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.

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.