Intervertebral disc disease: Degenerative changes in the discs located between vertebrae. The severity of the disorder is variable.
ntervertebral disc rupture in the thoracolumbar region presents with variable degrees of pain; however, characteristic gait abnormalities begin to develop and progress in usually predictable patterns. Early in the course of the disease a pet may simply refuse to walk or jump as it had previously, then an ataxic (“drunken sailor” or wobbly in the hind end) gait develops. The front limbs appear normal but the hind feet will often cross as the pet steps. The entire hind end of the dog may sway without any real degree of coordination. Next to be lost is full motor function in the hind end, and the pet often will exhibit variable weakness and refusal or inability to walk or stand. This precedes complete loss of hind limb motor function. Usually at the same time, conscious ability to urinate is lost and the pet loses the ability to void (empty) its bladder completely. Urine pooling in the urinary bladder often leads to a large distended bladder and subsequent urine dribbling. Finally pain perception is lost, which is a sign of severe cord injury that can carry a guarded to poor prognosis. Intervertebral disc disease in the neck commonly produces only neck pain without major loss of nerve function to the limbs. Most dogs will protect their neck from movement and walk with a stiff gait. They often refuse to flex or extend their neck to eat and sudden movements can cause them to cry out. More significant disc ruptures may produce the ataxic (“drunken sailor”) gait to front and hind limbs and lead to variable loss of ability to walk.
The intervertebral disc is composed of two different tissues that function together to absorb and dampen forces. The outer portion is fibrocartilage, called the annulus fibrosus, and functions to give support to the disc space and retain the inner portion. Within the annulus fibrosus is a mucoid (or soft centered) nucleus pulposus, which functions to absorb forces. The majority of intervertebral disc ruptures occur in chondrodystrophic breeds (tiny breed dogs like Dachshunds, Lhasa apsos, Pekinese, beagle, etc.). These breeds undergo an early type of disc degeneration (chondroid metaplasia) that leads to early mineralization of these discs and predisposes the disc to mechanical failure under traumatic and normal forces. During chondroid metaplasia, the nucleus pulposus becomes less mucoid and more like cartilage, then undergoes a secondary calcification. During traumatic events or due to weakening of the outer annulus fibrosus over time, the inner nucleus pulposus may rupture into the spinal canal and impact the spinal cord leading to compressive and concussive forces and progressive neuron loss. This type of complete protrusion of the calcified nucleus pulposus into the spinal canal has been classified as a Hansen's Type I Disc Herniation. In many of these dogs, chondroid metaplasia occurs at multiple disc spaces throughout the spine. Large breed dogs (nonchondrodystrophoid) are significantly less affected; however, they also undergo a type of disc degeneration. The center of the disc (nucleus pulposus) undergoes fibroid metaplasia, a condition where it becomes fibrous, like the outer lining of the disc (annulus fibrosus). Over time, this can begin to slowly bulge inward toward the spinal cord causing compression alone. These are classified as Hansen's Type II Disc Herniation.
Trauma, luxation, or fracture of the spine is always considered as a possible reason for neurologic dysfunction. Certain breeds of dogs have specific degenerative conditions that need to be considered. One example is German Shepherd dogs with degenerative myelopathy, a progressive deterioration of the spinal cord. Additionally, a fibrocartilagenous embolus (FCE) needs to be considered and neoplasia (cancer) of the spinal cord or spine is always considered in older patients. Certain infectious diseases may also produce similar clinical signs.
Prognosis varies significantly with the degree of injury and the location of the injury. Most intervertebral disc ruptures that present in dogs that are still walking or have motor function have an excellent chance to return to walking and normal or near normal function. Prognosis for return to good function is decreased if motor function is absent at the time of surgery. Return to function if deep pain perception is absent can be unlikely if the duration of insult has been prolonged. Some dogs treated for intervertebral disc rupture will have some degree of wobbliness while walking.
Conservative treatment with cage rest, confinement, and steroids is often only offered to patients that have only recently begun their first episode and the neurologic deficits are mild. Multiple different surgical procedures and approaches exist varying on the surgeon and the location of the lesion. The choice of exactly which procedure will be performed is made by the surgeon based on his or her experience and preferences. Surgical decompression of the spine via removal of the bone over the spinal canal is nearly always recommended.