Intervertebral disks are located between the vertebrae (bones of the spine). Each disk has two parts, a fibrous outer layer and the jelly-like interior. When disk herniation occurs, the interior either protrudes (bulges) or extrudes (ruptures) into the vertebral canal, where the spinal cord resides. The onset of herniations can be either acute or chronic. When the spinal cord is compressed by this disk material, the dog or cat experiences signs ranging from mild back or neck pain to paralysis, loss of sensation, and loss of bladder and bowel control. Disk disease can be serious; in some cases, paralysis or fecal and urinary incontinence may be permanent.
When disks mineralize, the mineralization starts centrally. When they become completely mineralized, they look a lot like a lens end on (like the one on the left in the x-ray image). If a mineralized disk herniates, it will leave a hollow shell or a partially hollow shell. Because mineralization starts centrally, a hollow shell is a sign of herniation. Note that this disk seen above has an increased opacity (looks whiter) in the IV foramen.
Intervertebral disk herniations are most common in the long, low chondrodystrophic breeds (e.g., dachshund, basset hound, beagle, Cocker spaniel, Shih Tzu, Lhasa apso, Pekingese, and corgi). It is a genetic predisposition due to the animal’s conformation. These low-slung dogs tend to get the bulging extrusions. Large breed dogs are more typically affected with protrusions. The degeneration weakens the disk, allowing it to herniate. However, disk herniations can also be caused by physical trauma (an accident, such as being hit by a car), or the onset of a disease (such as cancer).
Intervertebral disk disease sometimes occurs in cats, but it is not as common as it is in dogs.
Disks in the neck can have the same types of problems as do the disks in the back. If the herniations are mild to moderate, they cause only neck pain or a forelimb limp; if severe, all four limbs may be paralyzed. Age
In affected dogs of chondrodystrophic (long, low-slung) breeds, disk degeneration occurs within the first few months of life, but the actual herniation typically occurs suddenly at around 3 to 6 years of age. In non-chondrodystrophic breeds, the disk degeneration usually starts at age five, with the herniation occurring slowly over time (6 to 8 years of age).
Grading of Clinical Signs and Diagnosis
A neurological examination allows the severity of clinical signs to be graded as follows:
Grade 2: severely paraparetic (weak/wobbly); good voluntary motion still present in hindlimbs, but cannot get up without assistance
Grade 1: slight voluntary limb motion present
Grade 0: paraplegic (no voluntary motion present). This grade is further subdivided as to whether or not deep pain sensation is present.
Diagnosis may require radiographs of the spine, myelography (a type of imaging involving the injection of a contrast agent [a liquid that x-rays don’t go through] into the spinal canal to pinpoint the compressed area of spinal cord), CT or MRI scan, or other tests. A spinal tap under general anesthesia may be required to examine the cerebrospinal fluid for signs of other diseases.
Treatment and Prognosis
Mild cases may be managed medically. Confinement to a crate with minimal physical activity (no jumping, no running, no going up/down stairs, no playing, etc.) is necessary for several weeks. Pain medication, such as steroid-like drugs, muscle relaxants, or anti-inflammatories may be prescribed on a short-term basis. Paralyzed or chronically affected dogs usually require surgery, but the success of the surgery will vary, depending on the amount of damage that the spinal cord has incurred. More than 90% of the dogs who have the ability to sense pain in their hindlimbs will walk again after surgery; only about 60% of the dogs who have lost deep pain sensation will walk again. This surgery may require the expertise of a veterinary surgical specialist or neurosurgeon. Pet owners need to provide intensive care for the pet, no matter which treatment is used. Full recovery usually takes weeks to months.
The prognosis depends on the clinical signs, how long the problem has been present, and how the dog responds to treatment. Some animals respond to the medical/surgical treatment, and some end up with permanent paralysis and fecal/urinary incontinence. The majority of dogs treated appropriately do well long term.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 5/20/2014