= spinal cord displacement through anterior / lateral dura mater defect → uncommon cause of thoracic myelopathy

Cause: ? occult minor trauma / remote traumatic event (herniated / calcified disk may cause thinning / erosion / rupture of dura)

Mean age: 51 (range, 21–78) years; M÷F = 2÷3

• symptomatic for 1–20 years: Brown-Séquard syndrome (66%), chronic progressive paraparesis (30%), isolated sensory deficit (3%), ataxia, pain, spastic monoparesis
  Idiopathic spinal cord herniation often present with symptoms of Brown-Séquard syndrome, including ipsilateral upper motor neuron paralysis, loss of proprioception and contralateral loss of pain and temperature sensation

Location: T3–T7 ← anterior position of spinal cord with physiologic kyphosis; usually solitary lesion

Site: level of intervertebral disk >>vertebral body; spanning (usually) 1–2 / multiple vertebrae (rare)

• variable degree of cord deformity / kinking:
  • Type K: obvious kinking toward the ventral region
  • Type D:disappearance of spinal cord completely
  • Type P:protrusion of ventral aspect of spinal cord with full effacement of anterior subarachnoid space + little posterior cord kinking
  • Type C:central hiatus
  • Type L:lateral hiatus

Myelography:

• acute anterior kink of thoracic spinal cord with enlargement of dorsal subarachnoid space
• free flow of contrast material (cannot completely exclude space-occupying lesion as in wide-necked communicating arachnoid cyst)

N.B.: arachnoid cyst must be excluded!

CT myelography:

• “nuclear trail” sign = linear high-attenuation lesion in inferior endplate of adjacent vertebra ← posterior interosseous disk herniation
• soft tissue extending from apex of cord displacement through dural defect into epidural space
• ± concomitant scalloping of vertebral body

MR:

• obliteration of CSF space ventral to cord
• widened dorsal CSF space
• absence of solid / cystic mass posterior to cord
• small amount of extradural soft tissue extending from ventral apex of displaced cord into epidural space (occasionally)
• NO enhancement
• ± cord atrophy and high T2 signal intensity

Errors in diagnosis (common):

• intradural arachnoid cyst, extradural mass with cord compression, disk herniation with cord tethering

Rx: postoperatively improved neurologic symptoms (88%)

DDx: traumatic / iatrogenic cord herniation; CSF flow artifact