Myasthenia gravis

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Sections available in full article Introduction, Presenting signs, Age predisposition, Breed predisposition, Cost considerations, Pathogenesis, Etiology, Predisposing factors, Pathophysiology, Diagnosis, Presenting problems, Client history, Clinical signs, Diagnostic investigation, Confirmation of diagnosis, Gross autopsy findings, Histopathology findings, Differential diagnosis, Treatment, Initial symptomatic treatment, Standard treatment, Monitoring, Subsequent management, Sequelae, Prognosis, Expected response to treatment, Reasons for treatment failure, Sources, Publications, Vetstream contributor(s),
Contributors Dr Kyle Braund BVSc MVSc PhD FRCVS DipACVIM
Dr G Diane Shelton DVM PhD DipACVIM

Introduction

  • Cause : two forms: congenital (rare) and acquired (common). Congenital - structural or functional defect of acetylcholine receptors. Acquired - immune-mediated or paraneoplastic effect.
  • Signs : localized or generalized muscle weakness, may be exacerbated by exercise, regurgitation (megaesophagus); dysphagia (pharyngeal weakness): may be lameness, drooling, tremors.
  • Diagnosis : presumptive diagnosis by clinical signs and Tensilon test; acetylcholine receptor antibody test is confirmatory.
  • Treatment : treat underlying cause if present, anticholinesterases, low dose prednisone therapy if necessary. Rarely need other immunosuppression.
  • Prognosis : good in absence of severe aspiration pneumonia or dysphagia.

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

Diagnosis

Clinical signs

  • Muscle weakness that may be exacerbated by exercise.
  • Neurological examination normal.
  • Dilated esophagus.
  • Dysphagia.
  • Change in bark.
  • Decreased palpebral reflexes Neuro exam: palpebral reflexes.

Diagnosis

Differential diagnosis

  • Causes of weakness exacerbated by exercise:
    • Cardiovascular conditions.
    • Severe respiratory conditions.
    • Neoplasia.
    • General debilitation of chronic disease.
    • Other myopathic Myopathies or neuropathic Peripheral neuropathies disorders.
  • Causes of megaesophagus Megaesophagus.

Sequelae

Prognosis

  • Acquired myasthenia - good-fair.
  • Congenital myasthenia - guarded to poor.

Expected response to treatment

  • Variable response to anticholinesterase.
  • May be successful initially but then become refractory.
  • Spontaneous recovery occurs unless death results from severe aspiration pneumonia or neoplasia.
  • Treatment must be tailored to the needs of individual animals.
  • Anticholinesterase therapy and supportive care → complete remission → cessation of therapy.
  • Anticholinesterase therapy and low-dose corticosteroid therapy required in some cases → complete remission → cessation of therapy.

Reasons for treatment failure

  • Poor owner compliance Standard reasons for failure in a treatment.
  • Severe inhalation pneumonia Lung: aspiration pneumonia.
  • Severe dysphagia.
  • Overdosage with anticholinesterase.
  • Severe gut ulceration Gastric ulceration secondary to corticosteroid therapy.
  • Condition is, or becomes, refractory to anticholinesterase therapy (congenital myasthenia).

Sources

Publications

Refereed papers

  • Recent references from PubMed.
  • Shelton G D & Lindstrom J M (2001) Spontaneous remission in canine myasthenia gravis: Implications for assessing human MG therapies. Neurology 57, 2139-2141.
  • Wood S L, Rosenstein D S & Bebchuk T (2001) Myasthenia gravis and thymoma in a dog. Vet Rec 148 , 573-574.
  • 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.
  • Lipsitz D et al (1999) Inherited predisposition to myasthenia gravis in Newfoundlands. JAVMA 215 (7), 956-958.
  • 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, 1428-1431.
  • Klebanow E R (1992) Thymoma and acquired myasthenia gravis in the dog - case report and review of 13 additional cases. JAAHA 28 , 63-69.

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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