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Idiopathic herniation of the thoracic spinal cord

Since 1974, when Wortzman et al were the first to describe a case of idiopathic spinal cord herniation (ISCH), the number of reported cases has increased owing to magnetic resonance imaging (MRI) now is routinely available for patients with myelopathy and spinal surgeons are becoming more familiar with this clinical entity. This extremely rare herniation occurs exclusively in the thoracic spine, causing slowly progressive myelopathy. Diagnosis is based on ventral displacement of the spinal cord in the thoracic spine. MRI is the technique of choice to exclude a posterior arachnoid cyst, the most common mistaken diagnosis, and to recognize a spinal cord herniation when an anterior dural defect is present. A case of ISCH is reported and a Literature review of this clinical entity often mis-diagnosed has been obtained. (www.actabiomedica.it)


Introduction
Since 1974, when Wortzman et al were the first to describe a case of idiopathic spinal cord herniation (ISCH) (1), the concept of ISCH has gradually been appreciated, and the number of reported cases has increased owing to magnetic resonance imaging (MRI) now is routinely available for patients with myelopathy and spinal surgeons are becoming more familiar with this clinical entity. This extremely rare herniation occurs exclusively in the thoracic spine, causing slowly progressive myelopathy. Diagnosis is based on ventral displacement of the spinal cord observed on sagittal MRI. Surgical reduction of the herniated spinal cord usually improves the myelopathic condition (2). The idiopathic or spontaneous aetiologies are different from documented traumatic or postoperative causes (3). Despite the existence, to our knowledge, at least 133 ISCH cases (Table 1) have been reported in the international Literature, whose 14 in the radiological Literature, misdiagnosis and delayed diagnosis remain a major concern (1-60).
With this article we report another case of ISCH of the thoracic tract and provide a thorough review of the Literature about the clinical-radiological correlation to better recognize and characterize this entity.
However, to date, there is no radiological or pathological proof to confirm any of this theory.
Tekkok and Coworkers report that it is difficult to define criteria for distinguishing between traumatic and spontaneous cord herniation (51). Many patients, moreover, report a long time, often more than 30 years between the spinal trauma and the onset of symptoms and sometimes it is difficult to understand the relationship between traumatic event and the herniation (1,54).
According to Isu et al (23) an intradural arachnoid cyst, causing pressure and erosion, migrates throughout a dural fissure arising from a congenital defect, a mild trauma, sometimes unknown or an erosion of the dura by a herniated and calcified disc (1,26,30). Degenerative disc prolapse with transdural rupture of disc material, often calcified, has been also proposed as a potential cause of the dural defect (20).
Miyaguchi et al (33) reported a case of ISCH with documented intervertebral disc herniated and calcified as the cause of the ventral defect. About the pathophysiological mechanism leading to spinal cord herniation, some authors (17,29,34) report the role played by factors as cardiac pulsations, respiratory movements, and the physiological spinal curve. These factors support the contact between spinal cord and dura mater, resulting, over the time, in a total adhesion with generation of a tear of the dura which will be almost totally blocked by the spinal cord.
Even the pulsation of cerebral spinal fluid (CSF) on the dorsal side of the spinal cord, secondary to respiratory movements and cardiac pulsations, can contribute to generate the herniation, pushing the spinal cord into the extradural arachnoid cyst (Fig. 1) (30). Although Masuzawa et al describe this finding as an extradural arachnoiod cyst, Sioutos et al suggest that it should be classified as a meningeal diverticolum or an arachnoid pseudocyst (46). This pathogenetical mechanism could Some authors think that the pressure inside the dorsal arachnoid cyst, as it enlarges, could be enough to produce a progressive thinning of the ventral dura mater until a tear appears and the arachnoid herniates through it (Fig. 1) (21, 23). However, dorsal arachnoid cysts are reported only in 25% of ISCH so other mechanism have to be looked for the most part of the cases (21,23,32,46,47,53,60).
Moreover, as highlighted by White and Firth (60), an erosion of the dura due to the pressure by the arachnoid should be more common in intradural tumor than in arachnoid cysts. To strengthen the unknown trauma hypothesis, Tronnier et al (52), stress the presence of inflammatory changes observed in some of the cases reported by Literature, either in the epidural space (4,52), or in the arachnoid membrane (60).
Other authors (12,15,31,32,39,40,55), think the herniation of the neural tissue through the dural ventral membrane as consequent to a congenitally duplication of the dura mater.

