Spontaneous Intracranial Hypotension: A Systematic Imaging Approach for CSF Leak Localization and Management Based on MRI and Digital Subtraction Myelography

Discussion

Until recently, the cause of SIH had been unclear, and several etiologies had been postulated.¹⁰ With evidence presented here as well as that provided by other authors,^{1,2,5,11,17⇓–19} it now appears that these leaks are predominantly, if not exclusively, due to mechanical tears, CVFs, and leaking nerve root sleeves. For guidance of management and efficiency of discussion, we have categorized the CSF leaks we have encountered into 4 types based on the morphology and distance from the midline. This is similar to a classification scheme previously proposed by Schievink et al.⁵

Type 1 is caused by degenerative disc disease creating a mechanical tear in the ventral dura. Patients with these ventral dural punctures are well-recognized as representing a large segment of patients with SIH (48% in this consecutive series). All patients with type 1 are SLEC-P on MR imaging with or without intracranial findings and show a typical ventral hole in the thecal sac on DSM or dynamic CT myelography.^5,11,20 The site of CSF leakage is occasionally seen associated with a calcific “microspur” at the site of a degenerated disc.^11,21 Unless imaged in a dynamic fashion so that the initial flow of contrast agent is captured as it travels down the spine, the site of leakage will be obscured by mixing of contrast in the intradural and extradural compartments above and below the hole.

All type 2 CSF leaks were SLEC-P as well. A type 2 leak due to a more lateral tear in the dura was a less common cause of SLEC-P CSF leakage in our series. The SLECs associated with a type 2 leak may tend to be positioned more in the neural foramina and are not associated with degenerative disc disease. These leaks are thought to arise from the base of the nerve root relating to a pre-existing thinned or dehiscent area of the dura.¹¹ In our experience, these lateral dural dehiscent lesions are more commonly seen in women (4 of 4 in our series) and appear to be more amenable to treatment with multiple EBPs and bedrest, possibly due to the nature of the tear and it not being held open by a disc protrusion or substantiated by continued flow as in a CVF. This lesion was seen in 4 of 31 patients (13%).

The recent discovery of a CVF by Schievink et al¹⁸ provided an extraordinary step forward in the care of patients with SIH. We postulate that the dural defect initiating the formation of a CVF occurs more distally along the nerve root sleeve adjacent to an arachnoid granulation or perhaps where the nerve root sleeve is in close proximity to a venous plexus. In this environment, the extravasated fluid is more likely to find its way into a venous channel. We propose that it is in this manner that an embryonic pseudomeningocele, masquerading as a diverticulum, first insinuates into the venous system and heals, aberrantly creating a 1-way CVF, which we categorize as a type 3 CSF leak (Fig 4). This postulation also accounts for these CVFs being commonly but not invariably seen next to what appears to be a diverticulum. Once established, a CVF will not respond to nondirected EBP because it lies well beyond the epidural compartment. Whether it responds to directed regional EBP remains to be better defined. CT-guided focal fibrin patching may have some success in the hands of other authors; however, in our experience, it has not provided a durable cure. Surgical ligation of a CVF as described by Schievink et al,^5,22 in our experience, is the most efficacious treatment of these lesions.

The least common form of CSF leak we encountered in our series we have termed a type 4 leak. This results from a distal nerve root sleeve leak that does not fistulize into the venous system but rather tracks into and dissipates into the adjacent facial planes (Fig 4). A type 4 leak, because of its distal location, will present as a SLEC-N leak and will not be amenable to EBP. Whether this can be treated with CT-guided focal fibrin patching remains to be defined.

