Red flag symptoms
- Frightening hallucinations
- Symptoms suggestive of psychosis
- Neurological disorder
- Focal neurological deficit
- Recreational drug use
A hallucination, which may be defined as a sensory perception in the absence of an external stimulus, may be olfactory, auditory, visual, tactile or gustatory.
It has been reported that up to 30% of patients experiencing delirium may have visual hallucinations.1
Hallucinations are often thought of as manifestations of mental illness, but those without mental illness may experience them, especially gustatory and olfactory hallucinations.
Moreover, hypnagogic (occurring on falling asleep) and hypnopompic (occurring on waking) hallucinations can occur in all individuals.
This means hallucinations may not necessarily be pathological - recently bereaved people may report visual or auditory hallucinations.
Hallucinations may occur in the presence of increased external stimulation (for example, in the presence of others) or in conditions of decreased external stimulation.
Hallucinations may be the presenting feature in schizophrenia, conversion reactions and affective disorders. Auditory hallucinations are the most common hallucinations in schizophrenia, although visual hallucinations may also be reported.
Auditory hallucinations may accompany severe depression, with voices talking directly to the patient, the content of which is congruent to the patient's low mood. They may also be seen in manic states, and again congruent with the patient's mood.
An association between hallucinations and childhood sexual abuse has previously been described.2 Auditory hallucinations have also been reported in post-traumatic stress disorder.3
Auditory hallucinations resulting in self-harm may be seen in patients with borderline personality disorder.
Patients with unilateral and bilateral hearing loss may report auditory hallucinations. Auditory hallucinations may occur in substance-induced psychoses.
Hallucinations may occur in neurological conditions, such as epilepsy, migraines, Parkinson's disease and Lewy body dementia. They are more common in Parkinson's disease with advancing age, length of illness, cognitive impairment and depression.
In moderate to severe Alzheimer's disease, if hallucinations occur, they are most likely to be visual.
Visual hallucinations may result from cortical lesions that involve the occipital and temporoparietal areas. Olfactory and gustatory hallucinations may arise as a result of a temporal lobe lesion.
Retinal disease, age-related macular degeneration, glaucoma and cataracts may result in visual hallucinations.
Charles Bonnet syndrome describes the presence of visual hallucinations after loss of visual acuity, where there has been no cognitive impairment. It is thought this may be related to increased cortical compensation in the temporal cortex, striatum and thalamus.
Drug misuse may be causative in the case of alcohol misuse, cannabis and MDMA use. Vivid and colourful images may be seen following ingestion of psychoactive drugs.
Cocaine and amphetamine use may result in tactile hallucinations, such as the sensation of insects crawling on the skin. Flashbacks in the form of visual hallucinations may occur in the drug-free state.
Hallucinations associated with delirium tremens may involve visual hallucinations of animals; these hallucinations tend to be frightening.
- Sleep deprivation
- Schizophrenia, conversion reactions, affective disorders
- Neurological – epilepsy, migraines, Parkinson’s disease, Lewy body dementia
- Eye disease - retinal disease, age-related macular degeneration and cataracts
- Drug misuse
In patients presenting with hallucinations, a thorough history and neurological examination are required to assess the underlying cause and determine the prognosis.
Auditory hallucinations are the most common type overall, but in brain disorders, visual hallucinations are the most common.
It is important to assess the modality of the hallucinations and any associated factors. Assess any risk to the patient as a result of them - for example, ask about the nature of the hallucinations and if the patient is frightened of them.
This is particularly relevant if psychosis is suspected because there may be a risk of self-harm, including suicide.
Schneider's first-rank symptoms of schizophrenia consist of auditory hallucinations where there is thought echo and a running commentary discussing the patient in the third person.
According to the ICD-10 classification, these symptoms may support a diagnosis of schizophrenia.
Subclinical hallucinations may be seen in children and occur more frequently in those with conduct and/or emotional disorders.
Hallucinations are often thought to be the manifestation of psychiatric disease, but many other conditions may present with them.
Hallucinations may respond to antipsychotic medications and to treatment of the underlying cause. If they persist, CBT and supportive psychotherapy may be useful. Specialist neurological and/or psychiatric referral should be considered.
- Dr Kochhar is a GP in Bexhill-on-Sea, East Sussex
This is an updated version of an article that was first published in May 2015.
- Ali S, Patel M, Jabeen S et al. Insight into delirium. Innov Clin Neurosci 2011 Oct; 8(10): 25–34.
- Sheffield J, Williams L, Blackford J et al. Childhood sexual abuse increases risk of auditory hallucinations in psychotic disorders. Compr Psychiatry 2013; 54 (7): 1098-1104.
- Dorahy M, Palmer R. Auditory hallucinations in chronic trauma disorders: phenomenology and psychological mechanisms. Comprehensive guide to post-traumatic stress disorder.