Type I intervertebral disc disease

Clinical signs: Onset of neurological signs may be peracute (<1 hour), acute (<24 hours) or gradual (>24 hours). Dogs presented with peracute or acute thoracolumbar disc extrusions may manifest clinical signs of spinal shock or Schiff-Sherrington postures. These indicate acute and severe spinal cord injury but do not determine prognosis. The degree of neurological dysfunction is variable and affects prognosis. Clinical signs vary from spinal hyperaesthesia only to paraplegia with orwithoutpain perception. Dogs with back pain only are usually reluctant to walk and may show kyphosis. Dogs with back pain alone and no neurological deficits often have myelographic evidence of substantial spinal cord compression.

Neuroanatomical localization for thoracolumbar lesions is determined by intact (T3-L3) or hyporeflexive (L4-S3) spinal reflexes and by the site of paraspinal hyperaesthesia. Asymmetrical neurological deficits may be less reliable for determining the site of disc extrusion.

Pathogenesis: Hansen (1951) first classified intervertebral disc disease (IVDD) as type I and type II. Hansen type I IVDD is herniation of the nucleus pulposus through the annular fibres and extrusion of nuclear material into the spinal canal. Hansen type I IVDD is typically associated with chondroid disc degeneration. The disc extrudes through the dorsal annulus causing ventral, ventrolateral orcircumferential compression of the spinal cord.

Acute disc extrusion is characterized by the presence of soft disc material within the vertebral canal and extradural haemorrhage.

Chronic disc extrusion is characterized by extradural fibrous adhesions around the herniated disc material, which has often become a hard mineralized mass.

Hansen type I IVDD typically affects chondrodystrophoid dogs and has an acute onset. However, large non-chondrodystrophoid breeds of dog such as the Doberman Pinscherand Labrador Retriever, may also be affected ().

Hoerlein (1952) determined that IVDD accounted for 2.02% of all diseases diagnosed in dogs. Incidence of IVDD peaks at 4-6 years of age in chondrodystrophoid breeds and at 6-8 years in non-chondrodystrophoid breeds (). The Dachshund had the highest incidence of frequency followed in succession by the Pekingese, Welsh Corgi, Beagle, Lhasa Apso and Miniature Poodle ().

Hansen type I IVDD most commonly occurs within the thoracolumbar region of chondrodystrophoid breeds. The thoracolumbar junction (T12-T13 to L1-L2) accounted for the highest incidence of all disc lesions (). The incidence of thoracolumbar IVDD progressively decreases from T12-T13 caudally. The most common site for Hansen type I IVDD in large, non-chondrodystrophoid breeds is the interspace between L1 and L2 ().

Diagnosis: The initial diagnosis of thoracolumbar IVDD is obtained from the signalment, history and neurological examination. Differential diagnoses to be considered include trauma, FCE, discospondylitis, neoplasia and (meningo) myelitis. Diagnosis of thoracolumbar disc extrusion and/or protrusion is confirmed by radiography and surgery.

Survey spinal radiography can help to determine the diagnosis and site of a thoracolumbar disc extrusion if radiographic signs are well defined and consistent with neuroanatomical localization (see Chapter 5). Studies of dogs with surgically confirmed thoracolumbar IVDD showed that when identifying the site of disc extrusion survey radiography had an accuracy of 68-72%; but the percentage accuracy was higher with myelography (). Normal variants for the thoracolumbar spinal region include narrowing of the anticlinal disc space at T10-T11 and of the L4-L6 interspaces ().

As survey radiographs identify the correct site of disc extrusion in only about 70% of cases, further imaging, such as myelography, is strongly recommended by most neurosurgeons prior to surgery. Myelo-graphic contrast injection at the caudal lumbar region is preferred over the cerebellomedullary cistern for demonstrating thoracolumbar disc extrusion (). Longitudinal lesion localization by myelography for thoracolumbar IVDD () varies in accuracy from 40% to 97%, but is usually close to 90% ().

