Myasthenia Gravis

 

  • Background

    Pathophysiology

    • Auto-antibody to nicotinic ACh receptor (NAChR) at the neuromuscular junction.
    • B and T cell involvement.

    Epidemiology

    • 1/8000 prevalence.
    • 2 peaks: young females – associated with other autoimmune diseases (RA, SLE) and thymic hyperplasia – and old males – associated with thymic atrophy or thymoma (10%).

  • Signs and symptoms

    Fatigable muscle weakness usually in following order:

    • Extraocular: ptosis, diplopia. Presenting complaint in 50%.
    • Bulbar: dysarthria/phonia/phagia.
    • Face: myasthenic snarl on smiling.
    • Neck
    • Limb girdle i.e. proximal weakness.
    • Trunk

    Exacerbating factors:

    • Intercurrent illness: infection, ↓K+.
    • Over-treatment.
    • Altered climate.
    • Physiological stress: pregnancy, emotion, exercise.
    • Drugs: gentamycin, tetracycline, opioids, quinine, Ξ²-blockers.

    Signs:

    • Fatiguability: cannot hold eyes in upward gaze for more than a few seconds, voice becomes quiet on counting to 50.
    • Weak forced eye closure that is easily overcome.
    • Reflexes, bulk, tone, and sensation all normal.

  • Differential diagnosis

    • Polymyositis
    • Multiple sclerosis.
    • Motor neuron disease.
    • Guillain BarrΓ© syndrome.
    • Lambert-Eaton myasthenic syndrome.
    • Botulism. Causes ↓ACh release at NMJ.

  • Investigations

    Anti-AChR Ab:

    • 90% sensitive, or 70% in ocular-only disease. May also be +ve in Lambert-Eaton, thymoma, or small cell lung cancer.
    • If -ve, check MuSK Ab (+ve in ⅓), and anti-SM Ab.

    Special tests:

    • EMG, which should include repetitive nerve stimulation and single-fibre EMG.
    • Ice test: ice pack applied to closed eyelid for 2-5 minutes → ptosis improves in MG.
    • Tensilon test: tests response to sequential doses of short-acting IV cholinesterase inhibitor edrophonium, which should cause transient ↑power. No longer routine practice, and if it is done there should be resus facilities and atropine to hand.

    CT thorax to look for thymus changes.

  • Management

    • Long-term cholinesterase inhibitor e.g. pyridostigmine. Cholinergic side effects: salivation, lacrimation, sweats, miosis, diarrhoea and vomiting, colic.
    • Immunosuppression for relapses: prednisolone, azathioprine, methotrexate, methylprednisolone IV, or plasma exchange.
    • Thymectomy if thymoma present. Also beneficial in treatment-resistant MG even if thymoma absent.

  • Prognosis

    • Often follows a relapsing-remitting course.
    • Normal life expectancy for most with treatment.

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