Thymoma & Myasthenia Gravis

• • Thymic tumors occur in anterior mediastinum and include thymomas, lymphomas, Hodgkin lymphoma

  • Most common type often difficult to distinguish from lymphoma

  • 3 predominant cell types

  • -Lymphocytic (25%)

    -Epithelial (25%)

    -Lymphoepithelial (50%)

Myasthenia Gravis (MG)

• • Neuromuscular disorder characterized by weakness and fatigability of voluntary muscles

  • Decreased number of acetylcholine receptors at neuromuscular junctions

  • Believed to be autoimmune process

Epidemiology

• • 30% of patients with thymoma have MG

  • Thymoma develops in 15% of patients with MG

  • Thymomas associated with paraneoplastic syndromes including:

  • -Cytopenias

    -Red cell aplasias

    -Hypogammaglobulinemias

    -Autoimmune diseases, such as rheumatoid arthritis and lupus

  • MG is more commonly associated with lymphocytic variety

Symptoms and Signs

• • 50% of asymptomatic cases identified on chest film

  • Chest pain dysphagia, MG, dyspnea, or superior vena cava syndrome most common if symptomatic

  • Easy fatigability in patients with MG

Laboratory Findings

• • MG

  • -90% have serum antibodies against acetylcholine receptors

    -70% have germinal center formation on thymic biopsy

Imaging Findings

• • Chest film: Anterior mediastinal mass

  • CT scan: Useful in assessing extent of lesion

  • MRI: Can assess vascular invasion

• •Myasthenia Gravis (MG): Decremental response in muscular contraction to repeated stimulation with improvement after edrophonium administration (short-acting anticholinesterase)

  • Definitive diagnosis: Histologic tissue analysis

  • Do not biopsy small, well-encapsulated mediastinal masses

  • Complete excision, otherwise risk tumor seeding

Rule Out

• • Lymphoma

  • -Can be difficult to differentiate histologically from thymoma

  • Thymic carcinoma

  • -Very aggressive variant of thymic lesions

• • Tissue histology necessary to make diagnosis

Surgery

• • Treatment of choice for thymoma is total thymectomy

  • Performed via median sternotomy, trap door, or clamshell approach

  • Cervical incision not useful for malignant disease, only for benign disease

Indications

• • Stages I, II, III should be aggressively resected

  • En bloc resection with associated structures is warranted if complete resection is possible

  • Large (> 5 cm) neoadjuvant chemotherapy may shrink tumor

  • Large lesions with gross invasion-biopsy to confirm histologic diagnosis, then neoadjuvant therapy prior to resection

  • MG indication for early thymectomy

Contraindications

• • Lymphoma not indication for thymectomy

Medications

• • Postoperative radiation therapy indicated for stage II

Myasthenia Gravis (MG)

• • Anticholinesterase drugs initial treatment and used aggressively in postoperative period

  • Corticosteroids used in select cases

  • Plasmapheresis can minimize need for anticholinesterase agents

  • Avoid muscle relaxants and atropine

Treatment Monitoring

• • Response rate exceeds 70% with chemotherapy

  • 75% of patients with MG improve after thymectomy