Myasthenia Gravis - Epidemiology

Epidemiology

Myasthenia gravis occurs in all ethnic groups and both sexes. It most commonly affects women under 40 and people from 50 to 70 years old of either sex, but it has been known to occur at any age. Younger patients rarely have thymoma. The prevalence in the United States is estimated at 20 cases per 100,000. Risk factors are the female sex with ages 20 – 40, familial myasthenia gravis, D-penicillamine ingestion (drug-induced myasthenia), and having other autoimmune diseases.

Three types of myasthenic symptoms in children can be distinguished:

1. Neonatal: In 12% of the pregnancies with a mother with MG, she passes the antibodies to the infant through the placenta, causing neonatal myasthenia gravis. The symptoms will start in the first two days and disappear within a few weeks after birth. With the mother, it is not uncommon for the symptoms to even improve during pregnancy, but they might worsen after labor.
2. Congenital: Children of a healthy mother can, very rarely, develop myasthenic symptoms beginning at birth, congenital myasthenic syndrome or CMS. Other than myasthenia gravis, CMS is not caused by an autoimmune process, but due to synaptic malformation, which in turn is caused by genetic mutations. Thus, CMS is a hereditary disease. More than 11 different mutations have been identified, and the inheritance pattern is typically autosomal recessive.
3. Juvenile myasthenia gravis: myasthenia occurring in childhood, but after the peripartum period

The congenital myasthenias cause muscle weakness and fatigability similar to those of MG. The symptoms of CMS usually begin within the first two years of life, although in a few forms, patients can develop their first symptoms as late as the seventh decade of life. A diagnosis of CMS is suggested by the following:

• Onset of symptoms in infancy or childhood
• Weakness which increases as muscles tire
• A decremental EMG response, on low frequency, of the compound muscle action potential (CMAP)
• No anti-AChR or MuSK antibodies
• No response to immunosuppressant therapy
• Family history of symptoms which resemble CMS

The symptoms of CMS can vary from mild to severe. It is also common for patients with the same form, even members of the same family, to be affected to differing degrees. In most forms of CMS, weakness does not progress, and in some forms, the symptoms may diminish as the patient gets older. Only rarely do symptoms of CMS become worse with time.