Dural ossification is also known as osseous metaplasia. Dural ossification is a benign condition of bony plaques on the inner surface of the dura mater, common in the cervical and lumbar regions of small and large breed dogs >2 years of age (). These are identified as radiopaque lines that outline the spinal cord and are best visualized at the intervertebral foramina. Dural ossification is rarely associated with clinical disease.
Type II intervertebral disc disease
Clinical signs: The clinical signs of Hansen type II IVDD include slowly progressive pelvic limb weakness, reluctance to rise or jump on furniture and difficulty climbing stairs. Onset of clinical signs is considered chronic and progressive. Localization is focal with asymmetrical or symmetrical weakness. Paraspinal hyperaesthesia may or may not be present.
Pathogenesis: Hansen type II disc disease is annular protrusion caused by shifting of central nuclear material and is commonly associated with fibroid disc degeneration (). The annulus fibrosus slowly protrudes into the spinal canal to cause spinal cord compression. Type II IVDD usually occurs at the mobile points of the spinal column and is more common in older, non-chondrodystrophoid breeds of dog.
Diagnosis: The diagnosis may be suspected on routine spinal radiographs that show the presence of degenerative changes in the spinal column, such as spondylosis. Myelography, CT myelography or MRI are necessary to locate the spinal cord compression ().
Treatment and prognosis:
Conservative management: Medical therapy is indicated in animals with early onset of type II IVDD and mild deficits. It also is indicated in those animals that are concurrently afflicted with suspected degenerative myelopathy. Medical therapy involves administration of NSAIDs or prednisolone. The use of a muscle relaxant such as diazepam or methocarbamol should be considered in patients with spinal hyperaesthesia (). Clinical signs do not always respond to medical therapy and often return after discontinuation of these therapies. Surgical decompression may offer a better long-term outcome.
Surgical management: The type of surgical decompression depends upon the location of the lesion. A hemilaminectomy is performed for lesions in the thoracic spine and lumbar spine cranial to L5. A dorsal laminectomy is performed if the lesion is located in the lumbosacral area. Typically type II disc protrusions require more spinal cord manipulation to relieve the compression from the annulus. The protrusion is usually excised; however, decompression by laminectomy alone may also be adequate in cases where the disc material is irretrievable. Often the neurological status of the animal is worse after surgery but this is usually temporary. Surgery may not be beneficial for those dogs with severe clinical signs that have progressed over several months because of irreversible neuronal loss consequent to the chronic compression.
If surgery is instituted early, prognosis is usually fair to good when patients are considered refractory to medical therapy. If the disease has coursed for several months and is associated with severe neurological signs (i.e. paraplegia) the prognosis is considered guarded.
Feline intervertebral disc disease
Intervertebral disc extrusions in cats causing secondary clinical signs of cervical and thoracolumbar myelopathy have been documented. However, clinically significant IVDD degeneration in cats is rare when compared with dogs.
Clinical signs: Clinical signs due to disc disease in cats are not common; the signs may reflect a painful transverse myelopathy at any region of the spinal cord but the probability of clinically significant disc extrusion seems to be higher in the thoracolumbar and lumbar area. Clinically significant intervertebral disc extrusion has been reported in cats <5 years of age but is more common in middle-aged to older cats ().
Pathogenesis: Hansen type I and II IVDD has been observed in cats, with type II being the more common. However, these are usually discovered as incidental indings at necropsy (). Ventral annular protrusions and degenerative changes were commonly observed especially in the caudal thoracic and lumbar spinal regions (). Age-related studies of disc degeneration in cats between 10 weeks and 18 years of age, that had no : nical signs of disc disease, showed that dorsal degenerative changes were most marked in the thoracic region followed by the cervical spinal region; and that ventral degenerative changes were more marked in the lumbar region. It was also shown that most older cats with intervertebral disc protrusions had multiple lesions and that frequency increased with age. Hansen type I extrusions tended to predominate at the thoracolumbar junction (), although one study found the peak incidence at L4-L5 ().
Diagnosis: Similar techniques for the diagnosis of disc disease in dogs apply for cats ().
Treatment and prognosis: Medical and surgical techniques for the treatment of disc disease in dogs also apply for cats but there have been no large studies published that evalute and compare the relative success of medical or surgical therapy in cats. Response to surgical treatment for type I IVDD can be excellent although it is obviously dependent on the severity of the initial injury (). Decompressive surgical techniques promote a more rapid and complete clinical recovery and definitively determine the diagnosis.
Clinical signs: Spondylosis rarely causes neurological signs ().
