Pathophysiology and clinical signs of myasthenia gravis

1. Big picture

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction causing fluctuating, fatigable skeletal muscle weakness.

The key exam sentence:

Myasthenia gravis causes weakness that worsens with repeated use and during the day, and improves with rest.

The disease does not primarily affect sensation, reflexes, pupils, consciousness or the central nervous system. Therefore, the typical patient has:

• normal sensation;
• normal deep tendon reflexes;
• no Babinski sign;
• no pathological reflexes;
• normal pupils;
• fluctuating weakness without sensory loss.

The most important clinical groups affected are:

1. Ocular muscles → ptosis, diplopia
2. Bulbar muscles → dysarthria, dysphagia, dysphonia, chewing weakness
3. Limb and axial muscles → proximal weakness, neck weakness
4. Respiratory muscles → myasthenic crisis risk

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