A systematic review of 20 population studies found dramatic variation in dementia prevalence estimates across Russia, ranging from 0.5% to 81.6% depending on diagnostic method and age group. Administrative data yielded the lowest estimates, while Mini-Cog screening in adults 85 and older produced the highest rates. Parkinson's disease prevalence ranged from 0.017% to 0.31%, with neurologist assessments showing higher rates than other methods. The enormous disparities highlight critical gaps in Russia's neurological health surveillance. This variability isn't merely academic—it has profound implications for healthcare planning in the world's largest country facing rapid population aging. The findings suggest millions of older Russians may have undiagnosed neurodegenerative conditions, particularly in regions relying on administrative records rather than clinical screening. The review exposes how different diagnostic approaches can yield wildly different public health pictures, potentially leading to massive under-allocation of dementia care resources. However, this preprint awaiting peer review has significant limitations: most included studies lacked representative sampling and used non-standardized criteria. The authors conclude Russia urgently needs population-based longitudinal research with validated diagnostic tools to guide evidence-based policy for its aging population.