Idiopathic intracranial hypertension imaging approaches and the implications in patient management

Abstract

Idiopathic intracranial hypertension (IIH) represents a clinical disease entity without a clear etiology, that if left untreated, can result in severe outcomes, including permanent vision loss. For this reason, early diagnosis and treatment is necessary. Historically, the role of cross-sectional imaging has been to rule out secondary or emergent causes of increased intracranial pressure, including tumor, infection, hydrocephalus, or venous thrombosis. MRI and MRV, however, can serve as valuable imaging tools to not only rule out causes for secondary intracranial hypertension but can also detect indirect signs of IIH resultant from increased intracranial pressure, and demonstrate potentially treatable sinus venous stenosis. Digital subtraction venographic imaging also plays a central role in both diagnosis and treatment, providing enhanced anatomic delineation and temporal flow evaluation, quantitative assessment of the pressure gradient across a venous stenosis, treatment guidance, and immediate opportunity for endovascular therapy. In this review, we discuss the multiple modalities for imaging IIH, their limitations, and their contributions to the management of IIH.

Introduction

Increased intracranial pressure (IICP) can be caused by mass lesions, impaired venous drainage, increased cerebrospinal fluid (CSF) production or decreased reabsorption. Idiopathic intracranial hypertension (IIH) represents an entity where the etiology of IICP is unclear and not secondary to identifiable causal factors. ¹ The incidence of IIH has been estimated at 0.3–4.7 per 100,000 per year. ^2,3 IIH most frequently affects reproductive age females, with a strong association with obesity. In an analysis of more than 23,000 IIH patients, 82.4% were female, and the median age at diagnosis was 28 years (range 21–40). ³ In addition to obesity, weight gain over the preceding 2–12 months is also associated with IIH. ⁴

A number of correlations between IIH and pathophysiological processes and markers exist. ⁴ Dysregulation of CSF dynamics is a primary consideration secondary to blocked arachnoid granulation CSF resorption or obstruction of glymphatic or lymphatic drainage. ^4,5 Increasing attention has been placed on the association between IICP secondary to dural venous sinus stenosis and IIH; however, this is further discussed in later sections. Abnormalities of circulating inflammatory markers secreted by adipose tissue is a potential link between obesity and IIH, as leptin has been shown to be significantly higher in the CSF of IIH patients compared to normal controls (p = 0.001). ⁶ BMI correlated with leptin levels in normal controls, but not in IIH patients, indicating possible hypothalamic leptin resistance in obese IIH patients. Another study showed elevated CSF oligoclonal bands in IIH patients and significantly higher serum inflammatory marker levels (IFN-γ, IL-4, IL-10, IL-12, and IL-17) compared to multiple sclerosis patients and normal controls. ⁷ Androgen receptors and androgen-activating enzymes are expressed in human choroid plexus, and may regulate plexus sodium-potassium-ATPase activity. IIH patients have shown androgen excess, with elevated serum testosterone and CSF testosterone and androstenedione compared to matched patients with polycystic ovarian syndrome and simple obesity. ⁸ Furthermore, the obesity patterns in patients show associations with IIH; male IIH patients have significantly larger hip circumference and less abdominal fat compared to age and BMI-matched controls. ⁹ Increasing intraabdominal pressure led to the development of IICP in animal models. ¹⁰ Increased intrathoracic and intraabdominal pressure in the setting of central obesity are also present in IIH patients. ¹¹ A potential theory is central obesity contributes to central venous stenosis, leading to delayed cerebral venous drainage and increased intracranial venous pressure.

Presentation can be heterogeneous, with the most common symptoms including headache, diplopia, nausea, neck and back pain, pulsatile tinnitus, and with delayed treatment, progressive vision loss. ¹² Headache, with a migrainous profile, is the most common presenting symptom of IIH, making IIH differentiation challenging. There is actually no correlation between CSF pressure and presence and intensity of headaches. In fact, headaches can persist even after CSF pressure normalization. ¹³

With IIH, transient vision loss, lasting for a minute or less, presumably arises from raised optic nerve pressure leading to transient ischemia. Binocular horizontal diplopia is seen in 18–38% of cases, possibly resultant from sixth nerve palsy. ¹³ The length of the sixth cranial nerve and its ascending course as it enters Dorello’s canal have been suggested as reasons for dysfunction with IICP. Vision loss in IIH can present as field deficits, initially as an enlarged blind spot. Reduced visual acuity is seen in advanced disease, however, visual acuity can be affected even in mild disease. ¹³ Papilledema is present in all IIH patients, although may be asymmetric. When untreated, papilledema can cause optic atrophy.

