drain CSF into the venous circulation. A vein (curved arrow) also courses through the body of the AG, penetrates the arachnoid cap layer, and empties into the dural venous sinus. Graphic is used with permission from Amirsys Inc., Salt Lake City, Utah

AGs are present in approximately two-thirds of individuals in the population.^3–7,8,13 They frequently occur in close relation to veins penetrating the dural venous sinuses, which are postulated to form weak areas in the dura through which perivascular arachnoid extrusion can occur.^5,14 The dural covering at the base of the AG diminishes in thickness and regresses completely at its apex.^15,16 The AG core is supported by trabeculated collagenous soft tissue and is filled with CSF from the contiguous subarachnoid space. CSF passes through channels in a “cap” of arachnoid cells which marginate the apex of the AG. It is thought that CSF is ultimately actively transported via vacuoles across a membrane of arachnoid cells at the periphery of the cap layer into the venous circulation.^1,2 Compared with smaller AGs, larger granulations are more likely to contain fibrous soft tissue and internal veins.^5,8,12

AGs have long been recognized on imaging studies. They were first identified on skull radiography as smoothly marginated impressions on the inner table of the calvaria and on the venous phase of cerebral angiograms as ovoid filling defects within the dural venous sinuses.⁹ Subsequently, CT and MR imaging signals of the intra-AG contents showed characteristics paralleling those of CSF.^5,6 More recently, so-called giant AGs ranging from 1 to 2.4 cm have been reported.^{6,11,13,17–20}

CSF-like attenuation on CT or fluid that parallels all MR images has been a conventional diagnostic criterion for AGs, though isolated exceptions to this general rule have been reported in the literature. Ikushima et al³ showed that 10% of AGs averaging 5.1 mm in diameter were slightly hyperintense to CSF on FLAIR imaging. Recently, Leach et al⁶ noted in passing that for AGs with an average size of 8.1 × 9.4 × 10.0 mm, intra-AG fluid may occasionally be FLAIR hyperintense. They commented that this appearance “may be due to pulsation artifact from the adjacent sinus and differing CSF flow characteristics within the granulation.”

The cause of CSF-incongruent MR imaging signal intensity within structures that clearly contain normal CSF will likely remain unknown because these structures are not biopsied, and analysis of actual intra-arachnoidal CSF is not performed. We postulate that spin dephasing due to disordered flow may account for the dissimilarity of the intra-AG fluid when compared with CSF in the adjacent subarachnoid spaces and ventricles. Altered CSF dynamics may be accentuated by stromal tissue frequently found in larger AGs.^15,21 One AG showed lack of suppression of the fluid marginated by 2 intra-AG stromal tissue planes on FLAIR imaging (Fig 1). The same case showed complete suppression of fluid signal intensity within the remainder of the AG, suggesting the possibility that noncommunicating fluid within loculated cysts may also contribute to the dissimilarities between intra-AG fluid and CSF.

Vascular structures presumed to be veins were common in our series, supporting similar previously reported findings.^6,12,12 These were identified as linear flow voids or focal contrast enhancement in regions both entering and within the AGs and were present in 63% of our 19 AGs.

Nonvascular soft tissue has also been reported in giant AGs and was variously interpreted as stromal collagenous tissue, hypertrophic arachnoid mesangial cell proliferation, or invaginated brain tissue.^{3,5–8,22–24} Nonvascular gray matter isointensities were identified in 9/19 of our AGs (47%). Of these, 5 showed linear tissue planes or septations, which may represent fibrous stromal tissue within the AG. Another 3 AGs demonstrated well-demarcated pedunculated soft-tissue nodules at the base of the AG, which may represent focal arachnoid cell proliferation or small meningoencephaloceles within the body of the AG.

Although most AGs communicate with the dural venous sinuses, a minority are located in regions of the temporal bone and do not communicate directly with venous circulation. These temporal bone and occipital bone AGs are thought to enlarge with time in response to CSF pulsations, which may lead to cephalocele formation and CSF leaks when located adjacent to pneumatized regions of the anterior skull base.^25–29

Diffusion restriction, thought to be caused by intra-AG collagenous stromal tissue, has been reported in some larger AGs, though restricted diffusion was not seen in the single AG for which DWI imaging was available.^5,8

Many investigators posit a broad differential diagnosis of giant AGs in the dural venous sinuses and include dural venous sinus thrombosis, calvarial osseous lesions, meningiomas, metastases, arachnoid cysts, dermoids, epidermoids, and extra-axial hemangiomas, including papillary endothelial hyperplasia (Masson vegetant hemangioendothelioma).^9,10 With the exception of dural sinus thrombosis and meningioma, all these pathologies are rarely found in venous sinuses. Regardless of internal fluid and soft-tissue signals, all giant AGs are well-demarcated ovoid structures, differentiating them from dural venous sinus thrombus, which is typically elongated and sausage-shaped. Giant AGs do not enhance strongly and uniformly like typical neoplasms. MR imaging signal intensity within giant AGs is not fatlike, differentiating them from dermoids, and does not demonstrate restricted diffusion as do epidermoids.⁶

Although AGs are commonly differentiated from other pathologic entities by identifying intra-AG fluid paralleling CSF on all sequences, the present study suggests that fluid within most giant AGs does not consistently follow CSF on MR imaging. Because giant AGs contain fluid that does not always follow CSF and often contain vascular and stromal tissue, shape (round/ovoid), lack of solid contrast enhancement, and the absence of blooming artifacts are helpful features in differentiating giant AGs from more ominous pathology. Because we have demonstrated that MR imaging signal intensity in giant AGs is quite variable, we believe that the most definitive imaging study may be CT. In this small series, fluid in giant AGs measured CSF-like attenuation in all cases.

Our study is limited due to a nonrandom assortment of cases, lack of all conventional imaging sequences across the 17 patients, lack of original datasets for quantitative signal-intensity comparison, possible partial volume averaging artifacts, and lack of biopsy-proved results. Despite these limitations, our findings are, nevertheless, still striking. While all giant dural venous sinus AGs with CT imaging showed CSF-like attenuation, we found that nearly 80% of them did not follow CSF signal intensity on at least 1 MR image. Almost half had CSF-incongruent signal intensity on >1 series. FLAIR was the sequence that most commonly showed CSF-incongruent signal intensity (8/8, 100% of AGs), followed by precontrast T1WI (7/10, 70%), T2W1 (13/19, 68%), and postcontrast T1WI (8/14, 57%). Random sampling, quantitative region-of-interest signal-intensity analysis, and 1-mm sections in future studies would help confirm these findings.

The clinical significance of giant AGs is uncertain. While some large AGs may cause dural venous sinus pressure gradients and headaches, most are usually asymptomatic and incidental findings on imaging studies.^17,18,20 They should be distinguished from other more ominous pathologies such as thrombus and neoplasm, and invasive studies such as biopsy should be assiduously avoided.