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Myasthenia gravis

icanis

Introduction

  • Cause: two forms: congenital (rare) and acquired (common). Congenital - structural or functional defect of acetylcholine receptors. Acquired - immune-mediated or paraneoplastic effect.
  • Signs: episodic, localized or generalized muscle weakness, worsen by activity and might be relieved by rest, regurgitation (megaesophagus); dysphagia (pharyngeal weakness); voice change (laryngeal weakness); facial muscle group weakness; may be lameness, drooling, tremors. Can present so weak that rest does not improve.
  • Diagnosis: presumptive diagnosis by clinical signs and edrophonium chloride (Tensilon ) challenge test; acetylcholine receptor antibody test is confirmatory.
  • Treatment: treat underlying cause if present, anticholinesterases +/- immunomodulatory therapy.
  • Prognosis: overall guarded (overall 1-year mortality rate as high as 60%). Aspiration pneumonia main cause of death.

    Print off the owner factsheet Myasthenia gravis Myasthenia gravis to give to your client.

Pathogenesis

Etiology

  • Congenital: often inherited.
  • Acquired: immune-mediated or paraneoplastic.

Predisposing factors

General
  • Underlying genetic predisposition.
  • Stresses including surgical.
  • Infectious diseases (triggers?).
  • Hormonal.
  • Thymic neoplasia Thymoma.

Pathophysiology

  • Acquired: aberrant immune process → development of heterogenous group of autoantibodies (mostly immunoglobulin class G) produced against the nicotinic acetylcholine receptor (AChR) antibodies → binds to the alpha-subunit nicotinic acetylcholine receptors at the neuromuscular junction → decreased number of functional receptors by causing endocytosis of AChR, activating complement-mediated destruction of postsynaptic muscle cell membrane near AChR, decreasing synthesis and membrane incorporation of new AChR, and directly interfering with AChR function by bound antibody.
  • Congenital: structural or functional abnormality of acetylcholine receptors. Result of a single or combination of presynaptic (mutations in choline acetyltransferase gene), synaptic (mutation of the acetylcholine esterase collagen gene) or postsynaptic (mostly deficient synthesis of AChr) defect(s).
  • In both forms of disease decreased number of functional nicotinic acetylcholine receptors → failure or reduction of synaptic transmission → failure or reduction of muscle contraction.

Diagnosis

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Treatment

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bexfield N H, Watson P J, Herrtage M E (2006) Management of myasthenia gravis using cyclosporine in 2 dogs. JVIM 20 (6), 1487-1490 PubMed.
  • Engel A G, Ohno K, Sine S M (2003) Congenital masthenic sundromes: progress over the past decade. Muscle Nerve 27 (1), 4-25 PubMed.
  • Shelton G D & Lindstrom J M (2001) Spontaneous remission in canine myasthenia gravis: Implications for assessing human MG therapies. Neurology 57 (11), 2139-2141 PubMed.
  • Wood S L, Rosenstein D S & Bebchuk T (2001) Myasthenia gravis and thymoma in a dog. Vet Rec 148 (18), 573-574 PubMed.
  • Ridyard A E, Rhind S M, French A T, Munro E A C & Hill P B (2000) Myasthenia gravis associated with cutaneous lymphoma in a dog. JSAP 41 (8), 348-51 PubMed.
  • Lipsitz D et al (1999) Inherited predisposition to myasthenia gravis in Newfoundlands. JAVMA 215 (7), 956-958 PubMed.
  • Shelton G D, Schule A & Kass P H (1997) Risk factors for acquired myasthenia gravis in dogs: 1,154 cases (1991-1995). J Am Vet Med Assoc 211 (11), 1428-1431 PubMed.
  • Klebanow E R (1992) Thymoma and acquired myasthenia gravis in the dog - a case report and review of 13 additional cases. JAAHA 28 (1), 63-69 VetMedResource.

Other sources of information

  • Shelton G D (2002)Myasthenia gravis and disorders of neuromuscular transmission.In:Veterinary Clinics of North America. Small Animal Practice.Ed G D Shelton. Philadelphia: W B Saunders. pp 189-206.

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