DEFINITION

• A delusion is a fixed, false, and idiosyncratic belief and is one of the trilogy of psychotic symptoms: hallucinations, delusions, and thought disorder.
  • "Fixed" refers to the strength of the patient’s belief. The patient is certain, and not persuaded by any arguments to the contrary.
  • "False" relates to the veracity of the patient’s belief.
    • Beliefs about which the truth cannot be known with reasonable certainty, such as cultural or religious beliefs, should not be classified as delusions without careful consideration.
  • "Idiosyncratic" means the belief is characteristic of the individual patient.
    • This criterion becomes important when considering the prevalence of fixed, false beliefs in the general population.
    • Beliefs shared by a group of people (for instance, a belief in UFOs) are generally not delusions.
• Several of Schneider’s first rank symptoms (FRS) of schizophrenia are delusions. These include:
  • Delusions of control or being controlled
  • Thought withdrawal
  • Thought insertion
  • Mind reading
  • Delusional perception (ordinary perception given a bizarre interpretation)
• Delusions can be divided by theme (e.g. persecutor, grandiose), plausibility (bizarre, non-bizarre), relationship to mood (congruent or non-congruent), or along other dimensions. Themes described by the DSM 5 TR include:
  • Erotomanic - where the individual is convinced that another person (often of fame or higher status) is in love with them
  • Grandiose - where the individual believes they have a substantial, though unrecognized, talent, power, knowledge, discovery, or relationship with a divine or famous entity
  • Jealous - where the individual believes their romantic partner is unfaithful
  • Persecutory - where the individual believes they are systematically being conspired against, harassed, attacked, or otherwise obstructed from achieving their long-term goals
  • Somatic - related to bodily function or sensation
  • Mixed - for delusions where no single theme is dominant
  • Unspecified - for delusions that fall outside the above themes or are unclear
• Most patients with delusions have more than one.
• Delusions and hallucinations frequently co-occur.
• The time course of delusions is highly variable and depends on the underlying diagnosis.
• An "overvalued idea" is a belief which narrowly fails to satisfy the definition of a delusion. One example would be an anorexic patient’s belief that he or she is fat.
• "... madmen put wrong ideas together, and so make wrong propositions, but argue and reason right from them; but idiots make very few or no propositions, and reason scarce at all." John Locke[1]