History
In 1974, Wortzman et al. [9] reported the first case of an idiopathic spinal cord hernia. White and Firth [10] proposed that segmental ischemia of the anterior spinal artery is a crucial factor that leads to clinical symptoms in patients with spinal cord hernias in 1994. Borges et al. [11] in 1995 emphasised that the lateral spinothalamic tract is prone to dysfunction and that the corticospinal tract is also involved as the disease progresses. In addition, Tekkök [12] in 2000 reviewed the literature and concluded that SCH affects all age groups; however, it usually occurs in middle-aged people, particularly women, and is located mostly at the T3-T8 level [5]. In 2005, Francis et al. [13] reviewed 11 cases of traumatic spinal cord herniation and emphasised that these patients may have previously experienced spontaneous SCH and did not exclude trauma as a factor that worsened clinical symptoms. In 2020, some scholars found that the spinal cord gradually herniated from the dura for at least 3 years and up to 16 years of follow-up [14].
Pathogenesis
The mechanism of action of anterior spinal cord herniation (ASCH) is yet to be identified. Although some scholars have mentioned that congenital factors lead to ASCH, they have not been discussed in detail in the literature [5, 14, 15]. In the early literature, most scholars found that arachnoid cysts were inseparable from ASCH [16-19], and many studies only reported no obvious cysts. However, they observed thickened arachnoid membranes during surgery [10, 12, 20]. In contrast, Sioutos et al. [21] found that patients who underwent arachnoid cystectomy had no significant effect. Schultz et al. [22] argued that cases of ASCH were misattributed to the dorsal arachnoid ‘cyst’, leading to the ventral displacement of the spinal cord, which was an enlarged subarachnoid space. Since defects are most common in the thoracic spine, some scholars hypothesise that this may be because the spinal cord is closer to the ventral spinal canal at this level than the cervical or lumbar spine [4, 23]. In addition, spinal cord herniation at the thoracic spine level may be closely related to cardiopulmonary exercise. Continuous exercise of the heart and lungs may cause tissue adhesion or inflammatory reactions around the thoracic spinal cord, eventually resulting in progressive pathological changes in the thoracic spinal cord [24]. The specific mechanism of ASCH is yet to be identified; however, since the cerebrospinal fluid flowing through the defect can separate the two dura mater, a potential negative pressure space is formed between the SCH and the dura mater, which eventually leads to herniation of the spinal cord. Japanese scholars are more inclined to the theory of duplicated dura mater [25-27]; this situation has also been confirmed in cadavers [28]. Since part of the SCH occurred at the intervertebral disc between the upper and lower vertebral bodies, after careful study, it has been found that the herniation and calcification of the intervertebral disc adjacent to the dura mater may lead to thinning and erosion of the dura mater, particularly for larger calcified intervertebral discs, and eventually lead to damage or rupture of the dura [29, 30]. The transdural herniated disk fragment found in their case by Randhawa et al. [31] convinced them of this theory. Based on their rare case, Vanden et al. [32] speculated that the lateral SCH was caused by the traction of the dentate ligament connecting the dura mater to the spinal cord. Kizilay et al. [33] suspected ASCH in a patient after corpectomy and ossified posterior longitudinal ligament resection, and they suspected that this was a contributing factor to the occurrence of ASCH. However, Bartels et al. [34] disagreed that acquired factors cause ASCH. This is because the adult spinal cord has little potential to move. Therefore, the response of the spinal cord to injury is sensitive. The causes of ASCH, including intervertebral disc herniation and calcification, cannot explain the lack of obvious symptoms before the onset in some patients. Moreover, the trauma that can lead to dural defects should have a high-energy impact, with severe neurological impairment, and these conditions are challenging to detect clinically.
