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A unifying theory explaining venous sinus stenosis and recurrent stenosis following venous sinus stenting in patients with idiopathic intracranial hypertension
  1. Kyle M Fargen
  1. Neurological Surgery and Radiology, Wake Forest University, Winston-Salem, North Carolina, USA
  1. Correspondence to Dr Kyle M Fargen, Neurological Surgery and Radiology, Wake Forest University, Winston-Salem, NC 27157, USA; kylefargen{at}gmail.com

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Introduction

Idiopathic intracranial hypertension (IIH), or perhaps more accurately chronic intracranial venous hypertension syndrome (CIVHS),1 is a disabling condition often resulting in severe pressure headaches, visual symptoms, and tinnitus. In the last decade, venous sinus stenting (VSS) has emerged as a safe and effective treatment for patients with IIH with associated venous sinus stenosis and a documented trans-stenosis pressure gradient (TSPG). While meta-analyses of small retrospective series have demonstrated low rates of retreatment with high rates of headache and visual improvement,2–4 some recent studies have revealed that many patients (as high as 60%) develop symptom recurrence after VSS.5 There are very few published articles that provide neurointerventionists with recommendations regarding the selection and treatment of IIH patients6 and essentially no evidence-based recommendations are available regarding the specifics of stent construct lengths or sizes. Further, questions remain about the pathophysiologic mechanism at play and why recurrent stenosis develops. Experience gained from a high-volume VSS practice has provided important insights into these phenomena. Herein I propose a unifying theory that explains the pathophysiology of de novo and recurrent venous sinus stenosis in IIH patients, with attention given to how this may impact stent construct size and length.

Unifying theory summary

The proposed theory is centered around four core principles: (1) elevations in intracranial pressure (ICP) have a diffuse impact on venous sinuses and their calibers; (2) there is variability in susceptibility to ICP-mediated stenosis in different regions of the sinuses, with the lateral transverse sinus being most susceptible and the torcula being resistant; (3) venous sinus stenosis is mediated through a positive feedback loop resulting in upstream venous congestion and elevated intramural venous pressures; and (4) recurrent stenosis occurs after stenting due to the effect of ongoing ICP elevations on susceptible regions of the sinuses in the setting of lower intramural venous pressures, resulting in a new positive feedback loop. These principles are explored in detail in the sections that follow.

Pathophysiology of extramural venous sinus stenosis

There is a growing literature supporting a direct relationship between ICP, extramural sinus compression, and venous congestion. Per this hypothesis, increased ICP from an unclear inciting event causes extramural compression of the venous sinuses, leading to progressive outflow obstruction and resulting in intracranial venous congestion.7 8 Increasing venous pressures upstream of the stenosis results in further elevations in ICP in a positive feedback loop, as the majority of arachnoid granulations exist along the superior sagittal sinus (SSS) rostral to the region of outflow obstruction.9 One study suggested the development of a significant TSPG can be found with angiographic stenoses of only 30%–35%.10 Eventually, once the TSPG is severe enough, intramural venous pressures resist further extramural compression7 and an equilibrium is reached where high venous pressures and ICPs coexist. This feedback loop is temporarily disrupted by cerebrospinal fluid (CSF) removal, with a number of reports detailing a short-lived resolution of venous sinus stenosis with lowering of ICP by CSF diversion.11–13 VSS, unlike CSF removal, is thought to interrupt the positive feedback loop that occurs, resisting further escalations in SSS pressures and therefore ICP by preventing ongoing venous stenosis at that location. Importantly, however, VSS does not prevent or resolve the patient’s underlying mechanism that incited the initial ICP elevation. Often times ICP will remain elevated after VSS if central venous pressures remain high due to obesity or heart conditions. VSS will therefore only be as beneficial as the TSPG in which it is correcting, and in the best conditions will only lower ICP to a point as designated by venous pressures caudal to the stenosis.

While never studied in detail in cadaveric studies, the transverse sinus (TS) near the transverse-sigmoid sinus junction clearly exhibits a predilection for involvement in IIH. The TS at this site is likely exposed to extramural compression more so than any other location within the major dural venous sinuses, as the vast majority of CIVHS type 1 and 3 patients1 exhibit stenosis at this site, and often bilaterally.14 The compression is usually extramural with scalloping of the sinus toward the bony margin. Conceptually, it is therefore reasonable to assume that this point of weakness along the TS is the focal, isolated problematic site in IIH. Logic and experience, however, would indicate that the situation is much more complex.

