An illusion is a sensory perception with a basis in reality. An example is seeing a cat at the bottom of your back gate in the dark, only to realise it is a rubbish bag. Illusions are normal phenomenon occuring in certain situations:

• Where there is little sensory information: e.g in a darkened room full of shadows
• Where there is a state of physiological arousal: e.g. anxiety such as when walking home alone in a bad area.

A hallucination is a sensory perception in the absence of a basis in reality. An example is seeing a cat that doesn't exist in the middle of a well lit clear floor. Genuine hallucinations are usually a sign of significant pathology except for hallucinations while falling asleep (hyponogogic) or awkening (hypnopompic) are not uncommon with the classic being hearing one's name called.

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Auditory hallucinations: These may be simple (bangs, chimes or the like) or complex (music, voices etc.). Only voices are of any diagnostic significance and generally referred to as second person (referring to the patient as 'you') or third person (referring to the patient as he, she, or by name). There my be more than one 'voice' and it is possible that they may argue amongst themselves. An important part of auditory hallucinations is that the patient is convinced they are not their own thoughts - they are like sounds from outside the head.

With auditory hallucinations, it is important to ask about content. What do the voices say?

• Patients may hear their own thought spoken aloud - this may seem to be at the same time as the thought (gedankenlautwerden), or after the thought (écho de la pensée).
• They may hear a commentary on their own actions "he is going out the door now..."
• The may tell the patient he or she is worthless, or evil (positive voices that reassure are rare but not unknown)
• They may command the patient to do things - this has significant risk issues. Can the patient resist the voices? Do they tell him to harm others or himself?

Auditory hallucinations are mostly seen in schizophrenia and third person voices, an auditory commentary describing one's own actions (e.g. He's opening the dor, now he's waling down the hall...), or hearing one's own thoughts spoken aloud are particularly suggestive of this disorder. Drug induced psychosis is an important cause to rule out. Voices which denigrate the patient may suggest depression, especially if the patient feels these views are deserved.

Autoscopic hallucinations: these consist of seeing oneself as if from outside the body. They are ofen referred to as 'out of body' or 'near death' experiences.

Gustatory (taste) and Olfactory (smell) hallucinations: these are relatively uncommon. They may occure in schizophrenia, temporal lobe epilepsy or depression. It is important to exclude physical causes such as sinus infection or tumours of the olfactory pathway.

Tactile or Somatic hallucinations: These are hallucinations of the sense of touch. These can be simple (a touch) or complex (insects under the skin). Surface manifestations tend to be called tactile hallucinations while deeper ones (e.g. affecting the internal organs, or of sexual contact) are reffered to as somatic. Both types are suggestive of schizophrenia. Formication (the hallucination of insects under the skin) is common in cocaine intoxication.

Visual Hallucinations: The content is not generally diagnostic as auditory hallucinations can be. There are some specific subtypes: Lillipution (after the tiny race in Gulliver's Travels) hallucinations are those of small people / dwarves. Extracampine hallucinations are when a person hallucinates something outside the normal field of vision (e.g. behind the head, or in another room).

Bipolar disorder: additional symptoms in the manic phase include wild spending, increased libido, gambling, and delusions (usually grandiose). In the depressive phase symptoms are as depression.

Dementia: additional symptoms include memory loss; difficulty performing familiar tasks; problems with language; disorientation to time and place; poor or decreased judgement; problems with keeping track of things; misplacing things; changes in mood, behavior or personality; Loss of initiative.

Depression: additional symptoms include delusions (often nihilistic or of guilt), sleep dysfunction, altered appetite, loss of libido, anhedonia and a subjective low mood. Ask about suicidal ideation.

Grief reaction

Schizophrenia: this is a disease entity encompassing many many presentations. Always ask about suicidal ideation. Schneiders first rank symptoms comprise four characteristic hallucinations and 5 characteristic delusions. Any one of these is strongly suggestive of schizophrenia:

Four Hallucinations

• Auditory hallucinations in the third person (talking about the patient)
• Auditory hallucinations of a running commentary on one's own actions.
• Auditory hallucinations of hearing one's own thoughts spoken aloud
• Somatic hallucinations

Five Delusions

• Relating to thought: thought insertion, withdrawal, broadcast
• Delusional perception.
• Passivity phenomena (feeling of actions being controlled)

Substance related: intoxication, withdrawal or psychosis from the use of. Obviously a history of drug or alcohol use is vital in forming this diagnosis. Ask about suicidal ideation.