Intervertebral disk disease in dogs is a common cause of back pain, rear limb paralysis, and inability to walk or feel the back legs. Certain breeds including the dachshund, shih tzu, Pekingese and Beagle are commonly affected. Intervertebral disk can affect any part of the canine spine, however, this article will focus on the thoracolumbar (‘mid-back’) spine.
Anatomy of the Canine Spine
The canine vertebral column (spine) is made up of individual vertebrae (bones). There are seven cervical (neck) vertebrae, thirteen thoracic vertebrae (these have ribs), seven lumbar vertebrae (low back), three fused sacral vertebrae, and a variable number of caudal (tail) vertebrae. Most vertebrae are made up of a ‘body’ at the base, a pedicle and arch (side and top) and a dorsal spinous process. The vertebral canal is the ‘tunnel’ formed by the vertebrae that the spinal cord travels through and is protected by. Between most of the vertebral bodies (there are exceptions) is an intervertebral disk. The intervertebral disk is made up of the outer annulus fibrosus and the inner gelatinous nucleus pulposus. A ‘jelly doughnut’ analogy is often used in describing the intervertebral disk, in which the annulus fibrosus is the outer doughnut, while the nucleus pulposus is the inner jelly portion. The intervertebral disk acts as a ‘cushion’ between each vertebral body.
Pathogenesis and Clinical Signs of Intervertebral Disk Disease
In some breeds, the inner nucleus pulposus loses it’s ‘jelly-like’ properties and becomes dry and brittle. The dorsal (top) of the annulus fibrosus is thinner than the rest of the annulus fibrosus. In dogs that herniate (or ‘slip’) a disk, the nucleus pulposus extrudes dorsally from the annulus fibrosus and may forcibly strike the spinal cord and/or compress the spinal cord.
This leads to signs of intervertebral disk disease. Signs of thoracolumbar intervertebral disk disease include back pain, pelvic limb ataxia (walking wobbly), inability to stand, inability to move the rear legs, or even inability to feel the back legs. A grading scale is used in dogs:
- Grade 1) Pain only–these dogs are able to walk normally, but exhibit signs of pain including reluctance to move, reluctance to jump, shivering, crying, muscle spasms, and/or a tense abdomen.
- Grade 2) Ambulatory paraparesis–these dogs are able to walk, but are weak and wobbly in the rear legs. They may cross their back legs when walking, splay out, knuckle over or stumble in their back legs.
- Grade 3) Non-ambulatory paraparesis–these dogs are still able to move their legs and wag their tails, but are not strong enough to support their own weight and walk.
- Grade 4) Paraplegia–these dogs have no voluntary movement in the rear legs.
- Grade 5) Paraplegia with absent nociception (no ‘deep pain’)–in addition to being unable to move the back legs, they are unable to feel their back legs.
Diagnostics for Intervertebral Disk Disease
There are other diseases that can cause similar clinical signs of spinal cord disease including meningitis/myelitis, spinal tumors, trauma, infection, malformations, vascular problems and others. A thorough neurological examination can give the veterinarian an idea of which of these are more likely than others, but tests are necessary to accurately determine the cause. Spinal radiographs are useful for screening for disk infection and bony tumors, but typically are insufficient to diagnose intervertebral disk disease. Myelography is a technique that involves injecting contrast around the spinal cord to visualize it on radiographs. A CT scan is useful (often combined with a myelogram) and allows the body to be visualized in ‘slices’. Magnetic resonance imaging (MRI) is considered the imaging modality of choice when visualizing the soft tissues of the body, including the nervous system. High-field MRI offers several advantages over low-field MRI, CT and/or myelography.
Treatment of Intervertebral Disk Disease
Treatment depends on the severity of the signs, but can be divided into surgical and non-surgical options. For dogs with a first episode of back pain or mild pelvic limb ataxia, a conservative approach of cage rest and medications may be elected. Dogs with more severe signs (grade 2-5) or recurrent or persistent back pain that does not respond to rest and medications are best managed with surgery to decompress the spinal cord. After examination and MRI, the size, site and side of the disk herniation can be located. A hemilaminectomy (removal of one side of the side of the vertebrae) can be performed to allow the neurosurgeon to remove the herniated disk material and decompress the spinal cord.
Prognosis for Intervertebral Disk Disease
For dogs that are category 1-4, the chances of success are about 95% in the hands of an experienced neurosurgeon. For dogs in categories 1-4 that are managed non-surgically, the chances of regaining the ability to walk are about 50-60%. However, dogs that are category 1-4 that are managed without surgery have a much higher risk for recurrence, may take longer to improve, and are at risk for worsening when compared to those that are treated with surgery.
Dogs that are category 5 (paraplegic with no ‘deep pain’ perception) are a surgical emergency. The chances of recovery are significantly lower than those that can still feel their back legs. If surgery is to be successful, it should be performed as soon as reasonably possible and certainly within 48 hours of losing the ability to feel the back legs. Dogs that are paraplegic with absent nociception are at risk for myelomalacia, an extremely serious condition that is typically progressive and fatal.
Intervertebral disk disease is a common spinal cord disease in dogs that can cause pain, difficulty walking, or inability to feel the back legs. Diagnosis is based on examination and advanced imaging such as high-field MRI. Prognosis depends on the severity of signs and therapy chosen. Surgery often offers the best chances for long-term success.