Definition

Intervertebral discs degenerate by loss of water, cellular necrosis, and dystrophic calcification. Biomechanical properties of the disc deteriorate, and subsequently protrusion (Hansen type II) or extrusion (Hansen type I) of disc material occurs.

Pathophysiology

• Intervertebral disc degeneration is initiated by one or more factors including trauma, chronic overload, decreased spinal mobility, age-related deterioration in collagen and cartilage, and hypermobility.
• Hansen type I refers to acute extrusion of nucleus pulposus through the annulus fibrosis into the vertebral canal. Hansen type I lesions typically occur in chondrodystrophic dogs, but may occur in larger non-chondrodystrophic dogs as well. Onset of neurologic signs is usually acute and often severe.
• Hansen type II lesions involve gradual protrusion (bulging) of the dorsal annular fibers into the vertebral canal associated with fibroid degeneration of the disc. Hansen type II lesions typically occur in nonchondystrophic dogs. The most common case presentation is of gradual onset of neurologic dysfunction and slow progression.
• Some sources describe a Hansen type III injury where the nucleus pulposus herniates with such force as to enter the spinal cord. Hansen type III injuries are rare, but carry a poor prognosis because they are commonly associated with myelomalacia and loss of deep pain response.
• Acute disc extrusion results in extruded material causing direct trauma to the spinal cord and residual disc mass leading to extradural spinal cord compression. Trauma and spinal cord compression results in ischemia and spinal cord changes that vary from mild demyelination to necrosis of both gray and white matter; events at the cellular level include release of vasoactive substances, increased intracellular calcium, and increased free radical formation and lipid peroxides.
• Pain due to dural irritation, nerve root impingement, or possibly discogenic (annular pain receptors) in origin.
• Disc herniation is rare between T3 and T10 owing to the barrier of the intercapital ligament between the dorsal annulus and spinal cord.

Genetics

Early studies suggest a polygenetic model with no dominance or sex linkage.

Incidence/Prevalence

• Most common neurologic dysfunction in small animals; affects 2% of the canine population.
• Occurs less commonly in cats, but the exact incidence is unknown, and the disease is likely underdiagnosed or reported due to lack of suspicion as a differential diagnosis.
• In specific breeds, such as dachshunds, affected prevalence as high as 20% has been reported.
• Thoracolumbar disc disease comprises 85% of all disc herniations; 70% occur from T11 to L3.

Signalment

Signs

Causes

• Chondroid or fibroid degeneration of the thoracolumbar intervertebral discs.
• 15% of animals with spinal fractures have been reported to have disc extrusions in addition to the fracture/luxation.

Risk Factors

Type I disease most often affects chondrodystrophic breeds.

Differential Diagnosis

• Type I-trauma causing fracture/luxation, neoplasia, discospondylitis, fibrocartilaginous embolism; differentiated by history, survey radiography, and myelography.
• Type II-degenerative myelopathy, spinal neoplasia, discospondylitis, orthopedic disease; differentiated by history, radiography, and careful orthopedic and neurologic examination.

CBC/Biochemistry/Urinalysis

• Elevation of liver enzymes common if patient has received corticosteroids for pain or neurologic disease.
• Urine retention/incontinence increases risk of urinary tract infection characterized by leukocytes, protein, and bacteruria on urinalysis.

Other Laboratory Tests

CSF analysis performed routinely in conjunction with myelography if there is high suspicion of another disease process; may be normal but more typically shows mild to moderate increase in protein with or without pleocytosis.

Imaging

• Thoracolumbar spinal radiography with the patient anesthetized. Unanesthetized radiographs can be used to rule out neoplasia and other processes, but can be misleading for thoracolumbar disc disease.
• Survey radiographs rule out some other disease processes.
• Diagnostic radiographs taken under general anesthesia may reveal a narrowed or wedged disc space, collapsed articular facet space, and small intervertebral foramen with increased or mineralized density within the spinal canal.
• Accuracy and sensitivity of survey radiographs for determining a specific site of disc herniation is low.

