Thoracolumbar Intervertebral Disc Disease in Dogs

posted: 28/Aug/2017

Thoracolumbar Intervertebral Disc Disease in Dogs

Intervertebral disc disease (IVDD) is the most common cause of myelopathy in dogs.  The intervertebral disc consists of a central jelly-like incompressible nucleus pulposus and an outer lamellated annulus fibrosus consisting of collagen fibres that are oriented in such a way as to function as a ligament that permits limited movement in all planes.  Intervertebral discs account for a majority of the biomechanical stability of the vertebral column.  Intervertebral discs undergo degeneration with age, with regions of increased motion and stress such as the thoracolumbar junction undergoing the most profound changes.

Hansen type I degeneration (chondroid degeneration) is where the nucleus pulposus undergoes a progressive decrease in proteoglycan content, with consequent dehydration and accumulation of mineral.  This leads to a loss of ability to withstand pressure equally, and causes secondary degeneration and tearing in the annulus fibrosus.  In an acute episode of mechanical stress, the nucleus is expelled rapidly through a tear in the annulus (disc extrusion), usually dorsally.  The resultant injury consists of varying degrees of contusion and compression.

Hansen type II degeneration is where the nucleus pulposus is progressively dehydrated and replaced by fibrinoid tissue, with a consequent increase in stress transfer to the annulus fibrosus.  The annulus then undergoes a “wear and tear” degeneration that leads to rupture of fibres over a period of months to years, allowing the nucleus to cause protrusions of the dorsal aspect of the annulus, which compresses the spinal cord.

There is also a phenomenon associated with type I degeneration in which a peracute rupture of the annulus of a degenerative intervertebral disc occurs, along with spread of the calcified nucleus inside the epidural space over a large number of vertebrae, often without a notable centre of cord compression.  This was termed a type III extrusion as a subclass of the Hansen type I degeneration.

Another phenomenon is the low-volume/high-velocity disc extrusion, which is the peracute rupture of a normal intervertebral disc associated with extreme motion of the spinal column, often occurring in athletic dogs (and often confused with type III disc extrusions).

Ascending myelomalacia is a relatively rare phenomenon that can occur after acute nuclear extrusions.  Damage to the spinal cord progresses or “self-propagates” over a period of days following the initial insult, and through cranially directed progression, can lead to paralysis of the intercostal muscles and diaphragm, causing death.  Its pathogenesis is unknown, but thought to be due to excessive vascular damage and has been associated with type III extrusions.

Diagnostic work-up should include a complete physical examination, including neurological examination.  Determine if the patient is ambulatory – do not rely on the client’s word, but instead try to physically stand the dog up and determine if it is able to stand, and try to walk it forward.  Assess the patient for proprioceptive deficits and voluntary motor function.  If the patient has intact voluntary motor function, then it is unnecessary to assess for deep pain perception, as it will be present.  However, if the patient does not have voluntary motor function, then it is essential to assess for deep pain perception, and bladder function.  Note if the patient’s segmental spinal reflexes are normal, decreased (weak) or increased (exaggerated).  Determine if the patient has pain that localises to a specific area along the spine.  Finally, determine if the patient’s mentation and cranial nerve function are normal.

Radiography is best used as a screening diagnostic to rule out vertebral fracture/luxation, discospondylitis and other large osseous lesions.  Its accuracy for detecting intervertebral disc herniation is 51-61%.  Radiography does not allow identification of the spinal cord itself or determination of the distribution of disc material within the vertebral canal.  The presence of spondylosis deformans does not appear to be associated with the incidence of thoracolumbar disc extrusion.

Myelography does not allow direct identification of the spinal cord, but allows recognition of extradural compression.  Its accuracy for identifying the correct site of disc herniation is 84-98%.  Adverse effects include seizures (10-21% of dogs), myelopathy, apnoea, cardiac arrhythmias, meningitis, subarachnoid haemorrhage, and death.  At MVSC, we utilise myelography in the majority of our IVDD cases.

