Venous Stenting in Idiopathic Intracranial Hypertension (IIH)
Idiopathic Intracranial Hypertension (IIH) is a disease characterized by elevated intracranial pressure (ICP) without a cause that is associated with signs and symptoms, radiographic features, and lumbar puncture findings confined to increased ICP, in an alert and oriented patient.
IIH is seen predominantly in overweight women of childbearing age without any apparent cause (i.e., idiopathic) but can occur rarely in any age, either gender, and at any body weight. The modified Dandy Criteria define the diagnosis of IIH including: (1) increased ICP and associated symptoms, (2) without localizing neurological findings, (3) patient is alert and oriented, (4) normal neurodiagnostic studies except for radiographic signs of increased ICP (e.g., smooth- walled venous stenoses, empty sella , increased cerebrospinal fluid (CSF) in the optic nerve sheath, and globe flattening), and (5) no other cause of increased ICP is present. The initial treatment for IIH consists of weight loss and discontinuation of possible causal factors such as excessive vitamin A, steroids, or tetracyclines. Additionally, the diuretic acetazolamide (Diamox) has been validated in a large multicenter trial and is typically a part of the initial medical therapy for mild visual loss in IIH. Up to 10% of IIH patients, however, do not respond to or do not tolerate medical management and may require surgical treatment. In one prospective series of 50 patients with IIH 2.9% presented acutely with fulminant vision loss and required emergent or urgent surgery.  Each case of IIH requires a tailored approach. If progressive visual loss is the most pertinent problem, Optic Nerve Sheath Fenestration (ONSF) is generally the initial preferred surgery for patients who have failed maximum medical therapy. Alternatively, ICP lowering CSF shunting procedures (e.g., lumbo-peritoneal shunts and ventriculoperitoneal shunts), may provide better control of intractable and refractory headaches. Venous sinus shunting is a relatively new procedure being used to treat IIH. Initially several authors made the observation that there was stenosis at the junction of the transverse sinus and sagittal sinus in the majority of patients with IIH. Papers on venous sinus stenting procedures logically followed thereafter.
In 1995, King et. al. showed elevated venous pressures in the superior sagittal and transverse sinus in patients with IIH. In 2003, Farb et. al. showed stenosis at the junction of the transverse and sagittal sinus in the majority of patients with IIH. Using a special type of MRV called auto-triggered elliptic-centric-ordered (ATECO) three dimensional gadolinium-enhanced MR venography, they found that a venous conduit score (accounts for the degree of stenosis bilaterally) of <5.0 predicted IIH with a sensitivity and specificity of 93% each. 
Venous Stenting in IIH
It is yet to be fully determined whether the venous stenosis in IIH is secondary to increased ICP and merely a downstream effect of collapse of the venous sinus in response to elevated ICP. It has been hypothesized however that dural venous sinus stenosis is a direct driver toward the development of IIH. Venous sinus stenosis as a mechanism has been supported by the fact that other veno- occlusive diseases such as cerebral venous thrombosis and tumoral compression of the venous sinuses can present similar to IIH. On the other hand, it has been shown in other cases that there is reversal of venous hypertension and venous stenosis with reduction in ICP, and in some cases venous engorgement in the setting of elevated ICP. These findings are cited by critics who claim venous stenosis is a downstream effect of the elevated ICP in IIH.
Lublinsky et. al. developed a computer assisted detection (CAD) method for vessel cross section analysis and confirmed the narrowing of the transverse sinuses in patients with IIH as well as noting areas of highly stenotic segments This venous sinus narrowing was also supported by other demonstrations of generalized stenosis of bilateral transverse sinuses near the junction of the sigmoid sinus using CAD.
There is now considerable evidence to support venous sinus stenting (VSS) as potentially beneficial in the treatment of IIH. The leading theory to support why venous stenting can be therapeutic, is described by the self- limiting venous collapse feedback-loop model. This model is based on the idea that CSF pressure must remain higher than venous pressure to allow proper drainage into the arachnoid granulations. The venous sinuses are thus have a prismatic shape and adherent to the adjacent bone on one side to avoid compression by the higher CSF pressures. In individuals with compressible regions of the sinuses (ie. where the transverse sinus meets the sigmoid sinus), an elevation in CSF pressures by an inciting event such as weight gain or obstructive sleep apnea may cause stenosis of the venous sinuses. This in turn will lead to venous hypertension and prevent proper CSF drainage into the venous system. CSF will then back up causing further stenosis of the venous sinus; a dangerous positive feedback loop. Therapy, therefore, involves either reducing CSF pressure (e.g. acetazolamide, shunts, or lumbar puncture) or reducing venous pressure (venous stenting) to halt the positive feedback loop and decrease ICP.
