Myasthenia Gravis (MG) is a chronic autoimmune neuromuscular disorder that causes weakness in the skeletal muscles, which are responsible for voluntary movements such as breathing, walking, and swallowing. The hallmark of MG is muscle weakness that worsens with activity and improves after rest. It affects individuals of all ages but is more common in younger women and older men.
Causes
Myasthenia gravis is an autoimmune disorder, meaning the immune system mistakenly attacks healthy cells. In MG, antibodies produced by the immune system block or destroy the acetylcholine receptors at the neuromuscular junction—the area where nerve cells communicate with muscles. Acetylcholine is a neurotransmitter that enables muscle contraction, and when its receptors are blocked, muscles cannot receive signals properly, leading to weakness.
In some cases, MG may be associated with abnormalities of the thymus gland, which plays a role in immune function. The thymus gland may be abnormally large or contain tumors (thymomas), which can trigger the production of harmful antibodies.
Symptoms
The primary symptom of MG is muscle weakness that fluctuates and worsens with activity but improves with rest. The muscles most commonly affected are those controlling:
- Eye and eyelid movement: Leading to drooping eyelids (ptosis) and double vision (diplopia).
- Facial expression: Difficulty smiling, chewing, or swallowing.
- Speech: Slurred speech or nasal-sounding speech.
- Neck and limb muscles: Weakness in the arms, legs, and neck, leading to difficulty lifting objects, walking, or holding the head up.
- Respiratory muscles: In severe cases, MG can affect the muscles involved in breathing, causing a myasthenic crisis, which is a medical emergency.
Types of Myasthenia Gravis
- Ocular Myasthenia Gravis:
- Weakness is limited to the eye muscles, causing ptosis and diplopia.
- Some individuals with ocular MG eventually develop generalized MG, affecting other muscles as well.
- Generalized Myasthenia Gravis:
- Affects a broader range of muscles, including those involved in movement, speech, swallowing, and breathing.
Diagnosis
Diagnosing MG involves several steps and tests to confirm muscle weakness and the involvement of acetylcholine receptors:
- Physical and Neurological Examination:
- Evaluation of muscle strength, reflexes, coordination, and eye movements.
- Fatigability tests to observe muscle weakness after repeated use.
- Blood Tests:
- Detection of acetylcholine receptor antibodies (AChR antibodies) or muscle-specific kinase antibodies (MuSK antibodies), which are present in most people with MG.
- Electromyography (EMG):
- Tests the electrical activity of muscles to detect abnormal muscle responses to nerve stimulation.
- Tensilon Test:
- Injection of edrophonium (Tensilon), a drug that temporarily increases acetylcholine levels, may result in a brief improvement in muscle strength, suggesting MG.
- Imaging:
- A CT scan or MRI of the chest may be done to check for the presence of a thymoma or thymus gland enlargement.
- Pulmonary Function Tests:
- To evaluate the strength of the respiratory muscles, especially in severe cases or when a myasthenic crisis is suspected.
Prognosis
With appropriate treatment, most people with myasthenia gravis can manage their symptoms effectively and lead relatively normal lives. However, the disease is often unpredictable, and symptoms may fluctuate over time. Some individuals may experience periods of remission, while others may have persistent weakness.
Regular follow-up with a neurologist and adjustments in treatment are necessary to manage the condition long-term. Early intervention and treatment of myasthenic crises can prevent serious complications.