Surgical considerations for intervertebral disk disease (Proceedings)
Hoerlein determined that intervertebral disc disease (IVDD) accounted for 2.02% of all diseases diagnosed in dogs (Hoerlein 1952). Incidence of IVDD peaks at 4 to 6 years of age in chondrodystrophic breeds and at 6 to 8 years in nonchondrodystrophic breeds (Priester 1976). The Dachshund had the highest frequency followed in succession by the Pekingese, Welsh corgi, Beagle, Lhasa Apso and miniature poodle (Hoerlein 1952). Hansen first classified IVDD as type I and type II (Hansen 1951). Hansen type I is herniation of the nucleus pulposus through the annular fibers and extrusion of nuclear material into the spinal canal. Hansen type I IVDD is typically associated with chondroid disc degeneration and has an acute onset. The disc extrudes through the dorsal annulus, causing dorsal, dorsolateral or circumferential compression of the spinal cord. Acute disc extrusion is characterized by the presence of soft disc material within the vertebral canal and extradural hemorrhage. Chronic disc extrusion is characterized by extradural fibrous adhesions around the herniated disc material that has often become a hard mineralized mass. The thoracolumbar junction (T12-13 to L1-2) accounted for the highest incidence of all disc lesions (Hoerlein 1987). Large nonchondrodystrophic breeds of dog such as the Doberman pinscher and Labrador retriever may also be affected with Hansen type I IVDD (Cudia 1997). The most common site in large breed dogs is the interspace between L1 and L2 (Cudia 1997).
Clinical signsOnset of neurologic signs in dogs with type I IVDD may be peracute (less than 1 hour), acute (less than 24 hours), gradual (greater than 24 hours) and chronic (Coates 2000). 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 neurologic dysfunction is variable and affects prognosis. Clinical signs vary from spinal hyperesthesia only to paraplegia with or without pain perception. Dogs with back pain only are usually reluctant to walk and may show kyphosis. Dogs with back pain alone and no neurologic deficits often have myelographic evidence of substantial spinal cord compression. Neuroanatomic localization for thoracolumbar lesions is determined by intact (T3-L3) or hyporeflexive (L4-S3) spinal reflexes and by site of paraspinal hyperesthesia. Asymmetry of neurologic deficits to localize the side of the disc extrusion is less reliable (Schulz 1998).
The initial diagnosis of thoracolumbar IVDD is obtained from the signalment, history and neurologic examination. Diagnosis of thoracolumbar disc extrusion/protrusion is confirmed by imaging techniques and surgery. Survey spinal radiography can help to determine the diagnosis and site of thoracolumbar disc extrusion if roentogenic signs are well defined and consistent with neuroanatomic localization. In studies of dogs with surgically confirmed thoracolumbar IVDD, survey radiography had an accuracy of 68-72% in identifying the site of disc extrusion; whereas, accuracy of myelography was higher (Kirberger 1992; Olby 1994). Myelography is performed with isotonic, water soluble contrast. Myelographic 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 but in most cases is close to 90% (Kirberger 1994; Olby 1994). Computed tomography 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 (Olby 1999). MRI can provide multiplanar views of the cord compression allowing an accurate surgical approach and can help to identify associated vertebral canal hemorrhage to further assist with determining the extent of surgical decompression required. MRI can also identify parenchymal lesions such as edema or infarction that also may affect the prognosis (Ito 2005).