Q&A: Nervous system disorders were the top cause of health loss in 2021

According to a systematic analysis from researchers at American Academy of Neurology and IHME, in 2021, nervous system disorders were the top cause of health loss and impacted more than 180 million people. The leading causes of health loss were stroke, Alzheimer’s disease and other dementias, diabetic neuropathy, and migraine.

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Video transcript

This transcript has been lightly edited for clarity.

Why was it important to undertake this study?

The paper aims to provide the most comprehensive assessment, to date, of all conditions that impact the nervous system in the United States. To do so, this study built on a global paper published last year, in 2024. The present study uses the same methods but focuses on the United States and was able to provide state-specific estimates.

What’s novel about both papers, the current one and the one published last year, is the work spearheaded by Dr. Jamie Steinmetz and other coauthors, that identifies a broad set of conditions for neurological health loss as a feature, including conditions where neurological health loss may not even be the primary feature, but it is still a consequence, such as COVID-19.

It’s the comprehensiveness of this definition in capturing many impacts on the nervous system that sets this research apart.

What were the key findings of the study?

The study included 36 conditions identified as having neurological or nervous system impacts, and these conditions cover the full human lifespan.

In Figure 1, we have categorized the different conditions affecting the nervous system into different condition buckets. So, we have neurological disorders, neurodevelopmental disorders, and then a set of the congenital, neonatal, infectious diseases, and systemic conditions that can all have nervous system impacts. Within these we’re able to identify the prevalence of each one within the United States population in 2021.

So, for example, within neurological disorders, we see that tension-type headaches have the highest prevalence. Within neurodevelopmental disorders, we see the highest prevalence for ADHD or attention-deficit hyperactivity disorder. And then, within the third bucket, we see that diabetes has the highest prevalence.

Figure 2 shows, for 2021 in the United States, the breakdown of the total health loss expressed as DALYs, or disability-adjusted life years, of all the conditions considered into the components of DALYs, which are years lived with disability, or years of life lost. So, for example, looking at stroke, which has reached almost 4 million total DALYs, it is composed predominantly of years of life lost, with a smaller portion, around just under 1 million, being attributable to years lived with disability.

Other conditions, such as migraine or diabetes, incur health loss entirely through years lived with disability, because they don’t directly cause any mortality.

Taken in the aggregate, all these conditions of the nervous system constitute the top cause of health loss, or disability-adjusted life years, in the United States in 2021, coming up to 16.6 million DALYs.

Over half of the United States population, so 54%, are affected by a neurological disease or disorder, coming up to 180 million out of 333 million in 2021. The most prevalent among these conditions were tension-type headache, migraine, diabetic neuropathy. And the conditions that caused the most health loss, on the other hand, were stroke, Alzheimer’s and other dementias, diabetic neuropathy, as well as migraine.

The prevalence of the set of all these conditions has increased by 35% between 1990 and 2021, or the period covered by the study, and DALYs have increased during that time span by 55%. However, much of this change is driven by demographic shifts in our population. So, if we look at age-standardized rates, they show approximately stable prevalence over the period considered, and DALYs have decreased over that period by 4%.

The small decrease in DALYs is composed of a 20% decrease in years of life lost, or YLLs, and an increase of about 10% in years lived with disability as people live longer.

How do you explain the differences between states?

Discrepancies by state are driven, in large part, by age and sex composition differences. So, as we saw in the results, much of the variation in the impact of neurological disorders is driven by demographics. Additional factors can be the exposure to upstream risk factors for various individual conditions included in the bucket.

Figure 3 shows us two elements of the geographical variation of the health burden of conditions affecting the nervous system. The first is the color scale, which we can see at the bottom, corresponds to DALYs per 100,000 population, and these, of note, are age-standardized rates, so holding the population structure of each state constant, and holding it at the US overall population structure.

The other component that we’re able to see is the percentage change between 1990 and 2021, that has been shown in the DALYs for that specific state. So, in some cases, we can see for Alaska that there has been a 5% decrease in DALYs, for Hawaii, that number is 6%, a 6% decrease. In other states, like Washington, there is a 9% decrease.

And in very few states there may be a stable or even an increase in this trend. We can also see, as I mentioned before, that the lowest current rates of DALYs due to nervous system conditions are in New York, New Jersey, and Massachusetts. Meanwhile, the highest rates, age-standardized, of health loss, are in Mississippi, Alabama, and Louisiana.

How do you hope the findings of the study will be used?

Our hope is that policymakers and researchers will pay close attention to the fact that disorders affecting the nervous system are highly prevalent and affect over half of the United States population and are the leading cause of DALYs when compared to other condition aggregates, or condition categories. These findings highlight the importance of investing in neurological health, which might include more funding for research, preventive policies, early detection, as well as ensuring equitable access to neurological care.

Despite the high prevalence of these conditions, the United States already performs relatively well compared to other countries in preventing deaths. Stroke is a strong example of this: even though we may have higher stroke prevalence than other countries, we have relatively low stroke deaths. So, from here, we should aim to reduce the deaths of this category of conditions even further.

And, in particular, we should focus on preventing and mitigating the disability incurred by these conditions as people live longer as a result of our progress reducing mortality.