Page 1324 - Cote clinical veterinary advisor dogs and cats 4th

Myasthenia Gravis and Myasthenic Syndromes 669

ASSOCIATED DISORDERS Differential Diagnosis • IV fluids as needed
• In dogs, associated conditions include • Myopathies: polymyositis, degenerative • Nutritional support as needed: feedings
VetBooks.ir • Thymoma is common among cats with • Tick paralysis gastrostomy (preferred if megaesophagus Diseases and Disorders
with the head elevated or placement of a
myopathies
hypothyroidism, thymoma and other tumors,
hypoadrenocorticism, and thrombocytopenia.
• Acute idiopathic polyradiculoneuritis
present), nasogastric, or esophagostomy
acquired myasthenia gravis.
• Megaesophagus and aspiration can occur • Botulism feeding tube (pp. 1109, 1107, and 1106)
• Polyneuropathy
with myasthenia. • Metabolic disorders: hypokalemia, hypo- Chronic Treatment
glycemia, hypoadrenocorticism, hyperthy- Immunosuppressive therapy (for acquired
Clinical Presentation roidism in cats form only) is indicated when there is an
DISEASE FORMS/SUBTYPES • Orthopedic diseases: polyarthritis inadequate response to anticholinesterase
• Acquired: an autoimmune disorder char- • Focal acquired: megaesophagus (p. 642) medication. Prednisone is the initial drug of
acterized by circulating antibodies against choice (start at 0.5 mg/kg PO q 24h for 1-2
acetylcholine receptors at the neuromuscular Initial Database weeks, then increase to 2-4 mg/kg PO q 24h
junction; can occur in generalized, focal, or • CBC, serum biochemistry profile, urinalysis if needed; gradually taper if possible based on
acute fulminating forms to rule out metabolic causes of weakness clinical response). Azathioprine, cyclosporine,
• Congenital: an inborn defect in the neuro- • Thoracic radiographs to screen for mega- or mycophenolate mofetil can be added if
muscular junction esophagus, aspiration pneumonia there is an inadequate response to prednisone
• Thyroid, adrenal function testing or to allow decreased dose of prednisone in
HISTORY, CHIEF COMPLAINT • Edrophonium response test can help diagnose patients with severe glucocorticoid-related side
• Congenital: generalized weakness with or generalized myasthenia gravis. Administer effects.
without regurgitation/dysphagia, evident by edrophonium chloride 0.1-0.2 mg/kg IV
≈8 weeks of age during weakness. A positive response is Drug Interactions
• Generalized acquired: generalized limb weak- obvious but short-lived improvement in • Avoid drugs that impair neuromuscular
ness that can be precipitated by exercise, with strength within several minutes and sug- transmission, including ampicillin,
or without regurgitation/dysphagia gests the diagnosis. False-negative results aminoglycosides, fluoroquinolones, and
• Focal acquired: dysphagia/regurgitation or are common, and false-positive results phenothiazines.
facial weakness with no limb weakness are possible. Potential side effects include • Organophosphates can increase toxicity of
• Acute fulminating acquired: acute generalized dyspnea due to bronchial constriction and anticholinesterase drugs.
weakness and dyspnea due to respiratory secretions; treat with atropine. • In cats that develop myasthenia gravis while
muscle weakness taking methimazole, the methimazole should
Advanced or Confirmatory Testing be discontinued if possible.
PHYSICAL EXAM FINDINGS • Anti–acetylcholine receptor antibody test
• Generalized limb weakness is characterized by (p. 1298) for acquired myasthenia gravis Possible Complications
stiffness, tremor, and short-strided gait that • Electrodiagnostic testing (single-fiber electro- Aspiration pneumonia is the most common
may progress to inability to walk. Weakness myography and repetitive nerve stimulation) and serious complication in patients with
may be more severe in the pelvic limbs, is is useful for the diagnosis and for excluding pharyngeal/esophageal weakness.
often precipitated by 1-2 minutes of exercise, other causes of weakness.
and improves with rest. • Definitive diagnosis of congenital myasthenic Recommended Monitoring
• No ataxia and proprioceptive positioning is syndromes requires quantification of ace- • Client should monitor weakness and
usually normal when the patient’s weight is tylcholine receptors from muscle biopsy or dysphagia/regurgitation at home daily.
supported. Muscle atrophy is absent, and DNA testing. • Monitor anti–acetylcholinesterase receptor
tendon reflexes are usually preserved. antibody titer q 8-12 weeks in affected dogs.
• Weak palpebral reflex, especially in cats; TREATMENT Normalization of titer indicates remission.
palpebral may become progressively weaker
with rapid, repeated attempts to elicit the Treatment Overview PROGNOSIS & OUTCOME
reflex (decremental palpebral) Treatment goals include improving neuromus-
• Cats often have neck ventroflexion. cular transmission, administering supportive • Prognosis for the acquired form is good
• Abnormal lung sounds and fever possible if care, and immunosuppression (acquired form for patients without pharyngeal/esophageal
concurrent aspiration pneumonia only). weakness. Spontaneous remission occurs in
almost 90% of affected dogs but is uncom-
Etiology and Pathophysiology Acute General Treatment mon in cats.
• Congenital myasthenic syndromes are caused • Anticholinesterase drugs: pyridostigmine • Prognosis is guarded for patients with
by mutations in genes coding for components 0.5-3 mg/kg PO q 8-12h in dogs; 0.25 mg/ dysphagia/regurgitation because aspiration
of the neuromuscular junction. kg PO q 8-12h in cats. Start at low end of pneumonia is common and carries an ≈50%
• Acquired myasthenia gravis is caused by dose range, and titrate dose based on response 1-year mortality rate. If present, megaesopha-
circulating autoantibodies against the and side effects (hypersalivation, vomiting, gus may or may not resolve.
acetylcholine receptor. diarrhea). If dysphagia/regurgitation precludes • Relapse is rare but can be associated with
oral medication, neostigmine 0.04 mg/kg SQ stress (e.g., surgery) or vaccination.
DIAGNOSIS q 6h is an alternative. Human intravenous • The prognosis for the acute fulminating
immunoglobulin can be helpful for short-term form is poor; most affected dogs die from
Diagnostic Overview management of acute fulminating myasthenia. respiratory failure.
The diagnosis is suspected in one of four • Aspiration pneumonia (p. 793) is treated
contexts (see History, Chief Complaint above), with antibiotics, nebulization and coupage, PEARLS & CONSIDERATIONS
all characterized by weakness that often worsens and oxygen if necessary.
with exercise and resolves with rest. Measure- • Endotracheal intubation and ventilatory Comments
ment of serum anti–acetylcholine receptor support (p. 1185) may be necessary for the • Early diagnosis and treatment in an attempt
antibody levels is the diagnostic test of choice acute fulminating form or for patients with to avoid aspiration pneumonia improves
for acquired myasthenia gravis. severe aspiration pneumonia. outcome.
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