Page 1200 - Small Animal Internal Medicine, 6th Edition

1172 PART IX Nervous System and Neuromuscular Disorders

megaesophagus, neostigmine methylsulfate (0.04 mg/kg IM cats with MG. Because sequential antibody determinations
q6-8h) can be used. in an individual animal are correlated with disease progres-
VetBooks.ir ase treatment but then suddenly gets worse, it is important sion or remission, it is recommended that AChR antibody
If an animal appears to be responding to anticholinester-
concentrations be measured and monitored every 4 to 8
to determine whether the deterioration is due to underdos-
age of the anticholinesterase drug (myasthenic crisis) or weeks in animals treated for MG.
overdosage (cholinergic crisis). These are clinically indistin-
guishable, but administering one dose of a short-acting DYSAUTONOMIA
anticholinesterase (Neostigmine methylsulfate) allows the
clinician to distinguish between them. The animal in a myas- Dysautonomia is a polyneuropathy affecting sympathetic and
thenic crisis improves after anticholinesterase administra- parasympathetic nerves of the autonomic nervous system.
tion, whereas the condition of an animal in a cholinergic Historically, it was recognized as a problem of cats in the
crisis becomes transiently worse or does not change. United Kingdom, but since the late 1980s it more commonly
Acquired MG is an immune-mediated disease, and affects dogs in the Midwestern United States, particularly in
administration of glucocorticoids and other immunosup- rural Kansas, Missouri, Oklahoma, and Wyoming. The eti-
pressive drugs may be associated with a more rapid clinical ology is unknown, although toxic and autoimmune mecha-
response, a decrease in AChR antibody, and an improved nisms have been proposed. Clinical signs reflect failure of
outcome in some dogs. Ideally, immunosuppressive drugs autonomic function in multiple organ systems.
should be administered only to stable patients without aspi-
ration pneumonia. Glucocorticoids at standard immunosup- Clinical Features
pressive doses commonly cause transient worsening of The disease affects primarily young adult dogs from rural
muscular weakness in dogs with MG, so treatment should environments, with a median age of 18 months. Cats are
be initiated with a low dose (oral prednisone, 0.5 mg/kg/day) occasionally affected. Affected animals have a rapid onset of
and the dosage gradually increased over 2 to 4 weeks. Oral clinical signs that progress over days to weeks. Common
administration of azathioprine (Imuran, 2 mg/kg/day) or presenting complaints are vomiting or regurgitation, strain-
mycophenolate mofetil (CellCept, 10-20 mg/kg q12h) or ing to urinate, dribbling urine, photophobia, purulent nasal
Cyclosporine (5-6 mg/kg q12h) alone or in combination discharge, depression, and anorexia. Physical examination
with prednisone has been associated with a positive clinical findings include decreased or absent anal tone, dilated pupils
response in some dogs. that do not respond to light; dry nose, eyes, and mucous
If a thymoma is identified during initial evaluation of a membranes; and prolapse of the nictitating membranes. The
dog or cat with acquired MG, prompt surgical removal bladder may be distended and easy to express.
should be considered if the animal can be stabilized for
surgery. Megaesophagus, dysphagia, and aspiration pneu- Diagnosis
monia are all risk factors decreasing the odds of short-term Diagnosis is suspected on the basis of the observed clinical
survival. Many animals with MG will have a decrease in signs. Thoracic and abdominal radiographs may reveal
AChR antibody titer and dramatic resolution of their signs megaesophagus, aspiration pneumonia, generalized ileus,
after thymectomy. Adjunctive treatment may be required to constipation/obstipation, and a large distended urinary
prevent thymoma regrowth and recurrence of the signs of bladder. The bladder is easily expressed, suggesting dimin-
MG. ished urethral sphincter tone. Anal tone is usually decreased.
Pharmacologic testing can be used to support the diagnosis.
Prognosis When very dilute (0.05%-0.1%) pilocarpine (Isopto Carpine
Response to medical management of MG can be good if 1% [Alcon Laboratories] diluted with saline) is applied to the
aspiration pneumonia is not severe. Severe aspiration pneu- eye of a dog with dysautonomia, pupillary constriction and
monia, persistent megaesophagus, acute fulminating MG, nictitating membrane retraction will occur within 60 minutes
and the presence of a thymoma or another underlying neo- or less, documenting denervation hypersensitivity. There
plasm are all associated with a poor prognosis for recovery. should be no response when this solution is applied to the
Many affected dogs die of either acute fatal aspiration or eye of a normal dog or cat. Subcutaneous (SC) administra-
euthanasia within 12 months of diagnosis. Anticholinester- tion of bethanechol (0.04 mg/kg) may also enable an affected
ase drugs effectively control appendicular muscle weakness dog with a distended bladder and urine dribbling to void
in most animals, but their effect on esophageal and pharyn- normally. SC administration of atropine (0.04 mg/kg) does
geal function is variable. Response to various immunosup- not produce a change in heart rate in affected dogs. These
pressive protocols is difficult to determine because most dogs findings suggest the diagnosis of dysautonomia, but defini-
with acquired MG (>85%) will go into a spontaneous per- tive diagnosis requires the demonstration of lesions within
manent clinical and immunologic remission within 18 the autonomic nervous system at postmortem examination.
months after diagnosis (average, 6.4 months) regardless of Loss of nerve cell bodies results in decreased neuron density
the treatment used. Spontaneous remission is unlikely in in all autonomic ganglia, especially the pelvic, mesenteric,
animals with thymoma or other neoplastic disease and in and ciliary ganglia.