Myasthenia gravis

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ΕΤΥΜΟΛΟΓΙΑ

Latin. Myo - muscle, aesthenia - weakness, gravis - heavy/severe

Myasthenia gravis is an autoimmune disorder characterised by fatiguable weakness of skeletal muscles.

Contents

Aetiology

Pathophysiology

Myasthenic patients have a high incidence of other autoimmune disease (e.g. thyroid disease, rheumatological disease).

Animal passive transfer studies, and transient myasthenic-like syndrome in infants born to myasthenic mothers suggest an immune basis for the disease.

At present, two main antibody types have been implicated in the disease pathogenesis:

Anti-acetylcholine receptor antibody

These antibodies are thought to bind the post synaptic, nicotinic acetylcholine receptors, and can:

  • directly impair acetylcholine binding
  • disrupt the post-synaptic receptor via complement mediated injury
  • precipitate endocytosis of the receptors by the muscle

Anti-MuSK antibody

These patients are usually female, and usually have involvement of:

  • respiratory or bulbar muscles
  • neck, shoulder and respiratory weakness

MuSK (Muscle Specific Kinase) has an important role in postsynaptic differentiation and clustering of acetylcholine receptors, which is crucial for formation of the neuromuscular junction.

A third group of patients exist where it is not possible to identify circulating antibody to either MuSK or acetylcholine receptors. This third group is known as seronegative myasthenia gravis.

Clinical presentation

Usual presentation is variable exertional weakness of the skeletal muscles, eased with rest. The speed of onset is variable.

The condition may start with fatiguable ptosis, or other eye movement disorder. Weakness is also common in the head and neck, and bulbar muscles before spreading to the trunk and limb musculature.

A clinical classification system has been developed to assess response to treatment, and also for classification in research trials

Myasthenia Gravis Foundation of America Clinical Classification

Class I

Any eye muscle weakness Possible ptosis No other evidence of muscle weakness elsewhere

Class II

Eye muscle weakness of any severity Mild weakness of other muscles

  • Class IIa

Predominantly limb or axial muscles

  • Class IIb

Predominantly bulbar and/or respiratory muscles

Class III

Eye muscle weakness of any severity Moderate weakness of other muscles

  • Class IIIa

Predominantly limb or axial muscles

  • Class IIIb

Predominantly bulbar and/or respiratory muscles

Class IV

Eye muscle weakness of any severity Severe weakness of other muscles

  • Class IVa

Predominantly limb or axial muscles

  • Class IVb

Predominantly bulbar and/or respiratory muscles

(Can also include feeding tube without intubation)

Class V

Intubation to maintain airway

Signs

Generally, fatiguable weakness can be elicited although signs may be absent.

Bulbar muscles

Limb muscles

Respiratory muscles

Ocular muscles

Investigations

EMG allows separation from Lambert-Eaton myasthenic syndrome

Treatment

References

This article is a work in progress. Please feel free to contribute to it.

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