Graphical Abstract
Abstract
BACKGROUND AND PURPOSE: Spontaneous intracranial hypotension (SIH) can be caused by CSF-venous fistulas (CVFs), which often require a specialized lateral decubitus examination such as digital subtraction myelography (DSM) for diagnosis. DSM interpretations can be confounded by irregular nerve sheath diverticula at the cervicothoracic junction, potentially mimicking a true CVF. This study aimed to characterize anatomic variations of nerve sheaths at the cervicothoracic junction, in an effort to reduce the risk of misdiagnosis.
MATERIALS AND METHODS: We retrospectively identified 35 patients with low-risk Bern scores who were negative for CVF on DSM. Nerve sheaths at C6–C7, C7–T1, and T1–T2 were classified as normal (<5 mm), elongated linear (≥5 mm), linear-bulbous, linear-branching, or diverticulum. Results were obtained on both the left and right side for each patient.
RESULTS: Data were obtained for 34 patients. Among these, 74% (25/34) demonstrated at least 1 variant nerve sheath configuration. The most common site of variation was C7–T1 on the right (seen in 55%, 18/33), and the most frequent morphologic variant overall was an elongated linear sheath (28/198 levels; 40% of all variants).
CONCLUSIONS: Nerve sheath morphology at the cervicothoracic junction is frequently irregular, and these variants can resemble a CVF on DSM. Recognizing such normal anatomic variations is essential to avoid unwarranted interventions for suspected CVF in patients evaluated for SIH.
ABBREVIATIONS:
- CVF
- CSF-venous fistula
- DSM
- digital subtraction myelography
- SIH
- spontaneous intracranial hypotension
SUMMARY
PREVIOUS LITERATURE:
Prior studies have described the use of digital subtraction myelography (DSM) for the identification of CSF-venous fistulas (CVF). As more centers start to perform these procedures, it’s important for proceduralists to be aware of myelographic pitfalls that can mimic CVF.
KEY FINDINGS:
We found that 74% of patients who underwent DSM, without CVF, had irregular nerve sheaths at the cervicothoracic junction.
KNOWLEDGE ADVANCEMENT:
The irregular appearance of nerve sheaths at the cervicothoracic junction are a typical finding and should not be confused for a CVF.
Spontaneous intracranial hypotension (SIH) is a potentially debilitating disease requiring extensive work-up and cost, not only financial, but that of time, energy, and effort on both the part of the patient and the health care providers.1,2 Though cost can vary widely, the basics of work-up can include an MRI of the brain and likely spine, lumbar puncture, myelography, and potentially epidural blood patch,3 each of which result in a substantial time, financial, and mental-health burden. A cohort of 64 patients studied in the United Kingdom found that more than one-half of patients had to amend work duties due to SIH, two-thirds of patients reported that their condition affected their financial health, and more than one-quarter of patients reported losing their job.4 Furthermore, in a cohort of 234 patients with SIH, nearly one-half scored within the moderate depression range or worse on a Patient Health Questionnaire-9 patient depression questionnaire, with outcomes comparable with the general population after successful treatment.5
While headaches with an orthostatic or exertional component are the most common presenting symptoms, the involvement of less frequent or atypical symptoms poses a diagnostic challenge to suggest SIH. Patients can receive an inaccurate diagnosis (commonly migraine disorder), with true diagnosis being delayed by up to 13 months on average.3 Conventional brain MRI can detect sagging of the brain, dural thickening/enhancement, and venous distension to suggest SIH, but upwards of 19% of patients with SIH have a normal brain MRI despite presence of CSF leak.6
SIH almost always occurs from a spinal CSF leak.7 CSF-venous fistulas (CVFs) are becoming an increasingly prevalent recognized cause of SIH, especially in the absence of a longitudinally extensive extradural collection. CVFs are now believed to be the most common cause of SIH.8 Unfortunately, CVFs are occult on conventional CT and MRI and require lateral decubitus digital subtraction myelography (DSM) or CT myelography for diagnosis.9⇓⇓-12 Myelography imaging technique can also play a crucial role in visualization of CVF. In a study comparing DSM performed in the lateral decubitus versus prone positions, CVF was identified in 74% of patients imaged in the lateral decubitus position, as opposed to only 15% of patients imaged in the prone position.13 Furthermore, given the lack of sensitivity from the current diagnostic tools to diagnose SIH, not all patients who undergo lateral decubitus myelography are found to have a spinal CSF leak. More importantly, as more centers begin to perform lateral decubitus myelography for SIH,8 there is an increasing need to recognize pitfalls and potential CVF fake outs.
