The mechanism
Obsession (intrusive, unwanted, distressing thought/image/urge) → anxiety → compulsion (repetitive behaviour/mental act) → transient relief → negative reinforcement entrenches the cycle. Themes: contamination, checking, harm, symmetry/ordering, taboo (sexual/religious/aggressive) intrusions.
The discriminators that matter
- Obsession vs delusion: OCD is classically ego‑dystonic, resisted, and recognised as one's own — but ICD‑11/DSM‑5 specify an insight spectrum (good → poor → absent/delusional). Poor‑insight OCD is not schizophrenia; anchor on the obsession→compulsion structure.
- OCD vs GAD: GAD worries are about real‑life concerns; obsessions are intrusive, senseless, ego‑dystonic with ritualised neutralisation.
- OCD vs OCPD: OCD = ego‑dystonic symptoms; OCPD = ego‑syntonic perfectionism/rigidity as a personality style.
The risk nuance you must handle correctly
Intrusive thought ≠ intent. Harm/taboo obsessions (e.g., a new mother's intrusive thought of harming her infant) are typically ego‑dystonic and abhorrent to the patient — the opposite of psychotic command or genuine intent. Still perform a proper risk assessment, but don't misclassify OCD as homicidal risk (or vice‑versa: don't dismiss a genuine postpartum psychosis as "just OCD").
Management
First‑line CBT with ERP (the specific, evidence‑based modality) + SSRIs, often at higher doses and longer trials than for depression; clomipramine and augmentation for refractory cases.
Pitfalls
- Mislabelling poor‑insight OCD as psychosis (or the reverse).
- Under‑dosing/under‑duration of SSRIs; offering generic "CBT" instead of ERP.
Pearls
- 🎯 Ego‑dystonic + resisted + recognised‑as‑own = obsession; ownership/insight is the axis.
- 🩺 ERP, not reassurance — reassurance is itself a compulsion that maintains OCD.
Next → When the pathology is anchored to an event — trauma‑ and stressor‑related disorders (Concept 23).