Why does the spleen enlarge in individuals with infectious mononucleosis (mono)?

Spleen Enlargement in Infectious Mononucleosis

The spleen enlarges in infectious mononucleosis primarily due to lymphocytic infiltration and proliferation in response to Epstein-Barr virus (EBV) infection, causing splenomegaly in approximately 50% of cases. 1

Pathophysiology of Splenic Enlargement in Mono

EBV infection follows a two-step process that directly impacts the spleen:

1. Initial Infection and Viral Replication:

   • EBV primarily infects B lymphocytes through binding to CD21 receptors
   • Infected B cells undergo proliferation and activation
   • These activated lymphocytes accumulate in lymphoid tissues, including the spleen

2. Immune Response:

   • The body mounts a robust T-cell response against infected B cells
   • Atypical lymphocytes (primarily CD8+ T cells) are produced in large numbers
   • These lymphocytes infiltrate the spleen, causing significant enlargement

The spleen, as a major lymphoid organ, becomes a site of intense immune activity during EBV infection. This results in:

• Increased cellularity within the splenic tissue
• Expansion of white pulp (lymphoid component)
• Overall enlargement of the organ

Clinical Significance of Splenic Enlargement

Splenomegaly in infectious mononucleosis has important clinical implications:

• Prevalence: Occurs in approximately 50% of patients with infectious mononucleosis 1
• Timing: Typically develops within the first 2 weeks of illness
• Duration: May persist for 3-4 weeks, sometimes longer

Complications Related to Splenic Enlargement

1. Splenic Rupture:

   • Rare but serious complication (0.1-0.5% of cases) 1
   • Most likely to occur during the second or third week of illness
   • Can occur spontaneously without significant trauma 2
   • May present with:
     • Abdominal pain
     • Left upper quadrant tenderness
     • Referred left shoulder pain (Kehr's sign)
     • Hemodynamic instability in severe cases

2. Splenic Infarction:

   • Uncommon complication
   • Can sometimes precede other classic symptoms of mono 3
   • Presents with upper abdominal pain

Management Implications

The enlarged spleen in infectious mononucleosis requires specific management considerations:

• Activity Restriction:

  • Patients should avoid contact sports or strenuous exercise for at least 8 weeks or until splenomegaly resolves 1
  • This precaution is critical to prevent traumatic splenic rupture

• Monitoring:

  • Physical examination to assess splenic size
  • Imaging (ultrasound or CT) may be necessary in some cases to confirm resolution

• Return to Activity:

  • Athletes can return to contact sports when the spleen returns to normal size 4
  • Complete recovery of athletic performance may take 3-6 months 4

Conclusion

Splenomegaly in infectious mononucleosis results from the body's immune response to EBV infection, with lymphocytic infiltration causing significant enlargement. While usually self-limiting, the enlarged spleen poses a risk for rupture, necessitating activity restrictions until resolution. Understanding this pathophysiology helps guide appropriate management and prevent potentially life-threatening complications.