A 58-year-old woman presents with progressive ptosis, diplopia, and proximal muscle weakness that worsens with activity and improves with rest. Serology is positive for anti-acetylcholine receptor (AChR) antibodies. What is the most common site of involvement in myasthenia gravis?

Explanation

## Clinical Presentation of Myasthenia Gravis **Key Point:** Ocular involvement is the most common initial and most frequent site of neuromuscular junction pathology in myasthenia gravis, occurring in approximately 90% of all MG patients at some point during their disease course. ### Frequency of Site Involvement | Site | Frequency | Clinical Features | |------|-----------|-------------------| | **Ocular muscles** | ~90% (most common) | Ptosis, diplopia, blurred vision | | Bulbar muscles | ~15% | Dysarthria, dysphagia, facial weakness | | Respiratory muscles | ~10% | Respiratory failure, myasthenic crisis | | Limb girdle muscles | ~70% | Proximal weakness, fatigue with activity | ### Pathophysiology at the NMJ **High-Yield:** In seropositive MG (85% of generalized cases), anti-AChR antibodies bind to acetylcholine receptors at the neuromuscular junction, causing: 1. Complement-mediated destruction of the postsynaptic membrane 2. Blockade of acetylcholine binding 3. Increased receptor internalization and degradation 4. Reduced number of functional AChR molecules This leads to **failure of neuromuscular transmission**, particularly evident in muscles with high metabolic demand (extraocular muscles are among the most active in the body). ### Clinical Pearl Ocular myasthenia gravis (OMG) is often the presenting symptom. Approximately 50% of patients with initial ocular involvement will progress to generalized MG within 2 years. The extraocular muscles are exquisitely sensitive to AChR antibody-mediated dysfunction because they have: - Highest innervation density - Highest firing frequency - Limited acetylcholine reserve **Mnemonic for MG Presentation — OPTIC:** - **O**cular (ptosis, diplopia) — most common - **P**roximal weakness - **T**ime-dependent (worse with activity) - **I**mproves with rest - **C**ranial nerve involvement (bulbar) ### Why Ocular Muscles Are Most Vulnerable Extraocular muscles have the highest density of neuromuscular junctions and the most frequent action potentials of any skeletal muscle group. Even modest reduction in AChR function (from antibody binding) causes visible dysfunction before other muscle groups are affected.