Imaging of cerebral veins and sinuses has substantially advanced our understanding of disorders of intracranial pressure (ICP) and led to fundamental changes in the way the diseases are managed. The hypothesis linking venous outflow and IICP stems from the Monroe-Kellie Doctrine and is supported by the similar presentations of venous sinus thrombosis and IIH. ^14,15 A series of venography studies with manometry conclusively linked elevated cerebral venous pressures with IICP; ^16,17 however, the technique’s invasiveness limited its application for IIH screening.

The introduction of auto-triggered elliptic-centric-ordered three-dimensional gadolinium-enhanced magnetic resonance venography (MRV) allowed investigators to circumvent the limitations of older techniques and better investigate cerebral venous anatomy. ¹⁸ One of the earliest studies to use this technique found that narrowing of the dural venous sinuses on MRV was 93% sensitive and 90% specific for IIH. ¹⁹ Although techniques have varied among subsequent reports, ¹⁵ more recent studies have compared transverse sinus stenosis (TSS) to nine other MRI signs commonly associated with IIH and found it is the most sensitive and among the most specific findings. ^20,21

There is vigorous debate over whether TSS is caused by compression from cerebrospinal fluid (CSF) from IICP, or whether ICPs become elevated from impaired CSF resorption due to initial venous stenosis. ^15,22–26 Regardless, imaging has played a fundamental role in the diagnosis and interpretation of chronically IICP. In this review, we discuss the modalities for imaging intracranial hypertension, their limitations, and their contributions to the management of IICP.

Non-vascular MRI brain in intracranial hypertension

Historically, the role of CT and MRI for the evaluation of intracranial hypertension has been to rule out secondary or emergent causes of IICP, such as tumor, infection, hydrocephalus, or venous thrombosis. Exclusion of these pathologies with imaging is critical, as treatment for each entity differs, and the disease natural progression can result in poor patient outcomes, including debilitating morbidity and even mortality. Similarly, untreated or incorrectly treated IIH can cause poor outcomes.

Cross-sectional imaging, especially MR, may demonstrate several indirect findings suggestive of IIH. In these cases, there is no obvious lesion to explain IICP. Although etiologies of idiopathic cases are not well understood, the assumption is that there is an occult abnormality in vasomotor activity, increased venous pressure (due to increased central venous pressure or impaired venous outflow), and/or decreased CSF resorption. ^16,27,28 Although the cause may be visually occult on imaging, CT and MRI may demonstrate the sequelae of longstanding IICPs.

One of the more common findings is “empty” sella (Figure 1), characterized by a CSF-filled sellar space, with flattened pituitary along the sellar floor, concave superior pituitary margin, and elongated infundibulum. When present without an identified secondary cause, the finding is referred to as “primary empty sella”. ²⁹ Although empty sella may be incidental, the prevalence in IIH patients ranges from 70 to 94%, ²⁸ and the finding has been found to be 80% sensitive and 92% specific for IIH. ³⁰ The designation “partial” or “complete” empty sella depends on the degree of CSF filling of the sella (CSF filling <50% or>50%, respectively). The appearance is thought to be from herniation of the suprasellar space into the sella (arachnocele), with downward mass effect on the diaphragm sella. ²⁹ In some cases, the sella may be markedly enlarged, from osseous remodeling secondary to longstanding IICPs and CSF pulsation.

Figure 1.

MRI findings in IIH. 1A, 32-year-old female with difficulty walking and headache, who was diagnosed with idiopathic intracranial hypertension. T1 sagittal image (1A) demonstrates a sella that is >50% CSF-filled with peripheral displacement of the pituitary gland (white arrow), representing complete empty sella turcica. 1B, 28-year-old female presenting with double vision and nausea, who was diagnosed with idiopathic intracranial hypertension. T ₂-weighted axial image (1B) demonstrates bilateral distension of the optic nerve sheath complexes (white arrows) and flattening of the posterior sclera/intraocular protrusion of the optic nerve heads (black arrows).

