Enlargement of Dorello's Canal as a Novel Radiographic Marker of Idiopathic Intracranial Hypertension

Abstract

Objectives  The objective of this study is to compare the visibility and size of Dorello's canal (DC) on magnetic resonance imaging between patients with idiopathic intracranial hypertension (IIH) and control patients, for its evaluation as a potential novel marker for chronic increased intracranial pressure (ICP).

Design  Retrospective blinded case–control study.

Setting  Tertiary care academic center.

Participants  Fourteen patients with spontaneous cerebrospinal fluid (CSF) rhinorrhea and diagnosed IIH, as well as an equal number of age and gender-matched controls.

Main Outcome Measures  Radiographic presence or absence of visible CSF sleeve within DC as well as CSF sleeve width when present.

Results  Following review of 28 IIH canals and 28 control canals, IIH patients were significantly more likely to have a visible CSF sleeve within DC and to have a wider measured medial entrance to DC ( p  < 0.001).

Conclusion  Identification of CSF evagination into DC may serve as a reliable marker for increased ICP in the IIH population. This finding should be considered in the future as paradigms for diagnosis of IIH continue to evolve.

Introduction

Idiopathic intracranial hypertension (IIH), previously known as pseudotumor cerebri and benign intracranial hypertension, refers to the elevation of intracranial pressure (ICP) in the absence of an identifiable cause. These patients are at risk for development of spontaneous cerebrospinal fluid (CSF) leaks via either the anterior or lateral skull base. ¹ Various schemes have been identified to facilitate diagnosis of this entity, all of which rely on exclusion of other organic or anatomic causes of increased ICP. In general, definitive diagnosis of IIH requires symptoms of intracranial hypertension or papilledema, documented elevated opening pressure, normal CSF composition, and no evidence for other structural or physiologic causes for the observed ICP. ² Recent authors have suggested more broadly applying the diagnosis of the pseudotumor cerebri syndrome (PTCS), which would encompass both organic lesions and IIH.

Updated diagnostic criteria, published in 2013, allows for identification of suggested PTCS based on radiologic criteria alone. ³ Previously reported findings on imaging include a partial or complete empty sella, perioptic subarachnoid space dilatation, flattening of the posterior ocular globe, and bilateral transverse sinus stenosis. ^{4 5 6} More controversial reports include expansion of the foramen ovale, protrusion or enhancement of the optic nerve, and enlargement of Meckel's cave. ^{7 8 9 10} Given the invasiveness of lumbar puncture (LP) and subjectivity of papilledema, it has become prudent to search for radiographic signs that may aid in the early diagnosis of patients suspected of IIH. It has yet to be substantiated in the literature as to whether Dorello's canal (DC), which transmits the abducens nerve (CNVI) to the cavernous sinus, may undergo changes in the presence of increased ICP. This study evaluates the radiographic patency of DC on magnetic resonance imaging (MRI) as a marker for elevated ICP and its potential association with spontaneous CSF leaks (CSFLs) in the adult population.

Materials and Methods

This study was approved by the Institutional Review Board. Due to its retrospective nature and minimal foreseeable risk to patients, consent was not required. Fourteen patients with spontaneous CSF rhinorrhea and known IIH were selected for inclusion into the study, as well as 14 age and gender-matched controls who underwent pre- and postinfusion multisequence MRI, including high-resolution axial three-dimensional (3D) FIESTA (Fast Imaging Employing Steady-State Acquisition; GE; New York, United States) or CISS (Constructive Interference in Steady State; Siemens; Berlin, Germany) sequences with multiplanar reconstruction. All IIH patients had previously suffered from CSF rhinorrhea and were confirmed to have IIH based on the modified Dandy criteria. The criteria consists of signs/symptoms of increased ICP, lumbar opening pressure greater than 25 cmH ₂ O, normal CSF studies, and no localizing lesion on neuroimaging or neurologic exam. ¹¹ Controls were extracted from a list of all patients presenting to the primary author's clinic who underwent MRI of the temporal bones with FIESTA/CISS formatting. Exclusion criteria for controls included those with clinical suspicion for IIH, vision changes, chronic headaches, or intracranial mass lesions. Patients were de-identified upfront and data were stored on encrypted hard drives behind a locked door. Axial sections of 0.8 mm thickness through bilateral abducens nerves were reviewed by two fellowship-trained neuroradiologists, who were blinded as to the patient's IIH status or clinical indication for MRI. Imaging was evaluated for the presence or absence of visible CSF evagination into the proximal aspect of DC. When present, the width of this CSF sleeve was measured, and its width relative to CNVI was assessed ( Fig. 1 ). The absence of a CSF sleeve on imaging was considered 0 mm of CSF evagination, though in practice, slice thickness limits detection to 0.8 mm. Radiologists were permitted to review other formats of imaging; however, measurement of DC was limited to axial images, as this is the optimal format in which to measure CSF evagination into the canal. Interrater reliability was determined between the two neuroradiologists for the presence/absence of a CSF sleeve, as well as for the measured width of the CSF sleeve. The results from the two radiologists were combined and averaged. Student's t -test (two-tailed) was used for comparison of parametric data. All statistical analyses were performed using SPSS, version 24.0.0.0 (IBM SPSS Statistics for Macintosh, Version 24.0. Armonk, New York, United States). A p -value of 0.05 or less was considered significant for all statistical analyses.

