Six-Year Incidence of Blindness and Visual Impairment in Kenya: The Nakuru Eye Disease Cohort Study

Abstract

PurposeTo describe the cumulative 6-year incidence of visual impairment (VI) and blindness in an adult Kenyan population. The Nakuru Posterior Segment Eye Disease Study is a population-based sample of 4414 participants aged ≥50 years, enrolled in 2007–2008. Of these, 2170 (50%) were reexamined in 2013–2014.MethodsThe World Health Organization (WHO) and US definitions were used to calculate presenting visual acuity classifications based on logMAR visual acuity tests at baseline and follow-up. Detailed ophthalmic and anthropometric examinations as well as a questionnaire, which included past medical and ophthalmic history, were used to assess risk factors for study participation and vision loss. Cumulative incidence of VI and blindness, and factors associated with these outcomes, were estimated. Inverse probability weighting was used to adjust for nonparticipation.ResultsVisual acuity measurements were available for 2164 (99.7%) participants. Using WHO definitions, the 6-year cumulative incidence of VI was 11.9% (95%CI [confidence interval]: 10.3–13.8%) and blindness was 1.51% (95%CI: 1.0–2.2%); using the US classification, the cumulative incidence of blindness was 2.70% (95%CI: 1.8–3.2%). Incidence of VI increased strongly with older age, and independently with being diabetic. There are an estimated 21 new cases of VI per year in people aged ≥50 years per 1000 people, of whom 3 are blind. Therefore in Kenya we estimate that there are 92,000 new cases of VI in people aged ≥50 years per year, of whom 11,600 are blind, out of a total population of approximately 4.3 million people aged 50 and above.ConclusionsThe incidence of VI and blindness in this older Kenyan population was considerably higher than in comparable studies worldwide. A continued effort to strengthen the eye health system is necessary to support the growing unmet need in an aging and growing population.

Incidence and correction of vision impairment among elderly population in southern urban China

“…Comparisons between VI incidences should be made with caution in consideration of the difference in study population, design and definitions. Using the WHO criteria, incidence of PVI identified in our study (8.33%, ≥35 years old) was higher than that reported in Melbourne (1.38%, ≥40 years old), Shahroud (1.02%, 40‐64 years old) and Los Angeles (1.8%, ≥40 years old), but lower than that in Kenya (11.9%, ≥50 years old) . When compared with studies conducted in China, the Beijing Eye Study only reported incidence of best‐corrected VI (1.7%) .…”

“…Using the WHO criteria, incidence of PVI identified in our study (8.33%, ≥35 years old) was higher than that reported in Melbourne (1.38%, ≥40 years old), 11 Shahroud (1.02%, 40-64 years old) 12 and Los Angeles (1.8%, ≥40 years old), 22 but lower than that in Kenya (11.9%, ≥50 years old). 23 When compared with studies conducted in China, the Beijing Eye Study only reported incidence of best-corrected VI (1.7%). 10 The Liwan Eye Study, which is also based on an urban Southern Chinese population, reported a higher incidence of PVI compared with our study (WHO criteria, 12.4% vs 8.33%; US criteria, 20.6% vs 12.2%).…”

Systematic bias in real-world tonometry readings based on laterality?

“…[1] In undertaking a large population-based cohort study in Nakuru, Kenya, which included measurement of participants' IOP, it was noted that the IOP of right eyes were significantly higher than the fellow left eyes. [2][3][4] This same observation has been described by other studies that report IOP difference between right and left eyes, [5][6][7][8] despite no known physiological difference between right and left eyes that could explain this difference. If this research finding were also present in routine clinical practice, then a systematic bias in IOP measurement could lead to a systematic overtreatment of right eyes relative to left, which at a population level may have implications for clinical outcomes and resource allocation.…”

Assessment of Response Bias Is Neglected in Cross-Sectional Blindness Prevalence Surveys: A Review of Recent Surveys in Low- and Middle-Income Countries

“…While this potential limitation was overlooked by many studies, there were good examples of these issues being discussed in a way that enhances interpretation of the results. 13,18 A further limitation of adjusting prevalence estimates to a target population is the implicit assumption that blindness prevalence in responders and non-responders is the same. We found no examples of this being tested, but this assumption is conceivably not valid.…”