Nerve damage to the brain in MS can result in changes in thinking and physical ability. Why does this damage show up differently in different people? It could be down to each person's cognitive reserve.
Cognitive reserve refers to the difference between the amount of nerve damage that a brain has sustained and the impact the damage has had on thinking and behaviour. It explains how two people could have the same amount of physical damage to the brain, but it could affect them to different degrees. In multiple sclerosis, cognitive reserve may also explain why some people have brain lesions that show up on MRI scans but do not appear to affect them.
It is normal to lose nerves in the brain as we age. We also collect nerve damage from physical accidents or from medical situations like a stroke or a demyelinating disease like MS. Whatever the cause, some brains seem better able to cope with damage than others. They are able to re-route nerve signals around a damaged area because they have spare capacity in some way, or are better able to react flexibly.
Scientists are not certain exactly why some brains are more resilient. They might be physically larger, have better connected neurons or more efficient nerves, or simply have different ways of handling nerve signals. These features are partly genetic and partly down to your life experiences. Research has shown that your personality type and the way you think could also affect your cognitive reserve.
Normal ageing contributes to MS disease progression. Neurons naturally die off with age, and this loss of neurons might affect your reserve capacity. Where your brain might previously have been coping with an MS lesion, as the years pass, it may begin to struggle and you will notice an impact on your thinking or physical ability. This is also known as the 'time bomb' of progressive MS.
One way to think about cognitive reserve and MS is illustrated by the 'topographic model'. This is not a true representation of the way your brain works, but it can be a useful metaphor for some people.
Imagine the ocean floor. Mountains under the sea represent lesions in the brain or spinal cord, while the water level represents your cognitive reserve. The mountain peaks only break the surface when the water level drops. When the mountain peaks appear above the water, those would represent relapses or worsening disabilities. However, there are more mountains under the surface, representing clinically silent lesions.
Ageing means that the water level drops steadily, and fatigue or excess heat could result in a temporary water level drop. In both cases, this reveals more of the sea bed and the underlying damage to your brain.
Cognitive reserve can be boosted and maintained by keeping the brain healthy and active to allow it to cope better with MS. Factors like your education, IQ, occupation, how much you exercise and having social and intellectual hobbies have been shown to build reserve. These life experiences and activities encourage brain cells to remain flexible and healthy.
Depression tends to reduces your interest in social activity or hobbies, and so tends to reduce your cognitive reserve. This is another good reason to treat depression effectively, so you can get back to a more active and interesting lifestyle and protect your thinking ability long term.