Clinical features
ISCH is an rare clinical entity with almost 90 cases reported in the Literature (66) which typically occurs in middle-aged adults. The median age in all cases reported was 49.9 years with a range from 21 to 78 years and female preponderance (ratio: 2:1) (12,37,44,67).
The most common clinical presentation is a Brown-Séquard syndrome but spastic para-monoparesis, sensory dysfunction or sphincter dysfunction also can be observed (37,41,67). The symptoms are slowly progressive suggesting a long course resulting in an arachnoidal adhesions to the nervous tissue with secondary gliosis involving axons. Sudden onset of symptoms has been also reported (30,60).
The mean duration of the symptoms was 4.25 years (range from 1-12 years) for patients who came in with a Brown-Séquard syndrome and 5.34 years for those presenting with spastic paraparesis (37,51).
The first symptom is usually a progressive lowerextremity paraesthesia and weakness (54). According to the Literature (20,34,60). in the early period, not all of patients become aware of sensorial changes, so the majority of patients arrives at the clinical examination because of increasing difficulty in walking, frequent falls, progressive paraesthesia often preceded by a sensation of warm.
A increasing impotence is rare but may represent the onset of the symptoms (54). According to the Literature ISCH usually presents in more than 50 % of the cases with symptoms and signs as Brown-Sequard syndrome (3,14,26,30,34,39,41,46). The differential diagnosis of ISCH includes Demielinating Disorders, such as Multiple Sclerosis or a Transverse Myelitis (14,20,47).

Case Report
We report a case of ISCH in a 61-year-old man with history of a D7 explorative laminectomy for a suspicious of arachnoid cyst.
He was admitted to our institute with a clinical diagnosis of Brown-Séquard syndrome: weakness and paraplegia of the right lower extremity and paraesthesia of the controlateral lower extremity.
A neurological examination showed bilateral tendon reflexes hyperactive and the Babinski's sign was also bilaterally present. These data were suggestive of a spinal/medullar suffering below D9-D10.
The MRI of the dorsal spine T1-(performed before and after i.v. infusion of m.d.c.) and T2-weighted ( Fig. 2 and 3) showed a focal atrophy with a right ventral displacement of the thoracic spinal cord at the D6-D7 intervertebral level and a straightforward mushroom-shaped herniation of the spinal cord at the D7 level. A dural cyst, cranially to the herniation, was also found.
The diagnosis of ISCH has been established on the basis of the thin-section MRI of the dorsal spine findings. Surgical intervention was performed (reduction of the herniated spinal cord and duroplasty) with a posterior approach. The patient's postoperative course was uneventful with rapid improvement of the symptoms of the lower extremities within few months.

Imaging
The most part of the ISCH reported have been founded between T2 and T10, with high predominance (79% cfr Brugieres et al) between T4 and T8, and symptoms may appear before than an herniation becomes demonstrable by MRI (15,66,67). The MR presentation of ISCH may be characterized by ventral displacement of the thoracic spinal cord.
The herniation through a dural defect may mimic a epidural tumor either ventral or ventrolateral. There is no contrast enhancement. A secondary enlargement of the dorsal subarachnoid spaces is also present. This sign may mimic a dorsal an arachnoid cyst.
A myelography, which led Wortzmann et al (1) to a surgical treatment of the first case of ISCH, can provide only approximate information on a anterior and/  (14). Recently, advances in MRI have reduced the relevance of MCT in the ISCH diagnosis.
MRI typical findings of ISCH show on the sagittal scan an anterior S or C-shaped kink of the spinal cord with secondary enlargement of the dorsal subarachnoid space. On the sagittal plane a decreased spinal cord size (usually atrophic) can be also seen with spinal cord signal changes due to tethering.
The axial MRI images may show the dural defect in addition to the herniation but also arachnoid cyst and associated anomalies including scalloping of the vertebral body, spina bifida or other congenital deformities. Studying CSF dynamics by Phase-contrast cine MRI may be essential to detect a posterior compressing arachnoid cyst replacing MCT.
The most frequent misdiagnosis is: dorsal arachnoid cyst, enlargement of the dorsal subarachnoid space, extradural mass or compression, discal herniation or bulging with secondary spinal cord thinning.
The spinal cord appears typically abruptly deviated to the dorsal parts of the vertebral body at a localized area and the posterior subarachnoid space may be enlarged. These findings and the craniocaudal extent of the displacement are better shown on sagittal MRI scans (12). Hence, radiological techniques are crucial in ISCH diagnosis.

Management
Because of chronic progression of the symptoms, surgery represent the treatment of choice. The aim of surgery is to reduce the herniation, to repair the dural defect and to prevent recurrence.
After the herniation reducing, surgical treatment depends on type of dural defect. There are two main treatment strategies: a) closure of the defect if the nervous tissue is herniated in the epidural space or b) simply widening the aperture when a duplication of the dura mater or a ventral epidural cyst is present (3,12,14,23,56,59).

Conclusions
We reported ISCH findings, surgically confirmed, in one case. In addition, we reviewed the Literature of the cases reported of this clinical entity often misdiagnosed. MRI findings are a ventral displacement of the spinal cord in the thoracic spine. MRI is the technique of choice to exclude a posterior arachnoid cyst, the most common mistaken diagnosis, and to recognize a spinal cord herniation when an anterior dural defect is present.
Due to its rare occurrence, its mild, non-specific, and slowly progressive symptoms, it is important to keep in mind this differential diagnosis to achieve an early diagnosis and surgical treatment to prevent major neurological dysfunctions.

Conflict of Interest:
Each author declares that he or she has no commercial associations (e.g. consultancies, stock ownership, equity interest, patent/licensing arrangement etc.) that might pose a conflict of interest in connection with the submitted article