Measuring Cognitive Decline

By Jane Farrell

People whose brains have less myelin — the fatty tissue that insulates nerve cells — experience steeper declines in cognition over time, according to a National Institute on Aging (NIA) study published in Alzheimer’s & Dementia. The findings show the impact of myelin content as healthy people age and underscore myelin as a potential therapeutic target for neurodegenerative diseases, such as Alzheimer’s and mild cognitive impairment.

Myelin is an insulating layer that forms around nerves, including those in the brain. It helps neurons communicate more efficiently, maintain a healthy water balance, and respond to signals involved in memory formation and other cognitive functions, the NIA said in a news release. Previous studies have shown that myelin production slows as people age, but whether changes in myelin are linked to age-related changes in cognition remains unknown. However, some animal study evidence suggests changes in myelin may be responsible for cognitive deficits in neurodegenerative diseases.

For this study, the team of NIA scientists gathered data from 123 cognitively unimpaired adults enrolled in the Baltimore Longitudinal Study of Aging and from the Genetic and Epigenetic Signatures of Translational Aging Laboratory Testing study. The researchers tested several different types of cognition and used magnetic resonance imaging (MRI) brain scans to measure myelin content in the brain. They looked at two different MRI-based measurements to estimate the amount of myelin: myelin water fraction (MWF) and longitudinal relaxation rate (R1).

For both measurements, lower myelin was linked to greater cognitive decline over time. Lower myelin, as measured with MWF, was associated with a steeper decline in executive function, which includes skills such as planning, problem-solving, organization, and multitasking. Lower myelin as measured with R1 was associated with a faster decline in verbal fluency, which measures the ability to retrieve information from memory. These findings contribute to a foundation for further studies that explore how age-related changes in myelin are different in people who develop neurodegenerative diseases.

While overall this study establishes a meaningful relationship between myelin content and cognition, the authors noted some limitations, mainly that the cohort size was relatively small and included a higher proportion of men and White participants. Future research should reexamine the link in a larger, more diverse study cohort; explore the link between myelin and cognition in people with neurodegenerative diseases; and test whether promoting myelin production could be an effective strategy to prevent disease.