Types of Myasthenia Gravis

Myasthenia gravis (MG) comes in 2 main types, generalized and ocular. There are also subtypes of MG. People with MG can have multiple subtypes. Treatments can be tailored to fit a person’s type and subtypes.^1-4

Main types

Ocular

Ocular means “eye.” In ocular myasthenia gravis, the muscles that control the eyes and eyelids are weak and become tired with use. This can lead to droopy eyelids and double vision. About 15 percent of all people with MG have the ocular form.²

Generalized

Symptoms often begin in the eye and gradually spread to other muscle groups. When this happens it is called generalized MG. The muscles in the face and arms or legs are the areas most often affected. One out of every 10 people with generalized MG will develop weakness in the muscles that control breathing. When this happens it is called a myasthenic crisis.¹

If someone’s myasthenia gravis does not respond well to treatment, it may be called refractory MG. Only 15 percent of people with MG have refractory myasthenia gravis.³

Subtypes

Myasthenia gravis can also be divided into subtypes. These subtypes describe different features of the disease, such as:^1,4

• Age when symptoms began
• Cause
• Thymoma
• Types of antibodies present

Age when symptoms began

• Early-onset MG: Symptoms began when the person was under age 50
• Late-onset MG: Symptoms began when the person was over age 50
• Juvenile MG: Most often found in teenage girls. About 1 in 10 cases of MG are found in children or teens.

Cause of MG

• Autoimmune MG: Most people have this type of myasthenia gravis. It is caused by the immune system attacking healthy parts of the muscles that communicate with nerves.
• Transient neonatal MG: Some babies born to mothers with MG will have temporary muscle weakness. This usually wears off after a few weeks. These babies do not have a greater risk of developing MG later in life.
• Congenital myasthenic syndrome: In rare cases, someone will inherit MG but this is very rare. Symptoms usually begin in infancy and are lifelong.

Thymoma

A thymoma is a tumor on the thymus gland. About 1 out of 10 people with myasthenia gravis have a thymoma.⁴

Types of antibodies

With MG, the body creates chemicals called antibodies. These chemicals attack healthy parts of the muscle that communicate with nerves. This keeps the muscles from working correctly and leads to weakness. Doctors have found that people with MG create antibodies including:⁴

• Acetylcholine receptor (AChR)
• Muscle-specific kinase (MUSK)
• Lipoprotein-related protein 4 (LRP4)

How doctors classify

The Myasthenia Gravis (MG) Foundation of America divides MG into 5 main classes and several subclasses. This is how doctors talk about MG. People with MG generally do not use these terms. Here is what each class and subclass means:^2,4

• Class I: Eye muscles are the only muscles affected. May have trouble opening the eye. All other muscle strength is normal.
• Class II: Mild muscle weakness anywhere in the body. May also have eye muscle weakness of any severity.
• Class IIa: Mild muscle weakness that targets mostly the arms, legs, neck, and back muscles. The mouth and throat muscles may be less affected.
• Class IIb: Mild muscle weakness that mostly targets the mouth, throat, and breathing muscles. The arms, legs, neck, and back muscles may be less affected or equally affected.
• Class III: Medium weakness affecting any muscles other than the eyes. The eye muscles may also be weak.
• Class IIIa: Medium muscle weakness in the arms, legs, neck, or back. The mouth and throat muscles are less often affected.
• Class IIIb: Medium weakness in the mouth, throat, and breathing muscles. Muscles in the arms, legs, neck, or back may also be affected.
• Class IV: Severe weakness affecting any muscles other than the eyes. The eye muscles may also be weak.
• Class IVa: Severe weakness in the muscles of the arms, legs, neck,and back. Mouth and throat muscles are less severely affected.
• Class IVb: Severe weakness in the throat, mouth, and breathing muscles. Sometimes arms, legs, neck, and back muscles are less severely affected. The person may need a feeding tube but not help to breathe (intubation).
• Class V: The person needs help with breathing. This usually means the person needs intubation and a mechanical ventilator. This does not apply to routine recovery from surgery.