According to the modified Dandy criteria (1), the primary role of brain imaging in idiopathic intracranial hypertension (IIH) is to exclude other pathologies causing increased cerebrospinal fluid (CSF) pressure. However, since the earliest reports showing demineralization of the pituitary fossa on plain skull radiographs and small size of the ventricles on ventriculography in so-called “benign intracranial hypertension” (2), a constellation of more subtle findings suggestive of long-standing IIH gradually has emerged with modern neuroimaging. Their introduction in the most recent, albeit controversial (3,4 3,4), diagnostic criteria for the diagnosis of “possible IIH” (5) reflects the growing interest in the primary diagnosis of IIH rather than excluding an alternative condition (Table 1).
Magnetic resonance imaging (MRI) is the imaging of choice in IIH (5), in excluding other causes of increased intracranial pressure (ICP). MRI findings of raised ICP include empty sella flattening of the posterior globes, distention of the optic nerve sheaths, cerebellar tonsillar herniation, meningoceles, and transverse venous sinus stenosis (Table 2). Whether these findings truly assist clinicians in establishing the diagnosis of IIH or generate a number of overdiagnoses remains unclear.
This review provides a detailed description of the neuroimaging findings reported in IIH (Methods) and discusses their possible roles in the pathophysiology and diagnosis of this disorder.
Search Strategy and Selection Criteria
References for this review were identified by searches of PubMed from 1955 to January 2015, with the terms “idiopathic intracranial hypertension,” “pseudotumor cerebri,” “intracranial hypertension,” “benign intracranial hypertension,” “MRI,” “magnetic resonance venography (MRV),” “computed tomography (CT),” “CT venography (CTV),” “imaging,” and “CSF leak.” Additional references were identified by hand search of journals and relevant articles. Only articles published in English and in French were reviewed. The final reference list was generated on the basis of originality and relevance to the topics covered in the review. Priority was given to the most recent references.
When possible, we extracted the number of patients and, if available, the number of control subjects from each study for each radiological finding. When at least 2 studies used the same criteria to define a radiological finding, all patients from these studies were pooled to obtain a mean sensitivity with its 95% confidence interval (CI) by normal approximation to the binomial. When control subjects were available, the same method was used to obtain the mean specificity with its 95% CI. Our intention is to provide useful information to clinicians regarding the sensitivity and specificity, including their CI, of each radiologic finding by pooling several similar studies. However, differences among centers of MRI brand, protocol, data postprocessing, and, to a lesser extend, IIH criteria might reduce the scope of these results.
Empty sella is the most commonly reported imaging finding in patients with IIH, but it is also quite common in the general population. The term “empty sella” refers to complete or partial CSF filling of the sella turcica associated with lack of visible pituitary gland on mid-sagittal T1 MRI (Fig. 1) (6). Unlike secondary empty sella, in which the size of the pituitary gland is decreased with respect to the sella turcica (7), primary empty sella is believed to be related to an intrasellar herniation of arachnoid mater and CSF, which flattens the pituitary gland and remodels the sella turcica (7).
The sensitivity of empty sella in IIH ranges from 65% to 80%, depending on the definition used (8–12 8–12 8–12 8–12 8–12). With the commonly used system developed by Yuh et al (11) (Fig. 1), the pooled sensitivity is estimated to be 80% (95% CI: 71–89) (11,12 11,12). Severe empty sella (Category V) with lack of visible pituitary gland is uncommon, found in less than 10% of cases (11,12 11,12).
However, an empty sella is not specific to IIH and has been found in various causes of chronic intracranial hypertension such as cerebral venous sinus thrombosis and intracranial space-occupying lesions (11–13 11–13 11–13). In addition, it has been reported as an incidental radiologic finding in normal subjects (14), in whom it has the same appearance as in IIH patients (15). The specificity of empty sella in IIH ranges broadly across studies from 70% (10) to 100% (11). When using the definition based on the cross-sectional area of the sella (Fig. 1) (8–10 8–10 8–10), the pooled specificity is estimated at 83% (95% CI: 76–90).