Imaging features
The ASCH image was simple. Sagittal MRI generally reveals that the spinal cord is close to the ventral or ventrolateral dura, accompanied by a decrease in the ventral subarachnoid space and a larger dorsal subarachnoid space. Imaging findings, including vertebral body depression, intervertebral disc herniation, spinal cord atrophy, and ossification of the posterior longitudinal ligament, were occasionally observed, and axial MRI showed anterior or lateral protrusion of the spinal cord and a large subarachnoid space [35]. Furthermore, axial computed tomography clearly showed spinal cord deformation, degree of protrusion, and epidural arachnoid cysts [2]. According to the literature, no filling defect exists on the spinal cord’s dorsal side, and cerebrospinal fluid circulation is normal [36]. However, some researchers have found that the cerebrospinal fluid dynamics in arachnoid cysts are abnormal [37]. In 2009, Imagama et al. [15] were the first to classify patients with ASCH using MRI. In sagittal MRI, they identified the following three types of SCH at the level of the thoracic spine: type K, which has a prominent ventral spinal cord kink; type D, which is characterised by the complete disappearance of the spinal cord at the prominence; and type P, which has the anterior subarachnoid space on the ventral side of the thoracic cord and no spinal cord kinks. Based on axial MRI, SCH is further classified into the following types: central (C-type) and lateral (L-type).
Clinical manifestation
The clinical presentation of Brown-Séquard syndrome is the most documented clinical manifestation in the literature. Other symptoms include gait, sphincter, sexual dysfunction, progressive paraplegia, and sensory loss [38]. Lower extremity numbness is the most common initial symptom, which then progresses gradually [39].
Surgical management
Currently, surgery appears to be the only method to reverse or prevent neurological deterioration in patients with TASCH, and even those with a long medical history may benefit from surgery. However, some authors have emphasised the importance of early surgical treatment to improve the chances of a positive outcome [40-42]. Therefore, surgery aims to release the spinal cord and restore it to its normal anatomical position. Disease recurrence is prevented by closing the dural defect with direct sutures and filling it with a graft [43, 44]. However, the best surgical procedure is yet to be adopted. Currently, the following three surgical techniques are commonly used: closure of dural defects using sutures [18, 45], repair of defects using dural grafts [20, 46, 47], and widening of the dural defects [48, 49]. Tekkök [12] disagreed with the use of sutures to close dural defects, arguing that there is little space available for suturing below the ventral side of the spinal cord and that attempting such a procedure may increase the risk of neurological deterioration. Akutsu et al. [27] recommended using the Hammock method to facilitate the repair of dural defects, in which a triangular rubber surgical glove sleeve is inserted ventrally into the spinal cord and slid without pulling the spinal cord. On the opposite side, a ventral patch of polytetrafluoroethylene was inserted into the glove. Subsequently, the spinal cord was gently lifted with rubber gloves to confirm that the material had completely covered the dura mater defect before repair. They believed such an operation could reduce spinal cord injury to a greater extent, and clear vision was conducive to repairing the dura mater.
Nakamura et al. [26] proposed widening the dural defect, which is technically easier and safer than suturing and repair. They demonstrated the effectiveness of this approach at a follow-up of approximately 9.6 years. Watanabe et al. [39] used curved scissors to widen the dural defect and observed no recurrence during follow-up. Delgado-López et al. [50] found that excessive tension between defect edges could lead to suture loosening, necessitating the innovative use of special vascular clips to complete this procedure. Furthermore, Kasliwal et al. [51] used the paramedian approach of laminectomy and unilateral pedicle resection for the first time in a patient with a spinal cord herniation. They assumed that this approach was beneficial for the direct visualisation of the dural defect and prevention of recurrence. Simultaneously, damage to the spinal cord can be minimised. Herring et al. [6] applied this procedure to the clinic more skilfully and confirmed that the effect was improved after approximately 2 years of follow-up. Initially, a few scholars attempted to release the spinal cord and dura through the thoracic approach under direct microscopy and then sutured the dura [9, 11]. However, Borges et al. [11] disagreed with this surgical plan and believed that the lesion should be approached by posterolateral laminectomy. Although the angle of view is not as good as that of the anterior approach, arachnoid cysts can be treated concurrently.
