Hallucinations: Common features and causes

Approximately 5% of patients with epilepsy have occipital seizures, which almost always have visual manifestations. Epileptic visual hallucinations often are simple, brief, stereotyped, and fragmentary. They usually consist of small, brightly colored spots or shapes that flash.²² Complex visual hallucinations in epilepsy are similar to hypnagogic hallucinations but are rare. Intracranial electroencephalography recordings have shown that pathological excitation of visual cortical areas may be responsible for complex visual hallucinations in epilepsy.¹⁹

Dementia with Lewy bodies (DLB) is associated with visual hallucinations.²³ Visual hallucinations occur in >20% of patients with DLB.²⁴ Patients with DLB may see complex scenarios of people and items that are not present. Visual hallucinations have an 83% positive predictive value for distinguishing DLB from dementia of the Alzheimer’s type.²⁵ There is a strong correlation between Lewy bodies located in the amygdala and parahippocampus and well-formed visual hallucinations.²⁶

Visual hallucinations are common in Parkinson’s disease and may occur in up to one-half of patients.²⁷ Patients with Parkinson’s disease may experience hallucinations similar to those observed in DLB, which can range from seeing a person or animal to more complex, formed, and mobile people, animals, or objects.

Table 2

Common causes of visual hallucinations

Olfactory hallucinations

Also known as phantosmia, olfactory hallucinations involve smelling odors that are not derived from any physical stimulus. They can occur with several psychiatric conditions, including schizophrenia, depression, bipolar disorder, eating disorders, and substance abuse.²⁸ Olfactory hallucinations caused by epileptic activity are rare. They constitute approximately 0.9% of all auras and typically are described as unpleasant. Tumors that affect the medial temporal lobe and mesial temporal sclerosis are associated with olfactory hallucinations.²⁹ Olfactory hallucinations also have been reported in patients with multi-infarct dementia, Alzheimer’s disease, and alcoholic psychosyndromes. In patients with schizophrenia, the smell may be perceived as coming from an external source, whereas patients with depression may perceive the source as internal.³⁰ Patients who perceive that they are the source of an offensive odor—a condition known as olfactory reference syndrome—may wash excessively, overuse deodorants and perfumes, or become socially withdrawn.³⁰

Gustatory hallucinations

Patients with gustatory hallucinations may experience salivation, sensation of thirst, or taste alterations. These hallucinations can be observed when the sylvian fissure that extends to the insula is stimulated electrically.³¹ Similar to olfactory hallucinations, gustatory hallucinations are associated with temporal lobe disease and parietal operculum lesions.^31,32 Sinus diseases have been associated with olfactory and gustatory hallucinations.³³ Brief gustatory hallucinations can be elicited with stimulation of the right rolandic operculum, parietal operculum, amygdala, hippocampus, medial temporal gyrus, and anterior part of right temporal gyrus.³⁴

Tactile hallucinations

These hallucinations may include perceptions of insects crawling over or under the skin (formication) or simulation of pressure on skin.³⁵ They have been associated with substance abuse, toxicity, or withdrawal.²⁸ Tactile hallucinations are characteristic of cocaine or amphetamine intoxication.³⁵

Tactile hallucinations are a rare symptom of schizophrenia. Heveling and colleagues reported a case of a woman, age 68, with chronic schizophrenia who experienced touching and being touched by a “shadow man” several times a day in addition to auditory and visual hallucinations.³⁶ Her symptoms disappeared after 4 weeks of antipsychotic and mood stabilizer therapy.

Tactile hallucinations have been associated with obsessive-compulsive disorder (OCD).³⁷ Fontenelle and colleagues suggested that OCD and psychotic disorders may share dysfunctional dopaminergic circuits.³⁷

Somatic hallucinations

Patients who have somatic hallucinations report perceptions of abnormal body sensations or physical experiences. For example, a patient may have sense of not having a stomach while eating.³⁵

This type of hallucination has been associated with activation of postcentral gyrus, parietal operculum, insula, and inferior parietal lobule on stereoelectroencephalography.³⁴ In a study of cerebral blood flow in 20 geriatric patients with delusional disorder, somatic type who were experiencing somatic hallucinations, positron emission testing scan demonstrated increased perfusion in somatic sensory processing regions, particularly the left postcentral gyrus and the right paracentral lobule.³⁸ Other researchers have linked somatic hallucinations with activation in the primary somatosensory and posterior parietal cortex, areas that normally mediate tactile perception.³⁹

Related Resource

• Teeple RC, Caplan JP, Stern TA. Visual hallucinations: differential diagnosis and treatment. Prim Care Companion J Clin Psychiatry. 2009;11(1):26-32.