IIH may also lead to enlargement of other CSF spaces, including the subarachnoid space surrounding the optic nerves. An increased optic nerve sheath diameter (ONSD) (Figure 2), (and empty sella), is among the most reliable signs of IIH. ONSD (measured sonographically) strongly correlates with ICP, and may be used as a clinical surrogate for invasive intracranial pressure monitors. ^31–33 Although there is no consensus cut-off for an abnormally dilated optic nerve sheath, a diameter of 5–6 mm is often used, with variable sensitivity. ^32,34 One study reported a sensitivity between 72 and 80% and specificity of 96% for a diameter of 5.5–5.6 mm. ³² Of note, sensitivity may improve if diameter cut-offs are corrected for body-mass-index (BMI), as BMI is positively correlated with ONSD. ³³ Finally, most publications favor measuring the ONSD approximately 3 mm posterior to the globe, where the sheath is at its most compliant, and usually most dilated. ³¹

Figure 2.

27-year-old female, presenting with long standing headache, who was diagnosed with idiopathic intracranial hypertension. Axial (2A) and coronal (2B) 3D MIP MRV images show stenosis of the right distal transverse sinus (white arrows). Anterior-poster subtraction angiographic image (2C) demonstrates right transverse-sigmoid sinus stent placement (black arrows). Axial non-contrast CT head (2D) redemonstrates right transverse-sigmoid sinus stent (white arrow). Axial (2E) and coronal (2F) MIP MRV images demonstrate improved flow in the right distal transverse and proximal sigmoid venous sinus post-stenting (white arrows), when compared to the pre-stenting MRV (2A and 2B).

Severe IICP may lead to papilledema (an ophthalmologic diagnosis), which although often occult on imaging, may manifest (in addition to ONSD) as tortuous optic nerves with flattening of the optic disc, and inward protrusion and enhancement of the optic nerve papilla (Figure 2). Flattening of the optic disc markedly increases the likelihood of IIH, ³⁵ and in more extreme cases, there may even be “reversal” of the optic nerve head, with inward protrusion. These findings, although not sensitive, have been found to be highly specific for IIH, ³⁵ reinforcing that these signs are more likely to be seen in severe cases. IICP can also lead to vertical optic nerve tortuosity (S-shaped curve of the optic nerve on sagittal plane), which is a very specific finding for IIH (95%). ³⁶

Meningoceles at the petrous apex and prominent Meckel’s caves, while not sensitive, are a specific sign of IIH. ^32,37 Dilation of CSF spaces may also be observed surrounding the oculomotor nerve as it courses through the laterosellar space, surrounding the abducens nerve as it courses through Dorello’s canal, about the facial nerve at the geniculate fossa, and about the hypoglossal nerve at the hypoglossal canal. ^15,38

Although many of the subarachnoid spaces often dilate, hydrocephalus is not seen in IIH, suggesting that overproduction of CSF is an unlikely etiology of IICP in primary cases. In fact, older studies described slit-like ventricles in the setting of IIH, although this is no longer considered a dependable sign, ^32,35 and may have been due to the inclusion of younger patients in early studies. While IIH can lead to acquired tonsillar ectopia, in which the cerebellar tonsils herniate caudally through the foramen magnum, this finding may also be present in intracranial hypotension and is therefore not a reliable sign. Additionally, in such cases, it is important to differentiate between congenital tonsillar ectopia (Chiari one malformation), given the markedly disparate treatment. ¹⁵

It should also be mentioned that chronic IIH, with its associated dilated arachnoid spaces and presence ofmeningoceles/encephaloceles, can lead to CSF leak, and subsequent intracranial hypotension. When IIH is concomitant with a leak, pressures may artificially normalize, and traditional findings of IIH may coexist with those of intracranial hypotension. ³⁹

Dural venous thrombosis may show similar findings of intracranial hypertension as described above.