Though no exact false-positive rate regarding CVF seen on DSM has yet been reported in the literature, it is not unreasonable that in a subset of patients for whom CVF treatments are ineffective, there might actually have been a false-positive CVF seen on imaging. This is an especially important consideration at the cervicothoracic junction, where variation in appearance of nerve sheaths is a common finding, and could be mistaken for a CVF.
Under myelography, CVF presents as linear branching opacification, most often near the neural foramen, and often intermittent.14 We have found that at the cervicothoracic junction, nerve sheaths can take on a linear, elongated configuration. Anecdotally, we have seen cases of these irregular nerve sheaths erroneously reported as CVF. As such, the purpose of our work is to present illustrative cases of the types of anatomic variation in appearance of nonfistulous nerve root sheaths at the cervicothoracic junction. Our hope is that by defining normal anatomic variation, we can lessen the misdiagnosis and mistreatment in the already incredibly complex entity of SIH.
MATERIALS AND METHODS
This study was performed following approval by the local institutional review board. A retrospective review was performed of patients selected from an internal database with a low risk Bern score1 (Bern score mean 1.36, SD 0.8, range 0–2) and a DSM that did not find a spinal CSF leak. Patients were determined to be negative for CVF by 2 independent interpretations by neuroradiologists who specialize in SIH. Nerve sheaths at C6–C7, C7–T1, and T1–T2 on both the left and right were categorized as either normal (nerve sheath measurement of <5 mm on the AP view), elongated linear (nerve sheath measurement of ≥5 mm), linear then bulbous, linear then branching, or diverticulum (no linear component) as demonstrated in Fig 1. Data were collected by a total of 4 subspecialty trained neuroradiologists and 1 neuroradiology fellow. Patient images were excluded if they included a nondiagnostic evaluation due to incomplete intrathecal contrast, the level was not within the field of view, or a meningocele. Basic patient demographic data including age and sex were recorded.
Appearance of nerve sheaths at the cervicothoracic junction. Normal, defined as sheath length of 5 mm or less. Elongate linear, defined as sheath length of greater than 5 mm. Linear then bulbous, defined by elongated sheath with distal outpouching. Linear then branching, defined as elongated sheaths with distal branching components. Diverticulum, defined by focal outpouching of the nerve sheath without a proximal linear or elongated component.
DSM Technique
Typically, DSM took place over 2 days, in which patients were first imaged in the right lateral decubitus position and subsequently in the left lateral decubitus position.10 Patients were positioned with a cushion under their hip in an effort to achieve gradual downward angulation from their lumbar to cervical spine to facilitate flow of contrast within the intrathecal space. A 20- or 22-gauge spinal needle was used to access the thecal sac at L2–L3 or below by using sterile technique, and intrathecal positioning was confirmed by administration of 0.5 mL Iohexol (Omnipaque 300; GE Healthcare). In most cases, 5–10 mL of sterile saline was subsequently slowly infused into the thecal sac to allow for pressurization, followed by hand injection of 5–6 mL Omnipaque 300. Fluoroscopic imaging was first obtained from the lower cervical spine as the superior-most extent of imaging; the inferior-most extent was determined by patient positioning and body habitus. Following this, a second bolus of 5 mL Omnipaque 300 was injected while imaging from the needle access site as the inferior-most extent of the imaging field. The connecting tubing was then removed. The stylet was returned to the bore of the spinal needle and the system was then removed.
RESULTS
Thirty-five patients were reviewed. The mean age was 52 years (range, 26–79). Twenty-two patients (62.89%) were women. One patient was excluded due to insufficient intrathecal contrast caused by S-shaped thoracolumbar scoliotic curvature limiting flow of contrast, despite attempts of repositioning with pillow placement and table tilt. Five total levels among 3 patients were excluded due to not being within the field of view. One level in 1 patient was excluded due to a meningocele.
Data were analyzed from a total of 34 patients, at a total of 198 nerve sheath levels. In total, 70 levels (35%) had variant anatomy. Twenty-five patients (73.5%) demonstrated variant (non-normal) anatomy at 1 or more levels. Most commonly, variant anatomy was noted at C7–T1 on the right, with 55% (18/33) of patients demonstrating variation at this level (Table). The most common type of variant anatomy observed was an elongated linear nerve sheath, occurring at 14% (28/198) of levels, and representing 40% of variant anatomy (Fig 2, Table).
The observation rate of each specific variant type of nerve sheath at the cervicothoracic junction.