Our observations regarding the association of SLECs with differing types of CSF leaks is similar to experiences of other authors. Dobrocky et al¹¹ used DSM as well as conventional and dynamic CT myelography to locate the site of leakage in 14 patients. Their study included only patients with SLECs; thus, the authors did not identify any patients with CVFs in their series. Conversely, Schievink et al⁵ recently reported 14 cases of CVF, none of which were associated with SLEC. Kranz et al¹⁹ have reported their success in using dynamic CT myelography and DSM to identify 22 cases of CVF. In that study, the authors also noted that CVFs “often occur without epidural leak.” The 7 cases of CVF in our series were all seen in the thoracic region, were not associated with SLEC, were all paravertebral extending laterally from the neural foramen, were inconsistently associated with small diverticula (3 of 7), and, in our experience, were more prevalent on the left (5 of 7). These findings are similar to recently reported experience with CVF.^18,19,22

The relatively high prevalence of CVF (type 3 leak) in our series presented here is most likely due to the use of lateral decubitus DSM. Lateral DSM optimizes the visualization of the CVF by flooding the lateral gutters of the thecal sac with high-density contrast agent and profiling those gutters with direct frontal high-spatial and high-temporal-resolution myelography. Prone DSM is not optimal for detecting these laterally located CVFs and will commonly result in a false-negative DSM. Spinal MR imaging identified all of these patients as SLEC-N; thus, all were initially in the decubitus position for DSM, substantially increasing the yield of the DSM to identify the leak. This same strategy can no doubt be applied to dynamic CT myelography at those centers where that technique is favored over DSM.

Despite the foregoing, there still are a small number of patients who continue to have SIH in whom a CSF leak cannot be localized. There were 2 such patients in this report. Both patients were negative for SLEC, and both continue to show intracranial MR imaging stigmata. We suspect that these patients have a type 3 or perhaps type 4 leak, which our DSMs failed to identify.

A major question in the SIH arena is what to offer the rapidly growing population of patients with headache with brain and spine MR imaging negative for stigmata of CSF leak in whom a possible diagnosis of SIH continues to be considered. Such patients may still harbor a CSF leak despite normal findings on CSF manometry.^1,2,23 In this report, 3 symptomatic patients positive for SLEC with no intracranial findings underwent DSM (mandated by their SLEC) and were proved to have a leak. Therefore, we must allow the corollary to be true—that is, there must be a percentage of patients negative for SLEC (types 3 and 4) who are leaking slowly and thus not attaining the threshold to display intracranial MR imaging stigmata but are still nonetheless leaking and having symptoms. MR imaging, like CSF manometry, is not an adequate screening test for this disease. Moreover, as awareness of the syndrome of SIH increases the population of headache patients inferred to have a positional quality to their headache will undoubtedly grow while the low prevalence a true CSF leak in this population will further decline. A better screening test for SIH is needed. It is inefficient to offer exhaustive DSM with its discomfort, mildly invasive nature, radiation dose, and lack of disseminated expertise to this population of patients as the screening test for SIH.

There is clearly a role for EBP in patients positive for SLEC with SIH. Nine of 21 (43%) of all patients positive for SLEC were treated successfully with EBP (nondirected and directed) and did not require an operation. Determining early which patients positive for SLEC will do well with nondirected-versus-directed EBP and how many times EBP should be attempted before surgical referral is an area for further investigation. Conversely, in this report, 100% of the 10 patients negative for SLEC failed all forms of EBP. This observation is not surprising given the fact that all patients negative for SLEC were shown to have a leak that starts lateral to and beyond the confines of the spinal canal and thus predisposes an EBP to failure.

Once again, the strategy of using MR imaging to divide patients with SIH into SLEC-P and SLEC-N is prescriptive in that it also appears to foreshadow which patients may benefit from EBP before prone DSM and those who should go on to lateral decubitus DSM without further delay.

The limitations of this current report are obvious in that it represents the experience of essentially 1 neuroradiologist at 1 large neurosurgical referral center. A strong selection bias exists because all patients in this study had MR imaging findings of SIH. Despite an increasing awareness of SIH, the syndrome remains relatively uncommon, resulting in a limited number of neuroradiologists, anesthesiologists, and neurosurgeons skilled in the techniques of diagnosis and treatment of these patients. Dissemination of the technique of DSM as well as dynamic CT myelography, pooling of data across multiple centers, and development of a less invasive screening test for SIH would, no doubt, lead to a more efficient diagnosis and effective treatment of these patients.