Page 838 - Clinical Small Animal Internal Medicine

806 Section 8 Neurologic Disease

against RyR have been detected in both humans and dogs Corticosteroids may be used to treat MG if the patient
VetBooks.ir and in conjunction with AChR antibodies can cause a is not adequately responding to anticholinesterase ther-
apy alone. The optimal dosing regimen has not been
severe form of thymoma‐associated MG.
In people with MG, autoantibodies directed at stria-
suppressive doses are not recommended, especially in
tional muscle proteins (myosin, actin, alpha‐actinin, determined prospectively in the dog, but large immuno-
titin, sarcoplasmic reticulum) have been described. the face of aspiration pneumonia. In mild to moderately
They have also been described in the dog. Since these affected myasthenic dogs, a dose of 0.5 mg/kg daily to
proteins are normally not exposed to circulating anti- every other day has been recommended. There are
bodies they are thought to be nonpathogenic but may reports of high‐dose methylprednisolone therapy bene-
reflect muscle damage in cases where the AChR anti- fiting people but this must be used with caution in dogs.
body titer is negative, thereby supporting a diagnosis of Other immunosuppressants and immunomodulators
MG. Eighty percent of human patients with thymoma that have been described in case studies/series in the vet-
and MG, 24% of thymoma patients without MG, and erinary literature are azathioprine, ciclosporin A, and
30% of MG patients without thymoma have antistria- mycophenelate mofetil. These are uncontrolled retro-
tional antibodies. The incidence in dogs is not known. spective case series and although success was noted in
many of these cases, the use of these drugs should be tai-
lored to each individual case at this time. As an example,
Treatment
a retrospective study of 27 dogs with AChR‐positive
There are many reported treatments of MG but well‐ antibody titers reported that there was no beneficial
controlled prospective studies are not available. Many effect in dogs treated with pyridostigmine in combina-
canine patients will undergo spontaneous remission. tion with mycophenelate mofetil compared to pyri-
In one study, 47/53 (88.7%) AChR antibody‐positive dostigmine alone. Plasmapheresis has been described as
patients treated with anticholinesterase drugs alone a successful treatment in a single case report and more
went into remission over an average of 6.4 months. This recently has been described in several dogs that had
included focal, generalized, and fulminating forms. The therapeutic plasma exhange. These were serologically
presence of spontaneous remission needs to be consid- confirmed patients with marked generalized weakness,
ered when evaluating any treatment modality. Good megaesophagus, and aspiration pneumonia that were
clinical judgment should be used when determining the not adequately responding to traditional treatment.
therapeutic options in a case of MG. The presence or These dogs responded well to the therapy. Although
absence of dysphagia, esophageal weakness or megae- promising, this may have limited value due to cost and
sophagus may necessitate alternate feeding procedures the need for specialized equipment.
such as elevated feedings, a “Bailey Chair” (www. The management of severely compromised respiratory
baileychairs4dogs.com) or a gastrotomy tube. patients due to aspiration pneumonia and intercostal or
Anticholinesterase inhibitor (AChEI) drugs are the key diaphragm weakness usually needs to be done at facilities
therapy for MG and work by prolonging the action of with intensive care facilities and the ability to ventilate
acetylcholine at the neuromuscular junction. The dose patients if needed. Respiratory failure is the most com-
may need to be titrated to effect as these drugs have both mon cause of death in these patients. Antibiotics with
muscarinic and nicotinic effects. A return to full muscle the potential for neuromuscular blockade such as
strength is the goal. Animals can develop a cholinergic aminoglycosides and ampicillin should be avoided.
crisis (weakness, abdominal pain, salivation, and diarrhea) Serial monitoring of the AChR antibody titer should be
and need treatment with atropine and a decrease in dosage done in all patients and treatment should continue until
or discontinuation of the AChEI. The most commonly the antibody titer is negative. This would need to be inter-
used drugs are pyridostigmine bromide (Mestinon® preted in light of any concurrent immunosuppression.
1–3 mg/kg PO BID‐TID) and neostigmine bromide Treating acquired feline MG is very similar to the dog
(Prostigmin® 2 mg/kg/day divided to effect). Pyridostigmine but since there is a higher incidence of thymoma in cats,
is most commonly used because it has a longer duration all patients should have thoracic radiographs whether or
and fewer muscarinc effects. It is available as a tablet, not signs of megaesophagus or respiratory distress are
syrup, and timed‐release formulation. In critically ill present. The natural course of the disease in cats is also
patients or those with regurgitation that cannot take oral not known so we do not know if they also have sponta-
medication, a constant rate infusion (CRI) of pyridostig- neous remission. Cats can be treated with anticholinest-
mine bromide (0.01–0.03 mg/kg/h) may be used. The erase drugs but may also handle the adverse effects of
author has used neostigmine as a CRI at the same dosage corticosteroids better than dogs and therefore these
but found that it had many more adverse GI signs. may be more beneficial.