While the symptoms of delirium are numerous and varied, the causes of delirium fall into four basic categories: metabolic, toxic, structural, and infectious. Stated another way, the bases of delirium may be medical, chemical, surgical, or neurological. Many metabolic disorders, such as hypothyroidism, hyperthyroidism, hypokalemia, anoxia, etc. can cause delirium. For example, hypothyroidism (the thyroid gland emits reduced levels of thyroid hormones) brings about a change in emotional responsiveness, which can appear similar to depressive symptoms and cause a state of delirium. Other metabolic sources of delirium involve the dysfunction of the pituitary gland, pancreas, adrenal glands, and parathyroid glands. It should be noted that when a metabolic imbalance goes unattended, the brain may suffer irreparable damage.

One of the most frequent causes of delirium in the elderly is overmedication. The use of medications such as tricyclic antidepressants and antiparkinsonian medications can bring about an anticholinergic toxicity and subsequent delirium. In addition to the anticholinergic drugs, other drugs that can be the source of a delirium are:

anticonvulsants, used to treat epilepsy
antihypertensives, used to treat high blood pressure
cardiac glycosides, such as Digoxin, used to treat heart failure
cimetidine, used to reduce the production of stomach acid disulfiram, used in the treatment of alcoholism
insulin, used to treat diabetes
opiates, used to treat pain
phencyclidine (PCP), used originally as an anesthetic, but later removed from the market, now only produced and used illicitly
salicylates, basically found in aspirin
steroids, sometimes used to prevent muscle wasting in bedridden or other immobile patients

Additionally, systemic poisoning by chemicals or compounds such as carbon monoxide, lead, mercury, or other industrial chemicals can be the source of delirium.

Just as the ingestion of certain drugs may cause delirium in some patients, the withdrawal of drugs can also cause it. Alcohol is the most widely used and most well known of these drugs whose withdrawal symptoms may include delirium. Delirium onset from the abstinence of alcohol in a chronic user can begin within three days of cessation of drinking. The term delirium tremens is used to describe this form of delirium. The resulting symptoms of this delirium are similar in nature to other delirious states, but may be preceded by clear-headed auditory hallucinations. In other words, the delirium has not begun, but the patient may experience auditory hallucinations. Delirium tremens follow and can have ominous consequences with as many as 15% dying.

Some of the structural causes of delirium include vascular blockage, subdural hematoma, and brain tumors. Any of these can damage the brain, through oxygen deprivation or direct insult, and cause delirium. Some patients become delirious following surgery. This can be due to any of several factors, such as: effects of anesthesia, infections, or a metabolic imbalance.

Infectious diseases can also cause delirium. Commonly diagnosed diseases such as urinary tract infections, pneumonia, or fever from a viral infection can induce delirium. Additionally, diseases of the liver, kidney, lungs, and cardiovascular system can cause delirium. Finally, an infection, specific to the brain, can cause delirium. Even a deficiency of thiamin (vitamin B1) can be a trigger for delirium.

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