Computed tomography (CT) provides excellent bone detail, making it particularly useful in the setting of vertebral fractures or osseous neoplasia.  It is quick to perform, allows reconstruction in multiple planes, and provides enhanced detection of lateralised disc herniation.  CT is often combined with myelography for diagnosis of intervertebral disc disease.  Its accuracy for determining the site of compression is 90-100%.  At MVSC, we occasionally use CT myelography in selected IVDD cases.

MRI provides superior soft tissue contrast, which facilitates identification of structural changes within the spinal cord, epidural space, and intervertebral disc.  It allows classification of disc degeneration and disc herniation.  MRI can also be combined with myelography.  It can also sometimes be used to detect fibrocartilaginous embolism (FCE) lesions.  At MVSC, we occasionally use MRI for the diagnosis of IVDD, however it is more commonly used in cases where spinal neoplasia is deemed a probable cause.

Medical management of IVDD involves enforced (crate) rest to allow healing of the bulging or torn annulus fibrosus.  Analgesics with or without the addition of muscle relaxants are used to reduce pain, inflammation, and secondary spinal injury.  Physical rehabilitation should be considered to maintain and improve muscle mass, strength and range of motion.  The use of corticosteroids is controversial, and has been shown to be negatively associated with outcome and quality of life, and their adverse effects can lead to a 2% mortality rate due to pancreatitis, haemorrhagic gastroenteritis, and colonic perforation.  At MVSC, we do not recommend the use of corticosteroids for medical management of IVDD.  Medical management has a reported recurrence rate of 31-40%, and a failure rate of 14%.  For dogs that are non-ambulatory, surgical intervention is recommended, as it results in a higher percentage of return to function, and a faster return to function.

Surgical management of IVDD allows decompression of the spinal cord and evacuation of herniated or compressive disc material.  This can be accomplished by a hemilaminectomy (most commonly), dorsal laminectomy, pediculectomy, or mini-hemilaminectomy.  Up to 3 adjacent hemilaminectomies or 2 adjacent bilateral hemilaminectomies can be performed without compromising the stability of the vertebral column.  For dogs with intact nociception, successful surgical outcomes range from 72-100%.  For dogs with absent deep pain perception, surgery provides a 43-62% chance of return of ambulation.  Time elapsed after the loss of deep pain perception should not be used as the sole criterion for deciding whether or not a dog receives surgical intervention.  We believe that surgical intervention should always be offered, as long as the clients are aware that the chance of return of function is only 43-62%.  Dogs that regain deep pain perception within 2 weeks of surgery can have a good prognosis.  The outcome for dogs with annular protrusion may be worse than those with disc extrusion, possibly due to the more gradual progression of spinal cord compression with disc protrusions, with resultant spinal cord atrophy and potentially irreversible axonal damage.

Up to 15-20% of dogs that have had surgically treated thoracolumbar disc herniation may require subsequent medical or surgical intervention for another disc herniation.  Dachshunds have a higher rate of recurrence than other breeds.  The number of radiographically opacified disc spaces on radiography increases the risk of recurrence by 1.4 times in non-Dachshund breeds.  There is no evidence to suggest that the prognosis is worse after a second hemilaminectomy to treat recurrence at a second site.

Urinary bladder voiding disability is common following spinal injury and spinal surgery.  The character of the dysfunction depends on the location and severity of the lesion.  A common consequence is bacterial cystitis.  Other potential consequences include pyelonephritis, bladder atony, and bladder fibrosis with long-term distension.  If an upper motor neuron bladder is noted (large, firm bladder that is difficult to express and may overflow as intraluminal pressure overwhelms the urethral sphincters), phenoxybenzamine or prazosin can help, but phenoxybenzamine may take several days to take effect.  Skeletal muscle relaxants such as diazepam may also help to decrease the urethral sphincter tone and assist with bladder expression.  Urinary bladder distension should be prevented by indwelling urinary catheter placement, manual expression, or intermittent catheterisation. Manual expression and intermittent catheterisation have been advocated as a means of avoiding the increased risk of urinary tract infections resulting from indwelling catheterisation.  The use of prophylactic antibiotics is not recommended, as they can greatly increase the risk of infection, particularly with more resistant bacteria.