Venous sinus stenosis is a diffuse process

There is growing evidence that venous sinus stenosis is a diffuse process instead of a focal process. Recently, a study of angiographic venous sinus diameters demonstrated significantly smaller TS and SSS calibers in patients with IIH compared with patients without IIH. Notably, even in patients without IIH, SSS calibers increase as measured sequentially from rostral to caudal, but then reduce in caliber before joining the torcula; similarly, TS calibers are largest near the torcula and then decrease as the TS is measured more laterally.15 Further evidence stems from venographic studies on previously untreated patients presenting with concomitant TS and SSS stenosis with multifocal TSPG (figure 1). Clinical experience suggests that certain regions are highly susceptible to extramural compression, while others are moderately susceptible or resistant to stenosis. The lateral TS near the TS-sigmoid sinus junction is probably the most susceptible region of the venous sinuses to extramural compression; as high as 60% of IIH patients exhibit pathologic stenoses at this location.16 Other regions, such as the inferior SSS and the medial TS, are less frequently involved. In contrast, the torcula seems mostly resistant to external compression, while the sigmoid sinus also appears to be rigidly resistant. The anatomic reasons for these mixed susceptibilities have yet to be parsed out, but there does appear to be a diffuse effect of ICP on sinus caliber, with certain areas more susceptible to extramural compression and therefore preferentially affected (figure 2A and B).

Figure 1

Anteroposterior venogram of four different patients presenting with coexisting transverse sinus and caudal superior sagittal sinus stenosis (A–D). Measured venous pressures are shown.

Figure 2

Anteroposterior (A) and lateral (B) venography showing varying susceptibilities to extramural compression based on anatomic location. (C–E) Illustration of effect of stenting on venous transmural pressures and vein caliber. (C) Severe transverse sinus (TS) stenosis results in high upstream venous pressures (blue arrows) that resist extramural compression from high intracranial pressure (ICP) (black arrows) at susceptible sites. (D) Following TS stenting, elimination of the trans-stenosis pressure gradient (TSPG) and reduced upstream venous pressures no longer resist extramural compression during periods of high ICP and sinus stenosis now occurs in the medial TS, initiating a new positive feedback loop. Superior sagittal sinus (SSS) stenosis occurs but elevations in SSS pressure above the new stenosis (with resultant new TSPG) at the TS resist further SSS stenosis. (E) Following extension of the stent construct to the torcula and alleviation of the TSPG in the medial TS, reduced SSS pressures now no longer resist extramural compression during an inciting event of high ICP, resulting in initiation of a positive feedback loop and caudal SSS stenosis with development of a new TSPG.

Importantly, venous flow is retrograde and intramural venous pressures decrease as measured from the SSS caudally.16 While there are two potential major outflow corridors (right and left transverse-sigmoid sinus pathways) plus additional variable collateral channels, most patients with IIH exhibit codominant TS or right-side dominant (with a hypoplastic or aplastic left TS) anatomical types.15 In the presence of codominant or only slightly asymmetric corridors, bilateral TS stenosis is required to precipitate the positive feedback loop, because usually patency of one TS pathway is sufficient to prevent significant venous congestion. Alternatively, in patients with markedly asymmetric corridors, such as a contralateral aplastic TS, unilateral TS stenosis may be sufficient to precipitate this process.

Logically, intracranial venous stenosis that occurs more caudally (for instance, the TS) will have a greater impact on venous pressures upstream than more rostral stenoses, because more venous channels will be affected by venous congestion. Furthermore, TS stenosis will result in elevated intramural venous pressures in regions of the venous system upstream of the stenosis, which “push back” against extramural compression from elevated ICP and resist stenosis from developing in susceptible regions of the medial TS or SSS. This is probably why most patients present with isolated lateral TS stenosis. Following TS stenting, the most susceptible site to compression (TS) becomes rigidly fixed and incapable of narrowing and, as a result, the TSPG resolves, venous congestion diminishes, and rostral venous sinus pressures reduce accordingly. Additionally, venous pressures diminish in the untreated, contralateral TS (if present), preventing complimentary venous dilation. Now, with any new elevations in ICP, the upstream intramural venous pressures that were previously able to resist extramural compression at susceptible sites are now unable to “push back” against the compression, and new stenosis may occur. A new positive feedback loop occurs at this site, resulting in upstream venous congestion and recurrent elevations in ICP (figure 2).