CT or MRI are used alone or as an adjunct to myelography to more completely delineate lateralization of extruded disc material (). CT has been shown to be more accurate than myelography at identifying the major site of disc herniation and has the advantage of being a more rapid test with fewer side-effects than myelography (). MRI can provide multiplanar views of the cord compression allowing an accurate surgical approach and can help to identify associated vertebral canal haemorrhage and determining the extent of surgical decompression required. MRI can also identify parenchymal lesions, such as oedema or infarction, which may affect the prognosis.

Type I intervertebral disc disease: Treatment

Conservative management: Indications for non-surgical treatment of thoracolumbar IVDD include a first-time incident of spinal pain only, mild to moderate paraparesis and the financial constraints of the client. The last is the only reason for non-surgical treatment of a recumbent patient, which should always be considered a surgical candidate if possible. Dogs can be managed with strict cage rest for 4-6 weeks combined with pain relief using anti-inflammatory drugs, opioids and muscle relaxants. Gastrointestinal protectants also may be necessary with use of anti-inflammatory therapies. Never use non-steroidal anti-inflammatory drugs (NSAIDs) in combination with corticosteroids as gastric ulcers can result and in some cases these may lead to the death of the animal. Acupuncture also has been advocated as a treatment for pain management.

Dogs should be monitored closely for deterioration of neurological status. If pain persists or the neurological status worsens, surgical management is recommended. Success rates for conservative management of ambulatory dogs with pain only or mild paresis ranges from 82% to 100% (). Studies have shown that recovery rates in non-ambulatory dogs are lower and recurrence rates higher following conservative rather than surgical treatment. Methylprednisolone sodium succinate has been advocated as an adjunctive treatment of acute disc herniations causing paraplegia ().

Surgical management: Indications for surgical management of thoracolumbar IVDD include spinal pain or paresis unresponsive to medical therapy, recurrence or progression of clinical signs, paraplegia with intact deep pain perception and paraplegia without deep pain perception for <24-48 hours. Prolonged loss of deep pain perception (>48 hours) carries a poor prognosis and owners should be made aware of this prior to surgery. However, it is often difficult to know when deep pain perception was lost; in addition recovery has been observed in dogs that had surgery more than 5 days after the onset of paraplegia. Surgery includes spinal cord decompression by removal of extruded disc material. Chronicity of disc extrusion at the time of surgery may influence the ease with which extruded disc material can be removed.

Decompressive procedures for thoracolumbar IVDD include dorsal laminectomy (), hemilaminectomy () and pediculectomy (also termed mini-hemilaminectomy) (). These approaches are described in Chapter 21. There are advantages and disadvantages of each decompressive technique. Hemilaminectomy significantly improves retrieval of extruded disc material with minimal spinal cord manipulation; a clear advantage over pediculectomy and dorsal laminectomy. Pediculectomy is the least invasive and least destabilizing technique but these advantages may not be clinically significant except in cases that require a bilateral approach to the vertebral canal. Unilateral facetectomy and fenestration do not significantly destabilize the spine in lateral bending, which suggests that the articular facets of the thoracolumbar spine are more important to stiffness in axial rotation and extension ().

The type of decompressive procedure may not affect outcome; however, the ability to retrieve disc material depends on the decompressive procedure. The primary purpose of decompressive surgery is to provide adequate exposure to allow removal of disc material while minimizing spinal cord manipulation. Hemilaminectomy allows easier access to extruded disc material and the dorsolateral approach allows access to the disc spaces for fenestration. McKee (1992) reported retrieval of disc material in 93% of dogs that had hemilaminectomy compared with 40% of dogs that had dorsal laminectomy, although initial neurological recovery after hemilaminectomy was not significantly different when compared with that following dorsal laminectomy. Radical dorsal laminectomy (removal of pedicle-Funkquist A) has an increased risk ofconstrictive laminectomy membrane formation ().

