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Station 1: Definition and Diagnosis
Examiner: Define OCD and distinguish it from OCPD.
Expected answer: OCD is characterised by obsessions (intrusive, unwanted, recurrent thoughts/images causing marked anxiety) and/or compulsions (repetitive behaviours or mental acts performed to reduce anxiety from obsessions), time-consuming (over 1 hour/day) or causing significant distress/impairment. Key: ego-dystonic — the patient recognises thoughts as irrational and wants to stop.
OCPD (Obsessive-Compulsive Personality Disorder): ego-syntonic perfectionism, orderliness, control — the patient embraces these traits; no insight that they are problematic; no true obsessions or compulsions.
Station 2: Pathophysiology
Examiner: Describe the neurobiological basis of OCD.
Expected answer: Cortico-Striato-Thalamo-Cortical (CSTC) circuit dysfunction. The loop runs OFC to ACC to caudate (striatum) to globus pallidus to thalamus back to OFC. In OCD:
- Orbitofrontal cortex (OFC): hyperactive (generates "worry" signal)
- Anterior cingulate cortex (ACC): hyperactive (amplifies the emotional salience)
- Caudate nucleus: fails to filter the signal (normally suppresses irrelevant worries)
- Thalamus: relays the worry back to cortex, creating an endless loop
Neurochemical: serotonin deficiency (basis for SSRI treatment) and glutamate excess in the CSTC circuit (basis for glutamate modulators like memantine, ketamine).
Functional imaging (fMRI/PET): hyperactivity in OFC/ACC/caudate normalises after successful SSRI or ERP treatment — objective evidence of treatment response.
Station 3: Pharmacotherapy
Examiner: What is the pharmacological management of OCD and how does it differ from depression dosing?
Expected answer:
- First-line: SSRI at HIGH doses — fluoxetine 60 mg, sertraline 200 mg, paroxetine 60 mg, fluvoxamine 300 mg (significantly higher than depression doses)
- Onset: 8-12 weeks (longer than depression — 2-4 weeks)
- Second-line: Clomipramine 100-250 mg (TCA, ECG/LFT monitoring)
- Augmentation: Aripiprazole 5-15 mg or risperidone 0.5-2 mg if partial response to SSRI
- Maintenance: Continue for at least 12 months after remission; relapse rate 40-60% on discontinuation
- Glutamate modulators (emerging): Memantine, N-acetylcysteine, ketamine — for treatment-resistant cases
Station 4: Psychological Treatment
Examiner: Describe ERP and its evidence base.
Expected answer: Exposure and Response Prevention (ERP) is the gold-standard psychological therapy for OCD:
- Mechanism: Patient is systematically exposed to obsession-triggering stimuli (e.g., touching a "contaminated" doorknob) and prevented from performing the compulsion (washing). Anxiety initially spikes, then habituates through repeated exposure.
- Protocol: 13-20 weekly sessions of 60-90 minutes; therapist-led initially, then self-directed homework
- Efficacy: 60-80% improvement; comparable to SSRI; combination ERP + SSRI is superior to either alone for moderate-to-severe cases
- Key principle: The compulsion must be prevented — exposure alone without response prevention is ineffective
- NICE CG31: ERP is first-line for mild OCD; ERP + SSRI for moderate-to-severe
Station 5: Special Populations and Red Flags
Examiner: What is PANDAS and how would you manage sudden-onset OCD in a child?
Expected answer: PANDAS (Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal infections): sudden-onset OCD/tics in a child following Group A Streptococcal infection (pharyngitis). Mechanism: molecular mimicry — anti-streptococcal antibodies cross-react with basal ganglia neurons.
Diagnostic criteria (Swedo):
- Presence of OCD and/or tic disorder
- Prepubertal onset
- Episodic/sawtooth course (abrupt onset, dramatic symptom fluctuations)
- Temporal association with GAS infection (positive throat culture or elevated ASO titre/anti-DNase B)
- Neurological abnormalities (choreiform movements, hyperactivity)
Management: Standard OCD treatment (ERP + SSRI at lower doses) plus: treat the streptococcal infection with antibiotics (penicillin V); consider tonsillectomy for recurrent cases; IVIG/plasmapheresis for severe fulminant cases (controversial). Prophylactic antibiotics may prevent recurrence.
Red flags in OCD: suicidal ideation (lifetime rate up to 36%), comorbid depression, peripartum OCD with infant-harm intrusive thoughts (distinguish from postpartum psychosis), absent insight (treat with antipsychotic + SSRI, not as primary psychosis).