MCI can have a number of different possible causes. Some of these are treatable and some are not.
In some people, MCI is a 'pre-dementia' condition. This means that the brain diseases that cause dementia are already established. These diseases are not generally reversible and so, in time, these people's symptoms will worsen and their condition will progress from MCI to dementia.
For example, some people with MCI have mild memory loss that started gradually. These people are likely to develop Alzheimer's disease as their memory worsens.
Some people with MCI will turn out to have a different, often treatable, cause following assessment by a doctor. This could include depression, anxiety or stress. The same symptoms could also be caused by a physical illness (eg constipation, infection), poor eyesight or hearing, vitamin or thyroid deficiencies, or the side effects of medication. Where this is the case, the person will be diagnosed with this condition - a thyroid deficiency or depression, for example - rather than MCI.
A doctor will not always be able to say what is causing MCI, even after a thorough assessment. It may be necessary to wait a few months or more, to see how the person's symptoms develop.
How many people with MCI develop dementia?
People who have MCI are at an increased risk of going on to develop dementia. In research studies carried out in memory clinics, 10-15 per cent of people who had MCI with gradual memory loss went on to develop dementia - usually Alzheimer's disease - each year.
In studies carried out in other settings, the rates of 'conversion' from MCI to dementia are about half this level (about 5 per cent each year), but people with MCI still show a significantly increased risk of dementia.
Although MCI increases someone's risk of developing dementia, not everyone with MCI will get worse and develop dementia. Some people with MCI remain stable over time and some improve and no longer have any problems.
The outcome will generally depend on the cause of the MCI.
These different outcomes are one reason why some doctors find some aspects of MCI to be controversial.
Can we identify the people who will go on to develop dementia?
A lot of research has focused on trying to identify which people with MCI will go on to develop dementia. If this could be predicted, it would mean people could be offered a range of support earlier in the development of dementia. In the future, for example, researchers might develop drugs to prevent the progression of MCI to dementia.
Most research that looks at predicting which people with MCI will go on to develop dementia has focused on Alzheimer's disease. This is because it is the most common cause of dementia and also the best understood. There is good evidence that simple tests, based on learning lists of words, do a fairly good job of predicting those who will develop Alzheimer's.
Researchers have also tried to identify which people with MCI will go on to develop dementia due to Alzheimer's disease by using brain scans. These scans help to detect changes in brain structure and function.
A different approach to this problem is to measure the amount of certain proteins in the fluid that circulates around the brain and spinal cord. These proteins - called amyloid and tau - are analysed because they may indicate that the changes in the brain that take place in Alzheimer's disease are already underway.
While these are all promising areas of research, it is not yet possible to predict with certainty whether a person with MCI will develop Alzheimer's disease. Even a combination of memory tests, scans and protein level measurements cannot give a definite answer.
Research to identify which people with other forms of MCI (non-memory loss MCI) will go on to develop dementia is progressing, but is much less advanced.