Durotomy: This is ineffective as a treatment for acute compressive spinal cord trauma unless performed within 2 hours of the trauma occurring (). Durotomy allows visualization of the spinal cord parenchyma to determine the extent of swelling and the presence of myelomalacia. Absence of visual evidence of myelomalacia does not guarantee functional recovery; conversely, functional recovery may still occur despite presence of focal myelomalacia.

Fenestration: First described by Olsson (1951) fenestration has been advocated as a treatment and prophylactic procedure for disc disease. Surgical approaches for disc fenestration include dorsolateral, lateral and ventral incisions. The effectiveness of fenestration is related to the amount of nucleus pulposus removed (). Multiple disc fenestration is commonly performed atT11-T12through L3-L4; however, the more commonly affected disc interspaces for extrusion are T12-T13 to L2-L3.

Prognosis: Overall success rates after decompressive surgery range from 58.8% () to 95% (). However, the success of a surgical approach may depend on what criteria are used to define it, how long after the surgery the patient is assessed, as well as the outcome which the owners are willing to accept. Surgical success may be improvement of the patient’s pre-surgery neurological grade but may not mean that the patient is functionally normal, and residual signs, e.g. incontinence, can be unacceptable to many owners.

Differences in recovery rates of non-ambulatory dogs vary according to the severity of neurological dysfunction (neurological grade), time interval from initial clinical signs to surgery and speed of onset of signs ().

Neurological grade: Deep pain perception is considered the most important prognostic indicator for a functional recovery. In general the majority of dogs with intact deep pain perception, whether paraplegic or simply paraparetic, have an excellent prognosis particularly if treated surgically. Dogs with loss of deep pain perception for more than 24-48 hours prior to surgery have a poorer prognosis for return of function. Without surgery, or with delayed surgery, dogs with absence of deep pain perception have an extremely guarded prognosis, although duration of absence of deep pain perception prior to surgery as a prognostic indicator is controversial. Recovery rates for dogs with thoracolumbar IVDD and absent deep pain perception range from 0-76%. A recent study of 87 dogs with loss of deep pain perception reported 58% of the animals regained deep pain perception and the ability to walk (). In summary, dogs with absence of deep pain perception that have surgery within 12-36 hours have a better chance of more rapid and complete recovery than those with delayed surgery.

Dogs with more severe neurological dysfunction have a longer period of recovery (). The mean time from post-surgery to walking varied from 10 days for pain only or paraparetic dogs to 51.5 days for paraplegic dogs (). More recent long-term studies reported recovery times of 2-14 days for dogs that were either ambulatory or non-ambulatory with voluntary motor movement, and up to 4 weeks for paraplegic dogs ().

Onset and duration of clinical signs: There are many contradictory studies about the effect of (a) the speed of clinical sign onset and (b) the duration of the clinical signs prior to surgery, on the time taken for recovery and the final outcome.

In general it is agreed that rapid removal of extruded disc material facilitates a more complete and rapid recovery (). Dogs with a shorter duration of clinical signs prior to surgery and a gradual onset of neurological dysfunction (<48 hours) have a quicker recovery (). However, a recent study of 71 paraplegic dogs with intact deep pain sensation demonstrated that although a shorter duration of signs was indeed associated with a shorter recovery time, the rate of onset of clinical signs did not influence the recovery time. This study also reported that animals that showed clinical signs for more than 6 days took significantly longer to recover. However, the rate of clinical sign onset has been reported to influence the final outcome (). Similarly, Scott and McKee (1999) demonstrated that peracute onset of signs indicated a poorer prognosis for dogs with no deep pain perception. Knecht (1970) compared the outcome of dogs after hemilaminectomy with the duration of clinical signs and concluded that delay before surgery does not influence outcome in dogs with mild neurological dysfunction but does affect functional recovery in paraplegic dogs. When performed within 12 hours of clinical sign onset, hemilaminectomy in paraplegic dogs had a higher success rate.