Closing the gap: John Berdahl, MD, on the global cataract burden and the emerging dementia connection

Cataracts remain the leading cause of preventable blindness worldwide, yet the surgical gap continues to widen as population growth and aging outpace available care. In recognition of Cataract Awareness Month, Ophthalmology Times spoke with John Berdahl, MD, a board-certified ophthalmologist at Vance Thompson Vision in Sioux Falls, South Dakota, and a professor at the University of South Dakota, who is internationally recognized as a leader in cataract, cornea, glaucoma, and refractive surgery.

A longtime advocate for global eye care access, Berdahl has participated in surgical mission trips worldwide and serves in a leadership capacity with EyeCare America, which connects underserved patients with no-cost ophthalmologic care. In the following Q&A—the first of a 2-part conversation— Dr. Berdahl discusses the scale of the global cataract surgical gap, the structural interventions most likely to close it, and what emerging data linking cataract surgery to reduced dementia risk may mean for how surgeons counsel their patients.

This is part 1 of a 2-part conversation with Berdahl, part 2 will be available on June 12, 2026.

This interview has been edited for length and clarity.

Ophthalmology Times: In the context of Cataract Awareness Month and your work with EyeCare America and global surgical missions, how large is the current cataract surgical gap globally and domestically, and what structural interventions would most effectively close this gap?

John Berdahl, MD: Bottom line up front—this is the largest treatable disability burden in all of medicine, and the gap is getting wider, not narrower. That should bother us more than it does.

Cataracts is still the leading cause of blindness on the planet. In 2020, of roughly 43 million blind people worldwide, about 17 million were blind from cataract, nearly 40% of all blindness, with another 83.5 million carrying moderate-to-severe vision impairment from it. And here's the part that gets me; age-standardized cataract prevalence has fallen over the past 30 years, but the absolute number of cataract-blind people keeps climbing because population growth and aging are outpacing our surgical capacity. We're running up a down escalator.¹

The economics make the inaction tough to defend. Vision impairment costs the world around $411 billion a year in lost productivity, against an estimated $25 billion gap to address the unmet need.² That's a 16-to-1 return. There aren't many things in medicine with that kind of leverage.

What actually closes the gap? A few things, roughly in order of impact:

• Throughput. The scalable model is training local surgeons and standardizing high-volume, low-cost technique. Manual small-incision cataract surgery delivers visual outcomes and complication rates comparable to phaco, at a fraction of the cost and without the technology dependence.³ In low-resource settings, manual small incision cataract surgery is a great solution.
• Get patients to see the doctor. A huge chunk of cataract blindness is in people who live within reach of a surgeon but never come due to cost, fear, transport, or simply not knowing surgery exists.
• Domestically it's a different problem. We don't have a capacity shortage, we have an access-and-awareness gap, specifically disparities in who gets timely surgery. That's the reason EyeCare America exists; to connect seniors and underserved patients to care that's already right there.

Recent large observational studies have suggested an association between cataract extraction and reduced dementia incidence. How do you interpret this evidence mechanistically, and what methodologic limitations remain before a causal claim can be responsibly made?

Berdahl: The signal is real and surprisingly consistent, but we are not yet allowed to tell patients that cataract surgery prevents dementia.

The data really are striking. The landmark prospective study, Lee and colleagues in the Adult Changes in Thought cohort, found that cataract extraction was associated with a significantly reduced risk of dementia, hazard ratio .71, even after adjusting for education, race, sex, age, smoking, and APOE genotype.⁴ The UK Biobank work pointed the same direction, with surgery associated with lower all-cause dementia (HR, 0.63), and Alzheimer (HR, 0.40), versus the nonsurgical cataract group.⁵ Pooled across studies, cataract surgery associates with roughly a 26% relative reduction in dementia risk (RR, .74).⁶ When multiple cohorts on different continents land in the same place, you pay attention.

Mechanistically I think there are 3 plausible stories, but none are definitive and they aren't mutually exclusive:

1. Sensory deprivation: Degraded visual input means less cognitive stimulation and faster decline, restoring clear vision reverses some of that.
2. Blue light and circadian rhythm: A brunescent cataract filters the short-wavelength light that drives melanopsin-mediated circadian regulation, clearing the lens turns that back on.
3. Post-surgery improvement: Restored mobility and social engagement after surgery.

Although, we can't call this causal yet:

• Confounding by indication: Patients who get cataract surgery are healthier, more engaged with the medical system, and more functional than the ones who don't.
• Reverse causation: Early, undiagnosed dementia makes someone less likely to pursue elective surgery in the first place. So, some of the "protection" may just be that cognitively intact people are the ones who show up for the OR.
• It's not unanimous: One meta-analysis found the all-cause dementia association lost significance when a single large study was included and only became significant after excluding it.⁷ A result that swings on which studies you count is a result that needs more data.

I tell my patients that restoring your vision is reason enough to operate, and that the possible brain benefit is a bonus we can't promise but can hope for.

Read part 2 on June 12, 2026.

References
1. GBD Vision Loss Expert Group. "Global estimates on the number of people blind or visually impaired by cataract: a meta-analysis from 2000 to 2020." *Eye (London)*, 2024;38(11):2156-2172. (2020 blindness/MSVI figures; rising absolute burden despite falling age-standardized prevalence) https://www.nature.com/articles/s41433-024-02961-1
2. World Health Organization. Blindness and vision impairment. WHO fact sheet. Updated February 10, 2026. Accessed June 4, 2026. https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment
3. Velez G, Fortenbach CR, Oetting TA. Global causes of preventable blindness and vision impairment. EyeRounds.org. February 23, 2022. Accessed June 4, 2026. https://eyerounds.org/tutorials/global-blindness/index.htm
4. Lee CS, Gibbons LE, Lee AY, et al. Association between cataract extraction and development of dementia. JAMA Intern Med. 2022;182(2):134-141. doi:10.1001/jamainternmed.2021.6990
5. Ma LZ, Zhang YR, Li YZ, et al. Cataract, cataract surgery, and risk of incident dementia: a prospective cohort study of 300,823 participants. Biol Psychiatry. 2023;93(9):810-819. doi:10.1016/j.biopsych.2022.06.005
6. Liu X, Guan Z, Liang S, Feng S, Zhou Y. Associations of cataract, cataract surgery with dementia risk: a systematic review and meta-analysis of 448,140 participants. Eur J Clin Invest. 2024;54(2):e14113. doi:10.1111/eci.14113
7. Li Q, Zhu L. Association of cataract surgery with risk of dementia: a meta-analysis and systematic review of cohort studies with 720,075 participants. Arch Med Sci. 2024;20(5):1720-1726. doi:10.5114/aoms/193394