Transient neonatal MG is a rare type of the disease in which babies born to a mother with myasthenia gravis also will develop the disorder. It’s thought to occur in about 10%-20% of infants born to women with MG.

During pregnancy, the mother’s MG self-reactive antibodies can be passed on to the unborn child through the placenta, resulting in a similar autoimmune response in the fetus.

Symptoms are usually apparent shortly after birth and may include:

• muscle weakness
• impaired suckling or swallowing
• a weak cry or respiratory insufficiency.

While serious, these symptoms usually disappear within a few weeks or months with supportive treatment.

This type of MG can occur when a mother has active disease or is in remission. There are no reliable ways to predict which babies will be born with the condition, and no known interventions for preventing it.

Additional classifications

Myasthenia gravis patients also may be grouped according to various disease subtypes, which can influence a person’s prognosis, treatment plan, and clinical presentation — essentially, the symptoms a patient experiences.

Based on age at disease onset

While MG can occur at any age, it is most commonly diagnosed in adult women younger than age 40 and in men older than 60. Patients can be categorized based on when their symptoms began, as follows:

Early-onset myasthenia gravis symptoms start in adulthood, but among patients younger than age 50. This type of MG more often affects women than men.

Late-onset myasthenia gravis symptoms emerge in adults after age 50. This form is more commonly seen in men.

Juvenile myasthenia gravis symptoms are found in children and teens, most often in teenage girls. In the U.S., about 1 in 10 cases of MG are this subtype.

Symptoms of juvenile myasthenia gravis may be similar to those seen in adults, but are typically lifelong.

Based on antibodies

MG is caused by the immune system’s production of self-reactive antibodies that target proteins important for nerve-muscle communication.

The two main types of MG-causing antibodies target acetylcholine receptors, known as AChRs, and muscle-specific kinase, or MuSK. But their effects are disproportionate:

Those targeting AChRs are the most common type of MG-causing antibodies, present in about 85% of patients.

The ones against MuSK are found in about 6% of people with MG.

MuSK antibodies are often associated with more severe, rapidly progressing disease and a higher incidence of respiratory crises compared with anti-AChR antibodies.

Some patients may not test positive for either anti-AChR or anti-MuSK antibodies, and are thus considered to be double-seronegative.

These individuals also may or may not test positive for more rare types of MG-causing antibodies, such as those targeting the lipoprotein receptor-related protein 4 (LPR4) or agrin, which have been more recently identified as a potential cause of MG.

Based on thymoma

Abnormalities in the thymus gland are thought to be implicated in the production of the self-reactive antibodies that drive MG. About 10%-15% of people with MG have a thymoma, or a thymus tumor, that is thought to underlie their MG.

Patients with a thymoma have been reported to have more severe symptoms and higher levels of anti-AChR antibodies compared with those without a thymoma. Thymoma-associated MG is less common in people with late-onset disease.

Thymomas can be difficult to diagnose in MG patients, as they may not cause any relevant signs, apart from a persistent cough or chest pain.

A surgical procedure to remove the thymus gland, called a thymectomy, can be an effective treatment for people with thymoma-associated MG.

For further information on MG or other Rare Diseases in Canada, please go to www.raredisorders.ca