Infectious mononucleosis

A 19 year old college student presents with fever, severe fatigue, sore throat, and painful neck swelling for 1 week. Physical examination reveals tonsillar exudates, posterior cervical lymphadenopathy, and mild splenomegaly. Laboratory studies show lymphocytosis with atypical lymphocytes. The Monospot test is positive. Diagnosis?

Diagnosis is Infectious mononucleosis.

1. Definition

Infectious mononucleosis is a clinical syndrome most commonly caused by Epstein-Barr virus (EBV), characterized by fever, pharyngitis, lymphadenopathy, and atypical lymphocytosis.

2. Etiology

2.1 Common Cause

  1. Epstein-Barr virus (EBV) — most common cause

2.2 Mononucleosis-like Illnesses

  1. Cytomegalovirus (CMV)
  2. Acute HIV infection
  3. Toxoplasmosis
  4. Human herpesvirus 6 (HHV-6)
  5. Viral hepatitis including Hepatitis A
  6. Adenovirus

3. Pathophysiology

  1. EBV infects B lymphocytes via the CD21 (CR2) receptor
  2. Infected B cells trigger activation of cytotoxic CD8+ T cells
  3. Reactive CD8+ T cells appear as atypical lymphocytes on peripheral smear
  4. Lymphoid tissue hyperplasia causes lymphadenopathy and splenomegaly

4. Clinical Features

4.1 Classic Triad

  1. Fever
  2. Pharyngitis or sore throat
  3. Posterior cervical lymphadenopathy

4.2 Additional Features

  1. Severe fatigue
  2. Tonsillar enlargement with exudates
  3. Palatal petechiae
  4. Splenomegaly
  5. Hepatomegaly in some patients

5. Diagnosis

5.1 CBC

  1. Leukocytosis with lymphocytosis
  2. Atypical lymphocytosis

Important Point

  • Atypical lymphocytes are reactive CD8+ T cells

5.2 Monospot Test

  1. Detects heterophile antibodies
  2. Commonly positive in EBV infectious mononucleosis

Limitations

  1. May be falsely negative early in infection
  2. Less sensitive in young children

5.3 EBV Serology

  1. EBV viral capsid antigen (VCA) antibodies help confirm diagnosis

Typical Findings

  • Anti-VCA IgM → acute infection
  • Anti-VCA IgG → current or past infection

6. Management

6.1 Supportive Treatment

  1. Adequate rest
  2. Hydration
  3. Antipyretics and analgesics

6.2 Activity Restriction

  1. Avoid contact sports for at least 3 to 4 weeks and until splenomegaly resolves due to risk of splenic rupture

6.3 Important Drug Association

  1. Avoid amoxicillin and ampicillin
    • Can cause diffuse maculopapular rash in patients with EBV infectious mononucleosis

7. Complications

7.1 Hematologic Complications

  1. Autoimmune hemolytic anemia
  2. Thrombocytopenia

7.2 Splenic Complication

  1. Splenic rupture

7.3 Neurologic Complications

  1. Meningitis
  2. Encephalitis

7.4 Malignancy Associations

  1. Burkitt lymphoma
  2. Nasopharyngeal carcinoma
  3. Primary CNS lymphoma (especially in immunocompromised patients)

8. Key Clinical Insight

Fever, exudative pharyngitis, posterior cervical lymphadenopathy, atypical lymphocytosis, splenomegaly, and positive heterophile antibodies strongly suggest infectious mononucleosis

9. Exam Level Pearls

  1. Epstein-Barr virus is the most common cause of infectious mononucleosis
  2. EBV infects B cells via the CD21 (CR2) receptor
  3. Atypical lymphocytes are reactive CD8+ T cells
  4. Posterior cervical lymphadenopathy is characteristic
  5. Monospot test detects heterophile antibodies
  6. Amoxicillin classically causes diffuse maculopapular rash in EBV infection
  7. Splenic rupture is a serious complication
  8. Patients should avoid contact sports during acute illness
  9. EBV is associated with Burkitt lymphoma and nasopharyngeal carcinoma
By Prabin Duwadee at

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