Few morphometric studies of the sellar region in patients with IIH have been performed (6,12,16,17 6,12,16,17 6,12,16,17 6,12,16,17). Taken together, they suggest that enlargement and remodeling of the sella turcica is the main contributor to the empty sella appearance. Yet, some degree of compression of the pituitary gland appears likely with IIH as partial (6), and even complete (18) resolution of the “empty sella” appearance has been reported after treatment of IIH. Few studies mention how long it takes for empty sella to develop in a patient with IIH. Saindane et al (15), using cross-sectional data, demonstrated that the sellar width on average enlarges 1 mm per decade in patients with IIH, suggesting that the bony findings take years to develop.
The orbital findings in IIH mirror the mechanical deformations of the optic nerve sheath, lamina cribrosa, and posterior sclera secondary to elevated CSF pressure transmitted along the intraorbital optic nerve. Studies (8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20) have most often reported these findings based on subjective radiologic interpretation, likely explaining suboptimal reader agreement (21) and the wide range of sensitivity reported.
Posterior Globe flattening and Optic Nerve Head Protrusion
Posterior globe flattening and, in more severe cases, optic nerve head protrusion correspond to loss of normal curvature of the posterior sclera seen on axial MRI at the bulbar insertion of the optic nerve (Fig. 2) (10). On funduscopic examination, IIH patients with posterior globe flattening may have macular choroidal folds. In some patients, flattening of the posterior globes and choroidal folds may precede papilledema, emphasizing the importance of including intracranial hypertension, and especially IIH, in the differential diagnosis of “flat globe syndrome” (22).
The sensitivity of posterior globe flattening in IIH ranges from 43% to 85% and from 3% to 59% for optic nerve head protrusion and a pooled sensitivity of 66% (95% CI: 60–72) and 36% (95% CI: 26–44), respectively (8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20), and a pooled specificity of 98% (95% CI: 96–100) and 99% (95% CI: 98–100), respectively (8–10,20 8–10,20 8–10,20 8–10,20).
Posterior globe flattening likely reflects the gradient between the perioptic CSF and the intraocular pressures. It is not specific for IIH, as ocular hypotony displays a similar appearance on MRI (23).
Enhancement of the Optic Disc
Enhancement of the optic disc on postcontrast axial T1 orbital MRI (Fig. 3) is one of the less sensitive findings in IIH, likely because of the small size of the region of interest (24). Its sensitivity ranges from 2% to 50%, with a pooled sensitivity of 17% (95% CI: 11–23) (8–10,19 8–10,19 8–10,19 8–10,19). It is not specific for papilledema, and it has been reported in other causes of optic disc edema (25–27 25–27 25–27). Its specificity is very high, close to 100%, as it has so rarely been reported in normal controls (8–10 8–10 8–10).
Distention of the Optic Nerve Sheaths
Radiologic distension of the optic nerve sheaths [“optic nerve unfoldment” in anatomic studies (27)] results from increased ICP in the perioptic subarachnoid space (Fig. 4).
It is typically seen in IIH patients (28), with a wide ranging sensitivity from 45% to 89% (8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20), likely related to the use of inconsistent definitions. Applying the definition based on the absolute width of the perioptic CSF (CSF ring >2 mm), its pooled sensitivity is estimated at 58% (95% CI: 48–68) (9,10,12 9,10,12 9,10,12), and pooled specificity is estimated at 89% (95% CI: 85–95) (9,10 9,10).
Vertical Tortuosity of the Optic Nerve
The “kinked” appearance of the intraorbital portion of the optic nerve is visualized best on oblique sagittal views (Fig. 5). It is believed to be secondary to the distention of the sheath between the 2 ends of its intraorbital optic nerve, which are fixed at the scleral insertion and orbital apex (8).