Quantitative vascular imaging

Cerebral venous sinuses have a thin endothelial lined wall with no muscular layer that allows intrinsic and extrinsic pressure changes to affect its luminal caliber and cross-sectional configuration. ⁴⁰ Traditionally, clinical evaluation of cerebral venous sinuses has been qualitative. In this approach, the condition of sinuses is estimated visually based on the radiologist’s experience. ⁴¹ However, as the cerebral venous system is a dynamic apparatus, researchers have applied several quantitative imaging parameters on patients with normal and IICP to improve the diagnostic accuracy and to track physiologic changes and response to treatment. ⁴² Moreover, in IIH patients who are candidates for venous stenting, the quantitative measurements of the venous system can aid the interventionalist to choose the appropriate stent size, as oversizing or undersizing of stents may result in symptomatic overstretching of the sinus or incomplete expansion of the stenosis, respectively. ⁴⁰ These quantitative measurements may include but are not limited to volumetric dimensions, ^40,43–45 flow velocity, ⁴⁶ venous pressure, ⁴⁷ and mechanical stiffness of each sinus/region. ^48,49

Although CT venography and transcranial ultrasonography have been applied to obtain some quantitative information, ^50–54 the studies based on MRV or DSA were associated with more reliable results. ^40,55,56 West et al ⁵⁶ showed higher sensitivity of catheter angiography (sensitivity 0.81 at 34% stenosis threshold) and venography (sensitivity 0.92 at 31% stenosis threshold), relative to MRV/CTV (sensitivity 0.42) for significant pressure gradients on manometry. Boddu et al ⁴⁰ showed that the TOF-MRV sinus measurements in IIH are in good agreement with the IV US measurements with no significant variation. Additionally, using computer assisted methods whether in normal ⁵⁷ or IIH cases ^45,58 have provided promising results. Lublinski et al ⁴² developed an automated technique for precise cross-sectional analysis of dural sinuses before and after lumbar puncture in IIH patients. The technique which was validated by phantom models, and demonstrated a significant increase in cross-sectional area of major dural sinuses after lumbar puncture (p < 0.05). Dural sinus measurements with this rapid automated model had considerable accuracy with <3% of cross-sections requiring manual correction. In another study, a quantitative semi-automatic method for measuring transverse sinus stenosis showed 100% accuracy for differentiating 24 IIH patients from 24 normal controls and a better interobserver reproducibility (κ = 0.729) compared to qualitative radiologist assessment (κ = 0.467). ⁵⁹

There is pronounced heterogeneity in approaches to quantitative dural sinus measurement in the literature. Common variations included heterogeneity in the dural venous sinuses evaluated, ^18,46 variation of applied modalities ^40,56,60 and imaging parameters, ^40,61,62 use of intravenous contrast, ^63,64 comparison of pre- and post treatment measurements, ⁴³ and consideration of physiologic or developmental variations. ^65–67 Since there are a number of modalities available for venous imaging, standardized parameters for imaging modality acquisition and reconstruction as well as reliable quantitative biomarkers for each modality are needed.

MR venography

There is some controversy as to whether the presence of dural venous sinus stenosis represents a cause (poor venous drainage and increased proximal venous pressure) or effect (mechanical stenosis due to extrinsic increased pressure) of IIH. If IIH is due to impaired venous drainage in the setting of venous sinus stenosis, some may prefer a designation of secondary intracranial hypertension (rather than idiopathic). Regardless, dural venous sinus stenosis is a fairly common finding in the presence of what is usually referred to as IIH (Figure 3); in one study bilateral venous sinus stenosis was seen in approximately 93% of IIH cases, compared to 7% of normal controls. ¹⁹ Findings are usually characterized by smooth flattening and stenosis of the bilateral transverse dural venous sinuses, just proximal to the transverse-sigmoid junction. This is best appreciated on dedicated MR venography, or even 3D post-contrast T1 sequences, in which coronal and sagittal reconstructions can demonstrate flattening and concave margins of the transverse sinuses. ^{15,28,32,35,68} Associated findings include prominent or dilated occipital emissary veins. Regardless of etiology, symptomatic improvement is usually observed after transverse sinus stenting, with concurrent improvement in certain imaging findings, which may include resolution of optic nerve flattening/protrusion and decreased ONSD. ⁶⁸

Figure 3.

