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Quiz 4

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Case information
Chief complaint A 51 year old female presented with impaired right lower limb hemianaesthesia for temperature sensation since 1 year. Previously her complaints were affecting right lower limb up to the level of right knee but now the sensation abnormality had progressed up to right lower back with onset of mild right lower limb pain. Pin prick and tactile sensation were intact. She did not have any history of trauma or major illness. MRI thoraco-lumbar spine was performed and later CT myelography was also performed.
Age 51 yrs
Sex Female
Modality CT/MRI
System Musculoskeletal,Neurology
Case contributor
Dr. Drushi Patel

Gujarat Imagining Centre

Dr. Hemant Patel

Gujarat Imagining Centre

Dr. Keyur Mandaliya

Gujarat Imagining Centre

The answer is
Imaging findings

MRI findings (image A, B and C):

Focal distortion of the thoracic spinal cord is seen at T3 to T4 levels, appearing anteriorly displaced with resultant mild widening of dorsal CSF space giving 'Scalpel sign'. The thoracic cord at T3-T4 level is seen protruding through the dural sac at left antero-lateral aspect (at level of anterior funiculus) at T3-T4 level, suggests ventral cord herniation. No evident adjacent cord oedema is noted at the level of ventral cord herniation. It is associated with anterior intraspinal extradural CSF intensity loculated collection in cervico-thoracic region, most marked at T2 level. Disc osteophyte complex at T3-T4 level causing indentation over ventral aspect of dural theca.

CT myelography findings (image D):

Posterior osteophyte is seen at T3-T4 level Indenting dural theca. Intrathecal contrast is seen circumferentially opacifying the CSF space surrounding the cervico-thoracic cord. Thoracic spinal cord is seen anteriorly displaced and focally distorted at T3-T4 level with resultant widening of dorsal CSF space. Axial CT myelography images show focal ventral herniation of thoracic spinal cord at left anterior paramedian aspect, through the dural theca at T3-T4 level, just inferior to the above-mentioned posterior osteophyte at T3-T4 level. On delayed CT myelography images, contrast opacification of anterior epidural space extending from C6-C7 to T7- T8 level, most marked at T2 level suggest dural leak of contrast.

Discussion

Ventral cord herniation is a rare cause of cord myelopathy due to herniation of the thoracic cord through a dural defect. Usually, it is following surgery or trauma. Idiopathic cases are usually localized to dorsal cord and can be congenital or without noticeable antecedent history of trauma.
Clinical presentation can be variable, generally with features of myelopathy. However, progressive BrownSequard syndrome is a classical manifestation as in our case due to focal herniation of the cord. The underlying cause of ventral cord herniation is thought to be a dural defect allowing the subarachnoid space to communicate with the extradural space, which can be congenital or acquired. The thoracic cord, naturally closely applied to the ventral dura due to the normal thoracic kyphosis then 'plugs' the hole and gradually herniates through the defect. It is this normal anatomical relationship between the ventral theca and anterior thoracic cord which accounts for this entity only being encountered in the anterior aspect of the mid to upper thoracic spine. The distortion of the cord parenchyma, formation of adhesions, and possible vascular compromise, in turn, leads to myelopathy and neurological dysfunction.
The key imaging feature is focal distortion and rotation of the cord with no CSF seen between it and the ventral theca. Widening of the dorsal CSF space is known as the scalpel sign. In most instances, axial images would demonstrate the the cord focally bulging beyond the confines of the theca. MRI can identify the associated cord edematous changes if present. 
Small extradural CSF intensity collection may also be seen, thought to represent the bulging CSF-filled arachnoid layer. CT myelography can demonstrate the contrast leak through the defect as well as delineate the focal bulging of the cord through the defect as in our case. 
CT can also demonstrate a hyperdense focus at the site of herniation - posterior osteophyte - probable causative agent for the dural rent and resultant cord herniation. 
Surgery with division of adhesions and closure of the dural defect, which may require a dural graft/duroplasty, is curative. In most cases symptoms improve, however depending on the degree of pre-operative myelopathy complete recovery may not occur. 
MRI and CSF flow studies can help differentiate spinal arachnoid cyst and dorsal arachnoid web from ventral cord herniation.

References: 
1. M. Brus-Ramer, W.P. Dillon. Idiopathic Thoracic Spinal Cord Herniation: Retrospective Analysis Supporting a Mechanism of Discogenic Dural Injury and Subsequent Tamponade. (2012) American Journal of Neuroradiology. 33 (1): 52. doi:10.3174/ajnr.A2730
2. Watters MR, Stears JC, Osborn AG et-al. Transdural spinal cord herniation: imaging and clinical spectra. AJNR Am J Neuroradiol. 1998;19 (7): 1337-44. AJNR Am J Neuroradiol (abstract)
3. Dix JE, Griffitt W, Yates C et-al. Spontaneous thoracic spinal cord herniation through an anterior dural defect. AJNR Am J Neuroradiol. 1998;19 (7): 1345-8. AJNR Am J Neuroradiol (abstract)

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