Diagnostic Procedures

• MRI is now considered the diagnostic mode of choice when available due to the noninvasive nature and increased information about disease process and localization obtained. Presence of a hyperintense spinal cord lesion as long as the body of L2 on a T2-weighted image in deep-pain-negative dogs is associated with a poorer prognosis.
• CT with or without contrast is often diagnostic in chondrodystrophic dogs; dystrophic calcification of extruded material is easily visualized and lateralization can be easily determined. Contrast enhancement can different disc disease from neoplastic or granulomatous processes.
• Myelography performed with iohexol was previously the method of choice and is still indicated when CT or MRI is not available. The contrast agent is usually introduced at L5–L6. Positive diagnosis is an extradural mass lesion causing spinal cord compression adjacent to the affected disc; spinal cord swelling may be evidenced by thinning of contrast columns over several intervertebral spaces.
• Lateralization is more consistent when determined from both oblique and dorsoventral myelogram views..
• CSF analysis is an important addition when alternative causes of neuropathy are likely differential diagnoses or when imaging findings are equivocal or nondiagnostic.

Pathologic Findings

Appropriate Health Care

• Guidelines for therapy based on classification of clinical condition.
• Class 1-back pain only, first episode.
• Class 2-recurrent back pain, ataxia, mild paraparesis, motor ability good.
• Class 3-severe paraparesis, proprioceptive deficits, motor ability affected but still present.
• Class 4A-complete paralysis (no motor ability) with deep pain perception present.
• Class 4B-complete paralysis, no deep pain perception present.
• Class 1 patients treated medically unless pain persists.
• Class 2 patients treated medically initially with serial neurologic exam, surgery if patient remains static or condition declines.
• Classes 3 and 4A-immediate surgical therapy; prognosis good.
• Class 4B--surgical therapy and fair prognosis if within the first 12–48 hours of occurrence. Poor prognosis if deep pain perception has been absent for >48 hours.
• Serial neurologic examination important for all affected animals.

Nursing Care

• Absolute restricted confinement for 2–4 weeks or until ambulatory.
• Minimize spinal manipulation and support spine when handling patient.
• Ensure ability to urinate or consider bladder expression, intermittent catheterization, or indwelling urinary catheter for patients in classes 3–4B.
• Recumbent patients should be kept clean on padded bedding placed on elevated cage racks and turned frequently to prevent formation of decubital ulcers.
• Manual evacuation of the bowel or enemas may be necessary to promote defecation.
• Physical therapy with passive manipulation of rear limbs begun early followed by more intense therapy (hydrotherapy) for animals with neurologic deficits.
• Carts useful in many patients in promoting return to function; patient tolerance is limiting factor.

Activity

• Restricted movement most important part of medical management.
• Cage rest in hospital or enforced cage rest as an outpatient for 2–4 weeks for class 1 patients or postoperative animals.

Diet

Weight reduction if patient is obese.

Client Education

• Some degree of restricted activity may be important for the remainder of the animal's life since it has disc disease.
• Most animals in classes 1–4A have a good to excellent prognosis for return to function, i.e., ambulation with bowel and bladder continence; patients in class 4B have a poorer but not hopeless prognosis; percentages vary but up to approximately 50% may regain deep pain and some function.

Surgical Considerations

• Strongly indicated for animals in classes 3 and 4, also within the first 12–48 hours for class 4B dogs; also indicated for static or worsening class 1 and 2 dogs.
• Primary surgical goal is to relieve spinal cord compression by disc mass removal via hemilaminectomy, dorsal laminectomy, or pediculectomy; disc fenestration alone rarely indicated but is highly recommended as an adjunct to decompression at the primary site.
• Hemilaminectomy is the surgical procedure of choice for most thoracolumbar discs. Studies indicate improved return to function, minimized spinal destabilization, and decreased formation of compressive laminectomy scars in the long-term postoperative period.
• Foramenotomy and pediculectomy are similar to hemilaminectomy and may allow a more limited approach and decreased cord manipulation.
• Dorsal laminectomy is the procedure of choice for disc hernation in the region of the lumbosacral plexus or where extensive examination of the spinal cord is necessary.
• Recurrence of clinical signs in animals in the immediate postoperative period may be due to further extrusion of disc from the original site if unfenestrated. Later recurrence of clinical signs after recovery usually due to protrusion/extrusion at adjacent sites. Several studies indicate 8% long-term recurrence of clinical signs in dogs after initial hemilaminectomy and active site fenestration due to protrusion/extrusion at other sites.
• Surgical controversies remain over the issues of multiple site prophylactic disc fenestrations performed during the decompressive surgery and the timing and prognosis for return to function after surgery in patients showing no deep pain.

