Spontaneous Spinal CSF Leaks Stratified by Age, Body Mass Index, and Spinal Level

DISCUSSION

Our study shows that age, BMI, and spinal leak level vary by spinal CSF leak type. Patients with type 1 CSF leaks (dural tears) are typically younger (mean  age, 44.5 years) and have a normal BMI (mean, 24.3), while patients with type 3 CSF leaks (CVFs) are relatively older (mean age, 58.8 years) and have an elevated BMI (mean, 27.0). This easily obtained patient information could help provide guidance in the spinal CSF leak imaging pathway. Furthermore, type 1 CSF leaks were most common in the upper thoracic spine, and types 2 and 3 were most common in the lower thoracic spine.

Patients with type 1 CSF leaks had lower average BMIs than those patients with type 2 or 3 CSF leaks and also had lower BMIs than national averages. The type 1 patient characteristics match those in a smaller study that previously reported patient BMIs to be lower than the national average.⁸ Furthermore, a majority of type 1 CSF leaks in our study were usually secondary to calcified discs or microspurs (80%) that resulted in dural tears. The thoracic spine was the most likely location in our study (92%), and this is supported with another study.¹⁵ In our study, the upper thoracic spine at the T1–T2 level was the most common site (36%), and this high frequency of upper thoracic dural leaks was also reported in at least 1 other study.¹⁶ While thoracic disc-related leaks were observed more frequently in the upper thoracic spine, thoracic disc herniations, in general, are reported to more commonly occur in the lower thoracic spine, given that it has more mobility than the upper thoracic spine. Thoracic discs also have a high predisposition for calcification and dural tears.^17,18 We speculate that 1 possible reason that CSF leaks occur is due to the fixed biomechanics of the thoracic spine and the apposition of the dura to posterior vertebral bodies, given the normal kyphotic curvature.¹⁹ In our study, patients with type 1 CSF leaks had normal BMIs, and some patients had BMIs as low as 18. We postulate that a lower BMI may be accompanied by less spinal epidural fat, thus acting as less of a protective barrier to the calcified discs, thereby resulting in a dural tear. Another publication has also proposed the same possibility,⁸ but further studies are needed to verify this theory.

On the other hand, type 3 spinal CSF leaks generally occurred in overweight patients. It has been suggested that elevated spinal pressure or pre-existing idiopathic intracranial hypertension may be a potential etiology in some patients with CVFs, akin to the phenomenon that occurs with skull base leaks. This elevated pressure has also been postulated in the setting of de novo CVFs that occur after successful CVF treatment, whereby the elevated pressure is driving the creation of a new fistula.²⁰ Arachnoid granulations are closely related to paraspinal veins, and rupture of these granulations has been suggested as a possible inciting event of CVFs.²¹ Overweight patients have a higher susceptibility to elevated CSF pressures, possibly accounting for this arachnoid granulation rupture, and this could explain why patients with CVFs are more common in this population. While BMIs in this cohort were in the overweight category, they were slightly less compared with national averages, and we suspect that our patient population in Northern California has lower BMIs than in the rest of the country, but this information was not readily available for accurate comparison. Last, type 3 spinal leaks were most common on the right side and the lower thoracic spine. This laterality and spinal location correspond to information in another study,²² but a separate study has reported more CVFs on the left side.²¹

Type 2 spinal CSF leaks were the least common in our patient population. This finding contrasts with an older study that stated that type 2 leaks were the most common type of CSF leak.¹ In that study, it is possible that many of these type 2 leaks may have been CVFs arising from meningeal diverticula, which may not have been well-detected because decubitus myelographic techniques for CVFs were not discovered at that time.²³ In fact, CVFs were the most commonly detected CSF leak in our study, and with greater recognition of this entity, perhaps it will be the most common type of spinal leak. On the basis of the small number of patients in our study who had type 2 CSF leaks, it is difficult to make specific conclusions on age and BMI; however, the spinal levels in all 4 patients were the lower thoracic spine. This finding mirrors the most common location of CVFs, and we suspect that these 2 types of spinal leaks may have some similarities in pathogenesis.

The imaging pathway for CSF leak detection in patients with SIH varies per institution. In our spinal CSF leak program, after obtaining contrast-enhanced brain MR imaging, we perform a noncontrast total spine MR imaging with T2 fat-suppressed sequences to identify the presence or absence of an extradural collection. If there is a ventral extradural collection, we perform an ultrafast or dynamic prone CTM to identify the presumed dural tear. If the extradural collection is more eccentric on the MR imaging and there is concern for a ruptured meningeal diverticulum, we perform dynamic decubitus CTM to identify the ruptured meningeal diverticulum. If there is no extradural collection on the spine MR imaging, we perform decubitus CTM to identify the CVF.

While an imaging pathway such as this one usually identifies the specific CSF leak type and cause, determining the pretest probability of the spinal leak type based on age and BMI could potentially provide guidance in this imaging work-up. For example, type 1 spinal leaks were more common in the upper thoracic spine; therefore, an adequate Trendelenburg angle is needed when performing the dynamic myelography to ensure that the contrast flows to the upper thoracic spine. This knowledge can help minimize unnecessary scans in the lower thoracic spine. In fact, on the basis of this knowledge of type 1 leaks, we have changed our dynamic prone CTM technique. We used to inject a small volume of preservative-free iohexol contrast (Omnipaque 300; GE Healthcare) in the lumbar spine (1–2 mL) and then scan the total spine from caudal to cranial, cranial to caudal, and caudal to cranial. However, we observed that the contrast did not always traverse the upper thoracic spine, and there was often no leak in the lower spine; therefore, the patient had unnecessary radiation. Currently, we inject 3 mL of contrast with 1 acquisition of the total spine. If there is no spinal leak identified in the lower spine, we inject 2–3 mL more and then scan a more cranial part of the spine with some overlap of the normal area in 1 acquisition. We repeat this process until we identify the CSF leak, administering a maximum of 10 mL. This technique has resulted in less radiation to patients than our initial technique. Thus, understanding the pretest probability of the spinal CFS leak type and location can help with the diagnostic work-up.

The recognition of mean age and BMI in patients with spinal CSF leaks may also help when the diagnosis of SIH is equivocal. Because there is no absolute imaging test or clinical symptom to exclude the diagnosis of SIH, some patients may undergo many imaging examinations, myelograms, and treatments, even if the diagnosis of SIH and spinal CSF leak is not definitive. These demographic features of age and BMI may represent additional data points to help with decision-making in this difficult-yet-not uncommon scenario.

Our study has limitations, including its retrospective nature. Second, we did not obtain opening pressures consistently in our patients, which could help provide additional information. Nevertheless, it is known that opening pressures are normal in most patients with CSF leaks,²⁴ and this datum point may not have added noteworthy information. Last, most of our patients were from 1 geographic state, which could result in differences in body habitus. Nonetheless, our hospital is a major tertiary referral center for our multihospital network consisting of >20 hospitals over a very large area; therefore, the population did have some geographic diversity.