Idiopathic Thoracic Spinal Cord Herniation: Retrospective Analysis Supporting a Mechanism of Diskogenic Dural Injury and Subsequent Tamponade

Discussion

In this study we have addressed the pathogenesis of idiopathic tSCH through retrospective analysis. Coupled with this analysis is the description of 1 of our patients with MR imaging and surgically documented tSCH in whom severe postural headache, the cardinal symptom of SIH, resolved before she developed progressive myelopathy and imaging findings consistent with tSCH. The results of the retrospective analysis support the proposed pathogenesis that unrecognized trauma by herniated disk material or endplate osteophytes to the anterior dural surface in the thoracic spine leads to eventual herniation of the thoracic spinal cord through a dural defect. The report illustrates this pathogenesis and suggests a link to SIH.

Our analysis is consistent with the theory that a herniated thoracic disk or osteophyte pierces, lacerates, or progressively erodes the anterior thoracic dura. This defect permits CSF extravasation, which may resolve with time, to continue if untreated and result in SIH, or become tamponaded by the spinal cord. In greater than two-thirds of cases we reviewed, the apex of the spinal cord herniation point occurred at a disk level. We identified disk osteophytes in nearly one-third of cases, despite the fact that nearly all analyzed cases published only a single cross-section (at the point of herniation) of the rostrocaudal length of the thoracic spinal column.

The apparent inconsistency between the rate of disk level herniation and the rate of disk osteophytes can be attributed to significant motion of the spinal cord dura relative to the spinal column during flexion and extension movements. Thus, it is likely that the structure puncturing and/or lacerating the dural surface lies rostral or caudal to the site of the published cross-sectional image. This implies that the presence of disk osteophytes in the reviewed cases may be significantly under-represented. Assessment of extradural fluid was similarly limited by the amount of cross-sectional imaging, yet it too was present in almost one-third of cases, suggesting that extravasation of CSF through and/or around the dural defect continues despite the herniation of the spinal cord. If additional cross-sectional imaging immediately above and below the herniation point had been available, we presume that the rate of this finding would have been significantly greater.

The upper thoracic cord is a likely place for the anterior dural surface to be breached by calcified disk remnants because it is the spinal cord region most closely apposed to the posterior surface of the vertebral bodies and intervertebral disks. Barbagallo et al¹¹ reported that spinal cord herniation has never been reported at other spinal regions unless there was a history of significant trauma or iatrogenic injury. The relative positions of the spinal cord and dura change with flexion and extension of the spinal column. Such movement may permit osteophytes to contact and lacerate the dura. CSF may then exit the dura and migrate rostrally or caudally before the spinal cord herniation tamponades the defect. Consistent with the proposed pathogenesis, evidence of an HNP at the level of the thoracic vertebrae was present in >30% of cases we reviewed. Asymptomatic thoracic disk herniation has an estimated incidence of 11.1%–13.3%,¹⁷ whereas disk herniation at this level producing symptoms has an overall incidence of 1 per 1 000 000 patient-years.¹⁸ Most interesting, thoracic disk herniation has a high degree of calcification of herniated disk elements, and the herniations occur almost exclusively in the ventral or ventrolateral direction.¹⁸ Given the incidence of asymptomatic herniations at this level, we suspect that the recorded percentage of HNP would be significantly greater if additional cross-sectional imaging was available.

