Intervertebral disk: type 1 herniation

i canis

Contributor(s):

Rodney Bagley

Laurent Garosi

Harry Scott

Synonym(s): Hansen type 1 disk disease, Disk extrusion, Disk rupture

Introduction

Intervertebral disk disease (IVD) can occur in any area of the spinal cord caudal to C1-2.
Cause: chondroid metaplasia (degeneration) of nucleus pulposus characterized by loss of glycosaminoglycans and decrease in water content → loss of shock absorbency → nuclear material extrudes through all layers of the ruptured dorsal annulus fibrosus into the vertebral canal causing spinal cord compression. Often seen in chondrodystrophic breeds.
Signs: acute onset non-traumatic spinal cord pathology, pain → lack of deep pain sensation.
Diagnosis: neurological examination, radiography/myelography, advanced imaging.
Treatment: conservative (cage rest) or surgical decompression.
Prognosis: guarded if severe.

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General

Type I intervertebral disk abnormality is seen in chondrodystrophic breeds of dogs which have chondroid metaplasia of the nucleus pulposus beginning early in life (usually first 2 years of life - can begin as early as 2 months of age).
This metaplasia is characterized by a loss of glycosaminoglycans, an increase in collagen content, and a decrease in water content, resulting in loss of the hydroelastic properties of the disk and its ability to withstand pressure.
Chondroid metaplasia occurs along the entire vertebral column.
Intervertebral disk extrusion is typically associated with chondroid degeneration and involves the herniation of nuclear material through all layers of the ruptured annulus fibrosus into the vertebral canal.
The abnormal forces generated by the degenerate and mineralised nucleus pulposus cause tears to develop within the annulus fibrosus through which the disk extrude.
The extrusion causes dorsal, dorsolateral or circumferential compression of the spinal cord.
Acute disk extrusion is characterized by extradural hemorrhage and soft disk material.
Thoracolumbar area accounts for most cases causing upper motor neuron lesions in the pelvic limbs.

75-100% of chondrodystrophic puppies → disk degeneration to chondroid tissue → mineralized (90% by 2 years old).
Disk designed to dissipate concussive forces centrifugally.
Disk degeneration or excessive force → dorsal displacement of material and rupture of dorsal annulus → nucleus forced dorsally through the relatively thin dorsal annulus into vertebral canal → damaged cord parenchyma.
Extrusion of disk material causes primary spinal cord injury and associated neurological signs by concussion Spinal cord: concussion , compression, laceration and ischemic events.
Force of impact on the cord rather than the size is critical: small, high velocity explosive extrusion is far more damaging to the cord parenchyma than a less forceful impact from a larger, slower fragment.
Severe swelling and inflammation of the cord following impact → edema and autolysis → cord destruction propagates along the cord, ie ascending myelomalacia Diffuse progressive myelomalacia.
Autolytic processes may → hemorrhage, myelomalacia → euthanasia.

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