Drug(s) Of Choice

• NSAIDs or narcotics may be used as an analgesic in class 1 cases.
• Narcotic analgesics may be necessary postoperatively; hydromorphone (0.05–0.2 mg/kg IV or IM, SC q4h). Transdermal lidocaine or fentanyl may be useful modes of analgesic administration in some animals.
• Gabapentin (Neurotin), an NMDA receptor antagonist, may be useful at 5–10 mg/kg PO q12h in dogs with chronic pain sensitization.
• Methocarbamol (25–45 mg/kg q8h) may be useful in cases where muscle spasm is contributing to pain; more applicable with cervical disease.
• Bethanechol (5–15 mg/dog PO) and phenoxybenzamine (0.25 mg/kg PO q8–12h) or prazosin 1 mg/15 kg PO q8–12h variably helpful in managing bladder dysfunction associated with spinal cord lesion.
• Previous indications for methylprednisolone sodium succinate for treatment of acute pain are now controversial for efficacy. The high potential for side effects may outweigh the small gains from the drug.

Precautions

• Glucocorticoid use is discouraged in cases of IVDD where drug side effects such as gastrointestinal hemorrhage and intestinal perforation often far outweigh benefits. Potent glucocorticoids such as dexamethasone should never be used.
• Glucocorticoids are contraindicated within 48 hours of using an NSAID in dogs.
• Use of glucocorticoids without cage confinement may decrease pain, thereby encouraging excessive activity and leading to further disc herniation and deterioration of clinical condition.

Alternative Drug(s) and Therapies

• Acupuncture use has been described, but efficacy has yet to be determined.
• Chiropractic therapy has no proven benefits for IVDD and may potentially be harmful to the animal.
• Percutaneous therapeutic lasers do not reach the site of inflammation and are likely not efficacious.
• Discolysis by enzymatic injection or laser ablation described but not proven therapy in dogs.

Patient Monitoring

• Patients treated medically should be reevaluated two to three times daily for worsening neurologic signs for the first 48 hours after onset.
• If stable, reevaluate daily, then weekly, until clinical signs have resolved.
• Patients treated surgically are evaluated twice daily until improvement is noted; urinary bladder function or awaiting development of an autonomic bladder are the limiting factors for hospitalization.

Possible Complications

• Recurrence of signs associated with disc herniation at original or at new site.
• Deterioration of clinical signs with or without surgery; continued inflammatory processes initiated by the initial extrusion may result in myelomalacia or diminished function.
• Rarely, development of ascending or descending myelomalacia; occurs in class 4A or 4B dogs 3–5 days following injury and is characterized by variable and changing neurologic findings, possible fever, possible dyspnea; extreme pain, and progressive neurologic signs with the forward movement of the inflammation. Such animals are often euthanized when diagnosed.

Expected Course and Prognosis

• Overall prognosis for dogs in classes 1–4A good to excellent (85–95%); those treated conservatively may experience recurrence of clinical signs.
• Recurrence rates of dogs without fenestration at the time of laminectomy range from 5–30%.
• Dogs in class 4B have a variable (10–75%) chance of recovery; overall a guarded but seemingly favorable prognosis if surgery is performed within 48 hours and the animal is allowed sufficient time to recover.

Abbreviations

• CSF = cerebrospinal fluid
• CT = computed tomography
• IVDD = intervertebral disc disease
• LMN = lower motor neuron
• MRI = magnetic resonance imaging
• NSAID = nonsteroidal anti-inflammatory drug
• UMN = upper motor neuron

Author James K. Roush

Consulting Editor Walter C. Renberg

Client Education Handout Available Online