Based only on subjective impression and not quantifiable characteristics, its sensitivity ranges from 21% to 55%, with a pooled sensitivity of 43% (95% CI: 37–50) (8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20 8–10,12,19,20) and a pooled specificity of 90% (95% CI: 85–95) (9–10,20 9–10,20 9–10,20).
Optic Canal Size
The size of the optic canal rarely is measured with MRI in patients with IIH, and more easily performed on orbital CT. It has been suggested that optic canal may be larger on the side of the worse papilledema in selected patients with very asymmetric papilledema from IIH (29). It is unclear, however, whether a larger canal is congenital (and therefore may contribute to asymmetric papilledema by facilitating the transmission of CSF pressure to the lamina cribrosa, leading to worse papilledema) or may develop over time in patients with chronic untreated IIH because of bony erosion. This finding might shed light on some of the factors contributing to papilledema but requires further study.
CEREBELLAR TONSILLAR HERNIATION
Tonsillar herniation is the caudal displacement of the cerebellar tonsils ≥5 mm into the upper cervical spinal canal through the foramen magnum and is best visualized on mid-sagittal T1 MRI (Fig. 6) (30). Although tonsillar herniation is the hallmark of Chiari malformation Type I (CM1) (31) (Fig. 6B), these 2 conditions are not synonymous. Up to 15% of the normal population have a small degree herniation of the tonsils (≥2 mm and <5 mm) below the foramen magnum (tonsillar ectopia) (32). Cerebellar tonsillar herniation also has been reported after lumbar puncture and placement of a lumbar-peritoneal shunt. More recently, confusion has arisen with studies reporting patients with typical CM1 displaying radiologic findings of chronically increased ICP (33) and typical IIH patients with herniation of the cerebellar tonsils (some of whom met the criteria for CM1) (Fig. 6C, D) (34). It is unclear whether these patients have primary CM1 complicated by increased ICP, or IIH complicated by CM1-like tonsillar herniation, or both conditions. The distinction is very challenging in clinical practice and makes treatment more difficult (33).
The sensitivity of cerebellar tonsillar herniation in IIH ranges between 6% and 21% (9,34–36 9,34–36 9,34–36 9,34–36) with a pooled sensitivity of 16% (95% CI: 10–22) (9,34,36 9,34,36 9,34,36) and a pooled specificity of 95% (95% CI: 91–100) (9,34 9,34).
Although there is growing evidence of an association between IIH and cerebellar herniation, their relationship remains poorly understood. Progressive enlargement of the foramen magnum with secondary caudal descent of the tonsils might be hypothesized, as previously seen in thin bones of the skull base (37). However, one study (38) looking at the posterior fossa size in patients with IIH, patients with Chiari 1, and controls showed that the foramen magnum size was not different between IIH patients and controls. This finding would argue that the bony foramen magnum is not expanded in IIH.
Some authors have referred to a lower than usual position of the cerebellar tonsils in IIH as “tonsillar descent,” implying a dynamic process pushing the tonsils down, either because of low pressure below (such as in spontaneous intracranial hypotension) or high pressure above (such as in IIH). We believe that the tonsillar descent often observed in IIH is related to the increased ICP.
This association between tonsillar herniation and IIH is emerging, and the prevalence of IIH in tonsillar herniation and the direction of the causation, if any, between these 2 entities are still controversial.
MENINGOCELES AND MENINGOENCEPHALOCELES
Bony remodeling associated with IIH may lead to herniation of meninges (meningocele) or brain tissue (meningoencephalocele) through skull base foramina or areas of thin bone of the skull base. Meningoceles and meningoencephaloceles are best demonstrated on MRI with contrast (Fig. 7A, B) (39) and have been strongly associated with CSF leaks. In one study, 9/79 patients with IIH (11% [95% CI: 4–18]) had asymptomatic meningoceles, nearly all located in Meckel cave and in the petrous apex (Fig. 7C), vs. none in controls (40).
