Myasthenia gravis

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Thoracic radiographs demonstrating megaesophagus (arrows) and an asymmetric alveolar lung pattern affecting the left caudal and right middle lung lobes consistent with aspiration pneumonia due to Myasthenia gravis[1]

Myasthenia gravis (MG) is an acquired neurological disease characterised by muscle weakness and exercise intolerance in dogs.

Acquired MG is the result of autoantibodies against acetylcholine receptors located at neuromuscular junctions. It is more common in older German shepherd, Golden Retriever and Labrador Retrievers.

Myasthenia gravis may also develop as a paraneoplastic syndrome associated with thymoma.

These dogs present with exercise-induced stiffness, regurgitation and respiratory difficulty. Facial, pharyngeal, or esophageal weakness is common, and in many cases there is megaesophagus without generalized weakness (focal myasthenia). Aspiration pneumonia is a frequent complications.

Diagnosis is based on response to intravenous edrophonium (0.1-0.2 mg/kg) and detection of antibodies in serum.

A differential diagnosis would include exercise-induced collapse, botulism, idiopathic acute canine polyradiculoneuropathy and tick paralysis.

Treatment consists of anticholinesterase drugs, eg, pyridostigmine (1-3 mg/kg PO, bid-tid) or neostigmine (0.04 mg/kg, SC, qid)[2].

Pyridostigmine has also been used in conjunction with mycophenolate mofetil[3] and in conjunction with azathioprine and prednisolone in refractory MG cases[4].

Artificial ventilation may be required in critical cases[5].

Immunosuppressive dosages of prednisolone are recommended in animals that do not respond to anticholinesterase therapy[6].

Prognosis is generally good, and many dogs will undergo spontaneous remission, evident by a decrease in antibody titer. The prognosis is guarded for animals with aspiration pneumonia or persistent weakness.

References

  1. Richardson D (2011) Acquired myasthenia gravis in a poodle. Can Vet J 52(2):169-172
  2. Foy DS et al(2011) Cholinergic crisis after neostigmine administration in a dog with acquired focal myasthenia gravis. J Vet Emerg Crit Care (San Antonio) 21(5):547-551
  3. Dewey CW et al (2010) Mycophenolate mofetil treatment in dogs with serologically diagnosed acquired myasthenia gravis: 27 cases (1999-2008). J Am Vet Med Assoc 236(6):664-668
  4. Dewey CW et al (1999) Azathioprine therapy for acquired myasthenia gravis in five dogs. J Am Anim Hosp Assoc 35(5):396-402
  5. Rutter CR et al (2011) Outcome and medical management in dogs with lower motor neuron disease undergoing mechanical ventilation: 14 cases (2003-2009). J Vet Emerg Crit Care (San Antonio) 21(5):531-541
  6. Khorzad R et al (2011) Myasthenia gravis in dogs with an emphasis on treatment and critical care management. J Vet Emerg Crit Care (San Antonio) 21(3):193-208