The damage that multiple sclerosis (MS) causes in the brain isn’t visible on the outside, but it can cause a wide range of disruptive symptoms. Here’s what’s happening inside the MS brain, and how to help protect this crucial organ.
Multiple Sclerosis: It’s in Your Head
Multiple sclerosis (MS) is a disease of the central nervous system that causes damage to your brain, spinal cord, and optic nerves. It’s characterized by lesions, or areas of tissue damage that occur when your immune system behaves abnormally and attacks these areas.
While many symptoms of MS throughout the body can be caused by lesions in either the brain or the spinal cord, cognitive symptoms of MS — those related to your memory, language, and problem solving — are believed to be caused only by lesions in the brain.
Brain lesions are a hallmark of MS, but they’re not the only way MS can affect your brain function. MS can also contribute to brain atrophy, or shrinkage, over time — a process that occurs in all people as they age, but typically happens much more quickly in people with MS. Brain atrophy, in particular, can contribute to cognitive symptoms of MS.
What Happens When MS Attacks the Brain
In most people who have MS, symptoms of the disease arrive or get worse suddenly during flares of disease activity. During an MS flare, “What we think happens is that there is inflammation coming in from the bloodstream, and that’s the first step in a relapse or new lesion” in the brain, explains Anne Cross, MD, a professor of neurology and MS specialist at Washington University School of Medicine in St. Louis.
According to Dr. Cross, this process involves immune system cells and other substances entering the brain and not just acting on their own, but also “recruiting” local cells to secrete pro-inflammatory substances in the area. Some of these cells and substances attack myelin, a white fatty substance that covers nerve fibers in the brain and elsewhere.
A nerve fiber (also known as an axon) is a long, narrow part of a nerve cell (neuron) that extends from the main body of the cell. It transmits signals from the nerve cell to other cells and tissues. Depending on the specific cell or tissue it connects to, a nerve cell can play a role in any number of the body’s normal functions, from thinking and talking to walking and moving your arms.
Myelin acts as a protective layer on the outside of nerve fibers. When this layer is damaged, nerve fibers can become exposed, which may cause them to transmit signals erratically or less efficiently.
“If you have an active MS lesion, under the microscope it would have lots of inflammatory cells there, and it would have greatly reduced myelin, but the nerve fibers would mostly still be there,” says Cross.
How Brain Lesions Form in MS
Sometimes, loss of myelin in the brain does nothing to interfere with nerve signals, says Cross. “It depends on how much of it is lost in a bundle of nerve fibers,” she explains. But when enough myelin is lost from enough nerve fibers in an area, this lesion will be visible on a magnetic resonance imaging (MRI) scan of the brain — and MS symptoms may develop.
Even without medical treatment, brain lesions in MS don’t simply keep growing and growing. “The body calms down these lesions and surrounds them, and they stop,” says Cross.
If a lesion forms but doesn’t develop past a certain point, it may cause few or even no symptoms. “You might see a region that’s demyelinated, but the nerve fibers are still there and haven’t been damaged too much,” Cross explains. “And that person may have no functional deficits from that lesion.”
But in some lesions, the nerve fibers themselves become heavily damaged and die off as a result. That stops the affected nerve cells from sending signals and can result in a permanent loss of cognitive or physical function.
Brain Atrophy: Why Volume Loss Is a Concern
All people tend to lose brain volume as they age — a process known as atrophy. But in people who have MS, this process typically happens much faster.
It’s normal to lose 0.1 to 0.5 percent of brain volume each year as you age. However, in people with MS, this range is typically 0.5 to 1.35 percent, according to an article published in September 2016 in the journal Multiple Sclerosis and Related Disorders. This greater atrophy may begin even before an MS diagnosis.
When nerve fibers die off in significant numbers due to an MS lesion, myelin is lost from the areas of the brain outside that lesion. That’s because nerve fibers can be very long, extending from one area of the brain to another. A lesion may affect only a small portion of a nerve fiber at first, but when the nerve fiber dies, myelin is lost from the entire length of that fiber beyond the lesion.
There are two main types of tissue in the brain: gray matter and white matter. Gray matter consists of the main bodies of nerve cells, while white matter consists of the nerve fibers that extend from these bodies. White matter gets its color from the myelin that surrounds nerve fibers. So when myelin is lost in areas outside lesions, it tends to cause atrophy of white matter.
But brain atrophy due to MS isn’t limited to white matter. Loss of nerve fibers can lead to the death of entire nerve cells, which means loss of the main nerve cell bodies that make up gray matter. This gray matter atrophy “is more associated with functional consequences than white matter atrophy” is, says Cross.
Scientists are still trying to figure out which symptoms are likely to be caused by atrophy of particular areas of the brain. “The human brain is so interconnected that it’s difficult to say, ‘This dysfunction is due to that region,’” Cross explains. “Memory and cognition are particularly difficult to pin down to any particular region.” Continue reading this article in its entirety click here: What MS looks like in the Brain