Our case report suggests our proposed pathogenesis of tSCH and indicates a possible connection between tSCH and SIH. The patient presented developed a progressive myelopathy that, as others have reported, develops in the absence of known trauma or spinal manipulation. However, our patient reported a severe but transient postural headache that spontaneously resolved years before the development of myelopathy. Severe postural headache is 1 of the earliest and most prominent symptoms of intracranial hypotension, a disorder that occurs following therapeutic or traumatic dural perforation and subsequent leakage of CSF.¹⁹ In a small portion of cases, the symptoms develop spontaneously without known provocation; the primary defect is the loss of CSF volume through spinal CSF leaks.²⁰

From the proposed pathogenesis of tSCH and the case reported, we hypothesize that the tamponade of a dural defect by the spinal cord prevents or ameliorates the development of SIH. Once the dural defect is created, extravasation of CSF may continue until the fistula closes spontaneously or is closed by surrounding tissue or by a primary dural repair. Given CSF flow patterns, the overlying arachnoid may develop adhesions with the dural defect.⁷ Subsequently, the spinal cord or nerve roots may tamponade the dural defect and prevent further CSF outflow. If closure of the defect does not occur, continued loss of CSF volume may be sufficient to result in intracranial hypotension and the associated postural headache. By proposing a potential connection between tSCH and SIH, we hope not only to better understand their pathogenesis but also to increase clinical recognition of these syndromes.

During our review of previously published cases of tSCH, we found only 1 other mention of postural headache in the clinical history.⁶ An explanation of this absence is that headache in the setting of tSCH may have been regarded by authors as a nonspecific symptom with no association to the primary disorder. Conversely, this may reflect a lack of inquiry about this symptom or a delay between the resolution of headache and the development of SIH. As with our patient, the time between the SIH symptoms and tSCH-related myelopathy may be long enough that the patient fails to recall it or the physician fails to elicit the history. Indeed, only after close questioning did our patient describe postural headaches 20 years preceding the development of myelopathy due to thoracic cord herniation. This patient illustrates the temporal dissociation between resolution of the headache and the often insidious onset of the myelopathy secondary to spinal cord herniation. If our proposed pathogenesis of tSCH is correct, unreported headache in prior cases may also simply reflect the transient nature of headache as a symptom.

Alternative explanations for the pathogenesis of tSCH rely on congenital malformations of the spinal meninges. Duplication of dura²¹ and arachnoid cysts have been suggested in several cases as the cause of cord herniation and strangulation. If correct, one might expect these malformations to be scattered throughout the rostrocaudal extent of the spinal cord and to occur in all age groups. However, our review of the published literature agrees with the assertion by others¹¹ that there are no reports of idiopathic cord herniation at other spinal levels. In a recently published case report, CSF kinetic studies excluded the possibility of a posterior arachnoid cyst.²² This finding suggests that the presence of an “arachnoid cyst” identified on static imaging modalities may be an incorrect interpretation of enlarged posterior CSF spaces due to ventral tenting of the cord. Additionally, in a review by Pereira et al,²³ arachnoid cysts were present in only one-quarter of cases. These authors also reviewed several cases with evidence of meningeal inflammatory changes consistent with prior minor trauma, a pathogenesis proposed by Tronnier et al.³ The apparent restriction of cord herniation to the thoracic region thus points more strongly toward mechanical and anatomic factors specific to the thoracic cord.

Therapeutic implications of an identified mechanism for tSCH are significant. If tSCH develops according the proposed pathogenesis, then the symptom of postural headache that resolves spontaneously may serve as a harbinger for later development of severe focal neurologic sequelae. As noted in the case presented above, the SIH-like syndrome may precede the development of progressive myelopathy. The link to SIH, an infrequent but not rare disorder,¹⁹ implies that SIH and tSCH represent 2 different sequelae of a spontaneous breach of the spinal dura. In many patients, dural punctures or lacerations can be readily treated with a minimally invasive epidural blood patch, a procedure that provides almost instantaneous relief.¹⁹ Because the standard treatment for tSCH is laminectomy with open neurosurgical reduction of the herniated spinal tissue and dural repair, the consequences of delayed recognition are significant to treatment and patient recovery. In addition, follow-up imaging might be considered in patients presenting with resolution of SIH secondary to thoracic ventral leaks due to osteophytes or HNP. Although myelography was formerly required to identify tSCH, the advent and widespread use of high-resolution MR imaging provides a readily available alternative with comparatively few technical requirements.