Delirium is a rapidly developing disorder of disturbed attention that fluctuates with time.  Although the clinical presentation of delirium differs considerably from patient to patient, there are several characteristic features that help make the diagnosis.

Delirium Due to a General Medical Condition

The patient has a reduced level of consciousness and difficulty focusing, shifting or sustaining attention.

There has been a cognitive change (deficit of language, memory, orientation, perception) that a dementia cannot better explain.

These symptoms develop rapidly (hours to days) and tend to vary during the day.

History, physical examination or laboratory data suggest that a general medical condition has directly caused the condition.

Criteria for Substance Intoxication Delirium

The patient has a reduced level of consciousness and difficulty focusing, shifting or sustaining attention.

There has been a cognitive change (deficit of language, memory, orientation, perception) that a dementia cannot better explain.

These symptoms develop rapidly (hours to days) and tend to fluctuate during the day.

History, physical examination or laboratory data suggest that either:

The symptoms developed during Substance Intoxication or
They are caused by the use of a medication

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Criteria for Substance Withdrawal Delirium

The patient has a reduced level of consciousness and difficulty focusing, shifting or sustaining attention.

There has been a cognitive change (deficit of language, memory, orientation, perception) that a dementia cannot better explain.

These symptoms develop rapidly (hours to days) and tend to fluctuate during the day.

History, physical examination or laboratory data suggest that the symptoms developed during or shortly after Substance Withdrawal.

Criteria for Delirium Due to Multiple Etiologies

The patient has a reduced level of consciousness and difficulty focusing, shifting or sustaining attention.

There has been a cognitive change (deficit of language, memory, orientation, perception) that a dementia cannot better explain.

These symptoms develop rapidly (hours to days) and tend to fluctuate during the day.

These symptoms have more than one cause, as judged by history, physical examination or laboratory data.

Associated Features: 

Learning Problem
Dysarthria or Involuntary Movement
Hypoactivity
Psychosis
Euphoric Mood
Depressed Mood
Somatic or Sexual Dysfunction
Hyperactivity
Addiction
Sexually Deviant Behavior

Differential Diagnosis:

Some disorders have similar or even overlapping symptoms. The clinician, therefore, in his diagnostic attempt, has to differentiate against the following disorders which need to be ruled out to establish a precise diagnosis. The major problem in differential diagnosis is in distinguishing a delirium from an acute functional psychosis. The delirious patient is usually more acute and confused, and the hallucinations are usually more disorganized and are more likely to be visual.  Patients with functional psychoses usually don’t have confusion, disorientation, and illusions, and they are more likely to have a formal thought disorder.  It is wise to also check a patients personal and family history for serious psychiatric illness.

Schizophrenia.
Schizophreniform Disorder and other psychotic disorders.
Dementia.
Factitious Disorder with Psychological Symptoms.

Cause:

This common condition may be caused by physical illness, several causes simultaneously, or by unknown organic conditions. 

Treatment:

Most causes produce diffuse cerebral impairment and lie outside the Central Nervous System and is usually due to some form of deranged metabolism (infection, fever, hypoxia, hypoglycemia, medication side effects, drug withdrawal states, hepatic encephalopathy, postoperative changes), but also include Central Nervous System trauma. The objective of treatment is to control and/or reverse symptoms. Treatment varies with the specific condition causing delirium.

Medications that may be considered for use include:

Thiamine.
Anti-psychotics.
Beta-blockers if dementia is related to central nervous system lesions.
Serotonin-affecting drugs (lithium, trazodone, buspirone, clonazepam).
Dopamine blockers (such as haloperidol).
Fluoxetine, imipramine may be used to stabilize mood.
Stimulant drugs, may improve mood.