There are many, many causes of delirium, for example toxic or metabolic states. Drug withdrawal can be a common cause of delirium. Most people have heard of the DT's: delirium tremens Alcohol withdrawal causes this. There are metabolic encephalopathies. Delirium is a medical problem, not a functional or psychiatric problem, yet patients with delirium often end up misdiagnosed and on psych units.
There is bizarre problems and behavior associated with delirium, hallucinations: visual (seeing things) or tactile (feeling things crawling on your skin) are a common problem. Note that in a psychiatric disorder not a auditory (hearing voices) is the most common type of hallucination.
Liver, kidney and thyroid abnormalities are (metabolic) causes, infections, and drugs can be other causes. Basically there is an imbalance and the brains equilibrium with the rest of the body becomes out of whack.
A person with dementia may develop a delirium, for example a untreated urinary tract infection can cause this.
The person with delirium has a fluctuating level of consciousness, they may be in and out of being alert. It may fluctuate over the course of the day, becoming worse at night frequently. The person may be aggressive or violent and lash out.- hence a misdiagnosed psychiatric problem.
A person with delirium has not just an altered fluctuating level of consciousness, somnolence to agitation for example, but they can not attend. The persons attention span is altered.
Attention and level of consciousness are two distinguishing factors of delirium from dementia.
A person with dementia may be disoriented -not know who they are or you are, they may have an impaired memory, not remember things, and that part is the same as delirium.
But someone with dementia really does not have the same alternating level of consciousness, and they generally may not remember a thing, depending on how far the dementia has progressed, but they do have an attention span and they can attend. Example, if you give a demented person a strong of 5-7 numbers to repeat they can say them back, that is attention, not concentration or memory.
Concentration which would be saying the months of the year backwards or of course the classic, spell W O R L D backwards tests concentration. Concentration and memory and orientation are always typically impaired in delirium and dementia.
People with dementia should not have a delirium, but they are more sensitive to developing one. Example a younger person with a urinary tract infection and no dementia is not likely to develop delirium but someone more elderly and with dementia may be more likely.