A microcatheter is directed into the superior sagittal sinus (SSS) through the common femoral vein approach. Contrast venography with iodinated contrast material is performed after which intrasinus pressure differences between the proximal transverse and distal sigmoid sinuses are measured. 
Venous Sinus Stenting Procedure
Under general anesthesia, a stent is placed across the stenosis after the diagnostic angiogram demonstrates a significant difference in pressures between the proximal transverse and distal sigmoid sinuses.
Retrospective studies and a few uncontrolled, un-blinded prospective studies have suggested beneft to venous stenting in IIH. The outcomes below are represented by 22 studies published with 3 or more patients totaling 464 patients.
Lowering ICP is one general goal of IIH treatment. In one study, there were 90 patients with IIH treated with venous stenting who had both pre- and post- stent ICP measurements. Of these cases, the mean pre-stent ICP was 32.8cm H20 and the mean post- stent ICP was 17.6cm H20, representing a 46.3% decrease in ICP. This demonstrates a definitive drop in ICP. There were, however, differences in the timing of measurements following the procedures therefore the speed of ICP reduction is not clear.
Symptoms and signs after treatment for IIH with venous sinus stenting
- Headaches: 72.6% had resolution or improvement in headache after treatment.
- Transient Visual Obscurations(TVOs): 66.7% had complete resolution following stent placement. This is most likely a reflection of improvement in papilledema.
- Diplopia: 91.7% had resolution following stenting. Most likely secondary to relief of ICP-related sixth nerve palsy.
- Pulsatile tinnitus: 97% had resolution directly after stent placement.
Up to 60% of treated cases of IIH had complete resolution of papilledema and 26.9% had improvement in papilledema grading after stent placement. In four studies, Optical Coherence Tomography (OCT) was also used to quantitively follow papilledema improvement in patients with IIH and support the fundus findings. The mean pre- stent RNFL thickness was 225.4μm and the mean post- stent RNFL thickness was 87.6μm representing a mean reduction of 137μm. It is important, however, to interpret these findings in light of pseudo- normalization wherein a normalization of the RNFL may actually be a combination of RNFL thickening and atrophy. Analyzing the ganglion cell layer in addition to the RNFL would help uncover any pseudo- normalization, but there have yet to be studies which have adequately observed both.
Visual function after stenting treatment for IIH
- Visual Acuity: Mean visual acuity pre- stent was LogMAR 0.25 (20/36) and mean visual acuity post- stent was 0.136 (20/27). Papilledema rarely causes severe visual acuity loss; visual field assessments are more useful.
- Visual Fields: Visual field defects were reported in 172 eyes. After stenting, there was improvement in 122 eyes, no change in 38, and a worsening in 12. In three studies comprised of 71 eyes, a quantitative analysis of mean deviation (MD) was done showing an improvement in average MD by 3.29 dB.
As with any surgical treatment for IIH, there are complications with venous stenting. The most common is a headache ipsilateral to the shunt. This headache typically lasts for less than a week and is likely due to stretching of the dura. In-stent thrombosis has been reported cases where antiplatelet therapy was not started early enough, and there are reports of intracranial hemorrhages. There were also rare complications that occurred at the time of stent placement including femoral pseudoaneurysm, transient hearing loss (most likely from cochlear vein obstruction), allergic reaction to contrast, retroperitoneal hematoma, neck hematoma, contrast extravasation, stent migration, and ruptured ovarian cyst. Satti et. al. demonstrated that venous stenting had a higher rate of major complications (2.9%) when compared to ONSF (1.5%), but a lower rate when compared to CSF diversion (7.6%). 1.26% of patients developed recurrent in- stent stenosis while 4.26% developed stent adjacent stenosis. In the cases of stent adjacent stenosis, a new stent was placed within the newly narrowed region resulting in clinical improvement.
When to Stent
There are no set standard critera for when to use venous stenting for IIH treatment. Most authors consider using venous stenting when patients have failed or are intolerant to maximum medical therapy, have a documented venous sinus pressure gradient on manometry, and have failed, declined, or are not a good candidate for ONSF or a CSF diversion procedure.
Venous sinus stenting for IIH has been met with both enthusiasm and criticism. To date, much of what we know is based on retrospective studies and a handful of uncontrolled and un-blinded prospective studies. This leaves many questions such as the true rate of complications and how venous stenting compares to shunting procedures still left unanswered. Most agree that a head to head controlled clinical trial is necessary to answer these questions.
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