Eyes: Windows to peek at brain function in spinocerebellar ataxias

Written by Dr. Sriram Jayabal Edited by Dr. David Bushart

Eye movement deficits occur ubiquitously in spinocerebellar ataxias, even at early disease states, highlighting their clinical importance.

Imagine the different motor movements that you make in your everyday life. Many people think of actions that we perform using our hands and legs, such as reaching for objects or walking. Let’s zoom in on a different task: catching a baseball. You need to know where the ball is going to land so you can run to that spot, then guide your arms while diving, if need be, to catch the ball. For this to work perfectly, you need to see and follow the ball. Your eyes enable you to track the ball while it is moving. How can your eyes keep the ball in focus while you are running at full speed towards the spot where you expect the ball to land? Your eyes are equipped with muscles which enable the eyes to move so as to keep the visual scene in focus. These eye movements, as demanded by the needs of the current scenario, in this case, catching a baseball, are indispensable for us to see the world without any hindrance.

Woman with hand in a "C" shape in front of her face. She's focusing in on her eye.
The eyes may provide a window into spinocerebellar ataxia, even before other symptoms show up. Photo by fotografierende on Pexels.com

Which brain region gives us the power to do this?

The cerebellum, or “little brain”, which enables one to move their arms and legs precisely, also controls the way we move our eyes. Therefore, it is logical to posit that when cerebellum goes awry, it may lead to eye movement abnormalities. Several previous studies have shown this to be true in many spinocerebellar ataxias (SCAs), where non-gait symptoms such as eye movement abnormalities have been found to accompany gait deficits in advanced stages of the disease. However, recent work from pioneers in clinical ataxia research at the Harvard Medical School have shown that eye movement abnormalities are also commonly present in SCAs even in pre-symptomatic states. This study emphasizes the critical need to better document the history of eye movement deficits and track them throughout the progression of the disease. This will help researchers to develop better rating scales for ataxia.

In this study, a population of SCA patients (134 individuals) who exhibited different types of SCA (including SCA1, SCA2, SCA3, SCA5, SCA6, SCA7, SCA8 and SCA17) were assessed for eye movement abnormalities at different stages of the disease, from pre-symptomatic (with no gait deficits) to advanced stages (those who use a wheel-chair). First, it was found that ~78% of all pre-symptomatic individuals exhibited eye movement deficits, and these deficits became even more common as the disease progressed, where every single person in advanced stages exhibited eye movement deficits. Second, when researchers examined the eye movements closely, they found that different types of ataxia might cause different kinds of eye movement deficits. However, these results are only suggestive because of the small population size of early-stage SCA individuals in this study, and the types of assessments used. Therefore, future studies will require a larger population size and a thorough quantitative analysis of specific types of eye movement deficits to help characterize eye movement abnormalities in SCAs. Finally, the Brief Ataxia Rating Scale (BARS), a recently designed simple clinical test for ataxia, was further improved in this study to account for the clinically observed eye movement deficits in SCAs. With such a nuanced metric, an improved BARS score was found to correlate with the stage, severity and duration of the disease irrespective of the type of ataxia.

What does this mean for current ataxia scoring metrics?

By evaluating a large population of SCA individuals, this study has shown that eye movement deficits are common in many types of SCA. In addition, this study underscores that eye movement deficits are not only observed in advanced stages of the disease, but also, perhaps more importantly, are also seen in early stages before gait abnormalities are observed. This highlights the importance of the inclusion of eye movement deficits in the current ataxia rating scale so that these symptoms of the disease can also be recorded and tracked over the progression of the disease. Such thorough documentation of symptoms could help researchers identify markers that could be used in natural history and clinical trial studies. Furthermore, this study warrants a comprehensive quantitative measure of the eye movement abnormalities, which could help researchers identify different deficits in different types of SCAs. This could aid in the initial differential diagnosis of a patient’s specific type of SCA. Finally, this study also stresses the importance of assessing other non-gait deficits, such as speech deficits, that often accompany gait abnormalities. Perhaps, we should be heading towards a holistic ataxia rating scale which gives importance to both gait and non-gait symptoms for better documentation of ataxia progression, which will potentially help in future diagnosis and treatment.

Key Terms

Advanced Stages: Term used to describe patients who has been exhibiting symptoms for a long period of time. Their symptoms have developed to a stage where they are very severe and have a huge impact on a person’s quality of life.

Pre-symptomatic: Term used to describe patients have been confirmed through medical testing to have the mutation for a disease, but who are not yet showing any symptoms.

Conflict of Interest Statement

The authors and editor declare no conflict of interest.

Citation of Article Reviewed

Stephen, C.D & Schmahmann, J.D., Eye movement abnormalities are ubiquitous in the spinocerebellar ataxias. The Cerebellum, 2019, 1-7. (https://www.ncbi.nlm.nih.gov/pubmed/31175630)