Spine

 

Intervertebral Disc Disease


Structure and Function

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. Within the annulus fibrosus is a soft centered nucleus pulposus, which functions to absorb forces.


Intervertebral discs are subject to degenerative conditions and forces that predispose them to bulge or rupture over time. Disc rupture leads to two types of damage to the spinal cord - compression and concussion. Compression is the physical pressure exerted over time against the spinal cord which leads to slow degeneration and loss of nerve cells (neurons). Intervertebral disc rupture that is purely compressive usually begins slowly and leads to gradual worsening of neurologic function. Concussion force is the physical damage caused by a rapidly extruded disc impacting the spinal cord causing profound swelling and degeneration and loss of neurons. Purely concussive forces are usually rapidly progressive and have an acute onset. Most intervertebral disc ruptures are a combination of compressive and concussive forces that lead to the rapid degeneration of the spinal cord. The type of force, the degree of force applied to the spinal cord, and the duration that the force was applied will determine the extent of the damage and the loss of neurologic function.


Conscious proprioception is the ability to know where limbs are without seeing them. The neurons that control this are superficial on the spinal cord and relatively minor damage to these can lead to loss of coordination, strength and balance. Deeper within the spinal cord are the motor tracts that control coordinated movement of the limbs including walking. More significant damage leads to loss of walking and potentially loss of motor function. Deeper still within the spinal cord are the neurons that control pain perception. Severe damage can lead to entire loss of pain sensation. This can carry a very poor prognosis for recovery depending on the duration that pain perception has been lost.


Who's Affected?

The majority of intervertebral disc ruptures occur in chondrodystrophic breeds (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. 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.


Localization 

Intervertebral 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 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 gait to front and hind and lead to variable loss of ability to walk.


Intervertebral disc rupture is a surgical emergency. Prognosis for recovery significantly worsens as degree and duration of spinal injury increase. Any of the above described clinical signs are reasons for evaluation by a veterinary surgeon. The diagnostics 
associated with disc herniations and the surgical procedures for their treatment are regarded as specialty procedures.

 

The adjacent image is a myelogram of a dog with a herniated intervertebral disc.  Note the loss of dye contrast column indicating spinal cord compression caused by the ruptured intervertebral disc.

 

Left untreated, intervertebral disc rupture can lead to permanent loss of ability to walk and move the limbs. Most dogs that reach this point will also have no control of their urinary bladder and are susceptible to chronic urinary tract infections and urine scald. Additionally, without motor function, patients will be recumbent and can not turn themselves, which predisposes them to pressure sores.


Treatment and Convalescence

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. Surgical decompression of the spine via removal of the bone over the spinal canal is nearly always recommended.


A portion of the bone over the spinal canal has been removed (hemilaminectomy) in order to expose the spinal cord and the herniated disc. The disc can be seen compressing the spinal cord and the nerve root. The surgeon will next carefully remove disc material to decompress the spinal cord.


Postoperatively, most patients are kept significantly sedated and treated for pain for the first day and monitored for progression of neurologic dysfunction and seizures. Early postoperatively, most patients need urinary bladder expression and good nursing care that includes turning every 4 hours, good nutrition, and early rehabilitation (flexing and stretching of muscles and massage). The degree of nursing care is dictated in the long term by the degree of neurologic dysfunction and the response to surgical decompression of the spinal cord. Urinary bladder management may need to be done long term. Most of these dogs will require long term “life style” changes that include weight loss, the use of a harness, and prevention of traumatic activities like jumping or stair climbing.


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.

 

 

Spinal Trauma - Fractures and Luxations

External Fixation Techniques

Internal Fixation Techniques