Venous Anatomy and Occlusions

Main Text

Preamble

Dural venous sinus and cerebral vein occlusions are relatively rare, accounting for only 1% of all strokes. They are notoriously difficult to diagnose clinically and are frequently overlooked on imaging studies, as attention is focused on the arterial side of the cerebral circulation.

Familiarity with both normal venous anatomy and drainage patterns is essential for understanding the imaging appearance of sinovenous occlusive disease. Therefore, in this chapter, we first briefly review the normal gross and imaging anatomy of the cerebral venous system. Because ~ 1/2 of all venous occlusions result in parenchymal infarcts, we also discuss their drainage territories.

Normal Venous Anatomy and Drainage Patterns

Preamble

The intracranial venous system has two major components, the dural venous sinuses and the cerebral veins.

Dural Venous Sinuses

Dural sinuses and venous plexuses are endothelium-lined channels that are contained between the outer (periosteal) and inner (meningeal) dural layers.

Superior Sagittal Sinus

The superior sagittal sinus (SSS) is a large, curvilinear sinus that parallels the inner calvarial vault. It runs posteriorly in the midline at the junction of the falx cerebri with the calvarium (9-1). The SSS increases in diameter as it courses posteriorly, collecting a number of unnamed, small, superficial cortical veins and the larger anastomotic vein of Trolard (VofT).

Inferior Sagittal Sinus

Compared with the SSS, the inferior sagittal sinus (ISS) is a much smaller and more inconstant curvilinear channel that lies in the bottom of the falx cerebri. It terminates at the falcotentorial junction where it joins the vein of Galen (VofG) and basal veins of Rosenthal to form the straight sinus (SS).

Straight Sinus

The SS is formed by the junction of the ISS and VofG. It runs posteroinferiorly from its origin at the falcotentorial apex to the venous sinus confluence.

SS variants are relatively uncommon. A persistent falcine sinus is an unusual variant that is identified on 2% of normal CTAs. Here, a midline venous structure—the persistent falcine sinus—connects the ISS or VofG directly with the SSS.

Sinus Confluence and Transverse Sinuses

The SS terminates by joining the SSS and transverse sinuses (TSs) to form the venous sinus confluence (torcular Herophili). The TSs are contained between attachments of the tentorium cerebelli to the inner table of the skull. The TSs curve laterally and then turn inferiorly to become the sigmoid sinuses.

The two TSs are frequently asymmetric with the right side typically larger than the left. Hypoplastic or stenotic segments are present in 1/3 of the general population. Filling defects caused by arachnoid granulations (AGs) and fibrous septa are also common.

Sigmoid Sinuses and Jugular Bulbs

The sigmoid sinuses are the inferior continuations of the TSs. They follow a gentle S-shaped curve, descending behind the petrous temporal bone to terminate by becoming the internal jugular veins (IJVs). Side-to-side asymmetry of the sigmoid sinuses is common and normal.

Cavernous Sinus

The cavernous sinuses (CSs) are irregularly shaped, heavily trabeculated/compartmentalized venous sinuses that lie along the sides of the sella turcica and extend from the superior orbital fissures anteriorly [where they receive the superior ophthalmic veins (SOVs)] to the clivus and petrous apex posteriorly. The two CSs communicate extensively with each other via intercavernous venous plexuses.

The CSs contain the two cavernous internal carotid arteries (ICAs) and abducens (CNVI) nerves. CNIII, CNIV, CNV1, and CNV2 are actually within the lateral dural wall, not inside the CS proper (9-3).

The size and configuration of the CSs are relatively constant on imaging studies. The lateral walls normally appear straight or concave (not convex), and the venous blood enhances quite uniformly.

Arachnoid Granulations

The dural sinuses frequently contain AGs, CSF-containing projections that extend from the subarachnoid space (SAS) into dural venous sinuses (9-2). Although AGs can occur in all dural venous sinuses, the most common locations are the TS and SSS.

Cerebral Veins

The cerebral veins are subdivided into three groups: (1) Superficial (“cortical” or “external”) veins, (2) deep cerebral (“internal”) veins, and (3) brainstem/posterior fossa veins.

