OCD and related disorders appear in Paper B as part of the anxiety and obsessive-compulsive spectrum. The ICD-11 introduced a new grouping for obsessive-compulsive and related disorders, separating them from anxiety disorders. This change is examined. Body dysmorphic disorder, hoarding disorder, trichotillomania, and excoriation disorder are included in this grouping. NICE guidance CG31 (2005, updated 2024) provides the treatment algorithm.
ICD-11 Classification
ICD-11 created a separate chapter for obsessive-compulsive and related disorders, distinct from anxiety and fear-related disorders. This is an important change from ICD-10 (where OCD was classified under neurotic, stress-related, and somatoform disorders). Conditions included: OCD, body dysmorphic disorder (BDD), olfactory reference disorder, hypochondriasis (illness anxiety disorder), hoarding disorder, trichotillomania (hair-pulling disorder), and excoriation disorder (skin-picking).
OCD diagnostic criteria (ICD-11): Either obsessions, compulsions, or both. Obsessions are repetitive, intrusive thoughts, images, or impulses that are unwanted and cause anxiety. Compulsions are repetitive behaviours or mental acts that the person feels driven to perform in response to an obsession or according to rigid rules. The symptoms must be time-consuming (more than 1 hour per day), cause marked distress or functional impairment, and not be attributable to another medical condition. The person usually recognises the obsessions and compulsions as excessive or unreasonable (insight specifier: good/fair, poor, absent/delusional).
Body dysmorphic disorder: Preoccupation with perceived defects or flaws in physical appearance that are not observable or appear only slight to others. Repetitive behaviours (checking mirrors, comparing, camouflaging, seeking reassurance). A recall question confirmed: “Body dysmorphic disorder in adolescents” — increased risk of self-harm and suicide (NOT benign, NOT self-limiting).
NICE Treatment for OCD (CG31)
Stepped care for OCD: Step 1: Recognition and assessment (use Y-BOCS). Step 2: Mild functional impairment — brief CBT (less than 10 therapist hours) incorporating ERP, either individual or group, or guided self-help. Step 3: Moderate to severe functional impairment — more intensive CBT (more than 10 therapist hours) incorporating ERP, or an SSRI (first-line: fluoxetine, fluvoxamine, sertraline, paroxetine, citalopram) plus ongoing CBT/ERP. Step 4: Severe, complex, or treatment-resistant OCD — combined SSRI + CBT/ERP, multidisciplinary review. Consider augmentation with clomipramine (the TCA with the best evidence in OCD), or antipsychotic augmentation (risperidone, aripiprazole).
Key treatment points for the exam: ERP is the essential psychological intervention (not just general CBT). The key principle: exposure to the trigger without performing the compulsion leads to habituation and extinction of the anxiety response. Response prevention is the critical component. The SSRI dose for OCD is generally higher than for depression and takes longer to work (8-12 weeks for response, compared to 4-6 weeks for depression). Clomipramine is highly effective but less well-tolerated due to anticholinergic side effects. NICE does NOT recommend clomipramine as first-line — SSRIs are preferred.
Treatment-resistant OCD: NICE recommends adding an antipsychotic (risperidone has the best evidence, aripiprazole is also used). For severe refractory OCD, consider intensive inpatient or residential treatment. A recall question tested: “What is approved by NICE for refractory OCD?” — Anterior cingulotomy or DBS (deep brain stimulation) were the options, with DBS having more current evidence. Anterior cingulotomy is a neurosurgical ablation procedure that is rarely performed in the UK.
Comorbidity (Recall-Confirmed)
A recall question tested: “Most common comorbidity in OCD in adults?” — Major depressive disorder (NOT OCPD, GAD, or social anxiety). Another question: “Most common comorbidity in OCD in children?” — Depression again, not GAD. Depression is the most common comorbid condition in OCD across all age groups. Approximately 30-50% of OCD patients have lifetime major depression.
OCD vs OCPD (anankastic personality disorder): OCD involves ego-dystonic obsessions and compulsions (the patient recognises them as unreasonable, experiences distress). OCPD is ego-syntonic (the patient believes their perfectionism and orderliness are appropriate). OCPD is a personality pattern, not an anxiety disorder. This distinction is frequently examined.
PANDAS
Paediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections (PANDAS): Sudden onset of OCD symptoms and/or tics following group A streptococcal infection in children. The mechanism is cross-reactive antibodies against basal ganglia. The exam may ask about this as a phenomenon, though its clinical significance is debated and it is not a formal ICD-11 diagnosis. Sydenham’s chorea following rheumatic fever is a better-established post-streptococcal neuropsychiatric syndrome with OCD features.
High-Yield Recall Patterns
- ICD-11 change for OCD: Separate chapter from anxiety disorders, includes BDD, hoarding, trichotillomania, excoriation, hypochondriasis, olfactory reference disorder
- First-line psychological treatment for OCD: CBT incorporating ERP (Exposure and Response Prevention)
- First-line medication for OCD: SSRI at higher doses than depression (fluoxetine 40-60mg, sertraline 150-200mg, citalopram 40-60mg)
- Augmentation for treatment-resistant OCD: Antipsychotic (risperidone first-line evidence)
- BDD in adolescents: Increased suicide/self-harm risk, NOT benign
- OCD most common comorbidity: Major depressive disorder (adults and children)
- OCD vs OCPD: Ego-dystonic vs ego-syntonic. OCD has obsessions/compulsions, OCPD has perfectionism/orderliness.
- Clomipramine: Effective but anticholinergic burden limits use. Third-line.
- Y-BOCS: 10 items (5 obsessions + 5 compulsions), 0-40. Most common OCD severity scale.
PsychStar’s Paper B question bank covers OCD and related disorders with questions aligned to real exam difficulty. Start with 5 free questions at psychstar.io/try.