Learn The BasicsMyasthenia gravis (MG) is a rare autoimmune disease that causes muscle weakness. Autoimmune diseases are diseases in which the body’s immune system mistakenly attacks its own tissues.
MG can be divided into two groups based on the type of antibody the body produces. Antibodies are proteins normally produced in response to a foreign molecule, like a virus or bacteria.
MG can also be divided into two groups based on which area of the body it affects.
Types of MG based on antibody
Most people affected by MG have autoantibodies against a protein called an acetylcholine receptor (AChR), while others may have autoantibodies against a protein called a muscle-specific kinase (MuSK). Some people have neither type of antibody, which is called seronegative.
Anti-AChR-positive MG
In this type of MG, the body produces antibodies against the AChR, which is a protein found on the surface of muscle cells at the neuromuscular junction, or the connection between a neuron controlling movement (also called a motor neuron) and muscle fibers.
When a nerve signal is sent from the brain, it travels down a motor neuron and reaches the neuromuscular junction, and the electrical signal is converted into a chemical signal in the form of a neurotransmitter. Acetylcholine is one such neurotransmitter, which is released from the nerve cell ending at the neuromuscular junction and binds to the AChR on the muscle cell to transmit the signal to the muscle, which then contracts.
In the case of anti-AChR-positive MG, autoantibodies mistakenly recognize the AChR receptors as foreign and mark them for the immune system to destroy. This disrupts the signal transmission from nerves to muscles and leads to muscle weakness.
Anti-AChR-positive MG mainly affects the muscles in and around the shoulders, neck and face. However, there may be considerable variation from one person to another. In case the arms and legs are involved, muscle weakness more generally affects proximal muscles or those closest to the core of the body, like upper legs and hips and upper arms and shoulders. Less often, it can affect distal muscles or those further away from the center of the body, like hands, feet and the lower portions of limbs.
Anti-MuSK-positive MG
MuSK is a protein that plays a key role in the formation and maintenance of the neuromuscular junction.
When the immune system destroys the MuSK protein in anti-MuSK-positive MG, the signal transmission from the nerve cells to the muscle cells is disrupted.
This type of MG is usually more severe and symptoms progress more rapidly. It mainly affects women.
Seronegative MG
A small group of patients with MG have neither anti-AChR nor anti-MuSK antibodies in their blood. These patients are called seronegative or double-negative.
They are diagnosed based on clinical features and neurophysiological tests.
Types of MG based on affected area
MG can also be divided into two types based on the affected area: ocular or generalized MG.
Ocular MG
Ocular MG only affects the muscles that are responsible for moving the eyes and eyelids.
The main symptoms of this type of MG are droopy eyelids that may affect one or both eyes, double vision and difficulty focusing, which often worsen towards the end of the day when the muscles become fatigued.
Generalized MG
Generalized MG affects muscles throughout the body. In addition to visual symptoms, this type of MG may cause difficulty speaking and swallowing and difficulty walking due to muscle weakness in the arms or legs.
Reviewed by Debjyoti Talukdar, M.D., on June 4, 2025.