Infectious mononucleosis (IM) is a common illness in children and young adults caused primarily by the Epstein-Barr Virus (EBV). Transmission occurs primarily through sharing oral secretions, thus IM is known as the "kissing disease." Common clinical manifestations include fever, pharyngitis, posterior cervical lymphadenopathy, and splenomegaly. Atypical lymphocytosis and transaminase elevations are common, and the diagnosis of IM is confirmed with laboratory findings of a positive heterophile antibody ("Monospot"), polymerase chain reaction, or antibodies specific to EBV. Individuals with acute IM may be quite symptomatic and not feel well enough to participate in sports. Splenic enlargement is common, with rupture a relatively rare occurrence, typically occurring within a month of symptom onset, but this risk complicates sports participation, and is often the reason for restricting activity. The management of IM is primarily supportive, with no role for antivirals or corticosteroids. The variability of clinical presentation and the risk of splenic rupture in patients with IM present clinicians with challenging return to play/return to sport (RTS) decisions. This position statement updates the Evidence-Based Subject Review on Mononucleosis by the American Medical Society for Sports Medicine published in 2008 and reviews the epidemiology, clinical manifestations, laboratory assessment, and management including RTS for the athlete with IM. This statement also addresses complications, imaging, special considerations, diversity and equity considerations, and areas for future clinical research. Understanding the evidence regarding IM and sport is essential when communicating with athletes and their families and incorporating shared decision-making in the RTS decision.
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