This article originally appeared in the Winter 2023 issue of On The Level.
Summary by Divya Chari, MD, Harvard Medical School & Jeffrey Sharon, MD, UCSF Health
Brain fog is a common complaint for those with vestibular disorders. Forgetfulness, inattention, trouble concentrating, and errors in navigation all occur, leading to lost keys, forgotten names, and wrong turns. But why do vestibular problems cause brain fog?
THE VESTIBULAR SYSTEM AND THE BRAIN
The traditional view of the vestibular system focuses on two primary functions:
- keeping eyes steady as we move; and
- keeping bodies steady as we move.
If the vestibular system is there for clear vision and coordinated movement, why do so many experience trouble thinking? A number of research studies published in the last 20 years show that the vestibular system is far more complicated than we originally thought. These studies give us insight into connections between the vestibular system and the brain. They turn the traditional view of the vestibular system on its head (pun intended!).
The aim of our research was to collect and organize the evidence that vestibular problems can cause brain fog. In the paper, we summarize both animal and human data. Animals are easier to study because researchers can surgically remove or chemically destroy the peripheral vestibular system, or the part of the inner ear that contains the gravity sensors and movement sensors that tell our brain where we are in space. With human research, we study those with already damaged vestibular systems. While this does make the data messier, there are advantages as well, because humans are much more articulate than laboratory mice!
Numerous animal studies have shown that when the vestibular system is removed the animal doesn’t just have problems with keeping their eyes and bodies steady as they move, they also show specific patterns of brain dysfunction. They can’t form the mental maps necessary to navigate, and therefore they get lost and disoriented. They try and try but can’t find food or home without their vestibular system.
In humans, a number of different cognitive problems can be seen with vestibular disease. Studies have shown errors in navigation, short term memory, mental rotation (turning a picture of an object in your head), and attention. In most studies, people with one damaged vestibular system (keep in mind, we have two inner ears!) perform somewhat worse than healthy volunteers, and those without any vestibular system perform the worst.
When you study how the vestibular system is connected to different parts of the brain, the brain fog phenomenon makes sense. The vestibular system is directly plugged into the hippocampus, a brain area important for memory and navigation. One study showed that humans with vestibular damage had smaller hippocampi because of atrophy. Of note, we see brain fog with many different vestibular diseases, including vestibular hypofunction, vestibular migraine, and superior canal dehiscence syndrome.
Everyone agrees that more research is needed. We need to better understand brain fog and whether different diseases have different patterns of brain dysfunction. We also need better research to understand the most important question: with proper treatment, can we cure brain fog?