What are the causes of hallucinations?

Causes of Hallucinations

Hallucinations are primarily caused by medical conditions, neurological disorders, psychiatric disorders, substance use, and sensory deprivation, with treatment directed at the underlying etiology whenever possible. 1

Medical and Neurological Causes

• Acute Medical Conditions:

  • Infections (particularly UTIs and pneumonia) 2, 1
  • Metabolic disorders (electrolyte imbalances, organ dysfunction) 2
  • Endocrine disorders (thyroid dysfunction) 2, 1
  • Intoxication and withdrawal syndromes 2
  • Medications and drug toxicity 1

• Neurological Disorders:

  • Cerebrovascular disease (stroke, TIA) 2
  • Traumatic brain injury 2
  • Seizures (including status epilepticus and nonconvulsive seizures) 2
  • Intracranial mass effect or elevated intracranial pressure 2
  • Chronic subdural or subarachnoid hemorrhage 2
  • Meningitis or encephalitis 2
  • Hydrocephalus 2
  • Parkinson's disease (visual hallucinations predict dementia and rapid deterioration) 3

Psychiatric Disorders

• Primary Psychotic Disorders:

  • Schizophrenia (highest prevalence of hallucinations at 61.1%) 4
  • Schizoaffective disorder 2

• Mood Disorders:

  • Bipolar disorder (22.9% in mixed states, 11.2% in mania, 10.5% in bipolar depression) 4
  • Major depressive disorder with psychotic features (5.9%) 4

• Dementia:

  • Dementia with Lewy Bodies (up to 80% experience visual hallucinations) 1
  • Alzheimer's disease (hallucinations predict rapid cognitive decline) 3
  • Frontotemporal dementia (especially with C9orf72 mutation - 21-56% experience delusions and hallucinations) 2

Sensory Deprivation and Special Conditions

• Charles Bonnet Syndrome:

  • Occurs in 15-60% of patients with ophthalmologic disorders 2, 1
  • Characterized by recurrent, vivid visual hallucinations with preserved insight 2
  • Requires some degree of vision loss 2
  • Attributed to cortical-release phenomenon from lack of visual input 2, 1

• Other Sensory Deprivation:

  • Phantom limb syndrome 3

Substance-Related Causes

• Intoxication:

  • Alcohol 2, 4
  • Hallucinogens 2
  • Stimulants 2
  • Cannabis 2

• Withdrawal:

  • Alcohol withdrawal (delirium tremens) 2
  • Sedative-hypnotic withdrawal 2

Clinical Features That Help Distinguish Causes

• Timing and Onset:

  • Acute onset (minutes to days) suggests medical conditions 2
  • Late adolescence onset is more specific to schizophrenia 5

• Type of Hallucination:

  • Auditory hallucinations are most common across all disorders 4
  • Visual hallucinations are more common in medical/neurological conditions 4
  • Somatic hallucinations rank third in frequency 4
  • Olfactory hallucinations are strongly associated with delusions 4

• Associated Features:

  • Hallucinations with preserved insight suggest Charles Bonnet Syndrome 2
  • Hallucinations with fluctuating consciousness suggest delirium 2, 1
  • Hallucinations with cognitive fluctuations and parkinsonism suggest Lewy Body Dementia 1
  • Hallucinations with persecutory delusions are common in bipolar disorder 4

Diagnostic Approach

• Laboratory Assessment:

  • Complete blood count
  • Basic metabolic panel
  • Liver function tests
  • Urinalysis
  • Blood cultures (if febrile)
  • Thyroid function tests 1

• Neuroimaging:

  • Head CT without IV contrast is usually first-line 1
  • Brain MRI for unclear clinical picture, atypical presentation, or abnormal examination 1
  • Indicated for focal neurological deficits, abnormal exam, atypical presentation, first-episode psychosis without clear psychiatric cause, new/worsening headaches, head trauma, suspected stroke/seizure/infection, or altered consciousness 1

Common Pitfalls to Avoid

• Attributing hallucinations solely to psychiatric illness without ruling out medical causes 1
• Assuming hallucinations in elderly patients are due to age or pre-existing dementia 1
• Overlooking medication side effects as a potential cause 1
• Missing subtle infections (especially UTIs and pneumonia) in elderly patients 1
• Failing to recognize Charles Bonnet Syndrome in visually impaired patients 2

Remember that no single hallucination feature uniquely indicates schizophrenia (except perhaps late adolescence onset), as 95% of hallucination features in schizophrenia are shared with other psychiatric disorders, 85% with medical/neurological conditions, and 66% with substance-related conditions 5.