Tonofibrils seen under electron microscopy can differentiate thymoma from other tumors such as carcinoid, Hodgkin's, and seminoma. Patients are usually asymptomatic but can present with myasthenia gravis-related symptoms, substernal pain, dyspnea, or cough. Invasive tumors can produce compression effects such as superior vena cava syndrome.
Electron microscopy examination of tonofibrils can distinguish thymoma from other tumors like carcinoid, Hodgkin's, and seminoma.  While most patients are asymptomatic, they may experience myasthenia gravis-related symptoms, substernal pain, shortness of breath, or cough. Invasive thymoma can compress blood vessels, leading to superior vena cava syndrome.