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Feature October 2015 Issue

Intervertebral Disc Disease in Cats

This common canine condition is relatively rare in cats, but it does occur on occasion. When it does, this is what to expect regarding diagnosis and treatment.

Since becoming a feline-only practitioner, there are some illnesses that I rarely see now that I no longer treat dogs. Adrenal gland disorders — such as hyperadrenocorticism (Cushing’s disease) and hypoadrenocorticism (Addison’s disease) — are common in dogs, but almost unheard of in cats.

Hypothyroidism (thyroid hormone deficiency) is a very common canine disorder. Cats, however, almost never develop hypothyroidism. Instead, they get the opposite condition — hyperthyroidism, where there’s an excess amount of hormone as opposed to a deficiency.

Intervertebral disc disease (IVDD) is another one of these conditions. It is very common in dogs (with the Dachshund being the classic example), but quite rare in cats. However, it does happen and here is how it’s diagnosed and treated.

THINKSTOCK

X-rays can be helpful in diagnosing IVDD because they often allow the vet to rule out other potential disorders that could cause similar clinical signs.

The spinal column is comprised of individual bones called vertebrae. Between every vertebra (except for the first and second vertebrae in the neck), there is a structure called the intervertebral disc. The disc has been likened to a “jelly donut,” with a firm outer portion (the cake-like part of the donut, called the annulus fibrosus) and the gelatinous inner portion (the jelly in the donut, called the nucleus pulposus).

How it happens

In some breeds of dogs, the discs undergo degeneration as the dog ages. If the disc gets compressed, the gelatinous material inside can extrude (herniate) through the weakened outer portion of the disc and damage the spinal cord, which runs dorsal to (above) the vertebral column. Obesity and muscular fitness are other factors implicated in canine IVDD.

And breed plays an important role in canine IVDD. The dachshund reportedly has a 10 to 12 fold greater risk than all other breeds combined. Less is known about IVDD in cats, but a similar process is believed to occur, albeit less frequently.

Differing degrees of damage

The force and volume of the herniated disc material can damage and compress the spinal cord, with the degree of damage from an acute herniation varying from mild inflammation to irreversible destruction. This will determine the clinical signs when the cat presents for examination. Some cats with IVDD present with serious neurologic dysfunction, such as partial or total paralysis, mainly in the rear limbs.

Other cases are milder, and the cat may show spinal pain only, or show pain along with some wobbliness and an unsteady gait. Some discs, when they herniate, will extrude the disc material more toward one side of the spinal cord than the other, causing one limb to be markedly more affected than the other. This can often be misinterpreted as an orthopedic injury, which confounds the diagnosis. In some patients, spinal pain may be the only presenting clinical sign. Voluntary control of the bladder and colon may also be lost when the spinal cord is injured.

Lymphosarcoma, a type of cancer, is the most common disease affecting the spinal cord in cats, and is always on the differential list in cats with acute or progressive incoordination or paralysis. Aortic thromboembolism — a condition in which a small piece of a blood clot that has formed in the heart travels down the aorta and acutely obstructs the blood flow to the rear limbs — is a common condition in cats and it, too, is always on the list of possibilities for cats that present with acute rear limb paralysis.

Why the diagnosis is tricky

Making the diagnosis is easier in dogs than in cats. In dogs, the age and breed, description of the clinical signs and physical exam findings allow veterinarians to make a tentative diagnosis of IVDD. There is no age or breed predilection in cats. However, it seems to be more likely to occur in older cats (in one study, the mean age was 9.8 years).

The neurologic examination is an important part of the diagnostic evaluation. Although herniated discs can occur anywhere along the spinal column, they are most common in the lumbar (lower back) or thoracolumbar (the junction between the chest and lower back) region, and less common in cervical (neck) area. As a result, the front limbs are usually not affected, with gait, reflex and postural abnormalities being confined to the rear limbs.

Affected cats often show pain or hypersensitivity to palpation of the muscles along either side of the spine in the area of the herniated disc. The neurologic exam should conclude with assessment of pain sensation in the affected limbs. This is evaluated by pinching a toe with a hemostat. A lack of deep pain sensation warrants a more guarded prognosis. In dogs, loss of deep pain perception for longer than 48 hours is associated with a grave prognosis (less than a five percent recovery rate).

X-rays may be helpful in making the diagnosis of IVDD because they often allow the veterinarian to rule out other potential disorders that could cause similar clinical signs such as diskospondylitis (infection of the disc and adjacent vertebrae), cancer, and spinal fractures or dislocations. In some cases, X-rays may show changes that are clearly indicative of IVDD; however, plain radiographs often fail to accurately determine the site of the disc herniation and a more advanced imaging technique is required.

More advanced procedures

Myelography is a procedure in which a radio-opaque dye is injected into the fluid that bathes the spinal cord, allowing clear visualization of the cord, and also the identification of the area that is being compressed by the herniated disc. This procedure requires anesthesia. In recent years, computed tomography (CT) and magnetic resonance imaging (MRI) have also become common methods of confirming the diagnosis and precisely locating the site of spinal cord damage.

How it’s treated

Treatment for IVDD can be either medical or surgical. Deciding whether to proceed with surgery vs. attempting medical management can be challenging. In cases where the spinal cord is shown to be significantly compressed, surgery is clearly warranted. The most common surgery performed is a hemilaminectomy, in which a portion of the vertebra that overlies the area of spinal cord compression is removed. The compressed area of spinal cord is now decompressed. Any visible extruded disc material that is identified at surgery is removed.

Cats with milder clinical signs can sometimes be managed more conservatively. Rather than undergo surgery, they may be managed with anti-inflammatory medication, pain relievers and strict confinement. In either case, unless the spinal cord damage is severe, most cats have a good chance of returning to normal function. — Arnold Plotnick, DVM, DACVIM

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