Is it Mononucleosis or what is it?

Infectious mono, peripheral smear, low power showing atypical lymphocytosis.
Source: by Ed Uthman

Dear Dr. Mo: I’m a medical student and I wanted to ask about mononucleosis – in your office, when do you suspect it? It is confusing because when it presents with a sore throat I am not sure when to think “mono”. 

Dear reader: I understand your confusion and concern – they are both warranted. Timely, accurate diagnosis of infectious mononucleosis can help us target treatment, assure patient safety and provide an accurate prognosis. Let me quickly tell you when to be on alert for mono; and since you’re from the field, I’ll be free to use some technical lingo, so readers beware.

Mononucleosis is a viral illness most commonly found among patients 5 – 25 years of age and in particular those 16-20 among whom approximately 1 in 13 will present with sore throat and will have mononucleosis. If lymphadenopathy is absent in your clinical exam, the likelihood of mono is reduced but conversely, if the posterior cervical adenopathy, inguinal or axillary adenopathy, palatine petechiae and/or splenomegaly is present, the likelihood of mono increases.

As you rightfully pointed out in your question, symptoms are indeed of limited value for the diagnosis – sore throat, fever and fatigue are sensitive but quite non specific; also, the presence of atypical lymphocytosis can increase the likelihood of mononucleosis by a lot (although I am honestly not sure why are these lymphocytes called atypical for doing what a lymphocyte is supposed to be doing when presented with an antigen) but you have to expand your clinical mind here too as it could still be something else like CMV or Toxoplasmosis or any viral Hepatitis or even drugs like Fenitoin – these can all give you atypical lymphocytosis so you cannot really hang your hat on that either.

In conclusion, in adolescent and adult patients presenting with sore throat, look for the presence of posterior or cervical, inguinal or axillary lymphadenopathy, palatine petechiae, splenomegaly or/and atypical lymphocytosis as these are associated with an increased likelihood of mononucleosis. It will still likely be something else and more common but mono is a diagnosis you don’t want to miss, especially in an active teenager whose spleen gets enlarged and could potentially rupture and bleed out with any contact sports or high energy activity – rest is the most important treatment strategy to encourage in patients, in this and every other mono case.

Mono is something one doesn’t get rid of, the virus (EBV) stays with us for life and would typically be reactivated a couple more times in the future.

This was just a quick overview and I encourage you to discuss this topic further with your preceptors.

Yours in health,

Dr. Mo

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