18-year-old female presenting with diplopia and chronic headache. Axial T2 fat-saturated orbits (3A) shows bilateral intraocular protrusion of the optic discs and flattening of the posterior sclera (white arrows), and distension of the optic nerve sheath (short arrows). Sagittal post-contrast T1 MPRAGE (3B) shows partial empty sella (>50% filling of sella). Axial MIP 3D phase-contrast MRV (3C) shows stenosis of the bilateral transverse-sigmoid junction (short arrows), that is better delineated than the stenosis represented on axial 3D MIP TOF MRV reconstruction (3D). Anteroposterior view of left transverse sinus catheter venogram (3E) shows irregularity and narrowing of the left transverse-sigmoid sinus (short black arrow). Anteroposterior catheter venogram with injection of the superior sagittal sinus (3F), with superimposed venous pressures, shows a substantial transverse-to-sigmoid pressure gradient on the right (28) and left (27). The interventionalist decided to stent the left transverse-sigmoid sinus, as seen on axial non-contrast CT head (3G), because the left-sided transverse sinus was dominant, and improved flow on the left would most likely best improve venous drainage and normalize intracranial pressure. Post-stenting axial MIP 3D phase-contrast MRV (3 h) shows improved patency of the left transverse-sigmoid sinus.

There are two main non-contrast MRV techniques, time-of-flight (TOF-MRV) and Phase contrast (PC-MRV), as well as various gadolinium-enhanced 3D MRV (Gd-MRV) techniques. TOF-MRV has a short acquisition time and good spatial resolution (1 min 14 sec acquisition with 0.6 × 0.6×2.0 mm acquired resolution at 3.0T field strength). However, it is prone to flow dephasing for in-plane flow, resulting in artifactual loss of signal, mimicking occlusion or stenosis. With TOF-MRV techniques, multiplanar acquisitions can help reduce artifactual stenosis by imaging in a plane perpendicular to the dural sinuses, an approach that is most sensitive to flow representation. PC-MRV relies on velocity-induced phase shifts to represent blood flow, and obviates the need for post-contrast MRV acquisitions. PC-MRV has a GRE readout with a bipolar velocity-encoding gradient. ⁶⁹ The velocity-encoding (VENC) can be set depending on the expected flow velocity. If the VENC is set too low, aliasing will result, providing inaccurate measurements, however, if the VENC is set too high, the actual flow will occupy only a small portion of the spectrum, limiting flow velocity discrimination. The technique improves background tissue signal suppression, can quantify flow and demonstrate flow direction, does not mask T1 shortening of subacute thrombus, and is more capable of differentiating slow flow from stenosis or occlusions, when compared to TOF-MRV. ⁷⁰ It has been shown to be better for flow representation in the torcula and sigmoid sinuses compared to TOF-MRV. ⁷¹ Disadvantages of PC-MRV include longer acquisition times due to three directions of acquisition and challenges in accuracy if the incorrect VENC is selected with 3D-PC. Contrast-enhanced MRV has been shown to decrease the artifacts encountered with non-contrast MRV techniques and to more accurately detect sinus thrombosis. Meckel et al ⁷² showed that time-resolved contrast-enhanced MRV, despite lower spatial resolution (depending on the parameters and espoused temporal resolution), provides better image quality of dural venous sinuses compared to TOF-MRV. 3D gradient T ₁-weighted post-contrast sequences have been shown to be more accurate in the detection of venous thrombosis compared to TOF-MRV and standard 2D MRI sequences, while TOF-MRV is more accurate in venous thrombus detection compared to T ₁-weighted, T ₂-weighted and contrast-enhanced T ₁-weighted sequences, relative to DSA as the reference standard. ⁷³

Vessel wall imaging

Intracranial-vessel-wall-MRI (IVW) has been used clinically for arterial vasculopathy assessment and characterization, ^74–76 differentiation, ⁷⁷ association with symptoms in atherosclerosis ⁷⁵ and cerebral aneurysms, ⁷⁸ associations with vasospasm development after aneurysmal subarachnoid hemorrhage, ⁷⁴ and identification of etiology in cryptogenic stroke. ⁷⁹ Three-dimensional IVW techniques have inherent dark blood due to variable flip angle sweep, however additional pre-pulse suppression techniques can help improve blood suppression. IVW without pre-pulse blood suppression can suffer from artifactual wall enhancement on post-gadolinium acquisitions that may mimic pathology.

