Treating acute disk herniations (Proceedings)

Acute intervertebral disc herniations (Hansen type 1 intervertebral disc disease) are a common cause of devastating neurological signs in dogs. Although there is probably more information on outcome of acute disc herniations than any other disease in veterinary neurology, there is a lot of confusion as to the most appropriate therapy. This presentation will discuss the available information on the diagnosis and therapy of both types of disc disease and describe ongoing studies on therapy and pathogenesis.

Etiology and pathogenesis

This is a very common condition encountered primarily, but not exclusively in chondrodystrophoid dogs. Dachshunds account for nearly 50% of cases and other chondrodystrophoid breeds such as the Pekingese, Beagle, Shih Tzu, Bassett Hound and Cocker Spaniel are commonly affected. Large breed dogs such as the Labrador Retriever, German Shepherd Dog, Doberman Pinscher and Shar Pei can also be affected and cats suffer acute disc herniations occasionally. In this disease, chondroid degeneration of the intervertebral disc occurs with age. The disc dehydrates and the nucleus pulposus is invaded by hyaline cartilage and becomes mineralized. The degenerate disc loses its shock absorbing capacity, and the annulus fibrosus develops fissures and weakens. As a result, mineralized nuclear material acutely extrudes through the annulus to lie within the vertebral canal, causing both spinal cord compression and contusion. It takes time for these changes to occur and as a result, the peak age for acute disc herniations is between 3 and 6 years. The most common sites of acute disc herniations in the cervical spine are C2/3 – C4/5, and in the thoracolumbar spine are T11/12 to L1/2. They can occur from C2/3 to C7/T1 and from T9/10 to L7/S1.

Clinical findings

The signs reflect the location of the disc herniation; in the thoracolumbar spine, signs progress from spinal pain, to ataxia and paraparesis, paraplegia and then loss of pain perception. Most affected dogs have upper motor neuron (UMN) signs, and there is a cutaneous trunci reflex cut off just caudal to the lesion. 10-15% of dogs have lower motor neuron (LMN) signs reflecting a L3-L7 lesion. The degree of dysfunction for thoracolumbar disc herniations is graded as follows; Grade 1 - pain only; Grade 2 - conscious proprioceptive deficit, ataxia, paraparesis; Grade 3 – non-ambulatory paraparesis; Grade 4 - paraplegia with intact pelvic limb sensation; Grade 5 – as for grade 4 with loss of pain perception. Approximately 10% of dogs with grade 5 injuries will develop ascending myelomalacia: this disease is fatal and can be recognized by an ascending level of the cutaneous trunci reflex cut off, loss of pelvic limb reflexes, tetraparesis and respiratory failure. In the cervical spine, the most common sign is severe neck pain, which is often associated with a nerve root signature (thoracic limb lameness and holding the thoracic limb in flexion). With more severe injuries the animal may be tetraparetic or tetraplegic. The degree of dysfunction for cervical disc herniations is assessed as pain only, tetraparesis, tetraplegia and tetraplegia with hypoventilation.

Diagnosis

The diagnosis is suspected from characteristic clinical signs in a dog of typical signalment for the disease. Survey spinal radiographs may be suggestive but are accurate in identifying the exact location in only 50-60% of disc herniations and definitive diagnoses and surgical decisions should not be based on survey radiographs alone. Computed tomography (CT) identifies mineralized disc material safely, sensitively and quickly. However, if the disc material is not mineralized, it will not be visible on these images and so either myelography or magnetic resonance imaging (MRI) can be used.

