Extremely rare but its incidence is probably underestimated. Rare cause of progressive myelopathy. Hernia of the spinal cord through a defect in the dura mater. 
There are cases:

-         idiopathic: more often in women, between T4 and T7 (favoring effect: dorsal kyphosis). Cause: posterior arachnoid cyst ? repetitive, calcified herniation eroding the dura mater?

-         post-surgical

-         post-traumatic

Clinical signs are caused by strangulation of the spinal cord and correspond to a Brown-Sequard syndrome of gradual onset:  spastic monoparesis, unilateral piriformis syndrome, pain, ataxia, hypertonic disorders.

Diagnostic MRI: angulation of the spinal cord that is anterior in case of herniated thoracic or posterior in case of cervical hernia.

Treatment: surgery of the dura or enlargement of the dural defect to release the spinal cord.

Anesthetic implications: 

nobody knows whether a neuraxial block might exacerbate the neurologic clinical picture. One English neurosurgeon is convinced that a spinal block or an epidural block (including a blood patch) is contraindicated.

References : 

-        Goetti R, Wille D, Ktrezschmar U, Klein A, Scheer I. 
Idiopathic spinal cord herniation. 
JAMA Neurol 2013 ; 70 :125.

-        Malmood A, Ramaswamy K. 
Idiopathic thoracic spinal cord herniation : the unknown anaesthetic implications. 
Anaesth Intensive Care 2015 ; 43 : 787-8

Updated: September 2018