There are many different causes of hallucinations and many different types of hallucinations. Hallucinations can mean seeing things (visual hallucinations, hearing voices (auditory hallucinations), feeling or smelling things (somatic hallucinations and olfactory hallucinations) which cannot be seen or detected by anything else.  A hallucination can be caused by a variety of different illnesses such as schizophrenia and delirium, or by lesions such as a stroke or a tumor in the part of the brain associated with that particular sensation. Careful history and evaluation as to the exact nature of the hallucination can provide clues to the cause. It is important to identify the cause of a hallucination to initiate an appropriate treatment plan.  Hallucinations are classified as follows:

1. Auditory Hallucinations (hearing voices) – An auditory hallucination is when a person can hear sounds or voices which are not real. An auditory hallucination is caused by the increased firing of dopamine, a hormone associated with alertness and cognition in the temporal lobe which is a part of the brain where signals carrying sound, originating from the eardrums are processed and given meaning. For example, an eardrum may vibrate in response to the sound of a bell, but it is understood by the brain to be the sound of a bell in the temporal lobe.  Auditory hallucinations are the most common type of hallucinations. They can be caused by illnesses such as schizophrenia, delirium, drug use, major depressive disorder and sometimes alcohol withdraw.

The type of hallucination can sometimes give you a clue as to the cause:

• Schizophrenia – voices are typically whispering; often two or more voices talking about the person hearing the voices. Sometimes the voices can give commands such as ‘don’t forget to take your medication,’ and sometimes the voices can be heard as a running commentary ‘John is walking down the street,’ ‘John is talking to his neighbor.’ John is crossing the road’, etc. Although most people with schizophrenia take medication to make the voices disappear, some people can find the voices soothing or comforting and described as feeling lonely when the voices disappear. Sometimes in paranoid type schizophrenia, the voices can tell people that someone is out to hurt them and can be very distressing.
• Bipolar Disorder –  Bipolar disorder is described as suffering from periods of elevated mood followed by long periods of depression.  The voices in bipolar disorder can vary throughout the person’s illness. During the manic or elevated mood phase of bipolar disorder a person can hear voices telling them that they are a particular person on this Earth and during the depressive period of bipolar disorder a person may listen to a single voice telling them they are ‘no good,’ ‘ugly,’ ‘useless,’ or other negative things.
• Major depression with psychosis
• Substance intoxication: Voices heard when a person is acutely intoxicated and can be bizarre and vary in nature. Voices are more common when a person is high on stimulants like cocaine or amphetamines, while hallucinogenics such as mushrooms and LSD are more likely to cause visual hallucinations.
• Substance withdrawal: Hearing Voices can sometimes be a symptom of cocaine or alcohol withdrawal. Typically, the individual hears one voice saying degrading things similar to the presentation of major depression with psychosis.
• Endocrine (hormonal) causes of psychosis such as thyroid disease, Cushing’s disease (when the body is producing too much cortisol)
• Bereavement: Sometimes a grieving person may briefly hear the voice of a loved one calling out to them. This is common in bereavement especially the first six months after a person’s death.
• Infections affecting the temporal lobe such as ‘Temporal lobe encephalitis’ caused by Herpes Simplex Virus can cause auditory hallucinations.
• Medications such as ‘Levadopa’ a dopamine receptor agonist (activator) can cause auditory hallucinations. An overdose of Aspirin can cause a ringing in the ears.

2.Visual Hallucinations

Contrary to popular belief visual hallucinations are usually not seen in schizophrenia, and persistent visual hallucinations are more likely to be a symptom of organic brain disease. Causes of visual hallucinations include:

• Delirium: This is the most common cause of visual hallucinations, especially in the elderly. They can be caused by anesthetic agents, hypoxia (lack of oxygen to the brain), stroke, heart attacks,  infection, head trauma, tumor. The hallucinations usually subside when the cause of the delirium is treated.
• Dementia: Visual hallucinations can occur in the frontal lobe and Lewy body dementia. This is a type of dementia which affects the front of the brain.  Visual hallucinations do typically occur in Alzheimer’s disease or vascular dementia (blockage or small arteries in the brain).
• Brain Tumors: Tumors to the occipital lobe, which is a part of the brain in the back of the head which processes visual stimuli can cause visual hallucinations.
• Drugs: All type of anesthetics and recreational drugs can cause visual hallucinations. Drugs such as ketamine, PCP, LSD, magic mushrooms are especially known for causing people to ‘see things.’
• Hypnagogic hallucinations: Can occur just as a person falls asleep. They may present as seeing a ‘white figure,’ or a ghost.

3. Olfactory Hallucinations 

• Olfactory hallucinations are strange smells which are not real. The song ‘ ring a ring a rose, a pocket full of poses’ is a reference to the unexplained ‘sweet smell’ that individuals would experience after catching the virus known as the ‘black death’ during the middle ages in England. The sweet smelling sensation would be followed by sneezing, sickness and fever then death, which explains the rest of the rhyme ‘atishoo atishoo, we all fall!’
• Modern-day causes of olfactory hallucinations include brain tumors and seizures. A person may describe a strange smell before losing having a seizure consciousness.  Sometimes an olfactory hallucination (unexplained smell) can occur during a migraine as part of the ‘aura.’

4. Tactile hallucinations:

Tactile hallucinations are sensations on your skin. Tactile hallucinations are most commonly seen in schizophrenia and substance abuse.  Cocaine and heroin use can both cause tactile hallucinations. The most unpleasant type of tactile hallucination is a feeling of bugs crawling on your skin.

• Schizophrenia: People often describe a feeling of being touched, poked or prodded by an outside force. The most unpleasant sensation is that of bugs crawling on the skin.
• Ekbom’s syndrome: This is a feeling of parasitosis or insects crawling under the skin.
• Parkinson’s Disease: Parkinson’s disease usually affects elderly people. It can cause stiffness, rigidity, tremor, shuffling gait and a mask like faces. Michael J. Fox and Mohammad Ali are famous individuals affected with Parkinson’s disease.  People with late-stage Parkinson’s often describe tactile or auditory hallucinations. Interestingly, the hallucinations associated with Parkinson’s disease do not usually cause distress and are sometimes described as ‘amusing.’
• Phantom limb pain: This is when an amputee experiences pain in a limb or part of a limb which no longer exists or has been amputated.

Causes of hallucinations can be evaluated with a careful history, mental status exam and medical  workup

It is important not to assume that every person who hears voices or has a hallucination has schizophrenia or is taking drugs because sometimes the correct diagnosis or cause can be missed. Evaluating a person who has hallucinations involves careful history, mental status exam, lab work, brain imaging when indicated with CT or MRI, collateral from the family, CXR and urine toxicology.

Treatment of hallucinations includes treating the underlying cause and antipsychotics where needed. Contrary to popular belief, most types of hallucinations are usually reasonably easy to treat and respond fairly well to medication.

Gundu Reddy, MBBS, is a Board certified psychiatrist in New York City, New York. She is Medical Director of Gaba Telepsychiatry,