Quetiapine for Delusional Parasitosis
Quetiapine is an appropriate second-line treatment option for delusional parasitosis, with evidence supporting its use for symptomatic management of delusions, though pimozide has traditionally been considered first-line therapy. 1, 2
Treatment Algorithm for Delusional Parasitosis
First-Line Treatment
- Pimozide has historically been the treatment of choice for delusional parasitosis
- Effective dosage range: 1-10 mg/day
- Shown to be effective in small doses (2-8 mg/day) compared to placebo 3
- Monitor for extrapyramidal symptoms (EPS), QTc prolongation, and cardiotoxic effects
Second-Line Treatment (Atypical Antipsychotics)
Quetiapine is recommended as a second-line option 1
- Starting dose: 25 mg orally at bedtime
- Target dose: up to 200 mg/day in divided doses
- Advantages: Lower risk of EPS, sedating effect may help with agitation
- Monitor for metabolic side effects, orthostatic hypotension
Olanzapine may also be considered 1
- Starting dose: 2.5 mg/day
- Maximum dose: 10 mg/day in elderly; up to 20 mg/day in younger adults
- Sedation is a recognized side effect which may be advantageous
Risperidone is another alternative 2, 4
- Starting dose: 0.25-0.5 mg/day
- Maximum dose: 2 mg/day in elderly; up to 6 mg/day in younger adults
- Higher risk of EPS than quetiapine or olanzapine
Aripiprazole may offer benefit with fewer adverse events 1, 5
- Monitor for akathisia which is more common with this medication
Clinical Considerations
Patient Assessment
- Confirm diagnosis by ruling out actual infestations and organic causes
- Assess for comorbid psychiatric conditions that may influence treatment choice
- Evaluate cardiac status before initiating antipsychotics (especially pimozide)
- Consider age-related factors (elderly patients require lower doses)
Treatment Challenges
- The principal difficulty is establishing rapport and convincing patients to take medication 4
- Patients typically reject psychiatric referral and are convinced they have a dermatological problem 2
- Approach the patient with empathy while acknowledging their distress
Monitoring
- Follow up within 1-2 weeks to assess side effects and response 6
- Monitor for:
- QTc prolongation (especially with pimozide)
- Extrapyramidal symptoms
- Metabolic parameters (weight, glucose, lipids)
- Orthostatic changes and sedation
Evidence Summary
- A retrospective review of 20 cases showed clinical improvement with various atypical antipsychotics 5
- The evidence for pimozide as first-line is limited to one small non-randomized placebo-controlled trial 5
- Atypical antipsychotics have a safer side effect profile compared to traditional antipsychotics 4
- The ESMO clinical practice guidelines support the use of quetiapine for symptomatic management of delusions 1
Important Caveats
- Lowest effective dose should be used for the shortest possible duration to minimize risk of tardive dyskinesia
- Elderly patients require lower starting doses and slower titration
- Antipsychotics carry black box warnings regarding increased mortality in elderly patients with dementia
- Treatment may need to be continued for several months before attempting discontinuation
- Response rates vary, with better outcomes reported in younger patients with shorter duration of symptoms 7
When treating delusional parasitosis, quetiapine offers a reasonable balance of efficacy and tolerability as a second-line option, particularly in patients who cannot tolerate pimozide or who have risk factors for extrapyramidal side effects.