Intervertebral disc disease

What it is

Intervertebral disc disease (IVDD) is the displacement of degenerated disc material into the vertebral canal, where it compresses and bruises the spinal cord or nerve roots. Two classic forms are recognised (Hansen classification):

• Hansen type I (extrusion): early chondroid degeneration of the nucleus pulposus, which dehydrates and often mineralises. The degenerated nucleus ruptures through the dorsal anulus fibrosus and is pushed suddenly into the canal. Onset is usually acute.
• Hansen type II (protrusion): slower fibroid degeneration; the anulus bulges dorsally without rupturing. Onset is gradual and the course chronic.

Acute non-compressive nucleus pulposus extrusion, a high-velocity extrusion of healthy nucleus that bruises the cord without staying in the canal, is a related form.

Who it affects

Type I is typical of chondrodystrophic breeds (dachshund, French bulldog, beagle, Pekingese, shih tzu, Lhasa apso, cocker spaniel), usually from about 3 to 7 years of age, although discs start degenerating within the first year of life. Type II affects mainly older, non-chondrodystrophic large breeds such as the German shepherd and Labrador.

Anatomy involved

Each intervertebral disc (discus intervertebralis) joins two vertebral bodies and has a gelatinous centre, the nucleus pulposus, surrounded by the fibrous rings of the anulus fibrosus. The anulus is thinner dorsally, under the dorsal longitudinal ligament and the floor of the vertebral canal, which is why disc material moves dorsally toward the spinal cord. Most thoracolumbar herniations occur between T11 and L2, with a peak at T12–T13 and T13–L1. From T2 to T10 they are rare because the intercapital ligament, which joins the heads of each pair of ribs across the dorsal surface of the disc, reinforces the anulus. In the neck the C2–C3 disc is the most common site in small breeds, and the caudal cervical discs in large breeds.

Clinical signs

• Cervical: marked neck pain, low head carriage, reluctance to move the head, root signature (lameness of a thoracic limb); paresis is less frequent because the cervical canal is wide.
• Thoracolumbar: back pain with kyphosis, then pelvic limb ataxia, paresis or paraplegia with upper motor neuron signs when the lesion is between T3 and L3. In severe cases, loss of voluntary urination and of deep pain perception, which is the most important prognostic factor.

Diagnosis

• Neurological examination to localise the lesion and grade its severity.
• Survey radiographs are only suggestive (narrowed disc space, mineralised discs in situ, small intervertebral foramen) and serve to rule out fractures, discospondylitis or tumours.
• MRI is the method of choice; CT and myelography are alternatives to locate the compression and its side before surgery.

Treatment

Dogs with pain alone or mild paresis can be managed conservatively with strict confinement for several weeks, analgesia and gradual rehabilitation. Non-ambulatory dogs, those that worsen or do not respond, and those with recurrent pain are candidates for surgical decompression: hemilaminectomy or pediculectomy in the thoracolumbar region and ventral slot in the neck. Prognosis is good when deep pain perception is preserved and guarded when it has been lost; a small proportion of those dogs develop progressive myelomalacia, which is fatal.