Definition
• The presence of increased lymphoid follicles in an otherwise normal thymus
Clinical features
• Present in up to 75% of cases of myasthenia gravis. These patients present with muscle weakness, especially of the eyes and facial muscles, which worsens on repeated movement
• Can also be associated with other autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, and Hashimoto’s thyroiditis
• Postthymectomy, myasthenia gravis patients that have many germinal centers have a slow remission rate compared with those that have few or no germinal centers in their thymus
Pathology
Gross
• Usually normal size and weight
Histology
• Increased density of lymphoid follicles with germinal centers (2 to 19 per low-power field)
• Preserved thymic architecture
• May have a plasma cell infiltrate or increased reticulin fibers
Immunopathology/special stains
• Not contributory
Main differential diagnoses
• Thymoma: architecture not preserved
• True thymic hyperplasia: increased weight and size without lymphoid follicles
Fig 1 Lymphoid hyperplasia of the thymus. Low-power view showing multiple lymphoid follicles with active germinal centers.
Fig 2 Lymphoid hyperplasia of the thymus. Intermediate-to-high-power views of the lymphoid follicles.
Fig 3 Lymphoid hyperplasia of the thymus. Intermediate-to-high-power views of the lymphoid follicles.
Fig 4 Lymphoid hyperplasia of the thymus. Intermediate-to-high-power views of the lymphoid follicles.