Immunology · Year 2 · from Immunology
Case 3: Myasthenia Gravis
Patient Demographics
- Age: 32 years old
- Sex: Female
- Ethnicity: Asian
Chief Complaint
Double vision and difficulty chewing for 2 months
History of Present Illness
A 32-year-old woman presents with intermittent double vision (diplopia) and difficulty chewing tough foods that has progressed over 2 months. She first noticed drooping of her eyelids (ptosis) in the evening after long days at work, which initially improved after rest. Over time, she developed horizontal diplopia and her jaw feels "tired" after eating. She denies limb weakness, difficulty breathing, or swallowing problems. Symptoms consistently worsen throughout the day and improve after rest.
Past Medical History
- Graves disease diagnosed 3 years ago (treated with radioiodine ablation)
- Currently on levothyroxine
Physical Examination
- General: Alert woman, appears comfortable
- Vital Signs: Normal
- HEENT: Bilateral ptosis, worse on right; positive fatigue test (ptosis worsens with sustained upgaze); diplopia on lateral gaze
- Cranial nerves: Otherwise intact; no facial weakness; voice normal
- Neck: Well-healed thyroidectomy scar
- Motor: 5/5 strength in all extremities; no fatigability with repetitive testing
- Sensory: Intact
- Reflexes: 2+ throughout, symmetric
Diagnostic Testing
| Test | Result | Reference Range |
|---|---|---|
| Acetylcholine receptor antibodies | 18.5 nmol/L | <0.4 nmol/L |
| Anti-MuSK antibodies | Negative | Negative |
| TSH | 2.1 mIU/L | 0.4-4.0 mIU/L |
| Repetitive nerve stimulation | >10% decrement | <10% decrement |
| CT chest | 3 cm anterior mediastinal mass | Normal |
Diagnosis
Myasthenia Gravis (MG) with thymoma, ocular-predominant with bulbar involvement
Discussion
Myasthenia gravis is an antibody-mediated autoimmune disorder targeting the neuromuscular junction:
Immunopathogenesis:
- Anti-AChR antibodies (present in 85% of generalized MG) cause:
- Accelerated receptor internalization and degradation
- Complement-mediated destruction of postsynaptic membrane
- Direct blockade of acetylcholine binding
- Anti-MuSK antibodies (5-8%) cause impaired AChR clustering
- Results in reduced safety factor at neuromuscular junction
This patient demonstrates classic features:
- Fatigable weakness: Worse with use, improves with rest
- Diurnal variation: Worse as day progresses
- Preferential ocular/bulbar involvement: Ptosis, diplopia, jaw fatigue
- Association with other autoimmune disease: Graves disease
Thymoma and MG:
- Thymic abnormalities in 75% of MG patients
- Thymoma in 10-15% (requires surgical resection)
- Thymic hyperplasia in 60-65%
- Thymectomy improves outcomes even without thymoma
MG classification by affected muscle groups:
| Class | Description |
|---|---|
| I | Ocular only |
| II | Mild generalized |
| III | Moderate generalized |
| IV | Severe generalized |
| V | Requiring intubation |
Treatment
- Symptomatic: Pyridostigmine (acetylcholinesterase inhibitor)
- Immunosuppression: Prednisone, azathioprine, or mycophenolate for generalized disease
- Thymectomy: Indicated for thymoma (mandatory) and non-thymoma MG under age 65
- Crisis management: IVIG or plasmapheresis for myasthenic crisis
- Monitor for respiratory compromise: Forced vital capacity
- Avoid medications that worsen MG: Aminoglycosides, beta-blockers, quinolones, magnesium
Myasthenic crisis warning signs: Dyspnea, weak cough, difficulty swallowing secretions