Hallucinations are a symptom or feature and not a diagnosis. They may be auditory, visual, tactile, gustatory, or olfactory. Hallucinations and similar phenomena are often defined as follows:
  • Hallucination:
    • Sensory perception that has the compelling sense of reality of a true perception without external stimulation of the relevant sensory organ and is experienced as a sensation through that organ
    • Patients may or may not have insight that they are having the hallucination
  • Illusion:
    • Misperception or misinterpretation of a real external stimulus
  • Flashback:
    • Recurrence of a memory, feeling, or perceptual experience from the past that may have the compelling sense of reality
  • Pseudohallucination:
    • Hallucination that is not experienced by a sensory organ (i.e., voices inside head or “inner voice” as opposed to hearing voices)


Incidence and prevalence estimates:
  • Lifetime incidence of auditory hallucinations is 4–8% in general population (although some estimates are higher due to vague definitions or inclusion of pseudohallucinations)
  • More than 50% of elderly patients with dementia have paranoia or hallucinations


There are numerous causes of hallucinations. The following are common: (An exhaustive list is beyond the scope of this chapter)
  • Psychiatric:
    • Schizophrenia
    • Bipolar disorder, mania
    • Major depression
  • Acute intoxications:
    • Ethanol
    • Cannabis
    • Synthetic cannabinoids (i.e., K2, Spice)
    • Sympathomimetics:
      • Amphetamine
      • Methamphetamine
      • Cocaine
      • Synthetic cathinones (i.e., MDPV, “bath salts”)
    • NMDA antagonists:
      • Ketamine
      • PCP
      • Dextromethorphan
    • Serotonergic:
      • MDMA (Ecstasy)
      • LSD
      • Peyote cactus (mescaline)
      • Mushrooms (psilocybin)
      • 2C series (i.e., 2CB, 2CT-7)
      • 5-MeO series (i.e., 5-MeO-DMT)
    • Kappa opioid receptor agonist:
      • Salvia divinorum (cause synesthesias – i.e., hearing colors or smelling sounds)
    • Opiates
    • Inhalants:
      • Toluene
      • Nitrous oxide
  • Medications:
    • Anticholinergic agents
    • Steroids
    • Methylphenidate
  • Withdrawal:
    • Ethanol
    • Benzodiazepines
    • Barbiturates
    • GHB
  • Substance-induced disorders:
    • Methamphetamine-associated psychosis:
      • Prolonged duration of psychosis, auditory hallucinations, and recurrence without relapse of using drug
    • Cannabis-induced psychosis
  • Infectious:
    • Meningitis
    • Encephalitis
    • In patients with dementia, any infection (i.e., UTI, pneumonia) can trigger hallucinations
  • Metabolic:
    • Hypoglycemia
    • Electrolyte imbalances
    • Thyroid disease
    • Adrenal disease
    • Wilson's disease
    • Thiamine deficiency
  • Neurologic:
    • Seizures:
      • Partial simple or complex seizures can result in visual, auditory, olfactory, and gustatory hallucinations
    • Migraines
    • CNS hemorrhage or tumor
    • CVA
    • Tourette syndrome
    • Neurodegenerative disorders:
      • Parkinson
      • Dementia (Lewy body, Alzheimer)
      • HIV
  • Ocular:
    • Glaucoma
    • Macular degeneration
    • Charles Bonnet syndrome
  • Others:
    • Food, sensory, sleep deprivation
    • Fatigue, extreme stress
    • Heat-related illness
    • Religious and ritual activities
    • Falling asleep and awakening from sleep

Pediatric Considerations
Hallucinations are relatively common in children and adolescents and are often developmentally normal. Most children with hallucinations do not have psychosis. Hallucinations can occur as part of a delirium, such as from fever. As with the adult patient, carefully conduct a search for a medical or neurologic etiology

Geriatric Considerations
In the elderly patient, hallucinations are most often from an organic cause. They can commonly accompany dementia, depression, medication reactions, and substance abuse, and are often associated with agitation. Atypical antipsychotic agents are effective treatment for hallucinations with agitation in the elderly

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