Many people notice a change in their loved one after surgery, in particular that their memory appears to have gotten worse. This has led to questions about whether the general anaesthetic used during surgery can cause dementia.
Whilst there is a known short-term effect of anaesthesia on memory (called post-operative cognitive decline), studies that have investigated a link between dementia risk and general anaesthetics have found mixed results. These inconsistent findings make it difficult to definitively say whether or not general anaesthesia increases risk of dementia.
Why might general anaesthesia increase risk of dementia?
There have been several studies looking at how general anaesthetics affect brain cells. Some studies have found that certain types of general anaesthetics seem to lead to increased levels of the Alzheimer’s hallmark toxic clumps of amyloid and tau proteins in brain cells. These proteins are thought to cause damage to brain cells in Alzheimer’s disease.
Some researchers suggest that some types of general anaesthetics could prevent brain cells from properly repairing damage. In the brain, damaged cells signal to nearby ‘repair cells’, which trigger the immune system (a process known as inflammation) to protect and heal the damage. This signalling process is thought to be altered in the brains of older people, leading to too much inflammation and causing further damage to brain cells (for references see here and here).
Researchers also think that other factors play a role in how general anaesthetics affect the brain. For example, low levels of oxygen in the blood and lower body temperatures caused by anaesthesia may contribute towards damage to brain cells. Some studies also suggest that anaesthesia may make existing underlying dementia mechanisms worse, particularly in people at an increased genetic risk the condition.
What has the research found?
Studying the effect of general anaesthetics in the brain is difficult. Dementia can have a very slow development and there is a lack of studies that follow people for a long enough period of time. Looking in-depth at the mechanisms underlying the effects of general anaesthetics on brain cells would require very invasive procedures and are not possible to do in people. Many studies have looked at these mechanisms in mouse and fly brains.
However, there are some studies that have looked at the effects of anaesthetics in people. One study looked at a very large group of older people (around 130,000) who had been exposed to general anaesthesia in the past 7 years. They found that general anaesthesia was associated with higher risks of dementia. The older the person when they had surgery the more likely they were to have a higher risk of dementia. The researchers suggested that older brains could be less resistant to damage caused by anaesthesia.
However, another study based in Denmark analysed memory and thinking in around 8,500 pairs of twins and found surgery and exposure to general anaesthetics did not lead to higher dementia risk. The researchers suggest any potential relationship between surgery and cognitive decline was more likely to have been caused by the condition that the surgery addressed rather than the surgery itself. For example, issues with the heart and blood vessels are known to increase risk of dementia, and a person with these issues is also more likely to require surgery.
There is a link between surgery and short-term changes in thinking and memory, called post-operative delirium or post-operative cognitive decline. This condition particularly seems to affect older people. Some studies have also found that these short term changes may be associated with higher risk of dementia later in life but other studies have found no association.
How general anaesthetics may be related to short term changes in thinking and memory has also been investigated. One study found that some types of anaesthesia may lead to faster recovery from short term changes in the brain than others in older people. This result indicates general anaesthetic may play a role in thinking and memory immediately after surgery.
Surgery and dementia risk
It is difficult to say from observational studies what part of the surgery has led to a particular complication. Some say that the effect on memory could actually be caused by the surgery itself and not the anaesthetic.
Surgery tends to carry higher risks of complications for older people, such as infection. One reason for this that an older body may be less able to protect itself from and repair damage. Doctors sometimes measure this as ‘frailty’. Studies have shown that older patients with higher frailty are at higher risk of complications, short term thinking and memory problems, and longer stays in hospital after surgery.
Some researchers think that cell damage from surgery is more likely to trigger too much inflammation in older brains than general anaesthetics are. There is some evidence that surgery can result in inflammation, which is associated with cognitive decline. The exact mechanism causing this is not well understood yet.
Overall, complications with the research mean that it is not clear whether general anaesthetics are known to increase dementia risk. Anyone who is going to have surgery should talk to their doctor first about the possible risks and benefits.