Megalomania does not appear in the DSM, but can be a feature of narcissistic personality disorder. It's an odd convention of the DSM that personality disorders are not considered "major mental illnesses" in the manner of depression or schizophrenia, but are still registered in that compendium of psychological disorders. PDs are thought to develop early - by young adulthood, and to be pervasive and difficult to treat, so entrenched in the personality are they. They are not episodic in the way that other disorders are, and can make "co-morbid", or co-occurring, disorders worse and more intractable to treatment.

Two psychiatrists who appeared on a Lawrence O'Donnell segment on MSNBC in recent days agreed that Trump has NPD, is paranoid (may have Paranoid Personality Disorder or delusions) and psychopathic. Psychopathy is not in the DSM, but its traits and measurement are standard fare in the clinical realm. The disorder most like it in the DSM is Antisocial Personality Disorder...