The vertebral column, or backbone, consists of 34 individual bones called vertebrae. The vertebral column also includes the spinal cord and nerves, tendons, muscles, ligaments, intervertebral disks, and blood supply. The vertebral column protects the spinal cord and many internal organs, serves as a base of attachment for tendons and ligaments, provides structural support, connects the upper and lower body, and enables a wide range of body movement. The bones in the vertebral column also store minerals and produce red blood cells. The disks separate the vertebrae from each other. These "cushions" absorb the stress and shock that the body incurs during movement. IVD degeneration occurs in all breeds of dogs; however, it is observed most frequently in the chondrodystrophoid breeds (Dachshund, Pekingese, French bulldog, Beagle, Basset Hound, American Cocker Spaniel, Shih Tzu, Lhasa Apso, and generally dogs with short thick legs). The intervertebral disk protrusion, characterized by partial protrusion, is seen more commonly in the non-chondrodystrophoid dogs, and usually develops in dogs of 6-10 years of age.
Structure of Intervertebral Disc
The intervertebral disk consists of two regions: the outer layer called annulus fibrosus of cartilage-like material and the central region called nucleus pulposus. The central region is a gel-like structure in the young animal which becomes progressively dehydrated and less gel-like with age.
The exact cause of the degenerative process is not yet fully understood, but is commonly attributed to influence of genetic, hormonal, autoimmune (immune system disorder), and mechanical factors.
Disc degeneration takes one of the two forms depending on whether the dog is of a chondrodystrophoid or non-chondrodystrophoid breed. In chondrodystrophoid breeds the nucleus pulposus is gradually replaced by cartilage. By 1 year of age, 90-100% of chondrodystrophoid dogs have changes in the nucleus pulposus. In non-chondrodystrophoid breeds the nucleus pulposus is gradually replace with collagenous tissue. This starts later and progresses more slowly. Degenerative changes that occur in nucleus pulposus lead to reduction in the disk's shock absorption mechanism. Stress on the disk is not dissipated, and the failure of elasticity ultimately leads to disruption of the annulus fibrosus. 3,4
Type I Intervertebral Disc Disease
Total rupture of the annulus fibrosus will allow varying amounts of nucleus pulposus into the vertebral canal. This will cause direct pressure on the spinal cord or nerve root. This is called Hansen type I protrusion, or intervertebral disk extrusion, and is more common in chondrodystrophoid breeds. Compression of the spinal cord may be minimal, causing mild back or neck pain, to severe, causing paralysis, loss of sensation, and lack of bladder and bowel control. Damage to intervertebral disc may cause scoliosis.
Type II Intervertebral Disc Disease
Partial rupture of the annulus fibrosus will cause the disk to bulge causing a Hansen type II protrusion or intervertebral disk protrusion. These are most common in non-chondrodystrophoid breeds.
After extrusion of the disk material into the vertebral canal, continued leakage of the nucleus pulposus causes inflammation and clinical signs.
Signs of Intervertebral Disc Disease
Photo courtesy of Online Orthopaedics
If your dog has already developed protrusion, he might suddenly cry out in pain when you pet it about the head, or when exercising. You will notice some decrease in activity, since any sudden movement causes excruciating pain. Pain is the hallmark of cervical IVD protrusion and may be constant or occasional. In worst cases, there may be loss of conscious pain sensation. There may be forelimb lameness or neurological manifestations, ranging from mild weakness of all four limbs to four limb paralysis.
Classification based on the severity of clinical signs is used:
- Grade 1: spinal pain without neurologic manifestations
- Grade 2: a slight or partial paralysis but able to walk
- Grade 3: a slight or partial paralysis but unable to walk
- Grade 4: paralysis with deep pain sensation
- Grade 5: paralysis with loss of deep pain
Myelomalacia is softening of the spinal cord characterized by hemorrhagic necrosis and liquefaction of spinal cord tissue that can occur after acute spinal cord injury. Myelomalacia is a severe complication of thoracolumbar intervertebral disk herniation in dogs.5
Treatment of Intervertebral Disc Disease
There are more treatment for disk disease in dogs than for most of the other neurologic problems. The IVD can be treated using conservative therapy (cage rest, anti-inflammatory agents, muscle relaxants) and surgically. Dogs classified with Grade 1 and 2 have a mild protrusion or extrusion. Cage confinement is essential, without analgesics, unless pain is severe. Most dogs will improve within 1-6 weeks. A preventive surgery called disk fenestration. This procedure essentially drains the disks of fluids to prevent any possible future rupture. Many veterinarians agree that this procedure prevents disk episodes from occurring and relieves the pain.4
Some dogs classified as Grades 3, 4, and 5 will improve with medical treatment. However, there is evidence that good recovery rates are low (20-30%) and surgical decompression may be required. Dogs in these groups have severe lesions with swelling of the spinal cord and bleeding.
Medical decompression with corticosteroids combined with cage confinement may improve the rate of recovery. Dogs that do not respond to therapy initially, may require surgery. In animals without deep pain sensation, lack of improvement after 2 weeks of indicates a guarded prognosis and lack of improvement after 1 month indicates a grave prognosis for recovery. The situation seems easier if the dog is small. 2,4
It has been shown that the shorter the time between the onset of the signs and the time of surgery, the higher the success rate of decompressive surgery. Surgery allows the removal of the disk material and relieves pressure.
In addition, alternative treatment methods including chiropractic manipulations, acupuncture, and underwater treadmill therapy have been applied. 4
- Radiographic Interpretation for the Small Animal Clinician. Jerry M. Owens and Darryl N. Biery
- Canine Medicine and Therapeutics. Neil T. Gorman
- A Guide to Canine and Feline Orthopaedic Surgery. Hamish Denny, Steve Butterworth
- Clinical Medicine of the Dog and Cat. Michael Schaer
- Prevalence and Risk Factors for Presumptive Ascending/Descending Myelomalacia in Dogs after Thoracolumbar Intervertebral Disk Herniation. J Vet Intern Med. 2017 Mar-Apr; 31(2): 498–504