Superficial Cortical Veins

Between 8-12 unnamed superficial cortical veins course over the cerebral convexities, cross the SAS, pierce the arachnoid membrane and inner (meningeal) layer of the dura, and drain directly into the SSS (9-4). A dominant anastomotic superficial cortical vein, the vein of Trolard (VofT), may be present. The VofT courses upward from the sylvian fissure and over the convexity of the brain to join the SSS.

The superficial middle cerebral vein (SMCV) lies over the sylvian fissure and drains the brain surrounding the lateral cerebral fissure. The SMCV also collects tributaries from the temporal, frontal, and parietal operculae that overhang the sylvian fissure.

The deep middle cerebral vein (DMCV) collects tributaries from the insula and basal ganglia, then anastomoses with the basal vein of Rosenthal (BVR) (9-5). The BVR then curves laterally around the midbrain, courses posterosuperiorly, and then joins with the internal cerebral vein (ICV) to drain into the VofG (9-9A).

A prominent posterior anastomotic vein, the vein of Labbé, courses inferolaterally over the temporal lobe to drain into the TS (9-4).

All three named superficial anastomotic veins—the VofT, vein of Labbé, and SMCV—vary in size, maintaining a reciprocal relationship with each other. If one or two are dominant, the third anastomotic vein is usually hypoplastic or absent.

Deep Cerebral Veins

The deep cerebral (“internal”) veins are themselves subdivided into three groups: (1) Medullary veins, (2) subependymal veins, and (3) deep paramedian veins.

Medullary Veins

Innumerable small veins originate between 1-2 cm below the cortex and course straight through the white matter toward the ventricles where they terminate at right angles in subependymal veins (9-10). These veins are generally inapparent on CTV, but DSA (9-9)and contrast-enhanced MR may show faint linear stripes of contrast coursing toward the ventricles. 3T T2* susceptibility-weighted imaging (SWI) best depicts the medullary veins because the deoxygenated blood is paramagnetic. Here, they are seen as parallel hypointense black lines converging on subependymal veins that outline the lateral ventricles (9-11).

Subependymal Veins

These veins course under the ventricular ependyma, collecting blood from the basal ganglia and deep white matter (via the medullary veins) (9-6). Prominent, named veins include the septal veins, thalamostriate veins, and ICVs. The thalamostriate veins receive tributaries from the caudate nuclei and thalami, curving medially to unite with the septal veins near the foramen of Monro to form the paired ICVs (9-9).

Deep Paramedian Veins

The ICVs and VofG provide drainage for most of the deep brain structures. The ICVs are paired paramedian veins that course posteriorly in the cavum velum interpositum, the thin invagination of SAS that lies between the third ventricle and the fornices. The ICVs terminate by uniting with each other and the BVR to form the VofG. The VofG (great cerebral vein) curves posterosuperiorly under the corpus callosum splenium, uniting with the ISS to form the SS (9-1).

Venous Drainage Territories

The cerebral venous drainage territories are both less familiar and somewhat more variable than the major arterial distributions. Intracranial venous drainage follows four basic patterns: A peripheral (brain surface) pattern, a deep (central) pattern, an inferolateral (perisylvian) pattern, and a posterolateral (temporoparietal) pattern (9-12). Accurately diagnosing and delineating venous occlusions depends on understanding these specific venous drainage territories.

Peripheral (Surface) Brain Drainage

Most of the mid and upper surfaces of the cerebral hemispheres, together with their subjacent white matter, drain centrifugally (outward) via cortical veins into the SSS.

Deep (Central) Brain Drainage

The basal ganglia, thalami, and most of the hemispheric white matter all drain centripetally (inward) into the deep cerebral veins. The ICVs, VofG, and SS together drain virtually the entire central core of the brain.

The most medial aspects of the temporal lobes, primarily the uncus and the anteromedial hippocampus, also drain into the galenic system via the DMCVs and BVR.

Inferolateral (Perisylvian) Drainage

Parenchyma surrounding the sylvian (lateral cerebral) fissure consists of the frontal, parietal, and temporal opercula plus the insula. This perisylvian part of the brain drains via the SMCV into the sphenoparietal sinus and CS.