Stent-adjacent stenosis

New stenosis immediately upstream of implanted venous sinus stents (stent-adjacent stenosis) is well documented,17 18 with studies identifying increased rates in female patients and those with extramural sinus stenosis17 as well as those with higher ICP measured on lumbar puncture.18 One important recent observation that has gained traction in the neurointerventionist community is an increased incidence of stent-adjacent stenosis following VSS with larger stent diameters.19 Given the triangular shape of the venous sinuses,20 21 oversized cylindrical stents may theoretically lead to significant vessel stretching and resulting adjacent narrowing. If the stent terminates in a segment of vessel that is already susceptible to stenosis (figure 1), stent-adjacent stenosis is possible. Interestingly, however, one would expect immediate, detectable stenosis of the adjacent sinus following oversized stent implantation. Practically speaking, this phenomenon is only rarely observed intraprocedurally (figure 3).

Figure 3

A patient underwent venous sinus stenting (VSS) for right transverse sinus (TS) stenosis. Immediately following single stent placement (A), stent-adjacent stenosis was present with migration of the trans-stenosis pressure gradient (TSPG) upstream to between the terminal end of the stent and the torcula, which resolved following extension of the stent construct to the torcula (B)). Note additional stenosis of the caudal superior sagittal sinus (SSS) without associated TSPG, and reduction of collateral venous outflow through the left TS pathway consistent with unobstructed right TS outflow. Terminal ends of the stent construct are shown with yellow dashes.

Instead, patients more commonly develop insidious stent-adjacent stenosis that tends to manifest weeks after VSS and involves susceptible regions of the sinus. Most commonly, stent-adjacent stenosis occurs between the medial terminal end of the stent construct and the torcula, in concert with either a hypoplastic/aplastic or persistently stenosed contralateral TS (figure 4B). This observation occurs with high enough frequency that many experienced authors now stent from the TS-sigmoid sinus junction to the torcula in all patients to avoid stent-adjacent TS stenosis.5 17 Stent-adjacent SSS stenosis, rostral to an implanted SSS stent, has also been reported.5 17 Importantly, however, stent-adjacent stenosis in the sigmoid sinus below the stent construct has not been reported in the literature, nor has it been observed in 130 VSS procedures by the author. This fact further attests to the fact that certain regions of the sinus have a natural predilection for developing stenosis, while others (the sigmoid sinus and torcula) appear resistant to developing stenosis.

Figure 4

Examples of bifid superior sagittal sinus (SSS) anatomy in the setting of transverse sinus (TS) stenosis (black arrows) making ideal stent construct length unclear. (A) markedly asymmetric and rightward deviated bifid SSS. (B) patient stented elsewhere with symptom recurrence and stent-adjacent stenosis following TS stenting with significant recurrent trans-stenosis pressure gradient (TSPG) and a symmetric, bifid SSS. Note that contralateral TS stenosis remains persistent. Logic would suggest that the stent construct should be extended to the site marked as point 1 in both cases, but it is unclear if the elongated bifid segment from point 1 to 2 are resistant or remain susceptible to extramural compression and are at risk for potential de novo stenosis.

The presented unifying theory explains stent-adjacent stenosis through two potential phenomena. First, oversizing of stents may result in stenosis at adjacent sites by mechanically stretching the vein in a manner that directly results in venous stenosis (not through increasing susceptibility to extramural compression but by mechanically inducing intramural stent-adjacent stenosis), artificially triggering the positive feedback loop immediately upstream of the stent. Second, stent constructs that terminate in a susceptible region of the venous sinuses will make the adjacent region of sinus more likely to stenose with elevations in ICP due to reduced intramural venous pressures and therefore a greater transmural pressure effect of ICP.

