Clinical
Presentation: The patient is a 52-year-old female with weakness and spasticity in the left lower extremity and a loss of temperature sensation of right lower extremity.
Imaging Findings: At the level of T4/T5 the left aspect of the spinal cord deviates and appears to attach to the anterior aspect of the thecal sac on the left side. There is an anterior epidural CSF collection that extends both cephalad and caudally from this area, with the superior end at C6 and inferior end at T12 and has an AP diameter of 5mm. The change of the spinal cord with adherence to the anterior thecal sac is characteristic of a spinal cord herniation. This is associated with an anterior epidural CSF collection characteristic of an arachnoid cyst.
Figure 3: Axial CT myelogram showing anterolateral herniation of spinal cord. Also seen is arachnoid cyst to the right of the herniation.
Figure 4: Axial T2 MR.
Diagnosis: Spontaneous spinal cord herniation
Discussion: Spontaneous spinal cord herniation (SSCH) is a rare occurrence in which the thoracic spinal cord deviates ventrally without dorsal compression. Since 1974 when it was first recorded, there have been less than 100 reported cases in the literature, although SSCH is being increasingly reported and recognized over the past years. The majority of SSCH is either anteriorly or anterolaterally, and all recorded cases are located in the thoracic spinal column, with the majority located between T2 and T8. There is a left sided predominance. SSCH is not thought to be due to trauma, in which cases the cord tends to herniated dorsally, although the pathogenesis of SSCH is still unclear.
A proposed mechanism of SSCH is the possibility of a congenital dural defect, including a preexisting ventral pseudomeningocele, a meningeal diverticulum, or an extradural arachnoid cyst. Another explanation involves the duplication of the ventral dura, with the spinal cord herniating through the inner layer. Still others have theorized that there is an inciting infectious process which causes the cord to adhere to the dura, and subsequent CSF pulsations cause a dural defect at the site of adhesion and allows for herniation.
SSCH typically occurs in middle-ages adults with a slight female predominance. The clinical picture is of slowly progressive neurologic symptoms, most commonly the Brown-Sequard syndrome (75%), with highest level of sensory or motor deficits localizing the lesion to the thoracic region. Other presentations include paraplegia, spastic paraparesis, intractable leg pain, and low-pressure headaches.
Despite the availability of MRI scanners for preoperative assessment of myelopathy, misdiagnosis remains high. Widening of the dorsal subarachnoid space due to ventral herniation can be misinterpreted as dorsal intradural arachnoid cyst. Unlike dorsal arachnoid cyst, with ventral herniation intrathecal contrast medium on CT myelography will not show differential retention of contrast dorsal to the cord. Other misdiagnoses include astrocytoma, disc herniation with tethered cord, extradural compression, and transverse myelitis.
Sagittal MRI with high resolution is useful for detecting this condition, which will show an anterior C-shaped kink of the spinal cord together with secondary enlargement of the dorsal subarachnoid space. The spical cord may be thinned because of atrophy or may show signal change. Axial MRI may show dural defect in addition to the herniation. A CT myelogram will confirm SSCH by showing that there is no subarachnoid space ventral to the displaced spinal cord. This finding on CT myelogram is essential for confirming the diagnosis of spinal cord herniation. Most patients undergo surgery to reduce the herniated spinal cord into the thecal sac and prevent recurrence. A dorsal laminectomy is most often the procedure of choice. Most patients improve neurologically following surgery (61%), although few have complete disappearance of the preoperative signs and symptoms. Biopsy of herniated tissue taken during surgery is correlated with worse outcome of relief of symptoms..
References:
Darbar A, Krishnamurthy S, Holsapple JW, Hodge CJ Jr. Ventral thoracic spinal cord herniation: frequently misdiagnosed entity. Spine. 2006 Aug 1;31(17):E600-5. [Medline]
Dix JE, Griffitt W, Yates C, Johnson B. Spontaneous thoracic spinal cord herniation through an anterior dural defect. AJNR Am J Neuroradiol. 1998 Aug;19(7):1345-8. [Medline]
Najjar MW, Baeesa SS, Lingawi SS. Idiopathic spinal cord herniation: a new theory of pathogenesis. Surg Neurol. 2004 Aug;62(2):161-70; discussion 170-1. [Medline]
Tekkök IH. Spontaneous spinal cord herniation: case report and review of the literature. Neurosurgery. 2000 Feb;46(2):485-91; discussion 491-2. [Medline]