Posterolateral (Temporoparietal) Drainage

The posterior temporal lobes and inferolateral aspects of the parietal lobes drain via the sphenoparietal sinuses and anastomotic vein of Labbé into the TSs.

Cerebral Venous Thrombosis

Preamble

Dural venous sinus, superficial (cortical) vein, and deep vein occlusions are collectively termed cerebral venous thrombosis (CVT). CVT is an elusive diagnosis with a great diversity of causes and clinical presentations; it is also easily overlooked on imaging studies.

We begin with the most common intracranial venous occlusion: Dural sinus thrombosis (DST). We next discuss superficial vein thrombosis and follow with deep cerebral occlusions. We conclude the discussion with a consideration of CS thrombosis/thrombophlebitis.

Dural Sinus Thrombosis

Terminology

Cerebral DST is defined as thrombotic occlusion of one or more intracranial venous sinuses (9-13). DST can occur either in isolation or in combination with cortical &/or deep venous occlusions.

Etiology

The majority of CVTs are acquired disorders. Oral contraceptive use and pregnancy/puerperium are the most common causes. Others include trauma, infection (including COVID-19), inflammation, hypercoagulable states, such as heparin-induced thrombocytopenia and vaccine-induced immune thrombotic thrombocytopenia, elevated hemoglobin levels, severe dehydration, collagen-vascular disorders (such as antiphospholipid syndrome), vasculitis (such as Behçet syndrome), drugs, and Crohn disease. Between 20-35% of all patients with CVT have genetic thrombophilic disorders.

Pathology

When thrombus forms in a dural sinus, venous outflow is restricted. This results in venous congestion, elevated venous pressure, and hydrostatic displacement of fluid from capillaries into the extracellular spaces of the brain. The result is blood-brain barrier breakdown with vasogenic edema. If a frank venous infarct develops, cytotoxic edema ensues.

Location

The TS is the most commonly thrombosed dural venous sinus followed by the SSS.

Gross Pathology

In acute DST, the affected dural sinus appears distended by a soft, purplish clot that can be isolated to the sinus or may extend into adjacent cortical veins (9-14). In chronic DST, firm proliferative fibrous tissue fills the sinus and thickens the dura-arachnoid. Associated brain injury in DST varies from venous congestion to ischemia to petechial hemorrhages and frank hemorrhagic infarcts.

Clinical Issues

Epidemiology

CVTs represent between 1-2% of all acute strokes. Although CVT can occur at any age (from neonates to older adults), it is most commonly seen in young individuals. Nearly 80% of patients are younger than 50 years of age.

Demographics

CVT predominantly affects women (F:M = 3:1). A sex-specific risk factor (oral contraceptives, pregnancy, puerperium, and hormone replacement therapy) is present in nearly 2/3 of all women with CVT. Mean age at presentation is nearly a decade younger in women compared with men (34 years vs. 42 years).

Presentation

CVT causes two distinct pathophysiologic entities: Cortical venous infarction with focal neurologic symptoms and elevated intracranial pressure.

Nonfocal headache is the most common symptom, occurring in 70-90% of cases. Headache often slowly increases in severity over several days to weeks. Nearly 25% of patients present without focal neurologic findings.

Natural History

Many DSTs recanalize spontaneously without long-term sequelae. In some cases, a thrombosed or partially recanalized venous sinus forms an arteriovenous fistula in the adjacent dural wall.

Prompt recognition of DST has a significant impact on clinical outcome. Diagnostic delay—averaging seven days in large series—is associated with increased death and disability.

Imaging

Keys to the early neuroradiologic diagnosis of DST are (1) a high index of suspicion, (2) careful evaluation of dural sinus density/signal intensity and configuration, and (3) knowledge of normal venous drainage patterns.

CT Findings

NECT is normal in up to 25-30% of CVT cases, so a normal NECT scan doesnot exclude the diagnosis of CVT! Early signs are often subtle. Slight hyperdensity (9-17A)compared with the carotid arteries is seen in 50-60% of cases. Density measuring ≥ 70 HU is highly suggestive of DST (9-20A).

