Intervertebral Disc Degeneration

Introduction

The term 'Intervertebral Disc Disease' (IVDD) refers to several pathological processes involving the intervertebral discs. It is common in the dog, but the type of degeneration of the disc is normally related to the breed of dog. Although any disc in the spinal column may be affects it is normally the cervical, thoracic or lumbar discs that are affected as they are less supported by surrounding structures.

Hansen Type I IVDD

In chondrodystrophic breeds such as the dachshund and the bassett hound, the nucleus pulposus undergoes chondroid degeneration. This is normal in these breeds and changes such as mineralisation are normally apparent in more than one disc by the time the dog is one year old. However extrusion of the degenerate nuclei can occur at any point, causing acute spinal cord trauma. The initial impact of the nucleus causes contusion of the spinal cord, and the extruded nucleus can continue to compress the cord. This is known as Type I (Hansen) IVDD.

Type I IVDD most commonly occurs in small and chondrodystrophic breeds, but it can occur in any dog. It is most likely to occur in young to middle-aged dogs. Clinical signs are acute in onset. Neurolocalisation will depend on which disc is affected but the classic signs are a non-ambulatory pelvic limb para-paresis, poor paw placement and the presence of withdrawl and patellar reflexes.

Hansen Type II IVDD

Type II IVDD is more common in older, large breed dogs such as the German Shepard Dog. IVDD type II involves fibroid degeneration of the nucleus propulsus and protrusion of the annulus fibrosis. Over time the fibroid tissue in the disc is replaced by collagenous tissue. At the same time hypertrophy of the surrounding annulus occurs, causing progressive compression of the spinal cord. Mineralisation of the discs is rare.

Clinical signs are normally insidious but the dog may present acutely. It is common for the dog to only show signs of dynamic compression of the spinal cord, for example when jumping. As the onset of disease is normally gradual, so clinical signs are normally less severe. These include:

• Yelping (unprovoked or when handled)
• Reluctance to jump or climb
• Low head carriage and arched back
• Reluctance to move head and neck
• Restlessness and panting

Fibrocartilagenous Embolic Myopathic (FCE)

This vascular disease most is typically seen in young medium to large breed dogs but it can occur in any dog or cat. It occurs when a fibrocartilagenous emboli causes an infarction of the spinal cord parenchyma.

It is typically peracute in onset and clinical signs are non-progressive, with a classic history of a dog jumping for a toy, yelping, and collapsing with paresis. The disease is non-painful as no pain receptors are present in the spinal cord parenchyma.

'Explosive Disc'

Explosive disc is the low volume, high velocity extrusion of the normal nucleus propulsus. This causes an non-compressive contusion injury to the spinal cord. The signalment, clinical signs and history are normally similar to FCE, with the dog presenting with a non-progressive paresis of peracute onset.

Diagnosis

A thorough history, clinical exam and neurological exam should be performed on presentation. The neurological exam should reveal the severity and neuro-localisation of the disease. Motor function, proprioception, spinal reflex arcs, the location of spinal pain and most importantly the presence or absence of conscious pain perception can provide an overall picture of location and severity.

Once this has been achieved imaging modalities can be used to further localise and classify the lesion.

Treatment

With Type I IVDD you may consider immediate referral. Providing there are no cost constraints, prompt surgery to relieve the pressure on the spinal cord results in an improved prognosis and specialist refferal units are equipped to do this.

Radiography

Survey radiographs should be taken to identify any signs of intervertebraldisc prolapse. These signs include:

• A narrowing of the intervertebral disc space
• Small or opaque intervertebral foramen
• The presence of gas in the intervertebral foramen
• Calcificiation of disc material overlying the vertebral canal

Radiographs are normally taken when the animal in anaesthatised to allow appropriate positioning. Lateral and VD views should be taken.

Myelography

Myelography involves injection of non-ionic contrast medium into the subarachnoid space, which outlines the spinal cord. It should be performed in all cases which showed no clinical signs on survey radiographs, cases where the survey radiographs do not match the clinical signs, cases where the extent of the lesion in unknown and any potentially surgical cases. The procedure should be performed under general anaesthesia - often the animal will be moved directly to theatre following myelography. Myelography can identify the lesion and confirm IVDD:

1) Extradural

The contrast medium bulges towards the cord - this is the sign you would expect to see in IVDD.

2) Intramedullary

The contrast lines are further pushed apart by an intramedullary lesion. Common differentials are neoplasia and spinal cord oedema.

3) Intradural-extramedullary

The contrast medium surrounds the lesion in these cases, producing a fork-shape in the contrast line. This is most commonly caused by a nerve root tumour.

MRI

MRI is the gold-standard for identifying IVDD, however it is generally only available in referral hospitals and is very expensive.

• Associated with loss of water from the nucleus pulposus due to lowering of proteoglycan content
  • Reduces the cushioning function
  • Can lead to herniation of the nucleus pulposus through the annular rings and impinge on the spinal cord.
• Breeds
  • Dachshund (45-75% of cases), Pekingese, beagle, Lhaso apso and cocker spaniel
  • Probably partly because all are chondrodysplastic to a varying degree
    • Occurs in all disks simultaneously (cf: non chondrodysplastic dogs – degeneration in only one disk at a time).
  • Chondrodysplastic dogs -> chondroid metaplasia of the nucleus pulposus, with progressive calcification
    • Occurs at a young age … cf non-chondrodysplastic breeds , occurs as a senile change
    • Progressive decrease in amount of glycosaminoglycans, and changes from chondroitin sulphate to keratin sulphate over the first few years of life
    • Hansen type I protrusions – explosive and large
  • Non-chondrodysplastic dogs -> progressive fibrous metaplasia of the nucleaus pulposus, starting at the periphery
    • Increased amount of type I collagen (poor at withstanding compressive loads)
    • Can lead with time, to bulging of the remaining nucleus pulposus dorsally, as it becomes increasingly constricted by the annular fibrosa
    • Calcification is uncommon
    • Hansen type II protrusions -> small and progressive