INTERVERTEBRAL DISC DISEASE Overview
The intervertebral discs (the cushion that resides in the space between adjacent spinal vertebrae) are subject to a number of degenerative conditions and forces that predispose them to bulge or rupture over time. This 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 neurons (nerve cells). 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 nervous tissue in 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 and a "drunken sailor" ataxic gait. 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.
Most intervertebral disc ruptures occur in the thoracolumbar (where the ribs meet the lumbar vertebrae) region because of the anatomy of the canine spine and the forces experienced by a four legged stance. Rupture of discs in this region produces a characteristic group of clinical signs that must be interpreted by the surgeon to ensure evaluation of the region of interest. Disc rupture can occur anywhere along the spine, except between the first and second cervical vertebrae (where there is no disc). Classification of disc ruptures is generally grouped into large regions because rupture produces similar clinical signs irrespective of the location within in the region. The following groupings are described:
cervical vertebral 1-5 cervical vertebrae 6 through thoracic vertebrae 2 thoracic vertebrae 3 to lumbar vertebrae 3 lumbar vertebrae 4 through the sacrum.
Grouping these into regions is called neurolocalization and allows the surgeon to begin to plan which diagnostic tests and potential surgeries will be offered. Intervertebral disc rupture is generally thought to be a true surgical emergency and prognosis varies significantly with degree of function remaining when the pet was evaluated and surgically treated.
Causes
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.
Incidence and Prevalence
Chondrodystrophoid breed dogs (Dachshund, Pekinese, beagle, Lhasa apso, etc.) account for the vast majority of all intervertebral disc ruptures, with the Dachshund accounting for 45-70% of all cases. In these dogs, average onset of clinical signs is between 3-6 years of age, although radiographic evidence of disc calcification is usually evident by 2 years of age. Nonchondrodystrophoid dogs (Labrador Retrievers, German Shepherd Dogs, etc.) usually present between 5 and 12 years of age.
Thoracolumbar (thoracic vertebrae 11- 12, 12- 13, 13 to 1umbar vertebrae 1, and lumbar 1-2) account for 65% of all disc ruptures, while cervical (neck region) account for up to 18%.
Signs and Symptoms
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 (“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.
When to Seek Veterinary Surgical Advice
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 veterinarian and referral to a veterinary surgeon. The diagnostics associated with disc herniations and the surgical procedures for their treatment are regarded as specialty procedures. Your family veterinarian will likely want to refer you to a surgical specialist for management of the problem. To find an ACVS veterinary surgeon in your area, click here: Find a Surgeon.
Exam, Screening Tests and Imaging
Most veterinarians require initial bloodwork that includes a complete blood count (CBC), serum chemistry, and a urinalysis. These will ensure that your pet should be able to undergo general anesthesia. A good physical, orthopedic and neurologic examination will be performed as well. Survey radiographs of the spine are often performed to rule out fractures and luxation of the spine. Further diagnostic tests require general anesthesia and may include a myelogram, which is a positive contrast radiograph series where a dye is injected around to spinal cord to highlight any compression (Figure 1). Frequently, a computed tomography (CT) study, which is a radiograph that allows viewing of slices of the spinal cord and surrounding tissues, can follow the myelogram. Alternately or in addition to the CT, a magnetic resonance imaging (MRI) study may be performed. A cerebrospinal fluid (CSF) analysis may be performed at the time as the diagnostic imaging.
Figure 1. A lateral myelogram of a dachshund with a herniated intervertebral disc. Note the loss of contrast column indicating spinal cord compression caused by the ruptured intervertebral disc.
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Differential Diagnosis
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.
Complications Caused By The Disease
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 Options
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. (Figure 2)
Figure 2. An intraoperative photograph of the patient from Figure 1. 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 all of the offending disc material to decompress the spinal cord.
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Potential Complications Following Treatment
The myelogram procedure has a low risk of causing seizures in some patients, but most seizures are readily treated and last less than 24 hours. Some patients never improve after surgery and some may potentially worsen. This is usually because of inflammation associated with the surgery and the may also be a result of the severity of the disc rupture or the rupture of additional discs. Anytime surgery is performed, infection is risked. Many patients, especially Dachshunds, may have a recurrence of disc rupture in the future. General anesthesia always carries a risk of mortality.
Aftercare
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
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.
—Greg Marsolais, DVM, MS
Small Animal Resident
Editor: William Daly, DVM
Diplomate ACVS
Posted 8/13/2004
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