Dementia is not a specific disease, but a general term that describes a gradual decline of mental ability that affects your intellectual and social skills to the point where daily life becomes difficult. Dementia can affect your memory, language skills and decision-making ability; impair your judgment and make you feel disoriented; and it may also affect your personality.

Dementia can be caused by many diseases that affect the brain. The most common cause of dementia is Alzheimer's disease. Others include vascular, Lewy body, frontotemporal and mixed dementia. Types of dementia differ based on underlying causes and can affect specific symptoms and how it progresses.

Alzheimer's disease is the most common cause of dementia. It represents 60% to 80% of all dementia cases and affects about 5% of people over age 65. It occurs more often with advanced age, affecting 20% to 25% of people over the age of 80. Although research is ongoing and there are many promising theories, currently it is unclear exactly what causes Alzheimer's disease. Aging and genetics (family history) are considered the greatest risk factors for developing Alzheimer's disease.

Vascular dementia is caused by reduced blood flow that causes cells in the brain to die. This can happen when the blood vessels in the brain are blocked by blood clots or fatty deposits, such as during a stroke. Vascular dementia accounts for 15% to 20% of dementia cases. This disorder may result in sudden, stepwise decline or a more gradual loss of mental ability.

Lewy body dementia accounts for 5% to 15% of dementia cases. Lewy bodies are deposits of a protein that accumulate in the brain which can result in problems with mood, movement, thinking and behaviour. This type of dementia usually progresses quickly and symptoms often include visual hallucinations.

Frontotemporal dementia is caused by the breakdown of nerve cells in two specific parts of the brain, called the frontal and temporal lobes. It leads to changes in language abilities, personality, and behavior.

Dementia can also have mixed causes, especially at older ages. The most common mixed dementia is due to a combination of Alzheimer's disease and vascular dementia.

Diseases such as Huntington's disease, Parkinson's disease, and Creutzfeldt-Jakob disease may eventually lead to symptoms of dementia. Dementia may also be caused by a number of factors that can damage the brain, such as alcoholism and drug use.

In some cases of dementia, it may be reversible or improved once the underlying cause has been treated. Unfortunately, when dementia is caused by conditions such as Alzheimer's disease, brain injury, or blood vessel changes, the changes that occur are irreversible.

Many of us often experience forgetfulness. We may forget where we put the car keys or repeat the same story to a friend or family member. These behaviours are usually caused by the information overload of our busy, stressful lives – it doesn't mean that we're developing dementia. As people age, they may experience memory changes such as slowing of information processing. This type of change is normal and doesn't interfere with day-to-day functioning. By contrast, dementia is disabling and not a normal part of aging.

Although dementia often presents itself differently from person to person, common symptoms include:

• gradual loss of memory of recent events and lack of ability to learn new things
• increasing tendency to repeat oneself, misplacing objects, becoming confused and lost in familiar places
• slow disintegration of judgment and reasoning abilities
• increasing irritability, anxiety, depression, confusion, and restlessness
• increasing communication and language difficulties (forgetting words or substituting incorrect words)
• changes in personality, behaviour, or mood
• decreasing ability to focus and pay attention
• inability to plan and carry out tasks that require multiple steps (e.g. paying bills).

To be diagnosed with dementia, the symptoms have to be severe enough to affect a person's independence and their ability to perform everyday activities.

Symptoms of dementia can vary depending on the cause. For example, people with Lewy body dementia often have long-lasting visual hallucinations. Some forms of dementia also tends to affect people younger than others and progress faster.

Dementia is diagnosed by the history of symptoms and physical examination. Your doctor may ask you a series of questions to assess cognition, which involves functions of the brain related to memory, recall, decision-making, language, recognition of objects used on a daily basis, and following directions.

Brain scans can show changes in brain structure. Brain scans such as CT (computerized tomography) or MRI (magnetic resonance imaging) may be used to help identify other conditions (e.g., stroke) that can cause dementia.

Absolute confirmation of the diagnosis can only be made by examination of brain tissue after a biopsy or an autopsy after death.