Page 1176 - Small Animal Internal Medicine, 6th Edition

1148 PART IX Nervous System and Neuromuscular Disorders

and marsupialization of the diverticulum are associated with definitive treatment. Ventral decompression is performed if
a good prognosis for recovery if performed within 4 months the cervical vertebrae are affected, whereas hemilaminec-
VetBooks.ir of development of clinical signs and if neurologic deficits are tomy for decompression at the site is usually attempted for
type II disks in the thoracolumbar spine. Effective surgical
not severe.
Type II Intervertebral Disk Protrusion decompression is often challenging to achieve because of the
chronic nature of the lesion and the difficulty encountered
Fibroid degeneration of the IVD occurs in some dogs as in removal of the dorsal annulus. The goal of therapy is to
part of the aging process, and this can lead to prolapse of stabilize the animal’s neurologic status. The spinal cord has
a small amount of disk nucleus into the annulus fibrosus. usually undergone considerable chronic compression before
A fibrotic reaction ensues, resulting in a round, dome-like clinical signs appear, so full recovery is rare. A few dogs
dorsal bulging of the annulus so that it protrudes into the experience temporary or permanent worsening of clinical
spinal canal and causes slowly progressive spinal cord com- signs postoperatively.
pression (see Fig. 65.5). This type of disk protrusion (i.e.,
Hansen type II) is seen most commonly in aging large- Degenerative Myelopathy
breed nonchondrodystrophoid dogs, particularly German A degenerative disorder of the spinal cord white matter
Shepherd dogs, Labrador Retrievers, and Doberman Pin- characterized by widespread myelin and axon loss that is
schers, but it has also been recognized occasionally in most severe in the mid to caudal thoracic spinal cord occurs
small-breed dogs. most often in aging German Shepherd dogs. Affected dogs
initially have a slowly progressive, often asymmetric, non-
Clinical Features painful, proprioceptive ataxia and UMN spastic paresis pri-
Clinical signs result primarily from slowly progressive spinal marily affecting the pelvic limbs, suggesting a T3-L3 lesion.
cord compression, although spinal discomfort is apparent in DM has been recognized in German Shepherd dogs from
a few dogs. Thoracolumbar type II disk protrusion results 5 to 14 years of age and is occasionally seen in older dogs
in UMN signs to the rear limbs, with normal forelimbs. of other large breeds including Boxers, Chesapeake Bay
Cervical type II disk disease may be seen in Doberman Pin- Retrievers, and Rhodesian Ridgebacks, and in Pembroke
schers, particularly in association with CSM (i.e., wobbler Welsh Corgis.
syndrome). In these dogs thoracic and pelvic limbs are
affected, with UMN neurologic signs most prominent in Etiology
the pelvic limbs. The cause of the noninflammatory axonal degeneration
that occurs in DM is uncertain. Some have speculated that
Diagnosis deficiencies of nutrients or vitamins or vascular supply are
Slowly progressive signs of spinal cord dysfunction in an responsible for the histologic changes. Recently homozy-
older dog should prompt consideration of type II disk pro- gosity for a genetic mutation in the superoxide dismutase
trusion, articular cysts, DM, or neoplasia. Neurologic exami- 1 (SOD1) gene has been identified as a necessary condi-
nation localizes the lesion to a spinal cord region, but, tion for the development of DM in breeds with a high
because the site is not usually painful, spinal palpation rarely familial predisposition for the condition. This mutation,
results in more precise localization. Survey radiographs of together with the neuropathologic features of DM, sug-
the spine are normal in most affected dogs, but disk space gests that it is a neurodegenerative spinal cord disorder
narrowing, osteophyte production, and end-plate sclerosis similar to UMN-onset amyotrophic lateral sclerosis (ALS)
may be seen at the site of type II disk protrusion in some in people.
dogs. All of these are, however, common incidental findings,
but these abnormalities are common at multiple sites in older Clinical Features
large-breed dogs, so they may not help to further localize the Clinically, DM results in a slowly progressive UMN parapa-
lesion. Myelography or advanced imaging technique (i.e., resis and ataxia of the rear limbs. A loss of proprioception
CT, MRI) is necessary to determine the extent and location results in knuckling, wearing of the dorsal nail surfaces of
of the lesion and to distinguish type II disk protrusion from the digits of the rear limbs, and progressively worsening
spinal neoplasia and DM. posterior ataxia. Most affected large-breed dogs progress
from mild proprioceptive loss to nonambulatory UMN para-
Treatment paresis within 6 to 9 months. A more slowly progressive form
Restricted exercise, antiinflammatory drugs (NSAIDs or of DM has been identified in the Pembroke Welsh Corgi,
low-dose prednisone), and muscle relaxants will provide with a median duration of progression of signs over 18
relief in dogs that are uncomfortable when the affected site months. Although initially all dogs with DM show typical
is palpated or manipulated. The addition of methocarba- UMN paraparesis suggesting a T3-L3 lesion, if dogs are
mol or gabapentin may help to relieve cervical discomfort managed beyond the point of being unable to walk, their
from muscle spasms or pinched nerve roots. Neurologic signs slowly progress to flaccid (LMN) paresis with muscle
signs caused by spinal cord compression will continue to atrophy and loss of reflexes in all four limbs, suggesting
progress, however, and surgery is recommended as the widespread denervation.