Myasthenia gravis



Introduction
Epidemiology
Aetiology
Clinical features
Investigations
Management


Introduction

Myasthenia gravis is an autoimmune disorder resulting in insufficient functioning acetylcholine receptors. Antibodies to acetylcholine receptors are seen in 85-90% of cases. Myasthenia is more common in women (2:1)

Epidemiology

  • Incidence: 1.50 cases per 100,000 person-years
  • Peak incidence: 30-40 years
  • Sex ratio: 1:1
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+

Aetiology

Associations
  • thymomas in 15%
  • autoimmune disorders: pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE
  • thymic hyperplasia in 50-70%

Clinical features

The key feature is muscle fatigability - muscles become progressively weaker during periods of activity and slowly improve after periods of rest:

The most common exacerbating factor is exertion resulting in fatigability, which is the hallmark feature of myasthenia gravis . Symptoms become more marked during the day

The following drugs may exacerbate myasthenia:
  • penicillamine
  • quinidine, procainamide
  • beta-blockers
  • lithium
  • phenytoin
  • antibiotics: gentamicin, macrolides, quinolones, tetracyclines

Investigations

Investigations
  • single fibre electromyography: high sensitivity (92-100%)
  • CT thorax to exclude thymoma
  • CK normal
  • autoantibodies: around 85-90% of patients have antibodies to acetylcholine receptors. In the remaining patients, about about 40% are positive for anti-muscle-specific tyrosine kinase antibodies
  • Tensilon test: IV edrophonium reduces muscle weakness temporarily - not commonly used anymore due to the risk of cardiac arrhythmia

Management

Management
  • long-acting anticholinesterase inhibitors e.g. pyridostigmine
  • immunosuppression: prednisolone initially
  • thymectomy

Management of myasthenic crisis
  • plasmapheresis
  • intravenous immunoglobulins