When present, a hyperattenuating vein (cord sign) (9-31) (9-32)or dural venous sinus sign (9-15)is both a sensitive and specific sign of cerebral venous occlusive disease. Parenchymal edema with or without petechial hemorrhage in the territory drained by the thrombosed sinus is a helpful but indirect sign of DST.

In 70% of cases, CECT scans show an empty delta sign caused by enhancing dura surrounding nonenhancing thrombus (9-34) (9-23A). “Shaggy,” enlarged, or irregular veins suggest collateral venous drainage.

MR Findings

Diagnosing CVT on MR can be challenging because thrombus and normal venous flow can have similar signal intensities. The imaging appearance of DST also varies significantly with imaging sequence and clot age.

Acute DST

An acutely thrombosed sinus often appears moderately enlarged (fat sinus sign) and displays abnormally convex—not straight or concave—margins. The “flow void” of rapidly moving blood typically seen in large venous sinuses disappears (9-20C). Acute DST appears isointense with the underlying cortex on T1WI (9-20B).

As hemoglobin in blood clots rapidly desaturates to deoxyhemoglobin, it becomes very hypointense relative to brain on T2WI (9-20D). Therefore, the acute T2 “dark” DST mimics normal intrasinus “flow void.” Most acute thrombi are hyperintense on FLAIR. Venous congestion may cause brain swelling with hyperintense parenchymal changes on T2/FLAIR.

Acute venous clots “bloom”on T2* (GRE, SWI). SWI shows the profoundly hypointense clot and slow flow with deoxygenated blood in dilated cortical veins. The appearance may nonetheless be confusing, as normal-flowing but deoxygenated venous blood also appears hypointense.

Extensive acute (or longstanding chronic) DST may result in collateral venous drainage through the medullary (white matter) veins into the deep subependymal veins. The medullary veins enlarge and contain desaturated hemoglobin; thus, they are seen on T2* sequences as prominent linear hypointensities entering the subependymal veins at right angles.

When seen in cross section, T1 C+scans demonstrate an empty delta sign, similar to the appearance on CECT and CTA/CTV . Intrasinus thrombi usually appear as elongated cigar-shaped nonenhancing filling defects on axial T1 C+.

Coronal 2D unenhanced TOF MRV may demonstrate absent flow, especially if the thrombus is in the SSS. As the TSs often have hypoplastic segments, a “flow gap” must be interpreted with caution. Contrast-enhanced MRV has the highest sensitivity of all MR sequences for visualizing acute DST.

Late Acute DST

As the intrasinus thrombus organizes, the clot begins to exhibit T1 shortening and becomes progressively hyperintense.

With T2 prolongation, a thrombosed sinus progresses from appearing very hypointense to iso- and then hyperintense with brain on both T2WI and FLAIR. T2* can be misleading, as clot signal gradually approaches that of normal sinuses. Late acute DST continues to exhibit an empty delta sign on T1 C+.

Subacute DST

Subacute thrombus is hyperintense on all sequences (T1, T2, FLAIR, T2*) (9-26) (9-21).

Chronic DST

Clot signal in chronic DST is quite variable and depends on the degree of clot organization. Chronic organized, fibrotic thrombus eventually becomes isointense with brain on T1WI and remains isointense to moderately hyperintense on T2WI. As blood has resorbed and largely disappeared, there is little or no T2* “blooming.” CTA/CTV readily demonstrates nonenhancing thrombus within the intensely enhancing dura.

Longstanding cerebral venous sinus thrombosis may develop significant collateral drainage through the medullary veins. This is seen as tortuous, corkscrew, or “squiggly” white matter vessels on CTV and intraparenchymal “flow voids” on T2WI that enhance on T1 C+ scans. The enlarged collateral veins sometimes become so prominent that they mimic an arteriovenous malformation on SWI and DSA (9-28).

Dura-arachnoid thickening is also common in longstanding chronic DST. In some cases, the dural thickening becomes so pronounced that it appears very hypointense on T2WI.

