Why do you do a MRI of the mediastinum in patients with myasthenia gravis?

Myasthenia gravis is a neurological condition that manifests with muscle weakness and - importantly - fatigability. It particularly involves the muscles of the head and neck (although it can involve the limbs too), and its presenting symptoms are therefore usually diplopia, ptosis, dysarthria and nasal speech.

Mediastinal MRIs are indicated in all patients who are diagnosed with myasthenia gravis, and this is because 10% of myasthenia patients have thymomas (tumours of the thymus). These are mostly benign, but even in this state they can cause problems due to compression of adjacent structures:

• superior vena cava → superior vena cava syndrome
• oesophagus → dysphagia
• trachea → dyspnoea, cough

Some, however, are malignant.

Even in the absence of a thymoma, 85% of patients clinically improve after thymectomy (the reason for this isn't clear), and so many authorities recommend routinely removing it in all patients with generalised myasthenia gravis if you're anywhere between a teenager and 55 years old.