Dementia is not a specific disease. It's an overall term that describes a wide range of symptoms associated with a decline in memory or other thinking skills severe enough to reduce a person's ability to perform everyday activities. Alzheimer's disease. Vascular dementia, which occurs after a stroke, is the second most common dementia type. But there are many other conditions that can cause symptoms of dementia, including some that are reversible, such as thyroid problems and vitamin deficiencies. Dementia is a syndrome that involves severe loss of cognitive abilities as a result of disease or injury. Dementia caused by traumatic brain injury is often static, whereas dementia caused by neurodegenerative disorders, such as Alzheimer's disease, is usually progressive and can eventually be fatal. Dementia can be grouped based on the part of brain being affected. it is roughly true that the earliest symptoms in "cortical" dementia include difficulty with high-level behaviour’s such as memory, language, problem-solving and reasoning; these functions tend to be less impaired in "subcortical" dementia. Cortical dementia occurs because of damage in the cerebral cortex, the outer layer of the brain. They play a critical role in memory and language. The symptoms usually include severe memory loss. Alzheimer's and Creutzfeldt-Jakob disease are two forms of cortical dementia. Subcortical dementia occurs due to the damage to the part of the brain beneath the cortex. The person suffering from it can show changes in their speed of thinking. Usually, people with subcortical dementia don't have forgetfulness and language problems. Parkinson's disease, Huntington's disease, and HIV are two forms subcortical dementia.