A 52-year-old man presents to the psychiatry clinic with a 3-year history of firmly held belief that his neighbour is deliberately poisoning his food through the ventilation system. He reports checking his meals multiple times before eating and has installed air filters in his home. He denies hearing voices or seeing things others don't see. His affect is appropriate, and he maintains good hygiene and employment. Mental status examination reveals logical, coherent thought processes. There is no evidence of mood disorder, and cognitive function is intact. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Delusional Disorder, Persecutory Type ### Key Diagnostic Features **Key Point:** Delusional Disorder is characterized by the presence of one or more non-bizarre delusions in the absence of prominent hallucinations, negative symptoms, or significant functional impairment in non-delusional domains. ### Criterion Analysis | Feature | This Case | Delusional Disorder | Schizophrenia | |---------|-----------|-------------------|----------------| | Duration | 3 years | ≥1 month (typically years) | ≥6 months | | Hallucinations | Absent | Absent or rare | Prominent | | Negative symptoms | Absent | Absent | Present | | Functional impairment | Minimal (employed, hygiene intact) | Limited to delusional content | Widespread | | Thought organization | Logical, coherent | Logical outside delusion | Disorganized | | Mood symptoms | None mentioned | None | Often present | ### Why This Is Delusional Disorder 1. **Single, non-bizarre delusion** (poisoning via ventilation) — the patient has a specific, organized false belief that, while false, is not impossible in the real world. 2. **No hallucinations** — the patient denies auditory or visual hallucinations; this rules out schizophrenia. 3. **Preserved functioning** — he maintains employment and self-care; functional decline is limited to the delusional theme. 4. **Appropriate affect and logical thought** — outside the delusional content, cognition and emotional expression are normal. 5. **Duration ≥1 month** (here, 3 years) — meets the temporal criterion. ### Delusional Disorder Subtypes (DSM-5) **High-Yield:** The persecutory type is the most common subtype (50% of cases). The delusion involves belief that one is being conspired against, spied on, followed, or harassed. **Clinical Pearl:** Patients with Delusional Disorder often have better premorbid functioning and less cognitive decline than those with schizophrenia. They may seek medical help for the perceived threat (e.g., installing filters, visiting multiple doctors) rather than psychiatric help. ### Differential Considerations - **Schizophrenia, paranoid type:** Would require prominent hallucinations, negative symptoms (alogia, avolition), or disorganized speech — none present here. - **Brief Psychotic Disorder:** Duration is 1 day to 1 month; this case spans 3 years. - **OCD with poor insight:** OCD involves intrusive thoughts the patient recognizes as irrational; here, the belief is held with absolute conviction (true delusion). [cite:DSM-5 Diagnostic and Statistical Manual of Mental Disorders]