Principles of Prescribing Antipsychotics - In-Depth Notes

Learning Objectives

  • Understanding antipsychotic categorization and indications for prescription.
  • Mechanisms of action:
    • Dopamine system mechanism actions.
    • Functioning and limitations of antimuscarinic agents.
  • Differences between typical and atypical antipsychotics.
  • Identifying common risks and life-threatening risks associated with antipsychotics and management strategies.

Antipsychotic Categorization

1st Generation / Typical Antipsychotics:

  • Benperidol
  • Chlorpromazine
  • Flupentixol
  • Haloperidol
  • Trifluoperazine
  • Zuclopenthixol

2nd Generation / Atypical Antipsychotics:

  • Amisulpiride
  • Olanzapine
  • Paliperidone
  • Quetiapine
  • Risperidone
  • Aripiprazole (possibly a 3rd Gen)
  • Clozapine
  • Sulpiride (questionable classification)

Indications for Antipsychotics

General Indications:

  • Schizophrenia and other psychoses.
  • Short-term agitation and disturbed behavior control.
  • Mania/Bipolar disorder.
  • Short-term treatment of aggression in Alzheimer’s disease.

Specific Indications:

  • Clozapine & Quetiapine: Psychotic symptoms in Parkinson's Disease.
  • Olanzapine: Psychotic symptoms in Huntington’s Disease.
  • Benperidol: Deviant antisocial sexual behavior.
  • Quetiapine: Major depressive disorder (smaller doses).
  • Haloperidol & Chlorpromazine: Nausea & vomiting.
  • Chlorpromazine: Intractable hiccups.

Dopamine Pathways

Four Main Dopamine (DA) Pathways

  1. Nigrostriatal: Involved in motor control (Extrapyramidal neurons).
  2. Mesolimbic: Motivational and rewarding behavior; excess DA linked to positive symptoms of schizophrenia.
  3. Mesocortical: Less active in schizophrenia; associated with negative symptoms and cognitive dysfunction.
  4. Tuberoinfundibular: Regulates hormones related to sexual function.

Schizophrenia Associations

  • Mesolimbic: Excess DA leads to positive symptoms (e.g., hallucinations, delusions).
  • Mesocortical: Reduction in activity linked to cognitive deficits and negative symptoms.
  • Dorsolateral PFC: Related to cognitive problems.

Typical Antipsychotics and D2 Receptors

  • Block D2 receptors (D2 antagonists).
    • Mesolimbic Pathway: Reduces positive symptoms.
    • Mesocortical Pathway: Can worsen negative symptoms.
    • Nigrostriatal Pathway: Causes extra-pyramidal side effects (EPSEs).
    • Tuberoinfundibular Pathway: Can cause hormonal dysregulation.

Potentially Lethal Effects

  • Neuroleptic Malignant Syndrome: Can mimic agitation; includes symptoms needing careful monitoring.
  • Acute Dystonic Reaction: Muscle rigidity and potential airway constriction.

Extra-Pyramidal Side Effects (EPSEs)

  • Not solely due to blocking D2 receptors; linked to decreased dopamine leading to increased ACh.
  • Antimuscarinics: Can reduce some EPSEs but have limited effectiveness.
  • Risk factors include medications that enhance anticholinergic activity.

Antimuscarinic Side Effects

  • Common effects include:
    • Pupil dilation (photophobia)
    • Urinary retention
    • Dry mouth
    • Constipation
    • Confusion, especially in elderly
    • Cardiac irregularities
    • Referenced as "Can't see; can't pee; can't spit; can't…" for memorization.

Risk of Acute Obstructive Bowel

  • Due to muscarinic receptor blockade; risk of constipation leading to potential fatality.
  • Risk factors include:
    • Diet and hydration issues
    • Specific antipsychotic use (especially Clozapine)
    • Co-administration with other anticholinergic drugs.

Atypical Antipsychotics - Fast-Off Hypothesis

  • Measures dopamine receptor binding affinity:
    • Higher K value indicates faster dissociation from D2 receptors.
  • Second Generation: Generally faster dissociation compared to typicals.

Mechanisms of Atypicals

  • Block both D2 and 5HT2A receptors; alleged to alleviate some negative symptoms and avoid worsening others.
  • Promote downstream dopamine activities beneficial in mesolimbic pathway while limiting inhibition in others.

Other Interactions and Side Effects

  • Metabolic Syndrome: Weight gain and associated health risks.
  • Acquired Long QT Syndrome: Risk of drug-induced arrhythmia.

Aripiprazole: Unique Mechanism

  • Acts as a partial D2 receptor agonist: maintains some receptor activity to decrease adverse effects from typical antipsychotics.

Important Management Questions

  • Monitoring agranulocytosis risk in clozapine therapy.
  • Indications for depot antipsychotics use.
  • Innovations in antipsychotic treatments and monitoring outcomes over time.