Pathogenesis: Spondylosis deformans is characterized by formation of bony growths and bridges at the intervertebral spaces. The condition is a common radiographic finding in older dogs along the thoracic and lumbar spine (). Most likely spondylosis is associated with degeneration of the annulus fibrosus of the intervertebral disc. Presence of spondylosis deformans has been associated with type II IVDD and degenerative LSS; however, these diagnoses are often made independent of the presence of spondylosis.
Diagnosis: The lesion is radiographically characterized (). The osteophyte formation usually does not compress the neural tissue or encroach within the vertebral canal ().
Treatment and prognosis: NSAIDs or steroids have been reported to lessen spinal discomfort in animals with severe ankylosing spondylosis.
Spondylosis deformans is usually an incidental finding and is rarely associated with clinical signs, therefore the prognosis is good.
Spinal synovial cysts
Clinical signs: Clinical signs are consistent with a progressive myelopathy and include paraspinal hyperaesthesia.
Pathogenesis: Spinal synovial cysts occur in the cervical spine () of young large-breed dogs but more commonly involve the thoracolumbar region in older large-breed dogs (). Spinal extradural synovial cysts and ganglion cysts arise from the articular facets. Some authors have referred to both structures as intraspinal cysts due to the confusion of tissue origination (). Pathogenesis of synovial cysts has been associated with degenerative disease and trauma. Increased mechanical stress and joint motion may predispose the thoracolumbar junction to osteoarthritis and synovial cyst formation. Histopathology of the cyst reveals fibrous connective tissue with a synovial cell lining.
Diagnosis: Radiographic findings include degenerative changes and remodelling of the articular processes. Myelography demonstrates spinal cord compression especially on the ventrodorsal view with attenuation of the contrast column medial to the articular processes. Attenuation of the ventral and dorsal columns (giving an hourglass appearance) is also present on lateral projections. The lesion is better defined using CT myelography and MRI. Albumino-cytological dissociation is a consistent finding on lumbar CSF analysis.
Treatment and prognosis: The treatment often involves surgical decompression and excision of the cyst. Surgical intervention is indicated with severe neurological deficits and refractory pain. A hemilaminectomy is often performed if one side is affected. The cyst and any protruding disc material are removed.
Marked improvements in gait and neurological deficits occur post-surgically.
Calcinosis circumscripta (tumoral calcinosis) is an unusual disease that affects young dogs. Mineralization of the soft tissues of thespine (typically the ligamentum flavum) causes compression of the underlying spinal cord. The most common site of these lesions is dorsal to C1-C2 and a full discussion of this disease can be found in site. However, cranial thoracic lesions that cause spinal hyperaesthesia and paraparesis have been reported, in particular in German Shepherd Dogs.
Clinical signs: The major forms of mucopolysaccaridosis (MPS) seen in cats are type I, VI and VII and in dogs are type I, II, III A and B, VI and VII (). These diseases are characterized by multisystemic abnormalities which include skeletal, ocular, hepatic, splenic and CNS. The axial and appendicular skeletons are affected most severely causing facial and limb deformities. MPS VI causes bony proliferative lesions of the thoracolumbar spine leading to secondary compressive myelopathy, most commonly from T12-L2 (). Necropsy demonstrates bony fusion in the cervical, thoracic and lumbar vertebrae.
Pathogenesis: MPS comprises a group of lysosomal diseases that result from defects in metabolism of certain glycosaminoglycans or acidic mucopolysaccharides, which accumulate in connective tissue and brain. The genetic defect of MPS in dogs and cats is considered to be recessively inherited.
Diagnosis: A diagnosis of MPS is suspected based on clinical signs and signalment. Radiography of the spine reveals bony proliferation. The lesions are better defined by myelography and CT. Definitive diagnosis can be made by measuring lysosomal enzyme activity in leucocyte pellets, frozen liver, serum or cultured skin fibroblasts. DNA testing is available for some forms of MPS. The toluidine blue spottest for urinary sulphated glycosaminoglycans is positive. In some forms of MPS circulating neutrophils contain metachromatic granules when stained with toluidine blue.
Treatment and prognosis: Surgical correction of the spinal cord compression is performed by dorsal or hemilaminectomy to ameliorate signs of compressive myelopathy associated with MPS VI (). Some forms of MPS have been partially corrected after allogeneic bone marrow transplantation (). Haemopoietic stem cell gene therapy has produced clinical improvement in dogs with MPS VII but not in dogs with MPS I ().
The prognosis is considered guarded for dogs and cats with spinal cord compression secondary to MPS.