Basic anatomy of the spine

Intervertebral disc disease is the most common neurologic disorder diagnosed in veterinary patients.  Most disc herniations are near the junction of the thoracic and lumbar spine (T-L junction) however, cervical (neck) disc disease accounts for about 15% of all canine disc herniations.   The intervertebral disc functions as a shock absorber in between each  vertebral body (individual bones of the spine).  The disc is composed of  2 components, an outer fibrous shell (annulus fibrosis) and a gelatinous center (nucleus pulposis).   Herniation of the disc can occur in one of two ways.   The annulus can rupture extruding the  gelatinous center into the spinal canal (referred to as a Hansen type I) or the annulus can hypertrophy and thicken and compress the spinal cord ( referred to as a Hansen type II).  In general Hansen type I extrusions are more common in smaller breeds. Hansen type II herniations are more common in larger breeds and can be a component of a more complex syndrome  known as wobbler’s disease.

The left image shows a Hansen type II compression, while the image on the right shows the more common Hansen Type I.

CLINICAL SIGNS- The signs of a disc herniation vary with location (either T-L or cervical), degree of compression as a result of the herniation, and duration of time the compression has been present.   Disc herniations in the T-L region are generally acute in onset and the clinical signs can range from back pain, to hindlimb ataxia (stumbling and difficulty walking) in the rear limbs to rear limb paralysis.   Most animals suffering from cervical disc herniation  present for acute neck pain.  They often have a stilted gait, a reluctance to move their head and a lowered head stance.  About 10% of dogs will have tetraparesis (neurologic dysfunction of all 4 legs) and a portion of these animals will be non-ambulatory.  Some with dogs with cervical herniations may present for a one sided forelimb lameness known as a “root signature”.  This results from a disc herniation at the rear of the cervical spine impinging on the nerves providing function to the affected limb.

A myelogram showing deviation of the contrast (black arrow) diagnostic of a intervertebral disc herniation.

Diagnosis-    Plain radiographs (X-rays) may be helpful for diagnosing disc herniations, however not usually completely diagnostic.  A myelogram, or contrast study outlining the spinal canal, is generally required to visualize a herniated disc, identify the disc space affected, assess the degree of spinal cord compression and potentially rule out other forms of spinal cord compression not associated with disc herniation.

Treatment– Treatment options for disc herniations are often dictated by the patients history and presenting neurologic status.  Patients presenting for mild  pain or  lameness may respond to conservative management.  This is comprised of strict cage confinement for at least 4 weeks, anti-inflammatory drugs and muscle relaxants.  Patients treated conservatively often have recurrence of their clinical signs in the future.  Dogs presenting with intractable pain, neurologic deficits or recurrence following conservative treatment are candidates for surgical management.

Surgical Management-   The aim of surgery is to remove the disc herniation, eliminate spinal cord compression and allow room for the spinal cord to swell which is a normal part of the healing process.  Surgical correction of cervical herniations accomplished via a ventral slot procedure.  A ventral (underneath) approach to the spine is made and a small “slot”  is made through the affected disc space and apposing vertebral bodies.  For herniations in the thoracic and lumbar spine a Hemilaminectomy is performed where a portion of the top of the vertebrae is removed. Once access to the vertebral canal is gained, the herniated disc material can be removed.  Some dogs (the minority) will have some noticeable instability between vertebral bodies and will require stabilization and fusion of the affected disc space.

Prognosis-  The outcome following surgical repair is variable based on the severity of clinical signs on presentation.  Dogs with pain should show considerable improvement within a few days.  In patients with neurologic deficits, it may take several weeks to regain function.   Despite the proximity of vital neurologic structures, surgical complications are rare and the overall prognosis for a normal return to function is excellent