Intervertebral Disk Disease in Dogs

Intervertebral disk disease (IVDD) and chronic spinal cord compression myelopathy associated with IVDD in the dog were first described in the late 1800s.^1-3 From the early to middle l900s, significant works describing the clinical and radiographic manifestations of IVDD were published. In the 1950s, numerous reports described the predisposition for IVDD in chondrodystrophic dogs.¹ Hoerlein, Olsson, Hansen, Funkquist, and many others contributed significantly to the literature in the 1950s and 1960s, forming the foundations of our current medical and surgical therapies for IVDD.^1,4-9 Numerous publications concerning modification of surgical technique, radiologic diagnosis, pathology, pathophysiology, and incidence of IVDD have appeared since the 1970s.^10-13 Most of the current literature describes modifications of previous techniques, ancillary therapies, genetic studies,^14,15 and a concentration on physical therapy techniques.¹⁶

Today, IVDD is the most common clinical spinal disorder in dogs. IVDD is manifested by pain, a partial loss of limb function, paralysis, and sometimes a loss of feeling in the hind limbs. It occurs most frequently in chondrodystrophic dogs such as the dachshund, French bulldog, Pekingese, Shih Tzu, basset hound, and American cocker spaniel. IVDD can also occur in many other, nonchondrodystrophic breeds, such as the German shepherd and Labrador retriever. In addition, over the past several years, noncompressive forms of the disease have been more detailed in the literature.

IVDD Anatomy, Function, and Pathogenesis

The intervertebral disk is located between each vertebra except for the C1-C2 junction. The normal intervertebral disk consists of a fibrous outside ring (anulus fibrosus) and a soft gel-like center (nucleus pulposus). The function of the intervertebral disk is to connect the vertebrae, act as a shock absorber, and provide rotational stability to the vertebral column. Degeneration of the intervertebral disk occurs when the nucleus pulposus begins to dehydrate or lose water. A dehydrated disk no longer functions as a good shock absorber.

In the chondrodystrophic dog, degeneration of the intervertebral disk begins between 2 months and 2 years of age. By 1 year of age, 75% to 100% of all intervertebral disks have undergone degeneration in some chondrodystrophic dogs.¹ This type of degeneration occurs rapidly and is frequently followed by mineralization of the intervertebral disk. Recent evidence suggests that this rapid degenerative process is associated with expression of a fibroblast growth factor 4 retrogene on chromosome 12.^14,15 A different type of intervertebral disk degeneration occurs in nonchondrodystrophic dogs; it is a slow aging process most evident between 6 and 10 years of age and is rarely accompanied by mineralization. Some contend it is a similar process, just at a slower pace.¹⁷

Types and Signs of IVDD

One often pictures the middle-aged chondrodystrophic patient that presents with back pain and paraparesis or paraplegia resulting from an intervertebral disk extrusion (IVDE); however, other forms exist. Compressive forms consist of extrusion of the nucleus pulposus into the vertebral canal (Hansen type 1; FIGURE 1) and protrusion of the intervertebral disk into the canal (Hansen type 2; FIGURE 2). Noncompressive forms of the disease are fibrocartilaginous emboli (FCE; FIGURE 3), acute noncompressive nucleus pulposus extrusion (ANNPE; FIGURE 4), and hydrated nucleus pulposus extrusion (HNPE; FIGURE 5). These forms can only be definitively distinguished with advanced imaging.

Figure 1. Transverse computed tomography (CT) image of a massive Hansen type 1 cervical intervertebral disk extrusion in a chondrodystrophic dog (arrow). The compressive nuclear material in the vertebral canal and that still remains within the anulus is heavily mineralized and is readily seen on a CT scan. In chondrodystrophic dogs, the mineral content of the nucleus pulposus often allows diagnosis with the CT scan.

Figure 2A. Sagittal magnetic resonance image of a Hansen type 2 intervertebral disk protrusion in the cervical spine of a nonchondrodystrophic dog.

FIGURE 2B. Transverse magnetic resonance image of a Hansen type 2 intervertebral disk protrusion in the cervical spine of a nonchondrodystrophic dog. Note the distortion of the spinal cord caused by the type 2 compression on the transverse image (arrow).

Figure 3A. Sagittal T2-weighted magnetic resonance image of a fibrocartilaginous emboli in the cervical vertebral column of a nonchondrodystrophic dog.

FIGURE 3B. Transverse T2-FatSat (fat suppression) magnetic resonance image of a fibrocartilaginous emboli in the cervical vertebral column of a nonchondrodystrophic dog. Note the increased signal over a large area in the cervical intumescence (arrow).

