The most common cause of dementia worldwide is Alzheimer's Disease. This is probably followed by multi-infarct dementia, which is caused by multiple strokes.

Basically, anything which causes damage to the cerebral cortex can cause dementia. Head trauma, toxins, poisons and encephalitis may cause dementia-like symptoms.

The important thing to remember when faced with someone who has apparent dementia is to exclude delirium in that patient.

Just one of many psychiatric disorders.