Identification of ventrolateral intramedullary intervertebral disc herniation in a dog

A 10-year-old male cross-breed dog was brought to Nihon University Animal Hospital with a history of acute onset of paralysis in the pelvic limbs 13 days previously. Magnetic resonance imaging revealed an intramedullary linear tract in the spinal cord at the thoracic vertebrae 12-13 region, which appeared hyperintense on T2-weighted images, but was hypointense and isointense on T1-weighted images when compared with normal parenchyma of the spinal cord. A hemilaminectomy was performed and a blob of what appeared to be fibrous tissue was found adhering to the surface of the dura mater. The diameter of the blob was about 4 mm. A durotomy was performed over the affected area and chondroid material was found within the spinal cord. Material from the nucleus pulposus penetrated the dura mater from the ventral aspect of the spinal cord in previously reported intramedullary intervertebral disc herniation cases, but, in this case, penetration occurred from the left ventrolateral aspect and progressed through to the right lateral aspect, forming a visible blob of what appeared to be fibrous tissue on the surface of the dura mater at the exit point. To the best of our knowledge, this was the first case report of an intramedullary intervertebral disc herniation originating from the ventrolateral aspect of the spinal cord in a dog.