Swelling and discharge from the surgical wound are the most common surgical wound complications.  Other complications post-operatively include bleeding from the incision, seroma formation, wound dehiscence, and urinary tract infections.  Surgical risks include post-operative deterioration in neurological status (usually temporary), pneumonia due to prolonged recumbency, pressure sores and urine scalding due to prolonged recumbency, and recurrence.  Any dog that deteriorates neurologically during the post-operative period should be assessed for ascending myelomalacia, and viewed as a candidate for additional imaging studies to determine if additional surgery is indicated.

References:
Kerwin SC, Levine JM, Hicks DG: Thoracolumbar spine. In Tobias KM, Johnston SA, editors: Veterinary surgery: small animal, Missouri, 2012, Saunders, pp 449.
Griffin JF, Levine JM, Kerwin SC, et al: Canine thoracolumbar intervertebral disc disease: diagnosis, prognosis, and treatment. Compend Contin Educ Vet 31:E1, 2009.
Griffin JF, Levine JM, Kerwin SC: Canine thoracolumbar intervertebral disc disease: pathophysiology, neurologic examination, and emergency medical therapy. Compend Contin Educ Vet 31:E1, 2009.
Levine JM, Levine GJ, Johnson SI, et al: Evaluation of the success of medical management for presumptive thoracolumbar intervertebral disc herniation in dogs. Vet Surg 36:481, 2007.
Levine JM, Levine GJ, Boozer L, et al: Adverse effects and outcome associated with dexamethasone administration in dogs with acute thoracolumbar intervertebral disc herniation: 161 cases (2000–2006). J Am Vet Med Assoc 232:411, 2008.
Davis GJ, Brown DC: Prognostic indicators for time to ambulation after surgical decompression in nonambulatory dogs with acute thoracolumbar disc extrusions: 112 cases. Vet Surg 31:513, 2002.
Olby N, Levine J, Harris T, et al: Long-term functional outcome of dogs with severe injuries of the thoracolumbar spinal cord: 87 cases (1996–2001). J Am Vet Med Assoc 222:762, 2003.
Ruddle TL, Barnhart MD, Klocke NW, et al: Outcome and prognostic factors in non-ambulatory Hansen type I intervertebral disc extrusions: 308 cases. Vet Comp Orthop Traumatol 19:29, 2006.
Brisson B, Moffatt S, Swayne S, et al: Recurrence of thoracolumbar intervertebral disc extrusion in chondrodystrophic dogs after surgical decompression with or without prophylactic fenestration: 265 cases (1995–1999). J Am Vet Med Assoc 224:1808, 2004.
Mayhew PD, McLear RC, Ziemer LS, et al: Risk factors for recurrence of clinical signs associated with thoracolumbar intervertebral disc herniation in dogs: 229 cases (1994–2000). J Am Vet Med Assoc 225:1231, 2004.
Dhupa S, Glickman N, Waters DJ: Reoperative neurosurgery in dogs with thoracolumbar disc disease. Vet Surg 28:421, 1999.
Bubenik L, Hosgood G: Urinary tract infection in dogs with thoracolumbar intervertebral disc herniation and urinary bladder dysfunction managed by manual expression, indwelling catheterization or intermittent catheterization. Vet Surg 37:791, 2008.
Bubenik LJ, Hosgood GL, Waldron DR, et al: Frequency of urinary tract infection in catheterized dogs and comparison of bacterial culture and susceptibility testing results for catheterized and noncatheterized dogs with urinary tract infections. J Am Vet Med Assoc 231:893, 2007. Olby NJ, Halling KB, Glick TR: Rehabilitation for the neurologic patient. Vet Clin N Am Small Anim Pract 35:1389, 2005.




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