FORAMEN OVALE WIDENING
Widening of the foramen ovale, visualized best on axial high-resolution CT scan, likely is related to herniation of the contents of Meckel cave through the foramen ovale leading to bony remodeling. Its sensitivity and specificity are 50% and 80%, respectively, using a cutoff value of 30 mm2 (20). The utility of this finding on routine imaging for patients with presumed IIH remains unclear.
SPONTANEOUS CEREBROSPINAL FLUID LEAKS
IIH is an increasingly recognized etiology of spontaneous skull base CSF leaks in the otolaryngology and neurosurgery literature (37,41–48 37,41–48 37,41–48 37,41–48 37,41–48 37,41–48 37,41–48 37,41–48 37,41–48). Otolaryngologists have emphasized that spontaneous skull base CSF leaks might represent a variant of IIH in patients who do not develop typical symptoms and signs of IIH because the leak acts as a “natural” CSF diversion (43).
Noncontrast high-resolution CT scanning with thin sections allowing multiplanar reconstruction is the first-line imaging technique to localize bone defects at the skull base with a sensitivity up to 92% and a specificity of 100% (Fig. 8) (39). In difficult cases, such as those with multiple defects or a negative CT scan despite an active CSF leak, CT or MR cisternography (39,49 39,49) may help identify the site of the CSF leak (39).
CSF leaks are associated with meningoceles and meningoencephaloceles in 50%–100% of cases (39,45 39,45). When a CSF leak is diagnosed, brain MRI is recommended to help differentiate herniated intracranial contents from sequestered secretions within the paranasal sinuses (Fig. 7B). In addition, brain MRI allows screening for findings consistent with long-standing IIH, such as a partial or complete empty sella, reported in 66%–100% of cases with spontaneous CSF leaks (41,45,50 41,45,50 41,45,50). Interestingly, unlike in patients with spontaneous spinal CSF leaks, pachymeningeal enhancement and sagging of the brain are uncommon in patients with CSF leaks presumed to be secondary to IIH (51). This may be explained by the relative lack of low CSF pressure in these patients.
Early descriptions of patients with IIH emphasized the radiologic finding of “slit-like” ventricles as opposed to the dilated ventricles of patients with obstructive hydrocephalus. This was mostly reported before the era of modern neuroimaging when patients with suspected increased ICP were evaluated with plain skull radiographs, ventriculography, and pneumoencephalography (52). Although slitlike ventricles were reported in studies using older generation CT scan imaging (53), this finding has not been corroborated on more recent MRI studies (9,10,17,20 9,10,17,20 9,10,17,20 9,10,17,20).
CEREBRAL VENOUS SINUS IMAGING
The primary role of intracranial vascular imaging in patients suspected of IIH is to exclude cerebral venous sinus thrombosis (54) and, more rarely, dural fistulas. Improvement in MRI techniques has allowed reliable visualization of the transverse venous sinuses where focal stenosis was previously often overlooked in patients with IIH (55–57 55–57 55–57). More than a decade ago, Farb et al (55) used a new MRV technique (auto-triggered elliptic-centric-ordered three-dimensional gadolinium-enhanced MRV [ATECO MRV]) to demonstrate bilateral distal transverse sinus stenoses in IIH patients (Fig. 9) with a sensitivity and a specificity of 93%. This finding supported a role of venous hypertension in patients with IIH, as suggested by previous manometric studies which found pressure gradients across transverse venous sinus stenoses in patients with IIH (58,59 58,59). It has since been proposed that bilateral transverse sinus stenosis, or stenosis of a dominant transverse venous sinus, might lead to the following series of events: decreased venous outflow drainage, cerebral venous hypertension, CSF passive resorption impairment, further raising CSF pressure resulting in external compression of the distal portion of the transverse venous sinuses, leading to worsening of the venous stenosis and perpetuation of this vicious cycle (60,61 60,61). Such bilateral transverse venous sinus stenosis also is often found in patients with intracranial hypertension not due to IIH (13,33 13,33).
