Myasthenia Gravis Disease Background

Last updated: 05 May 2024

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Introduction 

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Introduction 

Myasthenia gravis (MG) is an uncommon neurological disorder caused by autoantibodies against the acetylcholine receptor (AChR), against a receptor-associated protein, muscle-specific tyrosine kinase (MuSK-Ab), or occasionally to the low-density lipoprotein receptor-related protein 4 (LRP4).  

Epidemiology 

Based on several studies, myasthenia gravis has an incidence rate of 1.7 to 21.3 cases per million people and a prevalence rate of 15 to 179 per million. Myasthenia gravis affects all ages but have a bimodal distribution based on sex and age. In younger patients, those younger than 40, females predominate. In the fifth decade of life, cases are equally distributed between males and females. After the age of 50, males are more commonly affected. 

Pathophysiology 

The presence of autoantibodies leads to an autoimmune attack at the muscle endplate, finally resulting to the failure of neuromuscular transmission and muscle weakness. 

Classification 

Autoimmune Myasthenia Gravis

Acetylcholine Receptor Myasthenia Gravis (AChR-MG)

Achr-MG is noted to be the most common form of MG. It is characterized by muscle weakness that is more predominant in the limbs and neck extensor than in bulbar and neck flexor muscles. There is also prominent ptosis and external ocular muscle weakness. In long-standing disease, wasting of proximal limb and ocular muscles may occur (myasthenic myopathy). In 65% of patients, there is some form of thymus pathology, 15% have thymoma. The risk of recurrence is low with AchR-MG.

Muscle-Specific Tyrosine Kinase Myasthenia Gravis (MuSK-MG)

Compared to AChR-MG, MuSK-MG predominantly affects the bulbar and neck flexor muscles than the limb and neck extensor muscles. Patients with MuSK-MG only have less prominent ptosis and extraocular muscle weakness compared to AChR-MG. With this type of MG, there is noted early wasting of facial and tongue muscles. In 10% of patients, there is noted thymic hyperplasia, and unlike AChR-MG, patients with MuSK-MG have high risk of recurrent crisis.

Other Forms of Myasthenia Gravis

Ocular Myasthenia Gravis

In this type, the disease is confined to the weakness of ocular muscles.

Generalized Myasthenia Gravis

In generalized MG, the weakness affects muscles other than the ocular muscles, but may still have some form of ocular weakness. The disease can be mild, moderate, or severe, characterized by respiratory involvement.

Anti-Lipoprotein Receptor Protein 4 (LRP4) Myasthenia Gravis

Patients with anti-LRP4 MG tend to be younger, more often to be female, and have a milder disease. With this type, there appears to be no association with thymic pathology, particularly thymoma.

Myasthenic Crisis or Relapse  

Relapse occurs when a patient with a previously controlled disease deteriorates. A crisis is considered severe if there is a need of respiratory assistance. This may be caused by reduction or withdrawal of previous therapy. Relapse or crisis may be elicited by stress (eg infection, surgery, hormonal factors) or by the introduction of a new drug (eg beta-blocker).