Understanding Idiopathic Spinal Cord Herniation – A Comprehensive Review of Imaging and Literature

Idiopathic spinal cord herniation (ISCH) is displacement of spinal cord through a dural or arachnoidal defect. Most patients present with back pain or myelopathy, paresthesia, and sensory or motor weakness. Imaging findings include anterior displacement of the cord with possible kink, no filling defect on CT myelography, and no restricted diffusion/mass lesion on magnetic resonance imaging. Abrupt kink in the spinal cord or widened cerebrospinal fluid (CSF) space can be caused by a variety of reasons. The differential considerations include arachnoid web, intradural extramedullary epidermoid or arachnoid cyst, abscess or cystic schwannoma. We discuss the features, imaging, differentials, and treatment of ISCH as a rare cause of such kink in the cord. While reading such cases, a radiologist should include the location, segments involved, cord signal abnormality, visible defect, scalpel sign or C–sign, ventral cord kink, nuclear trail sign, the ventral CSF space preservation, or obliteration and the type.


INTRODUCTION
Idiopathic spinal cord herniation (ISCH) is an uncommonly recognized cause of non-compressive myelopathy in which the spinal cord herniates through a small anterior or lateral dural defect. [1] ere is no associated history of trauma or surgery. Timely recognition is warranted to prevent permanent neurological deficits. Increased awareness of this clinical condition may result in a timely diagnosis and prompt treatment. In this review, we discuss clinical and an imaging characteristic of this rare pathology followed by a comprehensive review of literature.

MATERIALS AND METHODS
Clinical and imaging data of three patients with ISCH were analyzed.

Case 1
A 67-year-old female presented with complains slowly progressing bilateral leg weakness with associated chronic low back pain. ere was no bowel or bladder involvement. On examination, motor and sensory exams were normal. Lab work was unremarkable. Magnetic resonance imaging (MRI) was performed which demonstrated anterior displacement of the spinal cord with an associated kink at T6 level with cerebrospinal fluid (CSF) pulsation artifacts in the posterior subarachnoid space [ Figure 1]. High-resolution thin slice T2weighted imaging demonstrated subtle small focus of herniated cord through the dural defect [ Figure 1]. e findings favored spinal cord herniation instead of a posterior arachnoid cyst. To confirm the findings, a CT myelogram was performed which showed anterior cord kink at T6 with complete opacification of CSF posterior to the kinked and anteriorly displaced spinal cord confirming spinal cord herniation and excluding arachnoid cyst [ Figure 2].

Case 2
A 76-year-old female presented with progressive onset difficulty in walking, leg weakness, and gait ataxia. On clinical exam, she had a sensory level T9 and T8-9 myelopathy was suspected. Lab parameters were within normal limits. ere was no history of trauma. MRI thoracic spine demonstrated acute ventral kink in the thoracic spinal cord at T2-3 level [ Figure 3].

Case 3
A 52-year-old female presented with progressive onset lower extremity weakness and gait abnormalities. Neurological exam was within normal limits. Lab parameters were unremarkable. She denied a history of trauma. MRI thoracic spine demonstrated acute ventral kink in the thoracic spinal cord at T4-5 level [ Figure 4 a-c].

Etiology and pathogenesis
e etiopathogenesis of ISCH remains unclear; however, the focal dural defect has been blamed for the development of this rare clinical condition. [2,3] Several mechanisms have been postulated to cause these dural defects. Remote history of trauma or trivial/occult injury may lead to dural tears. [3] Herniated calcified disk may     cause thinning, erosion, or rupture of the dura. [3] In addition, it has been postulated that congenital duplication of dura can provide a potential space for the cord to herniate between the two layers of the dura. [4] ISCH exclusively involves the thoracic spinal cord. [3] is has been suggested due to a limited range of mobility of the thoracic spine and ventral spinal cord curve secondary to of physiologic kyphosis. [5] CLINICAL PRESENTATION ISCH presents in 22-71 years of age [6] with female predominance (M/F ratio of 3:2). [7] Most common clinical presentation is Brown-Séquard syndrome, in greater than half of the reported cases. [8] Other major early manifestations may include numbness and decreased temperature sensation in the legs, gait disturbances, pain, and incontinence. [3,8] IMAGING MRI is the imaging modality of choice for diagnosing ISCH. Usually, there is a single dural defect; however, there have been reports of two defects. [9] An abrupt anterior kink may be seen, called the "Scalpel Sign" [Figure 3 in our series] or there may be a gradual C-or S-shaped kink [ Figure 4 in our series]. [1,10] e herniated part of the cord might appear as a small mass with a signal [ Figure 1 in our series] or density similar to cord best seen on high-resolution MRI or CT myelography. [1,11] Occasionally, abnormal cord signal abnormality may be seen representing syrinx formation [ Figure 4 in our series] or myelomalacia of the spinal cord. [12] Scalloping of the posterior vertebral body may also be noticed. [12] e end plate irregularity and/or sclerosis or herniated disc calcification called "Nuclear trail Sign" can be seen sometimes on CT scan. [13] e visualization of nerve roots traversing the dorsal subarachnoid space at the level of the cord deformity helps to differentiate SCH from other pathologies posterior to cord displacing it anteriorly such as a subarachnoid cyst or epidermal cyst which instead displaces the nerve roots peripherally. [1] is is best appreciated on high-resolution T2-weighted images such as constructive interface in steady state or sampling perfection with applicationoptimized contrasts using different flip angle evolution. [14,15] When the findings on MRI are equivocal, a CT myelography should be performed. It shows free migration of contrast through the dural defect at the level of the herniated segment or may show the widened dorsal subarachnoid space. [12,[16][17][18] e phase contrast pulse cine MRI demonstrates normal pulsatile CSF flow in subarachnoid space posterior to herniated cord in ISCH differentiating it from posteriorly positioned intradural arachnoid cyst. [17] A classification system for ISCH has been reported based on the severity of herniation which helps in pre-operative planning and also in prognosticating patients. [11,19] 1. Type K: Kink toward the ventral region. 2. Type D: Cord disappears at the herniated site. 3. Type P: Protrusion of the cord in a way that the ventral subarachnoid space is effaced with very little posterior kink. 4. Type C: When the hiatus is central. 5. Type L: Hiatus is lateral.
e Type P has good post-operative recovery. Type C hiatus with bone defect have severe pre-operative symptoms and poor postoperative outcomes. [11] MANAGEMENT e treatment of ISCH includes conservative management or surgery depending on the clinical presentation. Surgery is recommended with severe neurological symptoms or progressive worsening with the aim to reduce the herniated cord and prevent its recurrence. [20,21] Surgery is laminectomy with intradural adhesiolysis and arachnoid band resection with verification of flow of CSF from cranial and dorsal ends of dural opening followed by closure of dural defect with fat or dural patch or widening of the dural defect. [15] Surveillance is recommended for either form of treatment. [22] CONCLUSION ISCH is a rare clinical entity with a displacement of the spinal cord through a dural or arachnoidal defect. Early recognition of this abnormality is necessary to prevent permanent neurological deficits. An important imaging finding is anterior displacement of the cord with a kink, without associated mass lesion.

Declaration of patient consent
e authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. e patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship
Nil.

Conflicts of interest
ere are no conflicts of interest.