FIGURE 4A. T2-weighted sagittal image of a dog with an acute onset of a thoracolumbar syndrome. An acute noncompressive nucleus pulposus extrusion is evident at the L1–L2 junction (arrow).

FIGURE 4B. Transverse image of a dog with an acute onset of a thoracolumbar syndrome. Note that the hydrated L1–L2 intervertebral disk has a reduced volume as compared to the adjacent intervertebral disk and the hyperintensity in the cord on the transverse image (arrow).

FIGURE 5A. T2-STIR (short tau inversion recovery) sagittal magnetic resonance image of a hydrated nucleus pulposus extrusion in a nonchondrodystrophic dog at C3-C4 (arrows). Note that the compression is minimal. A majority of these dogs improve or return to normal without surgical intervention.

FIGURE 5B. T2-weighted transverse magnetic resonance image of a hydrated nucleus pulposus extrusion in a nonchondrodystrophic dog at C3-C4 (arrows).

Hansen type 1 extrusions are the more common form. The signs seen in a dog with a type 1 IVDE or “rupture” vary with location, the onset of the problem (sudden versus slow or gradual), and whether the nucleus hits the spinal cord with a lot of force or if it slowly pushes its way into the vertebral canal. Often, with a longer duration of compression, recovery may be slower.¹⁷

What the owner may notice in their dog will also vary with the location and severity of the spinal cord injury. These signs may include not wanting to eat, a tight or tense abdomen, crying or yelping when moving or being picked up, and a reluctance to go up or down stairs, jump, or go for a walk. Other signs can include kyphosis, shaking or trembling, weak (wobbly) legs, or knuckling of the paws. Dogs with IVDD in the neck often hold the head down when walking, have muscle spasms in the neck, and cry out in pain when moved.

Diagnosis of IVDD

The diagnosis of IVDD is made by a combination of a physical and neurologic examination plus radiographs and cross-sectional imaging of the spine (computed tomography, magnetic resonance imaging [MRI], myelography; FIGURES 6 AND 7). While to the well-trained eye some conclusions can be made with well-positioned radiographs, this is not a dependable method of assessing the presence or absence of IVDD. MRI can be helpful in differentiating between type 1, type 2, FCE, ANNPE, and HNPE.

Figure 6. T1-weighted magnetic resonance image demonstrating a large type 1 intervertebral disk extrusion on the left in the lumbar region (arrow).

Figure 7A. A computed tomography myelogram of a type 1 intervertebral disk extrusion.

Figure 7B. A computed tomography myelogram of a type 1 intervertebral disk extrusion.

Therapy for IVDD

After a diagnosis of IVDD is made, treatment can be recommended. Many of the treatments can be beneficial when used properly. Medical therapy and surgical therapy or a combination thereof seek to alleviate the pain or neurologic deficits associated with IVDD. Many paralyzed patients that lose feeling in the legs can often be helped; however, when a condition termed progressive myelomalacia develops, there is potentially bleeding inside the spinal cord (FIGURE 8), and the condition becomes hopeless. Fortunately, the incidence of progressive myelomalacia is quite low; the condition occurs only in the subpopulation of dogs that are deep pain negative. As with many diseases, an early diagnosis and institution of therapy improve the chances for recovery.

Figure 8. A postmortem image of a canine lumbar spinal cord with myelomalacia.

Medical Treatment

Medical treatment is most often indicated for initial presentations with only pain or mild loss of limb function, if the owners cannot afford additional treatment, in patients considered anesthetic risks, and when diagnostic tests do not show compression of the spinal cord. Methods of medical treatment should include very strict confinement (usually for 4 weeks), pain relief, and muscle relaxants.

Commonly employed analgesics are acetaminophen, gabapentin, pregabalin, nonsteroidal anti-inflammatory drugs (NSAIDs), or steroids.^18,19 The authors do not routinely recommend NSAIDs because they have minimal to no effect on neuropathic pain,²⁰ although the authors concede they can be effective in other types of pain, including inflammatory and nociceptive pain, both of which can be associated with IVDD. In the authors’ experience, certain complications from NSAIDs in the surgical patient can be seen.²¹ Acupuncture is another form of therapy that may provide pain relief.

On occasion, corticosteroids are used at an anti-inflammatory dose of 0.5 to 1 mg/kg q24h. In the authors’ practice, this is generally used for 5 to 7 days, then stopped. No tapering is needed at this dose for this duration. Others may choose a longer administration period with tapering. If the patient has been on NSAIDs for any reason, a 72-hour washout is recommended before initiating corticosteroids.

Absolute strict confinement is mandatory for any IVDD patient being medically treated with corticosteroids to help prevent the patient from becoming too active and causing further injury.