Outcome
Most patients with anterior spinal cord hernias experience progressively worsening symptoms and eventually require surgery. The mean surgical interval was 5.2 years [5]. Surgical treatment of SCH generally stabilises or improves neurological symptoms. However, poor post-operative outcomes have been reported [52], with 75.9% of patients with Brown-Sequard syndrome improving after surgery compared with only 55% improvement in patients with paraplegia. In most cases, long-term post-operative follow-up revealed that the patient was in good and stable condition without recurrence of ASCH [26]. Only an insignificant percentage of patients may experience slight improvement [53]. Generally, those with isolated lower-extremity paraplegia appear to have a better prognosis than those with Brown-Sequard syndrome [12, 26]. A recent review by Summers et al. [54] revealed that 74% (119/159) of patients who underwent surgery improved significantly in post-operative clinical symptoms. Only 18% of patients experienced no improvement in any symptoms, and 8% experienced worsening post-operative examination results. Borges et al. [11] reported a case of a young woman who underwent successful surgery but relapsed 3 years later, implying that dural transplantation did not prevent relapse. During the follow-up, Gaudino et al. [55] found that three patients experienced neurological deterioration, such as headache and hearing loss, possibly due to pre-operative superficial siderosis. According to Regensburger et al. [14], the 6-month electrophysiological results of post-operative patients showed no significant improvement in MEPs; however, SEPs recovered.
Most patients who visited our hospital were middle-aged, and their clinical manifestations were similar to those previously reported. However, perhaps because the disease is relatively rare, our findings were inconsistent with those of previous studies in those patients with ASCH at our hospital who were primarily male. Hence, the symptoms and prognosis differed partly, which may be due to the different sites of the dural defect, degree of spinal cord herniation, and length of the disease course.
In contrast to the paramedian approach, which requires pediectomy and partial cost-transversectomy, the surgery was performed using the more widely accepted posterior method [56-59], and we considered laminectomy or extended hemisection.
Laminectomy is less disruptive to spinal stability, mainly when the dural defect is anterolateral. The posterior approach has sufficient space to mobilise the spinal cord, and no significant complications are associated with this approach. Therefore, it has been used in all 7 patients in this hospital over the past decade. In the early stages of the study, dorsal arachnoid cysts tended to cause the disease. However, we only observed that the dorsal arachnoid membrane of some patients was bulging during the operation and was filled with clear cerebrospinal fluid. In most other patients, the arachnoid membrane is similar to that in normal people, and only the enlarged subarachnoid space and the spinal cord herniated forward can be detected. Therefore, the pathogenesis of ASCH requires further investigation. Suturing the defect under direct vision is difficult due to the involvement of the dura mater that needs to be repaired in the ventral defect and the occlusion of the spinal cord. It inevitably requires the spinal cord to be stretched; however, in our experience, adjusting the microscope angle and gently returning the compressed spinal cord as much as possible to reduce spinal cord haemorrhage is more beneficial to the patient’s prognosis. Regarding dural repair, we preferred to use an artificial dura with a larger area than the defect for a watertight suture, which we believed was easy and fast and reduced the risk of disease recurrence. Bone wax is generally used to repair vertebral body defects because it has strong plasticity, can stop bleeding, and is inexpensive and easy to obtain. It has been reported in the literature that it can persist in the human body for a long time [60]; therefore, we assume it is very suitable for repairing eroded vertebral bodies. Following spinal cord restoration, it is necessary to pay attention to the watertight suture of the dorsal dura, which is closely related to the occurrence of postoperative complications. A tightly sutured dura greatly reduces the risk of cerebrospinal fluid leakage, difficult wound healing, and central nervous system infection. Regarding this, we consistently applied another layer of the artificial biological dural graft to the tightly sutured epidural space to reduce the risk of cerebrospinal fluid flowing out of the suture pinhole. After the operation, one patient had severe preoperative symptoms and the disease rapidly progressed. No surgical treatment was given, as the local hospital misdiagnosed him as a space-occupying lesion in the spinal canal. Spinal hernia was severe during the operation. Immediately after the operation, the muscle strength decreased to grade 1. It is also an important reason to consider ischemia-reperfusion injury caused by the long course of the disease. Therefore, our experience in terms of operation is that if the dentate ligament also has adhesion, it should preferably be released and should be operated gently so as not to damage the herniated spinal cord. Instructing the patient to rest and move less after surgery is an essential measure for surgical success. According to the patient's recent follow-up (44th months after the operation), muscle strength partially improved (recovered to grade 3), sensory disturbance was greatly improved, and dysuria problems were almost completely alleviated. As the patient's neurological functions continue to recover, the patient will be followed up later. The main limitation of this study is the sample size, which was large considering the rarity of the disease in clinical practice. However, the sample size will continue to increase with the addition of new cases and follow-up of the existing cases in future studies.