Conservative treatment for both thoracolumbar and cervical disc herniations

Conservative treatment is appropriate in dogs that still have motor function. Dogs should be strictly cage confined in a crate for a minimum of 2 weeks, taken out to urinate and defecate 3 – 4 times a day and at that time, passive range of motion exercises performed. After 2 weeks, the amount of controlled exercise the dog can do when it is taken out can slowly be increased with the dog on a leash, supported by a sling if needed and walking only. However they should still be confined when not being worked with. Pain can be managed with non-steroidal anti-inflammatory drugs such as carprofen or etogesic and opiates such as torbugesic and tramadol (can be given orally) or fentanyl. Muscle relaxants e.g. diazepam or methacarbamol are helpful with neck pain as much of the pain is due to muscle spasm. Anti-inflammatory doses of corticosteroids (0.25 - 0.5mg/kg/day of prednisone) can be used in dogs with neck pain once the diagnosis is confirmed, but are not typically necessary for back pain, and should never be used without cage confinement. The dog should be evaluated regularly for any deterioration in neurologic status, or lack of improvement over 2 weeks, both of which indicate treatment failure. If the dog has motor function it should be able to urinate on its own, but owners should be coached in palpation and expression of the bladder, and required to seek veterinary help if their dog doesn't urinate voluntarily twice a day. If improvement is seen after 2 weeks, the conservative route can be pursued and the dog is transitioned to normal activity (excluding jumping and twisting) by a gradual increase in exercise between the fifth through eighth weeks.

Surgical treatment

Hemilaminectomy and fenestration for thoracolumbar discs, ventral slot and fenestration for cervical discs

Surgery (hemilaminetomy and fenetration) is the treatment of choice for any non-ambulatory dog and a ventral slot is the treatment of choice for dogs with severe persistent neck pain or tetraparesis. As a general rule, surgery should be performed as soon as possible after the onset of neurological signs, especially in dogs with Grade 5 deficits where prognosis declines rapidly if surgery is not performed within 24-48 hours. Surgery is specifically recommended for: paraplegic dogs (grade 4); paraplegic dogs with loss of pain perception (grade 5) for less than 48h; deterioration or lack of response with non-surgical therapy; recurrence after previous treatment. Concurrent fenestration of discs T11/12 to L2/3 and C2/3 to C5/6 reduces the risk of a recurrence and is strongly recommended. Rehabilitation following surgery is an important part of the treatment.

Prognosis

There is a lot of information on the outcome of thoracolumbar disc herniations treated surgically and as a general rule, all dogs that have intact pain perception have the potential to recover with surgery. Dogs with loss of pain perception have an approximately 50-60% chance of recovery of pain perception and motor function over a 3 month period. Of these recovered dogs, approximately 40% have mild problems with fecal continence and 30% have problems with urinary continence. A further 25 % of deep pain negative dogs do not recover pain perception but will recover some useful motor function over a protracted period, although not continence. Non ambulatory dogs treated conservatively tend to take longer to recover, and may not recover to quite the same level as their surgically treated counterparts but still have a greater than 70% chance of a good recovery. The exception to this statement are the paraplegic dogs with loss of pain perception. Prognosis for these dogs without surgery is guarded and they should be treated as a surgical emergency. There are fewer numbers available for outcome of cervical disc herniations. In general, all dogs that have motor function should recover with surgery, and while a conservative approach is appropriate in dogs with cervical pain, severe cervical pain often does not resolve with conservative management and these dogs often ultimately need surgery. Dogs that are tetraplegic with respiratory compromise (hypoventilation) have a guarded prognosis unless mechanical ventilation can be provided.

New therapies

Methylprednisolone sodium succinate (MPSS) has been advocated as a free radical scavenging therapy for the contusive injury caused by a disc herniation based on the results of the human NASCIS trials. However, there is a lot of controversy over the conclusions of these trials, and it is well known that the high doses advocated can cause secondary complications that can be severe. More recently, polyethylene glycol (PEG) has been shown to be a safe adjunctive therapy for canine disc disease in a phase I clinical trial, but claims of efficacy are as yet unproven. A multicenter clinical trial funded by Morris Animal Foundation comparing the use of MPSS and PEG with a placebo in surgically treated dogs with acute disc herniations is currently underway and should help to provide answers on the best adjunctive medical therapy.

References

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