There is limited data on the application of IVW in IIH. Quan et al ⁶⁴ showed that contrast-enhanced IVW is more accurate compared with PC-MRV in assessing stenosis degree in IIH patients in 62 patients with suspected IIH. Intermodality agreement of contrast-enhanced IVW (weighted κ = 0.868) performed better than PC-MRV (weighted κ = 0.653) relative to DSA as the reference standard. The typical imaging finding of IIH on IVW is focal stenoses of the dural venous sinuses, similar though better appreciated compared to the findings on conventional MRI, due to increased spatial resolution.

IVW has also been applied to the evaluation of underlying pathologies that may contribute to intracranial hypertension, specifically venous thrombosis. Vessel wall imaging has shown the ability to detect and directly characterize vascular luminal thrombus, ⁸⁰ in contradistinction to luminal imaging techniques (MRA, CTA) that evaluate vascular flow perturbations.

The data on venous thrombus imaging using IVW is limited. Yang et al ⁸¹ compared imaging characteristics of thrombus on non-contrast 3D T ₁-weighted IVW between patients with early subacute and late subacute cerebral venous thrombus. IVW accurately identified 113/116 segments having venous thrombus (sensitivity 97.4%) and accurately depicted thrombus volume.

Invasive cerebral venous imaging

While noninvasive imaging can provide useful information for the evaluation of IIH, several key data points can only be obtained through invasive means. First among these is the opening pressure on lumbar puncture, as elevated pressure is the sine qua non-of IIH diagnosis. ⁸² According to the modified Dandy criteria, to diagnose IIH, opening pressure must exceed 25 cm H₂O on lumbar puncture performed in the lateral decubitus position. ^83,84 While opening pressures of 20–25 cm H₂O are considered non-diagnostic, patients with opening pressures less than 25 cm H₂O or with functioning CSF shunts do not have intracranial venous pressure gradients. ^83,85

Cerebral venography with manometry is pursued when patients presenting with IIH (meeting modified Dandy criteria) either fail medical management or present with fulminant disease including rapid vision loss. Pressure gradients are the actionable finding when considering endovascular therapy for IIH. ⁸⁶ Pressure gradients must be assessed by venous manometry over the entire dural venous sinus system using meticulous technique. ⁸⁶ This is performed using a closed circuit system cleared of any air with the manometer zeroed after being placed at the level of the right atrium. Pressures are best measured by accessing the distal-most position to interrogate with a microcatheter over a microwire; the microwire is then removed and the manometer re-zeroed. The microcatheter is then slowly withdrawn while pausing at each predefined site to obtain a measurement. Flow limitation is identified by the presence of a pressure gradient between two points on venous manometry. Identified venous stenoses with venous flow-limitation can then be treated with stenting. ⁸⁶ Mean gradients in patients with IIH have been reported from 12 to 30 mm H₂O, and a difference of 8 mm H₂O is generally used as the threshold for diagnosis of a gradient amenable to stenting. ^85,86

Invasive venography also allows for superior visualization of the venous system, particularly with respect to focal stenoses that may require stenting. Real-time dynamic assessment more accurately represents venous sinus flow, showing preferential sinus outflow and the proximity of stenosis to cortical veins. ⁸⁷ Finally, three-dimensional rotational venography allows for superior measurements of the venous system to select the most appropriate stent size for treatment. ⁸⁸

Catheter venography and manometry risk is low. In a series of 164 venograms via femoral vein for IIH, there were no access site or intracranial complications. ⁸⁵ Another series of 147 venograms via the upper extremity found no major complications and only 2 (1.4%) minor complications. ⁸⁹

Venous imaging for therapeutic options

CSF diversion has been a treatment mainstay for medically refractory IIH, but there has recently been a shift towards endovascular treatment (Figure 4). ^14,28 The first case series of venous sinus stenting for IIH yielded mixed results with 5 out of 12 patients not experiencing any benefit. ²⁵ However, these results may reflect patient selection issues as five patients had already undergone CSF diversion and many patients refused the lumbar punctures necessary to confirm IICPs prior to stent placement. Furthermore, subsequent research showed that patients with more severe stenosis (>70%) and elevated pressure gradients (>6–10 mm Hg) across the stenosis are more likely to benefit from transverse sinus stenting. ^{28,85,90–92}

Figure 4.

