Infectious mononucleosis: Clinical sciences

Infectious mononucleosis, or mono for short, is a viral illness primarily caused by human herpes virus-4, otherwise known as Ebstein Barr Virus, or EBV. The virus spreads through close personal contact, usually through saliva or respiratory secretions. Classically, this happens by sharing food or drinks, or by kissing, which is why mono is sometimes called the “kissing disease” and why it’s common in young adults. Once the virus reaches the new person’s mouth, it infects both epithelial cells and B cells in the oropharynx. It is then carried throughout the body, allowing it to infect other lymphoid tissues including the liver, spleen, and lymph nodes.

Now, if a patient presents with a chief concern suggesting infectious mononucleosis, your first step is to obtain a focused history and physical exam. Your patient might report fever, fatigue, myalgia, sore throat, and in some cases, nausea, and vomiting. On the other hand, physical exam typically reveals posterior cervical lymphadenopathy, palatal petechia, tonsillar exudates, and possibly splenomegaly.

Here’s a clinical pearl to keep in mind! Splenomegaly in infectious mononucleosis is a result of lymphocytic infiltration in the spleen. Some patients might note vague abdominal discomfort or referred pain of the left shoulder, while others might be completely asymptomatic. During physical examination, be sure to check for an enlarged spleen. Palpate below the left costal margin and feel for the splenic edge then percuss for dullness in the lowest intercostal space along the left axillary line.

At this point, suspect infectious mononucleosis and obtain a heterophile antibody test, also known as a monospot test. If the heterophile antibody test is positive, diagnose infectious mononucleosis. Here’s the catch! A false negative heterophile antibody test is common in the early course of the illness. So, if the test is negative but you still clinically suspect infectious mononucleosis, order a CBC with differential. You can consider getting a peripheral smear to look at the morphology of the blood cells as well.

If the CBC reveals an absolute lymphocyte count greater than or equal to 4000 per cubic millimeter or an elevated lymphocyte count of 50% or more; or the peripheral smear shows at least 10% atypical lymphocytes, you should get a viral capsid IgM test. If this test is positive, diagnose infectious mononucleosis. If it’s negative, consider an alternative diagnosis.