This is a very rare and severe complication of mononucleosis. Meningoencephalitis is a medical condition that resembles both meningitis (an infection or inflammation of the meninges) and encephalitis (an infection or inflammation of the brain).
The patient starts feeling neurological manifestations and severe neck stiffness and headache, and must visit a doctor immediately.
As the symptoms of mononucleosis can be very nonspecific, it can be confused with other diseases, including flu, dengue, tonsillitis, among others. So, if you have had some of these symptoms for more than a week, or they are severe, it is important to look for a doctor in order to obtain the proper diagnosis and treatment to avoid complications.