Myasthenia gravis (MG) is an autoimmune disease affecting neuromuscular junctions of the body. The disease typically presents with autoantibodies in the body (immune proteins produced by the body’s immune system) targeting the nerve-muscle junction to create an immunological attack.
This leads to degradation of neurological signaling for voluntary muscle movement leading to muscle weakness in MG patients. The muscles frequently affected are those of the eyes, extremities, and throat in the body. Muscle weakness in the respiratory pathways may result in severe respiratory failure which is also called a myasthenic crisis. The initial symptoms of MG include ocular or eye-related symptoms such as drooping of upper eyelids or ptosis, caused by muscle weakness, and/or double vision, which is seen in 80% of the cases.
Types
Myasthenia gravis may be clinically classified according to the profile of the autoantibodies, the location of the affected muscles (ocular versus generalized), the age of onset of symptoms, and thymic abnormalities. According to the Myasthenia Gravis Foundation of America (MGFA), MG may be divided into 5 main classes (Class I to V) based on the clinical features and disease severity, each of which carries different prognoses or responses to therapy. MG may also be classified based on the clinical features and the type of antibodies involved, each of which responds differently to treatment:
Early-onset MG: Age at onset less than 50 years with thymic hyperplasia (the enhancement of the size of the thymus gland due to an increase in the number of cells).
Late-onset MG: Age at onset greater than 50 years with thymic atrophy (degeneration of cells in the thymus gland).
Thymoma-associated MG
Global perspective
MG is a rare neurological disease that is more common in adults than in children. The incidence and prevalence of the disease vary significantly based on geographic location. A systemic review on 35 studies up to 2007, highlighted the incidence rate of MG patients that ranged from 1.7 to 21.3 persons per million person-years, along with a global rate of 5.3 per million person-years. The overall incidence of acetylcholine receptor antibody-positive MG ranges between 4 and 18 per million person-years worldwide. However, the MG incidence has increased worldwide over the past seven decades. The prevalence rates have steadily increased over the past 50 years, partly due to improvements in recognition, diagnosis, treatment, and an overall increase in life expectancy in the United States. Incidence rates (varied) in Europe ranged from 4.1 to 30 cases per million person-years. According to the report of the National Organization for Rare Disorders, the prevalence of autoimmune myasthenia gravis ranges from approximately 14 to 40 per 100,000 individuals in the United States, with approximately current 36,000 to 60,000 cases in the United States. In women, the age of onset is between 30 and 50 years. In the case of men, the age of onset is found to be between 60 to 89 years. Women are more commonly affected before age 40, and the female: male ratio for early-onset MG is 3:1. However, after the age of 50, women and men are found to be equally affected, whereas the male: female ratio becomes 3:2.
Indian perspective
In India, MG distribution is gender-biased with a skewness towards males over females. The age of onset of the disease in males is between 60-70 years, whereas, in females, the onset is much earlier as seen in the early 30s of their lives. Another study from India emphasizes that among the late-onset MG patients, the male preponderance is seen with thymoma-associated MG observed in older patients.
Signs and symptoms
Symptoms include muscle weakness which can be prominently observed when the patient is in a restful state. The MG patients easily get tired, exhausted, and stressed out under trivial conditions. The patients are also prone to infections.
Diagnosis
Initial diagnosis of myasthenia gravis can be difficult as weakness is a common symptom in many disorders. The diagnosis is based on: