A delusion is a deeply held belief by a patient about a fact that is not true, even when the patient has strong evidence of the truth. This is often a symptom of some type of brain damage, but is common in many types of mental illness as well. It differs from a hallucination in that it happens when the patient is otherwise lucid and that patient can often determine that hallucinations are not true. It differs from a psychotic break in that the patient can rationally discuss the delusion.
Unlike hallucinations, which do not necessarily indicate an illness, a delusion is always a symptom of an underlying illness.
Delusions fall into four general types:
- Bizarre - something that could not possibly be true
- Non-Bizarre - a delusion which, although demonstrably false, could be true
- Mood congruent - where the delusion is strongly related to the person's current mental state
- Mood neutral - where the delusion is independent of the person's mental state
From there, there are several different common types of delusions, such as the belief the patient can control things, that one's intimate partner is unfaithful, or that people are able to read the patient's thoughts. Most common is the belief the person is being persecuted in some way.
In Honeymoon, House realized that Mark Warner was suffering from delusions when he realized that Mark had not honeymooned in Paris like he said, but that his brain scan at the time showed that he truly believed he had. This led House to the correct diagnosis - acute intermittent porphyria.
In Guardian Angels, the patient, in addition to hallucinating that her mother was in the room with her, was under the apprehension that her mother was alive. Although the hallucinations continued during the first stages of treatment, the delusion stopped right away and the patient accepted that it was a hallucination.