Rate of normal versus variant anatomy by site
DISCUSSION
This study evaluated a common imaging pitfall on DSM, that of irregular nerve sheath diverticula at the cervicothoracic junction. The results found that 74% of patients have variation in appearance of 1 or more nerve sheaths at the cervicothoracic junction. These findings indicate that radiologists interpreting such examinations should be aware of this potential pitfall and not mistake such findings for CVFs (Fig 3).
Variations in imaging appearance of variant, nonfistulous nerve sheaths at the cervicothoracic junction on myelography. A and B, Normal, smooth layering contrast within the thecal sac without prominent nerve sheath diverticula. C, Elongated linear and linear bulbous type nerve sheaths. D, Elongated linear and elongated branching nerve sheaths. E, Diverticula and elongated linear nerve sheaths.
To the best of our knowledge, this is the first study to directly assess irregular nerve root sleeves at these sites. Previously, Kranz et al15 evaluated patients for thoracic and lumbar spinal diverticula and found that 68% of patients with SIH had diverticula compared with 44% of normal controls. Our study evaluating the cervicothoracic junction found that only 17% of patients had bulbous diverticula but, including all irregular nerve sheath types, there was an incidence of 74%.
Variation in the appearance of nerve sheaths at the cervicothoracic junction may be due to variation in meningeal encasement of the exiting dorsal and ventral nerve roots at this level. In analysis of cadaveric spinal nerve roots, it has been noted that in the cervical spine, the dorsal and ventral nerve roots exit through separate dural ostia. However, in the thoracic spine these ostia tend to be closer together, with a thicker and more elongated perineural sheath (Fig 4).16 There is much variation in cell layers contributing to coverage of nerve roots as they leave the thecal sac, and variations at the cervicothoracic junction may contribute to differences in imaging appearance on DSM.
Appearance of exiting nerve roots at the cervical and thoracic levels. Cross-sectional drawings depict the relationship of meningeal layers and connective tissues surrounding the exiting nerve root. In the cervical spine, the dorsal and ventral nerve roots exit through separate dural ostia.
On DSM, a true CVF appears as a filling linear, sometimes intermittent,14 branching opacification at the level of a contrast-opacified nerve root sheath (Fig 5). The fistula often appears as a “blush” of contrast, first slowly appearing, and then gradually draining away as the nerve sheath empties of contrast. Often, the associated nerve root has a diverticulum type morphology, with the fistulous connecting commonly arising from the diverticulum itself.17 Additionally, true CVFs can demonstrate pulsatility with respiration, with more robust opacification during inspiration.18,19
Examples of true CVFs at the cervicothoracic junction. A, A 61-year-old woman with linear bulbous diverticula at C7–T1 and T1–T2. Linear opacification arising from the thecal sac proper represents a true CSF-venous fistula at both levels. B, A 62-year-old woman with linear bulbous nerve sheath anatomy at C7–T1. Adjacent linear, branching opacification arising from the nerve sheath neck represents a true CSF-venous fistula (arrow).
Once appropriately identified, treatment options for CVFs include embolization,20 surgery,21 or fibrin glue injection,22 each of which report high success rates. Incorrectly diagnosing a patient with a CVF can lead to unnecessary treatment. This burden is known by CSF leak centers but has not yet been well captured in the literature.
Our study has limitations, first being that we cannot prove that patients in our study did not have a true CVF. No criteria currently exist to define the imaging characteristics of a true CVF on DSM, however the irregular cervicothoracic nerve sheaths are important pitfalls that DSM practitioners should be aware of. We did select patients with low-risk brain MRI, who had DSM performed and interpreted by neuroradiologists who specialize in SIH, and had the images read by 2 people to confirm agreement on the lack of CVF. Given that our data are from a single institution, this also could limit the generalizability to other centers. Finally, there is some potential overlap in the imaging appearance of linear then branching and linear then bulbous. Combined, this incidence of these 2 types is 43%. Nevertheless, they are still a subtype of irregular nerve sheath at the cervicothoracic junction.
CONCLUSIONS
There is a wide range of anatomic variation at the cervicothoracic junction, where nerve sheaths on DSM can take on elongated linear, linear then bulbous, linear then branching, or diverticular configurations. It is important to recognize these normal anatomic variants that occur in most patients, and to recognize that they do not represent true CVF. As utilization of DSM for the diagnosis of CVF continues to evolve and be adopted by more centers, it is ever more important to understand potential imaging pitfalls associated with work-up, so we can better treat our patients and minimize nonefficacious procedures.
Footnotes
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References
- Received March 25, 2025.
- Accepted after revision June 13, 2025.
- © 2025 by American Journal of Neuroradiology
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