In a recent study published in The Lancet Neurology, a group of researchers estimated global, regional, and national health loss due to 37 nervous system conditions and their associated risk factors from 1990 to 2021.
Study: Global, regional, and national burden of disorders affecting the nervous system, 1990–2021: a systematic analysis for the Global Burden of Disease Study 2021. Image Credit: Gorodenkoff/Shutterstock.com
Background
Nervous system conditions include a wide array of disorders impacting the brain, spinal cord, and peripheral nerves, affecting cognitive, sensory, and motor functions at various life stages. These include congenital, neurodevelopmental, cerebrovascular, neurodegenerative diseases, infections, immunological disorders, neuromuscular conditions, traumas, and cancers. Their causes, symptoms, and prognoses vary, ranging from curable or preventable to those causing lifelong disability or high fatality.
As life expectancy increases, so do age-related neurological disorders like Alzheimer's, stroke, and Parkinson's disease. The World Health Assembly's Intersectoral Global Action Plan on Epilepsy and Other Neurological Disorders (IGAP) aims to reduce the impact of these conditions. Further research is needed to uncover new treatments, understand evolving disease patterns, and develop preventive strategies for neurological disorders.
About the study
The Global Burden of Disease, Injuries, and Risk Factors Study (GBD) 2021 assessed a broad spectrum of conditions, from neurodevelopmental to those resulting from infections, capturing the extensive scope of neurological health loss across 204 countries, marking a crucial step in understanding global neurological burdens.
To isolate the neurological impact of these conditions, the study employed various methodologies, drawing on a wide array of data sources, including population-representative studies, surveys, and hospital records. Adjustments were made to align non-reference data with standard definitions through regression analyses, allowing for a more accurate estimation of disease prevalence.
Bayesian models played a crucial role in estimating incidence and prevalence over time, adjusting for differences in geography, age, and sex, and accounting for disease progression and remission rates. The calculation of prevalence involved aggregating cases across different severity levels, termed sequelae, to reflect various health outcomes. For conditions with both neurological and non-neurological aspects, only the neurological components were considered.
The study also introduced a comorbidity correction to avoid overestimating prevalence by accounting for the co-occurrence of conditions. Non-fatal burden was assessed through years lived with disability (YLDs), calculated by multiplying the prevalence of each sequela by its disability weight, while deaths and years of life lost (YLLs) were estimated for conditions directly causing mortality.
Furthermore, the study evaluated the contribution of preventable risk factors to disease burden, offering insights into potential areas for intervention. This involved estimating the impact of reducing exposure to certain risks to a theoretical minimum level. The analysis utilized advanced statistical methods to calculate mean estimates, uncertainty intervals, and percentage changes over time, providing a detailed picture of the evolving landscape of neurological health loss globally.
Study results
In 2021, an estimated 3.4 billion individuals, equating to 43.1% of the global population, were living with a nervous system condition, leading to 11.1 million deaths and generating 443 million disability-adjusted life years (DALYs), positioning these conditions as the primary contributor to global DALYs. This category included a broad array of conditions, with previously identified disorders accounting for 70.5% of the total neurological DALYs.
Notably, newly considered neurological conditions, neurodevelopmental and pediatric conditions, along with other conditions causing neurological health loss, significantly contributed to the overall burden. Despite being the leading cause of global DALYs, the analysis revealed a 27% reduction in age-standardized DALY rates from 1990 to 2021, indicating an overall improvement in managing these conditions.
Regionally, the burden of neurological conditions was unevenly distributed, with the highest age-standardized DALY rates observed in western and central sub-Saharan Africa, primarily driven by conditions affecting children under five. This age group faced the highest DALY rates globally, with neonatal encephalopathy and meningitis being predominant causes. Across different age groups, the leading causes of DALYs varied, highlighting the impact of specific conditions at different life stages.
Temporal trends from 1990 to 2021 showed varying patterns among individual conditions, with significant increases in age-standardized DALYs for conditions such as diabetic neuropathy, while others, like tetanus, saw substantial decreases. The emergence of coronavirus disease 2019 (COVID-19) added a new dimension to the neurological burden, with millions of cases experiencing long-term cognitive symptoms or Guillain–Barré syndrome. Interestingly, the distribution of neurological DALYs between sexes was uneven, with males generally bearing a higher burden, except in the oldest age groups where the rates were similar or higher for females.
The study also examined the contribution of various risk factors to the neurological burden, with high systolic blood pressure being the most significant risk factor for stroke and high fasting plasma glucose for Alzheimer's disease and other dementias. Other factors, such as smoking and air pollution, were also identified as significant contributors to the burden of various neurological conditions.
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