Differential Diagnosis

Normal dura and circulating blood are mildly hyperdense on NECT scans. If a dural venous sinus appears unusually hyperdense, compare it to density of the ICAs. A dural sinus measuring > 70 HU is likely thrombosed.

Asymmetry of the TSs is common. A hypoplastic segment is seen in 1/4 to 1/3 of imaged cases and may mimic DST. A high-splitting torcular can mimic an empty delta sign on CECT.

Giant AGs are focal, round or ovoid, CSF-like filling defects in the sinuses, whereas clots tend to be elongated, cigar-shaped lesions (see “Venous Occlusion Mimics” section later in the chapter).

Superficial Cerebral Vein Thrombosis

Superficial cerebral vein thrombosis (SCVT) can occur with (9-32) (9-41)or without DST.

Superficial Vein Thrombosis With DST

Imaging findings of SCVT with accompanying DST are similar to those of DST alone. Thrombus extends from the affected dural sinus into one or more draining cortical veins (9-35).

SSS occlusion with SCVT usually causes variable amounts of edema and petechial hemorrhage involving the cortex and subcortical white matter (9-48). If the anastomotic VofT is dominant, its occlusion may result in lobar hemorrhage. TS occlusion that extends into a dominant vein of Labbéoften causes extensive posterior temporal and anterior parietal hemorrhage (9-42) (9-45).

Superficial Vein Thrombosis Without DST

Isolated SCVT (iSVCT) without DST is rare, representing only 2-5% of all sinovenous occlusions. The clinical outcome of iSVCT is generally good.

iSCVT usually presents with a nonspecific headache. Approximately 10% of patients report sudden onset of a “thunderclap” headache that clinically mimics aneurysmal subarachnoid hemorrhage.

Symptoms, such as focal neurologic deficits, seizures, and impaired consciousness, are less common than with dural sinus or deep vein thrombosis.

The imaging diagnosis of iSCVT can be problematic. NECT is usually negative, although some cases may demonstrate focal convexal subarachnoid hemorrhage or a cord sign, representing a hyperdense thrombosed vein.

CTA/CTV or DSA may show a thin, round or tubular layer of contrast surrounding the thrombus.

The MR diagnosis of SCVT—whether with or without dural sinus involvement—is difficult to establish using only standard T1- and T2-weighted sequences. Acute thrombi are isointense with brain on T1WI and hypointense on T2WI, making them difficult to distinguish from normal “flow voids.”

FLAIR may demonstrate focal convexal subarachnoid hemorrhage seen as hyperintense sulcal CSF. Cortical-subcortical hyperintensities consistent with vasogenic edema are common associated findings.

Intraluminal thrombus can sometimes be seen as a linear hyperintensity on FLAIR or DWI. Venous ischemia may result in transient diffusion restriction.

T2* (GRE, SWI) sequences are key to the noninvasive diagnosis of SCVT (9-48). With a sensitivity of > 95%, they are by far the best imaging sequences for detecting thrombosed cortical veins. A well-delineated tubular hypointensity with “blooming” of hemoglobin degradation products within the clot is observed at all stages of evolution, persisting for weeks. Patchy or petechial hemorrhages in the underlying cortex and subcortical white matter are common, as is associated convexal subarachnoid hemorrhage.

Superficial vein thrombosis in the absence of DST can be problematic to diagnose. If the thrombosed vein is overlooked, iSCVT can mimic neoplasm, infection, or other more ominous pathologies.

Cavernous Sinus Thrombosis/Thrombophlebitis

Cavernous thrombosis/thrombophlebitis is the most common form of septic cerebral venous sinus thrombosis, a rare but potentially lethal condition with significant morbidity and high mortality.

Terminology

CS thrombosis (CST) is a blood clot in the CS. If it occurs in conjunction with sinus infection, it is termed CS thrombophlebitis (9-51).

Etiology and Pathology

The CS is composed of numerous heavily trabeculated venous spaces that have numerous valveless communications with veins in the orbit, face, and neck. Infection can thus spread easily through these venous conduits into the CS.

CST usually occurs as a complication of sinusitis or other midface infection. Staphylococcus aureus is the most frequent pathogen. Other less common agents include anaerobes and angioinvasive fungal infections.

