10.1 Introduction to Myasthenia Gravis

Overview of Myasthenia Gravis

Myasthenia gravis is an autoimmune disorder that disrupts communication between nerves and muscles at the neuromuscular junction. Because the body's own immune system destroys the receptors muscles need to receive signals, patients experience progressive weakness and fatigue in voluntary muscle groups. Understanding this mechanism is central to grasping why the medications covered later in this unit actually work.

Symptoms range from droopy eyelids to life-threatening breathing difficulty. Diagnosis relies on antibody blood tests, pharmacologic challenge tests, and electrophysiologic studies. The sections below break down the mechanism, symptoms, causes, and diagnostic approach.

Mechanism of Myasthenia Gravis

The core problem in myasthenia gravis is an autoimmune attack on acetylcholine receptors (AChRs) at the neuromuscular junction. Here's how the process unfolds:

1. The immune system produces antibodies that bind to AChRs on the postsynaptic membrane (the muscle side of the junction).
2. These antibodies destroy or block the receptors, reducing the total number of functional AChRs available.
3. When the presynaptic nerve terminal releases acetylcholine (ACh), there are fewer receptors for it to bind to.
4. With fewer successful ACh-receptor interactions, the muscle fiber can't generate a strong enough action potential to contract normally.
5. The muscle requires stronger and more frequent nerve impulses just to produce a basic contraction.

This is why weakness worsens with repetitive use (like typing, chewing, or climbing stairs) and improves with rest. During rest, ACh accumulates and has a better chance of binding to the remaining receptors. During sustained activity, ACh gets used up faster than it can compensate for the receptor deficit.

Myasthenia gravis primarily affects skeletal muscles, the muscles responsible for voluntary movement. Smooth muscle and cardiac muscle are not targeted.

Symptoms of Myasthenia Gravis

Symptoms fluctuate throughout the day and tend to worsen with activity. They can be grouped by the muscle regions affected:

• Ocular symptoms are often the earliest to appear:
  • Ptosis (drooping eyelids)
  • Diplopia (double vision)
• Bulbar symptoms involve muscles of the face, mouth, and throat:
  • Dysarthria (slurred or nasal-sounding speech)
  • Dysphagia (difficulty swallowing, which raises aspiration risk)
  • General facial muscle weakness, sometimes giving a flat or expressionless appearance
• Limb weakness tends to affect proximal muscles more than distal muscles. That means patients struggle more with climbing stairs, lifting objects overhead, or raising their arms than with fine motor tasks like buttoning a shirt.
• Respiratory muscle weakness is the most dangerous manifestation:
  • Dyspnea (shortness of breath)
  • Orthopnea (difficulty breathing when lying flat)
  • Severe cases can progress to myasthenic crisis, a medical emergency requiring ventilatory support

The hallmark pattern to remember: weakness gets worse with activity and better with rest.

Causes and Diagnosis of Myasthenia Gravis

Causes

The exact trigger for myasthenia gravis remains unknown, but two key factors are established:

• Autoimmune dysfunction accounts for most cases. A combination of genetic predisposition and environmental factors likely contributes, though no single cause has been identified.
• Thymoma (a tumor of the thymus gland) is present in about 10–15% of patients. The thymus plays a role in immune system development, and abnormalities in this gland may trigger the production of anti-AChR antibodies. This is why thymectomy (surgical removal of the thymus) is sometimes part of the treatment plan.

Diagnostic Tests

1. Acetylcholine receptor antibody test — A blood test that detects antibodies against AChRs. It's positive in about 80–90% of patients with generalized myasthenia gravis. A negative result doesn't rule out the disease, since some patients have antibodies against other targets (like MuSK).

2. Edrophonium (Tensilon) test — A short-acting acetylcholinesterase inhibitor is given intravenously. It temporarily prevents the breakdown of ACh, increasing ACh levels at the junction. If the patient shows a brief, noticeable improvement in muscle strength within seconds, the test is considered positive. (Note: this test is used less frequently now due to availability issues and the risk of cholinergic side effects.)

3. Repetitive nerve stimulation EMG — Electrodes measure the electrical response of a muscle while the nerve is stimulated repeatedly. In myasthenia gravis, you'll see a decremental response, meaning each successive stimulation produces a weaker muscle action potential. This reflects the progressive failure of neuromuscular transmission with repeated use.

4. Single-fiber EMG — The most sensitive test for detecting impaired neuromuscular transmission. It measures the timing variability ("jitter") between nerve impulse and muscle fiber response. Increased jitter and intermittent blocking of action potentials are characteristic findings in myasthenia gravis.

Autoimmune and Neurotransmitter Aspects

Myasthenia gravis belongs to a broader category of autoimmune diseases where the immune system attacks the body's own tissues. What makes it distinct is where that attack happens: specifically at the neuromuscular junction, targeting the acetylcholine receptor.

The thymus gland is thought to play a central role. In many myasthenia gravis patients, the thymus is abnormally enlarged (thymic hyperplasia) or contains a thymoma. The thymus normally helps train immune cells to distinguish "self" from "non-self." When this process goes wrong, the immune system may begin producing antibodies against AChRs.

The neurotransmitter at the center of this disease is acetylcholine (ACh). In a healthy neuromuscular junction, ACh crosses the synaptic cleft, binds to AChRs, and triggers muscle contraction. In myasthenia gravis, the reduced number of functional receptors means ACh can't do its job effectively. This is exactly why acetylcholinesterase inhibitors (covered in the next section of this unit) are a first-line treatment: they slow the breakdown of ACh, giving it more time to find and bind to the remaining receptors.