Discussion

Intervertebral disk disease is relatively rare in cats, and few articles on feline intervertebral disk disease have been published in the veterinary literature. The incidence of intervertebral disk disease in cats in one university study was 0.12% (vs. an overall intervertebral disk disease incidence in dogs of 2%).¹ The apparent rarity of feline intervertebral disk disease may prevent practitioners from including it on their differential diagnoses lists for cats with signs of spinal cord disease.

Signs of intervertebral disk disease typically manifest in middle-aged to older cats. Like dogs, cats have two recognized manifestations of intervertebral disk disease: intervertebral disk extrusion (Hansen's type I) and intervertebral disk protrusion (Hansen's type II).² Disk protrusions in cats appear to be an age-related (in middle-aged and older cats) phenomenon associated with fibroid disk degeneration.^1,3 In cats, chronic disk protrusions have been identified most commonly in the cervical spine but also appear frequently in the midlumbar spine.^2,3 Disk protrusions frequently occur at more than one disk space, and the number of disks affected tends to increase with age.^1,4 The presence of disk protrusions in cats does not always correlate with clinical neurologic disease.⁵

The clinical signs of a myelopathy include spinal hyperesthesia, proprioceptive deficits, ataxia, paraparesis or paraplegia, tetraparesis or tetraplegia, urinary or fecal incontinence or retention, sensory deficits, or a flaccid tail. Broad differential diagnoses to consider in cats exhibiting signs consistent with spinal cord disease include neoplasia, trauma, infectious or inflammatory diseases, fibrocartilaginous embolism, or intervertebral disk disease.^1,2,4,5

Lymphosarcoma has been reported to be the most common cause of spinal cord disease in cats, followed closely by spinal trauma.^1,4 Lymphosarcoma is most common in young, FeLV-positive cats. Spinal trauma is usually the cause of spinal cord disease in cats allowed frequent outdoor access. Infectious causes to consider include toxoplasmosis, FIP, cryptococcosis, and coccidioidomycosis. Coccidioidomycosis, a regional disease, should be considered in patients living in or traveling through the southwestern United States. Fibrocartilaginous embolism and discospondylitis have been reported in cats but are relatively uncommon.² Aortic thromboembolism and bilateral cranial cruciate ligament rupture can mimic signs of a myelopathy and should be ruled out. Pay careful attention to physical examination findings, including cardiac auscultation, palpation of peripheral pulses, and limb temperature, and orthopedic examination findings to help identify the correct cause of the clinical signs.⁴ Place intervertebral disk disease higher on the differential diagnoses list in middle-aged-to-older, FeLV-negative, indoor-only cats with clinical signs of a myelopathy.¹

In cats presenting with a myelopathy, perform a complete blood count, a serum chemistry profile including creatine kinase activity, a urinalysis, and FeLV antigen and FIV antibody tests and measure the serum thyroxine (T₄) concentration. Consider testing for FIP, toxoplasmosis, and other regional diseases based on your examination findings and your assessment of each animal's risk. Spinal survey radiography, spinal fluid analysis, myelography, CT, and magnetic resonance imaging are used to localize and identify the causes of spinal cord disease. In cats with intervertebral disk disease, spinal imaging helps identify the degree of spinal cord compression and aids in planning surgical spinal cord decompression.

In practices in which diagnostic imaging is limited to plain radiography, obtain orthogonal views of the spine in an anesthetized animal. Findings consistent with intervertebral disk herniation include narrowing of the intervertebral disk space, mineralization of the intervertebral disk, and opacification of the intervertebral foramen.^1,3 Degeneration of the articular facets may also be present. Normal radiographic findings do not preclude a diagnosis of intervertebral disk herniation.