Page 836 - Clinical Small Animal Internal Medicine

804 Section 8 Neurologic Disease

the absence of generalized muscle weakness. One study ● antibody cross‐linked AChRs result in increased inter-
VetBooks.ir demonstrated that 43% of dogs with a confirmed diagnosis nalization of AChRs, a decreased receptor lifespan and
a decrease in the total number of AChRs
of MG (positive AChR antibody titer) did not have clini-
direct inhibition of AChR function.
cally detectable limb muscle weakness. The remaining
57% of dogs had generalized weakness, with 13% of these ● A familial acquired MG has been reported in the
having no esophageal or pharyngeal dysfunction. Newfoundland and Great Dane.
The classic presentation of a dog with MG is generalized
muscle weakness that worsens with activity but not all
animals display this exercise intolerance. Dogs are more Diagnosis of Acquired Myasthenia Gravis
likely than humans or cats to develop megaesophagus The diagnosis of MG cannot be made on clinical signs
because the canine esophagus contains more skeletal alone and should be considered in any animal with signs
muscle than smooth muscle. If megaesophagus is pre- of neuromuscular disease. Other diseases that may have
sent there is a higher risk of aspiration pneumonia. similar clinical presenting signs include inflammatory
An acute fulminating form of MG (<5% of cases) has and noninflammatory myopathies, polyneuropathy,
been described. These dogs present with acute onset of especially acute ones such as polyradiculoneuritis,
rapidly developing signs. They are usually tetraparetic/ endocrine disorders such as hypoadrenocorticism or
plegic with marked respiratory distress due to a combi- other diseases of neuromuscular transmission such as
nation of aspiration pneumonia and respiratory muscle tick paralysis, botulism, and organophosphate toxicity.
weakness. These animals require intensive care and A minimum database including a complete blood count
may need ventilatory support. For this reason, acute (CBC), chemistry profile with creatine kinase (CK),
fulminating MG has a higher mortality rate. This form urinalysis and potentially a thyroid panel should be done
has also been associated with a higher rate of thymoma. to rule out other conditions resulting in or contributing
Thirty to 50% of dogs with thymoma may develop MG. to weakness. Thoracic radiographs may identify meg-
In order for these cases to resolve, thymectomy is aesophagus, aspiration pneumonia, or a cranial medi-
recommended. astinal mass.
The clinical signs of MG in cats are also variable, with The use of the short‐acting anticholinesterase drug
the two most common clinical signs being generalized edrophonium chloride (Tensilon®) (dogs 0.1–0.2 mg/kg
weakness without megaesophagus and generalized IV and cats 0.25–0.5 mg total dose) may provide a
weakness associated with a cranial mediastinal mass. presumptive diagnosis of MG but there are both false‐
The incidence of megaesophagus and dysphagia was positive and false‐negative results. An overdose of edro-
lower compared to dogs. Myasthenia gravis was also phonium or its use in a nonmyasthenic patient may
documented in five hyperthyroid cats. These cats cause parasympathetic overstimulation and necessitate
developed weakness 2–4 months after treatment with the use of atropine sulfate. Cats should be pretreated
methimazole and were seropositive (AChR antibodies). with atropine as they appear to be more sensitive to
It was postulated that the drug caused a reversible lack of edrophonium. Dramatic improvement in muscle
tolerance to self‐AChRs strength after edrophonium administration provides a
presumptive diagnosis of MG but other myopathic or
neuropathic disorders may also show an improvement.
Acquired Myasthenia Gravis Dogs with fulminant MG may not have enough AChRs
available to elicit a noticeable response. In dogs with
Acquired MG is an immune‐mediated disorder in which focal MG, there may not be a positive response to the
autoantibodies (typically IgG) against postsynaptic nico- edrophonium chloride test or it may be hard to judge
tinic AChRs of skeletal muscle are produced, resulting in the improvement, for instance in a dog with megae-
impaired neuromuscular transmission. sophagus alone. Unfortunately edrophonium chloride is
In acquired MG, there is a T lymphocyte‐dependent
production of specific autoantibodies directed against no longer clinically available therefore this type of test-
ing would be limited to clinicians who have any remain-
the main immunogenic region (MIR) of the nicotinic ing supply of edrophonium. It is the authors clinical
ACHR, which is located at the extracellular tip of the experience that injectable neostigmine can be used as a
α1‐subunits, allowing antibodies to cross‐link AChRs diagnostic challenge as well. Standard dosing (dogs
and induce antigenic modulation. These antibody 0.04 mg/kg IM) is used and the time to respond skin may
complexes result in immune‐mediated destruction of be up to 30 minutes.
the postsynaptic receptor via three main mechanisms:
Electrodiagnostic testing may also be useful to add
there is complement‐dependent lysis of the postsynap- supportive evidence to the diagnosis and exclude other
●
tic membrane caused by antibodies bound to AChRs neuromuscular disorders. Electromyography (EMG) and