Infectious mononucleosis

Overview

•Caused by Epstein-Barr virus (EBV) - transmitted via saliva

•Peak incidence 15-24 years; incubation 4-6 weeks

Presentation

•Classic triad: fever + pharyngitis/tonsillitis (grey-white exudate) + posterior cervical lymphadenopathy

•Splenomegaly - up to 50% of cases, typically second week

•Palatal petechiae - specific sign at hard/soft palate junction

•Periorbital oedema - early feature

•Maculopapular rash - 5-10% spontaneously; rises to ~80-90% if amoxicillin or ampicillin given (immune complex-mediated, NOT true penicillin allergy)

•Profound fatigue - may persist weeks to months

⚠️

The amoxicillin rash in IM is NOT a true penicillin allergy - it is immune complex-mediated and specific to EBV infection. Do NOT label the patient as penicillin-allergic, but document and explain clearly.

Investigations

🥇 First-line

•FBC - lymphocytosis (>50% lymphocytes); atypical lymphocytes (Downey cells = activated CD8+ T cells); mild thrombocytopenia and neutropenia common

•Monospot (heterophile antibody test) - positive in ~85% by second week; lower sensitivity in first week and in children under 12

•LFTs - mildly raised transaminases in up to 80% of cases

🥈 Second-line

•EBV-specific serology (VCA IgM, VCA IgG, EBNA IgG) - if Monospot negative but suspicion remains high; VCA IgM = acute primary infection

•throat swab - exclude concurrent Group A Streptococcal pharyngitis (co-exists in up to 30%)

•abdominal ultrasound - if splenomegaly suspected or before return to sport

🎯

A negative Monospot in the first week does NOT exclude IM. Repeat at 1 week or send EBV serology. Monospot is unreliable in children under 12 - use EBV serology directly.

Differential Diagnosis

•CMV mononucleosis - clinically identical but Monospot-negative; confirm with CMV serology

•Acute HIV seroconversion - mononucleosis-like illness; always consider in sexually active patient with pharyngitis and lymphadenopathy

•Toxoplasmosis, viral hepatitis

Management

•First-line (supportive): rest, oral hydration, paracetamol or ibuprofen for fever and pain

•Avoid *amoxicillin and ampicillin** - if concurrent streptococcal infection confirmed, use phenoxymethylpenicillin*** (penicillin V)

•Activity restriction - avoid contact sport and strenuous activity for at least 3-4 weeks (or until splenomegaly resolved) to reduce risk of splenic rupture

🥈 Second-line

•*corticosteroids (e.g. prednisolone - indicated for impending airway obstruction from tonsillar enlargement, severe thrombocytopenia, or haemolytic anaemia; NOT for uncomplicated IM

🥉 Third-line

•hospital admission - for severe airway compromise, splenic rupture, or significant neurological complications

Complications

•Splenic rupture - <1% but life-threatening; spontaneous or after minor trauma

•Airway compromise - massive tonsillar enlargement; rare but genuine emergency

•Neurological - meningoencephalitis (~1%), facial nerve palsy, Guillain-Barré, transverse myelitis

•Haematological - autoimmune haemolytic anaemia, thrombocytopenia

•Post-infectious fatigue - prolonged fatigue in a significant minority; small number develop chronic fatigue syndrome-like picture

•EBV-associated malignancies - Burkitt lymphoma, Hodgkin lymphoma, nasopharyngeal carcinoma, post-transplant lymphoproliferative disease

🚨

Splenic rupture is the most immediately life-threatening complication. Suspect it when: sudden severe left upper quadrant or generalised abdominal pain + shoulder-tip pain (Kehr's sign - diaphragmatic irritation) + haemodynamic instability. This is a surgical emergency.

Prognosis

•Most recover fully within 2-4 weeks; fatigue may persist for several months

•Return to contact sport only once splenomegaly excluded or resolved