A visual hallucination is a visual percept experienced when awake that is not elicited by an external stimulus. Historically, hallucinations have been synonymous with psychiatric disease, most notably schizophrenia; however, over recent decades, hallucinations have been categorized based on their underlying etiology as psychodynamic (primary psychiatric), psychophysiologic (primary neurologic/structural), and psychobiochemical (neurotransmitter dysfunction).1 Presently, visual hallucinations are known to be caused by a wide variety of primary psychiatric, neurologic, ophthalmologic, and chemically-mediated conditions. Despite these causes, clinically differentiating the characteristics and qualities of visual hallucinations is often a lesser-known skillset among clinicians. The utility of this skillset is important for the clinician’s ability to differentiate the expected and unexpected characteristics of visual hallucinations in patients with both known and unknown neuropsychiatric conditions.
Though many primary psychiatric and neurologic conditions have been associated with and/or known to cause visual hallucinations, this review focuses on the following grouped causes:
- Primary psychiatric causes: psychiatric disorders with psychotic features and delirium; and
- Primary neurologic causes: neurodegenerative disease/dementias, seizure disorders, migraine disorders, vision loss, peduncular hallucinosis, and hypnagogic/hypnopompic phenomena.
Because the accepted definition of visual hallucinations excludes visual percepts elicited by external stimuli, drug-induced hallucinations would not qualify for either of these categories. Additionally, most studies reporting on the effects of drug-induced hallucinations did not control for underlying comorbid psychiatric conditions, dementia, or delirium, and thus the results cannot be attributed to the drug alone, nor is it possible to identify reliable trends in the properties of the hallucinations.2 The goals of this review are to characterize visual hallucinations experienced as a result of primary psychiatric and primary neurologic conditions and describe key grouping and differentiating features to help guide the diagnosis.
Visual hallucinations in the general population
A review of 6 studies (N = 42,519) reported that the prevalence of visual hallucinations in the general population is 7.3%.3 The prevalence decreases to 6% when visual hallucinations arising from physical illness or drug/chemical consumption are excluded. The prevalence of visual hallucinations in the general population has been associated with comorbid anxiety, stress, bereavement, and psychotic pathology.4,5 Regarding the age of occurrence of visual hallucinations in the general population, there appears to be a bimodal distribution.3 One peak appears in later adolescence and early adulthood, which corresponds with higher rates of psychosis, and another peak occurs late in life, which corresponds to a higher prevalence of neurodegenerative conditions and visual impairment.
Primary psychiatric causes
Most studies of visual hallucinations in primary psychiatric conditions have specifically evaluated patients with schizophrenia and mood disorders with psychotic features.6,7 In a review of 29 studies (N = 5,873) that specifically examined visual hallucinations in individuals diagnosed with schizophrenia, Waters et al3 found a wide range of reported prevalence (4% to 65%) and a weighted mean prevalence of 27%. In contrast, the prevalence of auditory hallucinations in these participants ranged from 25% to 86%, with a weighted mean of 59%.3
Hallucinations are a known but less common symptom of mood disorders that present with psychotic features.8 Waters et al3 also examined the prevalence of visual and auditory hallucinations in mood disorders (including mania, bipolar disorder, and depression) reported in 12 studies (N = 2,892).3 They found the prevalence of visual hallucinations in patients with mood disorders ranged from 6% to 27%, with a weighted mean of 15%, compared to the weighted mean of 28% who experienced auditory hallucinations. Visual hallucinations in primary psychiatric conditions are associated with more severe disease, longer hospitalizations, and poorer prognoses.9-11
Visual hallucinations of psychosis
In patients with psychotic symptoms, the characteristics of the visually hallucinated entity as well as the cognitive and emotional perception of the hallucinations are notably different than in patients with other, nonpsychiatric causes of visual hallucations.3
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