29 year old female presenting with worsening vision and headache for 5 months, with known diagnosis of IIH. Axial T ₂-weighted fat-saturated sequence of the orbits (4A) shows distension of the optic nerve sheath complexes, flattening of the sclera and protrusion at the optic discs on both sides. Sagittal T ₁-weighted sequence (4B) shows partially empty sella, with flattening of the pituitary along the inferior aspect of the sella. Axial non-contrast CT head (4C) shows placement of ventricular shunt catheter for CSF diversion.

There is convergence of angiographic, ⁵⁶ MRV, ⁵⁵ and modeling ⁹³ data suggesting that 30–40% stenosis of the transverse sinus is clinically significant. One study using catheter angiography found an increase in the pressure gradient of 3.5 mm Hg with every 10% increase in stenosis. ⁵⁶ However, appropriate management of moderate TSS (30–70%) in patients with medically refractory IIH has been extensively debated. ^{26,28,93–95}

Many studies have shown that TSS improves following CSF diversion, and therefore, they argue that stenosis is secondary to IICPs. ^26,95 Others have shown an immediate and sustained drop in ICPs (and central venous pressure) following transverse sinus stenting, ^23,91,92 suggesting that stenosis may be the cause of IIH in these patients. The general consensus is that there is a positive feedback loop between IICPs and venous sinus stenosis, and although it is unclear which is the primary etiology, breaking the cycle at either point can be therapeutic. ^15,24 The ultimate decision of whether to pursue CSF diversion or endovascular stenting should consider the empirical data regarding long-term patient outcomes. IIH symptoms initially respond well to both shunting and stenting, but patients who undergo shunting are more likely to experience a return of symptoms, require revision surgery (55% vs  13%), and incur twice the healthcare costs as with stenting. ²⁸ On the other hand, stenting requires patients to be on anticoagulation or antiplatelet medications, which carries risk.

Patients require follow-up imaging after stent deployment, particularly if symptoms return. Symptom recurrence may indicate adjacent-stent stenosis, which may respond to re-stenting. ^90,91 Adjacent-stent stenosis may reflect increased compliance of the transverse sinus, ⁹³ perhaps due to wall dehiscence, ⁹⁶ and may be avoided by initial long-construct stenting from the torcula into the sigmoid sinus. MRV is a good screening tool with 100% sensitivity in detecting stent stenosis ≥50%. ⁶³ Once identified, catheter angiography can confirm the diagnosis. In-stent stenosis and cortical vein thrombosis are concerns, ²³ although these are rare and typically asymptomatic. ^87,97,98

Non-Invasive treatments

Non-invasive treatments for IIH are typically pursued when there are no vision complications at presentation. Weight loss is a principal approach to treating the underlying disease, however, acetazolamide can be used to lower ICP by inhibiting carbonic anhydrase and stunting choroid plexus CSF production. ¹³ Topiramate is another medication gaining traction for IIH treatment, owing to its antimigraine, appetite suppression, and carbonic anhydrase inhibition functions. ⁹⁹

Optic Nerve Sheath Fenestration

In addition to CSF diversion and venous sinus stenting, optic nerve sheath fenestration (ONSF) is pursued when there is rapid or progressive visual impairment. ¹³ ONSF is thought to result in fewer major complications than CSF diversion (1.5% vs  7.6%). ¹⁰⁰ A meta-analysis ¹⁰⁰ showed that ONSF had lower rates of headache improvement (44% vs  80%), but better rates of papilledema (80% vs  70%) and vision improvement (59% vs  54%). Venous sinus stenting, however, showed higher rates of headache, vision, and papilledema improvement (83%, 78%, and 97%, respectively) than other surgical options.

Conclusion

Advances in imaging of the cerebral venous system has substantially improved our understanding of intracranial hypertension and led to fundamental changes in its management. MRI and MRV are readily accessible imaging tools for initial evaluation and confirmation of intracranial hypertension, detection of secondary causes, and initial diagnosis of idiopathic intracranial hypertension. Invasive techniques allow for superior assessment of the intracranial venous system, particularly evaluation of focal stenoses that may require stenting. Quantitative imaging parameters may improve diagnostic accuracy and track response to treatment.