Delusional disorder is the diagnosis for persistent delusions lasting at least 1 month, in the absence of other psychotic features (no prominent hallucinations beyond those related to the delusion theme, no disorganized speech, no grossly disorganized behavior, no negative symptoms). Function in domains outside the delusion is largely preserved. The delusion is non-bizarre (plausible if untrue, though some patients have bizarre delusions).
DSM-5 subtypes based on delusional content: erotomanic (belief that another person is in love with the patient), grandiose (inflated self-importance, special powers), jealous (Othello syndrome — belief that partner is unfaithful, despite contrary evidence), persecutory (belief of being targeted, harassed, plotted against — most common), somatic (delusion about bodily function or appearance), mixed.
The clinical picture: the patient typically functions well in many life domains. They hold a stable job, maintain some relationships, manage finances, appear well-organized in casual encounter. The delusion is encapsulated — present but compartmentalized. They may present for legal problems (acting on persecutory beliefs), medical problems (somatic delusions driving repeated workups), or relationship problems (jealous delusions destroying marriages) rather than for "psychiatric" symptoms.
The challenges: patients rarely seek psychiatric care for the delusion itself — they don't think they have a delusion. Insight is typically poor for the specific belief, intact for other matters. Antipsychotic response is modest. Engagement in treatment is difficult given the patient's view that there is no problem with them. Many patients refuse psychiatric care entirely.
Jealous subtype (Othello syndrome) deserves special attention because it carries elevated violence risk. The delusion of infidelity, combined with intense affective response and behavioral pursuit, has produced intimate partner violence and homicide. Risk assessment important. Also elevated in older patients with cognitive impairment — sometimes a presenting feature of dementia.
Treatment: antipsychotics modestly effective. Pimozide historically used for some delusional disorder patients (particularly somatic subtype). Atypical antipsychotics (risperidone, olanzapine) more common in modern practice. Treatment over months sometimes produces meaningful improvement. Engagement is often as important as medication — building enough trust to maintain ongoing care.
Differential considerations: distinguish from paranoid personality disorder (lifelong pervasive trait without specific systematized delusion), from schizophrenia (other psychotic features present, functional decline more pervasive), from substance-induced psychosis (toxicology), from medical/neurologic causes including dementia (cognitive testing, imaging if indicated).
When you encounter a patient with persistent encapsulated delusions and preserved function elsewhere, delusional disorder is the diagnosis. The treatment is challenging; the alliance-building and ongoing engagement often matter more than the specific medication choice. Patience and predictability win where confrontation loses.