If the patient cannot voluntarily urinate, it is important for owners to learn how to express or evacuate the patient’s bladder at least 3 times a day. The area of confinement (usually a carrier or a small cage) should have plenty of soft, absorbent padding.

Surgical Treatment

Surgical treatment is indicated when the spinal cord is compressed or when intractable pain can be correlated with IVDD. Surgical treatment is highly successful (90% with cervical and with thoracolumbar [TL] or lumbosacral [LS] with intact nociception); its advantage over medical treatment is the completeness and rapidity of recovery. Patients with compressive myelopathy in the TL or LS region with absent nociception have at least a 50% to 60% chance of recovery if surgically managed within the first 48 hours.²² In surgery, hemilaminectomies are most common in the TL area (FIGURE 9), dorsal laminectomies in the LS area, and ventral slot decompressions in the cervical area (FIGURE 10). The average postsurgical stay in the hospital is approximately 1 week.

Figure 9A. An illustration of a thoracolumbar hemilaminectomy.

Figure 9B. An intraoperative photo of a thoracolumbar hemilaminectomy.

Figure 10. A completed cervical ventral slot.

Prophylactic fenestration of the offending disks is routine in TL IVDD surgery, and many surgeons fenestrate adjacent, dehydrated disks in the area.²³ In the cervical spine, prophylactic fenestrations of the intervertebral disk C2-C3 through C5-C6 are routinely performed in chondrodystrophic dogs in conjunction with ventral slot decompression.²⁴ In 1 publication, the recurrence rate of IVDEs seems particularly high in the French bulldog,²⁵ perhaps giving another reason for prophylactic fenestrations in this breed. One recent publication suggests that the combination of an extensive hemilaminectomy decompression and durotomy may improve the chance of recovery in dogs with TL-IVDEs and absent nociception²⁶; however, other papers have not shown similar results.^27,28

The use of corticosteroids as a part of the treatment regimen remains a point of controversy. None of the studies in veterinary neurosurgery have satisfactorily concluded the answer. It is known that the use of dexamethasone does seem to predispose patients to urinary tract infections and gastrointestinal issues.²⁹ Methylprednisolone sodium succinate (MPSS) is an injectable corticosteroid that has both glucocorticoid activity and strong oxygen free radical scavenging properties and is considered by some as an effective drug with minimal side effects to be used in conjunction with surgery. A dose of 20 mg/kg given once intraoperatively is a standard protocol by the authors in dogs that have not received NSAIDs and have no preexisting gastrointestinal issues. Other authors have cited increased morbidity with the use of MPSS, usually when given in much higher doses over the course of 24 to 48 hours^30,31; however, Olby et al cited that many of the dogs in these studies had also received NSAIDs.³² In this particular study, which evaluated the use of MPSS in dogs without nociception, Olby et al stated that adjunctive medical treatment of surgically decompressed acute TL intervertebral disk herniation with MPSS is safe with use of the protocols outlined, but the study did not find a benefit in the deep pain–negative dogs evaluated.³² The use of gastrointestinal protectants is always advised when using this drug,³¹ and MPSS should never be used in conjunction with NSAIDs in dogs. In human neurosurgery, the controversy is the same³³; however, in some studies the intraoperative one-time dose of MPSS does show benefit.³⁴ In some studies in humans, one-time use of MPSS has a very minimal chance of side effects and has been shown to reduce hospital stays and the rate of infections.^34-36 Other studies do not agree; therefore, the controversy remains. Certainly, there are no studies to support benefits of long-term use of corticosteroids in canine spinal surgery patients.

IVDD Postoperative Care

After surgery, or after recovery following medical treatment, physical rehabilitation is advised by many neurosurgeons, but this is also controversial. Physical rehabilitation is recommended on an individual basis, and even if it does not always result in an improved ability to walk, it may improve the quality of life. This can range from simple exercises an owner can do at home to a program that includes massage therapy and swimming exercises at a physical rehabilitation unit.

Aftercare is managed according to the pet’s needs. In addition to helping a patient regain the ability to walk, it is most important for the aftercare treatment to include managing bladder function. Keeping the bladder empty and using certain drugs (e.g., bethanechol, diazepam) to stimulate the ability to urinate are very important. Acupuncture is another form of aftercare recommended by some.

Summary

Canine IVDD is the most common spinal disorder seen in small animal practice and can occur in the cervical, TL, or LS region. Many manifestations of this disorder can occur, and the Hansen type 1 TL-IVDE is usually the most severe. Surgical therapy is highly successful and recommended for the compressive forms of this disorder, especially when presented as an acute, progressive episode. Successful outcomes are augmented by an early and complete diagnosis and careful attention to the details of recommended therapeutic measures; outcomes are often enhanced in some ways by postoperative physical rehabilitation.