De novo stenosis

Less commonly, remote stenosis may occur distant from the terminal ends of a stent, usually in the inferior SSS after stenting across the TS to the torcula.22 The stenosis that occurs is usually just rostral to the confluens in the S1 segment, above the resistant zone as shown in figure 1. Via a similar phenomenon, worsening stenosis of the contralateral (unstented) TS has been reported7 and has been occasionally observed in the clinical practice of the author following dominant TS stenting. This observation naturally suggests that stenosis occurring remote from the terminal ends of the stent construct is unlikely to be related to oversizing of stents, but instead is more likely secondary to a natural susceptibility to extramural compression from elevated ICP in the setting of lower venous intramural pressures compared with pre-stenting. figure 5 demonstrates two patients who developed SSS stenosis following TS stenting; the first patient developed intraprocedural remote (de novo) stenosis immediately after TS stent implantation, while the second returned with worsening symptoms 3 months after initial TS stenting with new remote SSS stenosis. The phenomenon of new stenosis developing remote from the terminal ends of the stent is the most supportive evidence for a diffuse effect of ICP on venous calibers.

Figure 5

De novo stenosis of the superior sagittal sinus (SSS) after right transverse sinus (TS) stenting. The first patient with severe right TS stenosis (A and B) underwent stenting of the right TS to the torcula and developed intraprocedural, de novo stenosis of the caudal SSS, remote from the terminal end of the stent construct, with trans-stenosis pressure gradient (TSPG) (C). The TSPG resolved following SSS stenting (D). A second patient with severe right TS stenosis and no appreciable SSS stenosis (E) underwent right TS stenting to the torcula (F). After recurrence of symptoms 3 months later, venography demonstrated de novo SSS stenosis with TSPG (G) which resolved following SSS stenting (H). Terminal ends of the stent constructs are shown with yellow dashes.

Role of recurrent ICP elevations

The presented theory of venous sinus stenosis in IIH is predicated on an inciting event that causes elevations in ICP that then induce the positive feedback loop, resulting in progressive stenosis. While a plethora of factors or associations have been identified in the literature, including a number of medications or hormones, the most common risk factor for IIH is obesity.23 There is a clear association with obesity and elevated central venous pressures as well as obstructive sleep apnea (OSA). OSA occurs in about 50%–60% of patients who are obese,24 and OSA and IIH commonly coexist.25 Marked elevations in ICP have been documented during nocturnal episodes in patients with OSA.26 27 In the author’s experience, the majority of patients of normal weight with IIH and venous sinus stenosis have either intramural venous sinus stenosis (from arachnoid granulations, for instance) or connective tissue disorders, most commonly Ehlers–Danlos syndrome (EDS). EDS is a rare condition defined by hypersensitivity and ligamentous laxity which probably make the dural sinuses and jugular veins more susceptible to compression.

A reasonable conclusion is that in many patients with IIH, obesity and OSA may cause nightly elevations in ICP while recumbent that drive the positive feedback loop to occur, leading to extramural compression and venous stenosis. While other activities (straining, for instance) or physiologic/pharmacologic factors could also be at play, nighttime ICP elevations seem most likely as a prominent, recurring instigator of the cycle. This ongoing predilection for high ICP to develop in IIH patients provides an explanation for the natural tendency for patients to “fail” stent treatment, as VSS is only a focal disruption of the positive feedback loop but does not resolve the underlying inciting mechanisms that drive episodic ICP elevations or the diffuse susceptibility of the venous sinuses to narrow under high ICP. Further evidence for higher ICP as a risk factor for recurrent stenosis following VSS indirectly supports this argument.18 Following stenting, recurrent ICP elevations, particularly in those with a predilection for high ICP, drive the feedback loop to occur in other susceptible regions of the sinus, which now are even more susceptible as the lower venous pressures within those segments are not adequate to resist extramural forces.