Assessing the sensitivity of bilateral transverse venous sinus stenosis is challenging because of inconsistent definitions across studies (55,62–65 55,62–65 55,62–65 55,62–65 55,62–65) and the use of various venous imaging techniques, including flow-sensitive MRV (9,62,65 9,62,65 9,62,65), contrast-enhanced MRV (55,64,66 55,64,66 55,64,66), and CTV (63,67 63,67). Using contrast-enhanced MRV, the pooled sensitivity of bilateral transverse venous sinus stenosis is estimated to be 97% (95% CI: 93–100) (55,64,66 55,64,66 55,64,66). However, transverse venous sinus stenosis can be found incidentally in normal subjects (68).
It has been pointed out that, depending on the definition of transverse venous sinus stenosis, 100% of patients with IIH have bilateral stenosis on MRV (64). Furthermore, in the absence of bilateral venous stenosis on MRV of patients with presumed IIH, clinicians should reconsider the diagnosis of IIH. This point also was emphasized in the most recently proposed modified Dandy criteria (Table 1) (5), in which bilateral transverse venous sinus stenosis was introduced as a minor diagnostic criterion of IIH.
BRAIN IMAGING IN PEDIATRIC INTRACRANIAL HYPERTENSION
There are few reports of the imaging findings of pediatric IIH patients and little is known regarding the potential pitfalls of applying the findings reported in adults with IIH to children.
A study evaluating the optic nerve sheaths of pediatric IIH patients on MRI found larger diameters in comparison with normal children (69). One cross-sectional study conducted in pediatric IIH (70) demonstrated that no single finding other than vertical tortuosity of the optic nerves was found to be statistically different from controls, whereas a subsequent cross-sectional study (71) showed that many neuroimaging findings (empty sella, flattening of posterior globe, intraocular protrusion of the optic nerve head, optic nerve sheath distension, and tortuosity of the optic nerve) were more prevalent in pediatric patients with IIH than in controls. The use of general anesthesia in 50% of patients in the former study (70), vs. 14% in the latter study (71), may have elevated the PCO2, and thereby transiently raised the CSF pressure in the control group, possibly explaining the lack of difference reported in the former study. One study (72) found that the prevalence of empty sella in children without increased ICP was lower than that in adults (1.2%, 95% CI: 0.1–2.3). However, no adults were recruited into a control group. All 4 children with IIH in that study had an empty sella.
To our knowledge, no studies have directly compared the prevalence of neuroimaging findings in adults with IIH to children with IIH. However, we identified 2 IIH studies using the same imaging criteria; one included 16 adults (8) and the other 23 children (70). We analyzed the data of each (Fisher exact test) and found that the neuroimaging findings were not statistically different in adult and pediatric IIH, except for empty sella, which was less prevalent in children (26% vs. 69%, P = 0.01).
DIAGNOSTIC VALUE OF THE COMBINATION OF VARIOUS IMAGING FINDINGS IN INTRACRANIAL HYPERTENSION
None of the imaging findings discussed have shown, in isolation, both high sensitivity and specificity for IIH. Although many reports have addressed the diagnostic value of each finding separately on cross-sectional studies, only 2 have focused on the combination of some of these findings in adults with IIH (9,10 9,10).
However, inconsistent results have emerged, as the methods were not comparable. One study (9) reported that any single finding, including a partially empty sella, flattening of the posterior globe, or bilateral transverse sinus stenosis, increased the sensitivity for IIH to ≈85% while maintaining a high specificity (≈95%). However, any combination of 2 of these findings increased the specificity to 100%, but lowered the sensitivity between ≈35% and ≈55%, depending on the combination. Conversely, the other study (10) showed that the combination of a totally empty sella and/or flattening of the posterior globe added little value, but bilateral transverse venous sinus stenoses were not included, limiting the scope of these results.