Fig. 1.

Measurement of Dorello's canal. (Left) Axial MRI FIESTA sequence with arrow demonstrating normal entry of the sixth cranial nerve into Dorello's canal with no CSF evagination observed. (Right) Axial MRI FIESTA sequence with CSF evagination into Dorello's canal measuring 7 mm in medial–lateral width. CSF, cerebrospinal fluid; FIESTA, Fast Imaging Employing Steady-State Acquisition; MRI, magnetic resonance imaging.

Results

A total of 14 IIH patients and 14 age and gender-matched controls were included into the study, allowing for evaluation of 56 total DCs. All included patients were female. Demographic and radiologic data are shown in Table 1 . The mean age of the IIH and control group was 52.2 and 51.8 years, respectively. Mean body mass index (BMI) in the IIH cohort was 41.6, while the control group had a mean BMI of 32.4 ( p  = 0.03). Twenty-five IIH canals (89.3%) had visible CSF evagination into DC compared with five control canals (17.9%; p  < 0.001). Similarly, when the medial–lateral width of CSF evagination into DC was visible, IIH canals demonstrated a mean CSF sleeve width of 3.86 mm, while control canals demonstrated a mean CSF sleeve width of 2.51 mm ( p  = 0.003). Finally, patients were evaluated for whether total width of CSF evagination was greater than twice the width of the included CNVI. In this analysis, 21 IIH canals (75%) showed this configuration versus 4 in control patients (14.3%; p  < 0.001). Five patients between the two cohorts had a canal visible only unilaterally. There was no significant difference between each radiologist's CSF sleeve width measurements in the IIH and control groups, indicating interrater reliability ( p  = 0.17 and p  = 0.88).

Table 1. Patient demographics and radiographic measurements.

	Female	Mean age (y)	Overweight or obese	Mean BMI (kg/m 2 )	Mean CSF width (mm)	Visible CSF sleeve
IIH patients	100%	52.2	92.8%	41.6	3.86	89.3%
Control patients	100%	51.8	85.7%	32.4	2.51	17.9%

Abbreviations: BMI, body mass index; CSF, cerebrospinal fluid; IIH, idiopathic intracranial hypertension.

Discussion

DC is a short intracranial passage which transmits both the inferior petrosal sinus and the abducens nerve (CNVI), connecting the cisternal segment of CNVI to the cavernous segment. It is best viewed as an invagination of the petroclival dura mater, from the petroclival entrance point and terminating at the posterior cavernous sinus. The canal is bounded superiorly by Gruber's (superior petrosphenoidal) ligament, laterally by the petrous apex, and medially by the clivus. ¹² Various authors have attempted to measure the canal using microanatomic dissection. Perhaps, Gruber himself first measured DC as 6 to 12 mm in length and 1 to 3 mm in diameter, while other authors have measured the canal as 4 to 13 mm in length and 0.5 to 3 mm in diameter. ^{13 14}

Due to the small size of DC in normal patients, it is generally not visualized using standard MRI scanning protocols. Patients in this study who had previously diagnosed IIH with CSFL were significantly more likely to have an observable DC on axial MRI when compared with matched controls. Similarly, the measured width of CSF evagination was significantly correlated with the presence of known IIH and previous history of spontaneous CSFL. This paradigm is consistent with previous reports that have postulated chronic increased CSF pressures as a cause of gradual dilation of other intracranial osseofibrous structures. ^{7 10}

Previous authors have anecdotally linked enlargement of DC with IIH when discussing enlargement of all CSF spaces in this cohort. ¹⁵ Historical papers intended to radiographically measure DC have often reported length of CSF evagination. Previous reports by Ono et al have used 3D fast, asymmetrical, spin echo MRI to provide detailed radiographic analysis of DC. ¹⁶ Using this technique, the authors were able to visualize CSF evagination within the canal in 77% of normal healthy patients. The length of CSF evagination into the canal was 1 to 2 mm in most patients. Similarly, the use of 3D-CISS MR cisternography by Yousry et al allowed for visualization of CSF evagination into DC in 94% of healthy patients, with length of evagination evenly distributed among <1, 1 to 2, and >2 mm. ¹⁷ Our cohort was evaluated using a slice thickness of 0.8 mm, with a far lower proportion of visualized CSF sleeves in control patients. This may be due to differences in slice thickness or multiplanar reconstruction, as the protocol used in this study is the one used clinically at our institution. Since DC and its CSF sleeve can also be visualized in patients without IIH, relative width of the CSF sleeve was compared with that of the CNVI as it enters the canal. In comparing the width of the nerve to that of the CSF sleeve, we found that if the CSF sleeve is greater than twice the width of the nerve, this too was highly associated with the presence of IIH, and may prove to be more specific to IIH than presence of a CSF sleeve alone. Additionally, five patients were observed to have visible DCs on only one side. This suggests that there is anatomic or local factor at play in the degree of osseofibrous remodeling that occurs in response to increased CSF pressures.

