Delusional disorder can appear in several ways

Dear Doctor K: I hear the phrase “you’re delusional” used so casually. But what does it mean when someone is really, clinically delusional?

Answer: If I told you I could substitute for Derek Jeter at shortstop for the New York Yankees, you could fairly call me delusional. (Actually, it might be a good thing if I could convince the Yankees to do that: I’m a Boston Red Sox fan, and my presence in the Yankee lineup would surely improve the Red Sox’ chances.)

A delusion is a false belief that is based on an incorrect interpretation of reality. A person with delusional disorder will firmly hold on to a false belief despite clear evidence to the contrary.

Delusions can be caused by mental illnesses called psychoses. These include schizophrenia and bipolar disorder. Delusions also can occur in degenerative brain conditions, particularly Alzheimer’s disease.

Delusions often are beliefs that could not possibly be true. If I believed that I were Joan of Arc, or that the moon was going to smash into the Earth tomorrow, those would be delusions.

Delusions also can be about something that is not happening, but that could occur in reality. The CIA could really be tapping my telephone, for example: I have a phone, and the CIA has the ability to tap it. If I devoutly believed that the CIA was tapping my phone, yet couldn’t give you a plausible explanation for why it would want to do that, I would likely be delusional.

In a condition called delusional disorder, a person with otherwise normal thinking continues to hold one very focused delusion. I might be functioning perfectly normally and expressing no strange thoughts ... except the repeated belief that Tony Bennett was going to ask me to sing with him at a benefit concert in the near future.

There are several types of delusions often seen in delusional disorder (and other types of psychosis):

Erotic: delusion of a special, loving relationship with another person, usually someone famous or of higher standing. (This kind of delusion is sometimes at the root of stalking behavior.)

Grandiose: delusion that the person has a special power or ability, or a special relationship with a powerful person or figure, such as the president or a celebrity.

Jealous: delusion that a sexual partner is being unfaithful.

Persecutory: delusion that the person is being threatened or maltreated.

Somatic: delusion of having a physical illness or defect.

In some people with delusional disorder, the delusion may continue for years, though it may vary in its intensity and significance. In other people, the disorder lasts only a few months.

People with other types of psychosis besides delusional disorder often have multiple, loosely connected delusions: The CIA is tapping their phone and someone (the CIA?) has placed wires in their head to control their brain.

Anyone expressing delusions has a serious underlying mental or neurological disorder. Medications and psychotherapy can help. The hardest part is getting someone who is delusional to recognize that he or she may need help. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.