Stent construct guidance

The theory presented, if true, has implications for stent construct design. First, this theory supports stenting with a construct that extends from one resistant zone to another resistant zone, instead of terminating the construct in a susceptible region to prevent stent-adjacent stenosis. Practically speaking, this means in most cases it would be advisable to stent from the TS-sigmoid sinus junction to the torcula, as recommended by some authors.17 Second, in patients with TS stenosis but additional SSS stenosis and at least a marginal TSPG across the inferior SSS, interventionists should consider additionally performing upfront stenting of the SSS as well as the TS. Third, it is important that proceduralists measure venous pressures in the rostral SSS and torcula, not just in the TS, when identifying potential candidates for stenting. Repeating manometry in the SSS and stepwise caudally is especially important following stent implantation to ensure upstream de novo stenoses have not developed. Fourth, avoidance of oversized stents is important, with most patients suitably treated with 6–8 mm diameter stents in the TS, and 6 mm or less in the SSS. Fifth, there is increasing evidence supporting a relationship between factors that influence ICP and venous pressures, particularly as it pertains to making clinical decisions regarding stenting. Given the pronounced effect of end-tidal carbon dioxide and blood pressure on venous sinus pressures,28 29 as well as the influence of general anesthesia on TSPG,30 31 candidacy for VSS should be determined under conscious sedation, and before and after stent implantation manometry should be performed under general anesthesia while under tightly controlled, physiologic conditions. Finally, and arguably most importantly, stent failure is predicated not only on the implanted construct but by lifestyle modification and medical management that prevents ongoing elevations in ICP that can lead to further development of adjacent or remote stenosis. Interventions including continuous positive airway pressure (CPAP) for OSA, weight loss, and medications such as acetazolamide, topiramate, or furosemide are strongly recommended. Greater attention to reducing recurrent elevations in ICP may be the best way to ensure VSS success.

Challenges and considerations going forward

A bifid SSS, where the SSS splits into two corridors above the torcula, poses interesting questions that challenge this theory and its practical implications. Recently, our group identified bifid SSS anatomy in 9% of sampled patients, defined by a SSS that bifurcates more than 1 cm rostral to the inferior margin of the junction of the transverse sinuses. Among this group, the mean height and width of the bifid SSS was 1.7 cm and 2.3 cm, respectively, but height ranged as high as 5.0 cm with a width between the SSS corridors as wide as 4.8 cm.15 This anatomical variability poses interesting questions regarding where the resistant and susceptible zones exist. In the author’s experience, there have been no cases of SSS de novo stenosis that have developed following stenting from the TS to the torcula in patients with a bifid SSS, however these patients are relatively uncommon. Whether this indicates the resistant zone extends throughout the length of the bifid SSS is not clear (figure 4).

Further challenging this theory is the lack of cadaveric studies providing hard evidence of differential susceptibility to compression along the sinuses. Studies exploring the elasticity or compliance of these channels in patients with IIH compared with those without IIH are necessary to understand the inherent predilections for developing stenosis in the IIH population. In addition, multicenter registries evaluating risk factors for stent failure, with specific focus on sinus diameters, outflow anatomy, and venous pressures pre- and post-stenting, are necessary. Identifying patients that may be higher risk for de novo SSS stenosis based on anatomic or physiologic factors may assist practitioners in choosing which patients to perform upfront SSS stenting in addition to TS stenting. Further, there have been cases of recurrent upstream stenosis occurring following stenting of the SSS.17 At what landmark is the SSS no longer susceptible to stenosis, or at what point are the majority of arachnoid granulations no longer upstream of the stenosis indicating the venous sinus pressure–CSF pressure differential is no longer a consideration? Finally, most practitioners currently stent the dominant TS only, as contralateral TS stenting is often unnecessary, even after stent failures. Whether bilateral upfront TS stenting in codominant or minimally asymmetric patients may potentially reduce the incidence of remote stenosis in the SSS is unclear.

Conclusions

Clinical experience and available evidence suggest that venous stenosis in IIH is a diffuse process. Different regions of the venous sinuses appear to harbor differential susceptibilities to extramural compression from elevated ICP. This theory may explain why stent-adjacent and remote stenoses develop following VSS. Understanding this theory may help guide stent construct length and sizing in IIH patients being treated with VSS.

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No additional data are available.

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Footnotes

  • Contributors The author is the sole contributor to the article and is responsible for its contents.

  • Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

  • Competing interests None declared.

  • Provenance and peer review Not commissioned; externally peer reviewed.