The mechanism by which IIH produces increased T2 fluid signal within osseofibrous structures such as Meckel's cave and DC remains poorly understood. Debate continues as to whether osseofibrous erosion and expansion, or compression of parenchymal structures leads to the increased T2 signal observed in these patients. The clinical sequelae of chronic intracranial hypertension, including skull base thinning and CSFL, would argue for the former; however, further volumetric studies of these sites must be done to clarify this point. In regard to our finding of increased width of DC in these patients, we propose that relative compression of the cavernous sinus may account for some of the observed effect. Degnan et al. reported that compression of the cavernous sinus is a common finding in patients with IIH, and it may be true that even a small amount of anterosuperior compression may allow detection of the canal on standard MRI protocols. ¹⁸

Despite increasing prevalence due to thence increases, however, to 3.5 per 100,000 persons when limited to females of childbearing age. ^{19 20} Our cohort displayed an expected female preponderance as 100% of IIH/CSFL patients were female; however, their age of first CSFL was 52.2 years, while the average age of IIH diagnosis is 30 years. ²¹ As these patients presented with CSFL as their chief complaint, this is consistent with previous reports that have suggested that the erosive changes observed with IIH often take decades to present clinically. ^{22 23 24} The mean BMI in our IIH/CSFL was 41.61, qualifying as morbid obesity. Again, this is in agreement with previous reports, which have shown a strong association between IIH and obesity; however, it should be noted that the mean BMI of our control cohort also qualified as obese. ²⁵

Symptoms of IIH include headache (94%), transient visual disturbances (68%), and pulse-synchronous tinnitus (58%), among others. ²⁶ As part of the diagnostic criteria for IIH other, more likely, diagnoses leading to intracranial hypertension must first be excluded. This requires cross-sectional imaging to be performed regardless of exam findings and opening pressure, with MRI being the preferred modality. Many radiographic signs for increased ICP have been identified in the literature. Despite this, few attempts have been made to elucidate their relative predictive value. Positive predictive value has been postulated to be highest when flattening of the posterior globe, enhancement of the optic nerves, or bilateral transverse sinus stenosis is present. ^{4 8 9 27 28} One author reported sensitivity and specificity of the latter to be 93% when measured on contrast-enhanced magnetic resonance venography. Other earlier indicators of intracranial hypertension such as “slit-like” ventricles and effaced cisterns have fallen out of favor due to poor predictive value.

Recently, Tuncel et al attempted to standardize a radiologic scoring system that could be predictive of opening pressure on LP ²⁵ . ²⁹ The authors included the aforementioned criteria, as well as less well-studied indicators such as optical tortuosity and radiographic swelling of the optic disc; however, they were unable to prove a correlation between their scoring system and measured opening pressures. Despite this, it is our belief that a well-designed radiographic scoring system, which incorporates the more sensitive and specific MRI findings may prove to be a useful noninvasive screening tool for patients in whom IIH is suspected.

This study has several limitations. Some authors have argued that CSFL offers a natural “outlet valve” for elevated ICP, and that radiographic compression of soft tissue structures is greater in patients without CSFL. This study did not include a cohort of patients who suffer from IIH without CSFL, in whom radiographic findings of the soft tissues may be even more pronounced. Additionally, our measurement of DC entrance width is a proxy for enlargement and increased T2 signal. Higher resolution volumetric or area-of-interest analysis would provide additional insight into the effects of longstanding intracranial hypertension, and may prove even more predictive of IIH. Similarly, the absence of a detectable DC in the majority of our control patients makes it difficult to compare true length measurements—this is again limited by the sensitivity of MRI scanning protocols used in the clinical setting. Despite these shortcomings, this study represents the first to distinguish enlargement of DC as a marker for increased ICP in the setting of IIH.

Conclusion

IIH is a poorly understood but serious condition commonly associated with spontaneous CSF rhinorrhea and elevated ICP. Invasive and costly methods of diagnosis remain the norm, thus calling for new, more cost-effective, and less-invasive screening procedures. This study is the first to demonstrate enlargement of DC as a surrogate marker for increased ICP in the IIH population. Further studies are required to confirm these novel observations, and incorporation of multiple imaging findings into a predictive model may someday offer a noninvasive option for diagnosis of IIH.