One study (70) addressed the combination of imaging findings in pediatric IIH and demonstrated that a combination of ≥3 imaging findings (flattening of posterior globe, optic nerve sheath distension, intraocular protrusion of the optic nerve head, tortuosity of the optic nerve, and empty sella) had a sensitivity of ≈45% and a specificity of 95% for the diagnosis of IIH in children.
INCIDENTAL FINDING OF NEUROIMAGING SIGNS SUGGESTING INTRACRANIAL HYPERTENSION
Because of the increasing prevalence of IIH paralleling the increasing prevalence of obesity, clinicians other than neuro-ophthalmologists are becoming more aware of IIH. Although this increased awareness is to be encouraged, it has had the unfortunate effect of overdiagnosis of IIH in young obese women with chronic headaches or anomalous optic nerves, many without clearly documented papilledema. Furthermore, no study has addressed the predictive value of the imaging findings of IIH, which have also been reported in healthy subjects, patients with secondary causes of increased ICP (13), and patients with primary headache syndromes (65,73 65,73). Clinicians must be aware that these imaging findings are not diagnostic of IIH and should not prompt invasive or costly investigations when discovered incidentally (74), unless the patient also has papilledema or other compelling symptoms or signs of elevated ICP.
While a variety of neuroimaging findings may be suggestive of chronically elevated ICP, none are indicative of whether the ICP in a patient is normal or abnormal. In addition, it is unclear how long it takes for most neuroimaging signs of raised ICP to develop and how often reversibility of these signs is observed once ICP is normalized.
Various approaches to noninvasively estimate ICP using imaging are currently under investigation (75). Transcranial Doppler ultrasound has been used to measure blood flow in major brain arteries, most commonly the middle cerebral artery. This technique can provide measurements of mean blood flow velocity and pulsatility index, a measure of relative vascular pulsatility (76). However, there is a complex relationship between pulsatility index and ICP that makes clinical application of this technique difficult. Phase contrast MRI gated to the cardiac cycle (77) enables quantitative measurement of CSF and blood flow and pulsatility. By varying the sensitivity of the technique, high velocity flow in arteries, or low velocity flow in veins and CSF, can be assessed (78). Estimates of intracranial compliance have been obtained based on relative distribution of arterial, venous, and CSF pulsatility at the craniocervical junction (79). This technique has been applied to patients with IIH (80) and has suggested reduced spinal canal compliance in patients with IIH compared with control individuals. Future studies using this technique and others under development correlating noninvasive measures directly to opening pressure measurements have the potential to validate an MRI-based method for estimating ICP in a clinically relevant fashion.
Emergent brain imaging (ideally brain MRI with and without contrast) and venous imaging with contrast (MRV or CTV) are the first-line tests in patients in whom IIH is suspected to exclude a space-occupying process, obstructive hydrocephalus, or cerebral venous sinus thrombosis. Although the diagnosis of IIH remains a diagnosis of exclusion, specific imaging findings on brain MRI and MRV may suggest IIH and prompt further investigations (Table 2). The most recently proposed modified diagnostic criteria for IIH (5) include these imaging findings in detail but do not require their presence to make a definite diagnosis of IIH. The description of such findings on routine MRI reports may result in overdiagnosis of IIH and must therefore be interpreted with caution. Because of the high sensitivity of some of these findings, we believe that their absence, rather than their presence, may be more important in establishing the diagnosis of IIH. If absent, the clinician should consider alternate diagnoses. However, it is likely that some of these findings such as empty sella and bony remodeling require some time to develop in patients with IIH and, therefore, may be absent initially.
STATEMENT OF AUTHORSHIP
Category 1: a. Conception and design, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse. b. Acquisition of data, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse. c. Analysis and interpretation of data, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse. Category 2: a. Drafting the manuscript, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse. b. Revising it for intellectual content, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse. Category 3: a. Final approval of the completed manuscript, S. Bidot, A. M. Saindane, J. H. Peragallo, B. B. Bruce, N. J. Newman, and V. Biousse.
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