Otomastoiditis, odontogenic disease, trauma, and neoplasm are less frequent causes of CST.

Clinical Issues

Epidemiology

CST without trauma, infection, or multiple other dural venous sinus occlusions is extremely rare.

Presentation

Headache, especially in the CNV1 and CNV2 distributions, and fever are usually the earliest symptoms. Orbital pain with edema, chemosis, proptosis, ophthalmoplegia, and visual loss is present in 80-100% of cases.

Natural History

Untreated CST can be fatal. Even with antibiotics, the mortality rate of CS thrombophlebitis is 25-30%.

Imaging

CT Findings

CS thrombophlebitis causes proptosis, “dirty” orbital fat, periorbital edema, sinusitis, and lateral bulging of the CS walls and may demonstrate a thrombosed superior ophthalmic vein (SOV) on NECT. CECT scans demonstrate multiple irregular filling defects in the expanded CS and SOVs (9-51A).

MR Findings

MR scans show enlarged CSs with convex lateral margins. Acute thrombus is isointense with brain on T1WI and demonstrates variable hypointensity on T2WI (9-51B). Nonenhancing filling defects within the enhancing dural walls of the CS and thrombosed orbital veins on T1 C+ are the definitive imaging findings in CST (9-51C).

Look for the normal intracavernous carotid artery “flow void,” as CS thrombophlebitis can lead to thrombosis or pseudoaneurysm formation.

Angiography

Nonvisualization of the CS on DSA can be a normal finding and does not indicate the presence of CST.

Differential Diagnosis

The differential diagnosis of CST includes neoplasm, carotid-cavernous fistula, and inflammatory disorders. Neoplasms (e.g., lymphoma, metastases) enhance uniformly. Carotid-cavernous fistula causes “flow voids,” and inflammatory disorders (e.g., sarcoid, inflammatory pseudotumors) enhance strongly and uniformly.

Deep Cerebral Venous Thrombosis

Deep CVT (DCVT) is a potentially life-threatening disorder with a combined mortality/disability rate of 25%.

Etiology and Pathology

The deep cerebral venous system (the ICVs and BVR, together with their tributaries, the VofG, and SS) is involved in ~ 10-15% of all patients with cerebral venoocclusive disease (9-52).

DCVT can occur either alone or in combination with other sinovenous occlusions. Because the cerebral venous system lacks valves and tunica muscularis, thrombosis in one sinus can easily extend to adjacent sinuses. Isolated DCVT is present in 20% of cases while 80% have additional thrombi in other sinuses. DCVT is almost always bilateral and results in symmetric venous congestion/infarction of the basal ganglia and thalami.

Clinical Issues

Initial symptoms of DCVT are variable and nonspecific, making diagnosis difficult. Most patients present with headache (80%) followed by rapid neurologic deterioration and impaired consciousness (70%). Focal neurologic findings are frequently absent, and subacute presentation is the rule, not the exception.

Deep system involvement, either isolated or in association with other venous sinuses, is an independent predictor of death and disability.

Imaging

Early NECT findings may be subtle. Hyperdense ICVs and SSs resemble a contrast-enhanced scan (9-58A). Hypodense “fading” or “disappearing” thalami with effacement of the border between the deep gray nuclei and internal capsule are key but nonspecific findings of DCVT (9-55A).

MR is the imaging modality of choice. Acute thrombus is isointense on T1WI and hypointense on T2WI (pseudo-“flow void”). Venous congestion causes hyperintensity with swelling of the thalami and basal ganglia on T2/FLAIR in 70% of cases (9-58A).

The most sensitive sequence is T2* GRE on which acute clots show distinct “blooming.” Venous congestion in the medullary and subependymal veins is also hypointense due to slow flow and hemoglobin deoxygenation.

CTA/CTV and DSA show absent opacification in deep venous drainage system (9-58C), and MRV shows absence of flow.

Differential Diagnosis

DEEP VENOUS THROMBOSIS

Pathology

• Uncommon (15% of CVTs)

• Usually both ICVs ± VofG, SS involved

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