Infectious Mononucleosis
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Direct answer
Infectious mononucleosis is an Epstein-Barr virus infection of B lymphocytes that classically causes fever, pharyngitis and generalised lymphadenopathy, often with splenomegaly and atypical lymphocytes in the blood. It mainly affects adolescents and young adults, and young children often have milder or atypical illness. Management is supportive, and the key exam point is to avoid aminopenicillins, which trigger a rash in a high proportion of patients with EBV disease.
What you must remember
- Receptor and cell: EBV enters B cells through the CD21 receptor, and the atypical lymphocytes seen in blood are reactive CD8-positive T cells.
- Classic triad: fever, exudative pharyngitis and posterior cervical lymphadenopathy, with hepatosplenomegaly common.
- Atypical lymphocytes: large cells with abundant cytoplasm and indented nuclei that support the diagnosis but do not confirm it alone.
- Paul-Bunnell (heterophile) test: IgM heterophile antibodies agglutinate sheep or horse red cells, but sensitivity is poor in young children and early in illness.
- Confirmation: EBV-specific serology, especially IgM antibody to viral capsid antigen, is reliable when the heterophile test is negative.
- Complications: splenic rupture, airway obstruction from tonsillar swelling, haemolytic anaemia, thrombocytopenia, hepatitis and Guillain-Barré syndrome.
- Treatment: supportive care; steroids are reserved for airway obstruction or severe haemolysis or thrombocytopenia, and routine aciclovir has no proven benefit.
A stepwise approach to a teenager with a sore throat and nodes
A 15-year-old has five days of fever, exudative tonsillitis, tender posterior neck nodes and marked fatigue. The work-up follows a fixed order.
- Examine the spleen and throat. Palpate for splenomegaly and check the tonsils and palate, since tonsillar swelling can narrow the airway.
- Send a blood smear and heterophile test. Atypical lymphocytes and a positive heterophile result support the diagnosis in an older child.
- Move to EBV serology if the screen is negative. In younger children, or early in illness, check EBV VCA IgM, which is the more dependable test.
- Avoid the trigger drugs. Do not give ampicillin or amoxicillin for a presumed streptococcal sore throat, because the rash of EBV disease is mistaken for allergy.
- Monitor and advise. Check liver enzymes, haemoglobin and platelets, and advise avoidance of contact sport and heavy lifting for several weeks because of splenic rupture risk.
The point of this sequence is that a positive heterophile test in an older child can close the case quickly, while a negative result in a young child should prompt EBV serology rather than reassurance. Tonsillar swelling and splenomegaly are the findings that change management, so they must be recorded at the first visit.
Where the exam tries to catch you
Four beliefs cost marks on this topic.
- Trap: labelling the amoxicillin rash as a true penicillin allergy Correct: the rash is usually not an allergy, but aminopenicillins should be avoided during this illness.
- Trap: treating a negative heterophile test as excluding EBV Correct: the test is insensitive in young children and early disease, so EBV-specific serology settles the question.
- Trap: giving aciclovir or steroids routinely Correct: management is supportive, and steroids are kept for airway obstruction or severe haematological complications.
- Trap: allowing contact sport once the fever settles Correct: splenic rupture can occur after recovery, so restriction continues for several weeks.
Frequently asked questions
Which test confirms infectious mononucleosis in a young child?
Infectious mononucleosis in a young child is confirmed by EBV-specific serology, mainly IgM antibody to viral capsid antigen. The heterophile antibody test is often negative in young children, so a negative result does not exclude the disease.
Why does ampicillin cause a rash in infectious mononucleosis?
The ampicillin rash in infectious mononucleosis is a maculopapular eruption seen in many patients with EBV who take ampicillin or amoxicillin. It is generally not considered a true IgE-mediated allergy, so the drug is avoided during the illness rather than recorded as a lifelong penicillin allergy.
Which complication of infectious mononucleosis requires restriction of contact sport?
Splenic rupture is the complication that requires restriction of contact sport and heavy lifting in infectious mononucleosis. The enlarged spleen is vulnerable to trauma, and the risk persists for several weeks after the fever settles.
When are corticosteroids used in infectious mononucleosis?
Corticosteroids in infectious mononucleosis are reserved for airway obstruction from tonsillar swelling, or for severe haemolytic anaemia or thrombocytopenia. They are not used routinely for uncomplicated disease, where supportive care is sufficient.
Does infectious mononucleosis need antiviral treatment?
Routine antiviral treatment is not recommended for infectious mononucleosis, because uncomplicated disease resolves with supportive care. Aciclovir has no proven benefit in otherwise healthy children with this illness.