10.1 Introduction to Myasthenia Gravis

Myasthenia gravis is an autoimmune disorder that messes with your muscles. It attacks the connection between nerves and muscles, making it hard for your brain to tell your body to move. This can lead to weakness and fatigue in various muscle groups.

The disease causes a range of symptoms, from droopy eyelids to trouble breathing. It's diagnosed through blood tests, nerve stimulation, and muscle activity measurements. Understanding how it works helps explain why rest can improve symptoms and why certain treatments are effective.

Overview of Myasthenia Gravis

Mechanism of myasthenia gravis

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• Autoimmune disorder targeting the neuromuscular junction
  • Antibodies attack and destroy acetylcholine receptors (AChRs) on the postsynaptic membrane of the neuromuscular junction reducing the number of functional AChRs
• Impaired neuromuscular transmission results from decreased sensitivity to acetylcholine (ACh) released by the presynaptic nerve terminal
  • Reduced ability of ACh to bind to AChRs and generate action potentials in the muscle fiber
• Muscle weakness and fatigue occur due to insufficient muscle fiber activation from impaired neuromuscular transmission
  • Muscles require stronger and more frequent nerve impulses to contract effectively
  • Weakness worsens with repetitive muscle use (typing) and improves with rest
• Primarily affects skeletal muscles, which are responsible for voluntary movement

Symptoms of myasthenia gravis

• Ocular symptoms manifest as diplopia (double vision) and ptosis (drooping eyelids)
• Bulbar symptoms include
  • Dysarthria (slurred or nasal speech)
  • Dysphagia (difficulty swallowing)
  • Facial muscle weakness
• Limb weakness affects proximal muscle groups more than distal causing fatigue with repetitive movements
  • Difficulty climbing stairs, lifting objects, or raising arms above the head
• Respiratory muscle weakness leads to
  • Dyspnea (shortness of breath)
  • Orthopnea (breathing difficulty when lying flat)
• Fluctuating nature of symptoms where weakness worsens with activity (chewing) and improves with rest

Causes and diagnosis of myasthenia gravis

• Causes
  • Autoimmune disorder (most common)
    • Exact trigger unknown but genetic predisposition and environmental factors may play a role
  • Thymoma (tumor of the thymus gland) present in 10-15% of patients may trigger autoimmune response
• Diagnostic tests
  1. Acetylcholine receptor antibody test detects presence of antibodies against AChRs in the blood
     • Positive in 80-90% of patients with generalized myasthenia gravis
  2. Edrophonium (Tensilon) test uses a short-acting acetylcholinesterase inhibitor administered intravenously to temporarily increase ACh levels at the neuromuscular junction
     • Positive test shows brief improvement in muscle strength
  3. Electromyography (EMG) with repetitive nerve stimulation measures electrical activity in muscles during nerve stimulation
     • Decremental response in muscle action potential with repetitive stimulation suggests myasthenia gravis
  4. Single-fiber EMG is a highly sensitive test for detecting impaired neuromuscular transmission
     • Abnormal jitter and blocking of muscle fiber action potentials occur in myasthenia gravis

Autoimmune and Neurotransmitter Aspects

• Myasthenia gravis is one of several autoimmune diseases where the immune system mistakenly attacks the body's own tissues
• The disease affects the transmission of neurotransmitters (specifically acetylcholine) at the neuromuscular junction
• The thymus gland, which plays a crucial role in the immune system, may be involved in the production of antibodies that attack AChRs