Original Investigation | Ophthalmology
Incidence of Visually Impairing Cataracts Among Older Adults
in Kenya
Andrew Bastawrous, BSc (Hons), MRCOphth, PhD; Wanjiku Mathenge, PhD; John Nkurikiye, MD; KevinWing, PhD; Hillary Rono, MMed, MPHEC;
Michael Gichangi, MPHEC; Helen A. Weiss, MSc, DPhil; David Macleod; Allen Foster, FRCS; Matthew Burton, PhD, FRCOphth; Hannah Kuper, ScD
Abstract
IMPORTANCE Half of all the cases of blindness worldwide are associated with cataract. Cataract
disproportionately affects people living in low- and middle-income countries and persons of
African descent.
OBJECTIVE To estimate the 6-year cumulative incidence of visually impairing cataract in adult
participants in the Nakuru Eye Disease Cohort Study in Kenya.
DESIGN, SETTING, AND PARTICIPANTS This secondary analysis of the Nakuru Eye Disease Cohort
Study was conducted from February 2016 to April 2016. This cohort comprised citizens of Nakuru,
Kenya, aged 50 years or older who consented to participate in the initial or baseline survey from
January 2007 to November 2008, as well as the follow-up conducted from January 2013 to March
2014. All participants at baseline (n = 4364) and follow-up (n = 2159) underwent ophthalmic
examination.
MAIN OUTCOMES AND MEASURES Six-year cumulative incidence of visually impairing cataract,
risk factors of incidence, population estimates, and required cataract surgical rates to manage
incident visually impairing cataract.
RESULTS In total, 4364 individuals (with a mean [SD] age of 63.4 [10.5] years and with 2275 women
[52.1%]) had complete eye examinations at baseline, and 2159 participants (with a mean [SD] age of
62.5 [9.3] years and with 1140 men [52.8%]) were followed up 6 years later. The 6-year cumulative
incidence of visually significant cataract in either eye was 251.9 per 1000 (95% CI, 228.5-276.8), with
an increase with age from 128.9 (95% CI, 107.9-153.2) per 1000 for the group aged 50 to 59 years to
624.5 (95% CI, 493.1-739.9) per 1000 for the group aged 80 years or older. This equated to an annual
incidence of visually significant cataract of 45.0 per 1000 people aged 50 years or older.
Multivariable analysis showed alcohol consumption (risk ratio [RR], 1.4; 95% CI, 1.1-1.8), diabetes (RR,
1.7; 95% CI, 1.3-2.3), educational level, and increasing age (RR, 3.8; 95% CI, 2.6-5.5 for those aged
ⱖ80 years) were associated with incident visually impairing cataract. Extrapolations to all people
aged 50 years or older in Kenya indicated that 148 280 (95% CI, 134 510-162 950) individuals might
develop new visually impairing cataract in either eye (visual acuity <6/18 in the worse-seeing eye) and
that 9540 (95% CI, 6610-13 750) might become cataract blind in both eyes (visual acuity <3/60 in
better-seeing eye).
CONCLUSIONS AND RELEVANCE Adults in Kenya appeared to have a high incidence of visually
impairing cataract, making cataract a priority for blindness prevention programs in the region;
surgical interventions and awareness of these services are also required.
JAMA Network Open. 2019;2(6):e196354. doi:10.1001/jamanetworkopen.2019.6354
Key Points
Question How many new people per
year become visually impaired from
cataract in Kenya?
Findings In this secondary analysis of
the Nakuru Eye Disease Cohort Study of
4364 participants at baseline and 2159
participants at follow-up, the 6-year
cumulative incidence of visually
significant cataract in either eye was
251.9 per 1000, with the incidence
increasing with age among those aged
50 to 59 years and those 80 years
or older.
Meaning In Kenya, reducing the burden
of sight loss from cataract is a national
priority, given its high incidence among
older adults; the cataract surgical rate
needs to be at the level of the incident
rate to prevent the prevalence of
blindness and visual impairment from
increasing.
+Supplemental content
Author affiliations and article information are
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Open Access. This is an open access article distributed under the terms of the CC-BY License.
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Introduction
The prevalence and incidence of cataract are known to increase with advancing age, and the
magnitude of visually impairing cataract is expected to continue to grow with the aging populations
and longer life expectancies worldwide.
1
Half of all cases of blindness worldwide are associated with
cataract.
2
Cataract disproportionately affects people living in low- and middle-income countries and
persons of African descent.
2,3
Multiple population-based studies have been conducted of the
prevalence of cataract in sub-Saharan Africa,
4
and they have found a considerable variation in
prevalence across the continent. However, surveys have routinely shown that cataract is the
condition most associated with blindness or visual impairment in sub-Saharan Africa.
4
Previous studies of the overall incidence of blindness and visual impairment,
5
macular
degeneration,
6
diabetes and diabetic retinopathy,
7
and glaucoma
8
used data from the Nakuru Eye
Disease Cohort Study. This cohort study of adults aged 50 years or older living in Nakuru, a city in the
Rift Valley region in Kenya, was deemed to be a regionally and nationally representative sample that
could inform the eye care needs and priorities of the entire country. The same cohort study serves as
the source for this current analysis, which characterizes the incidence of visual impairment
associated with cataract.
Management of cataract involves the surgical removal of the lens and insertion of an intraocular
lens and is considered one of the most cost-effective health interventions worldwide.
9
Identifying
the cataract surgical rate needed to control the cataract blindness rate depends on estimating the
incidence of cataract. However, the only incidence data on cataract from populations of African
descent come from outside the African continent. The best estimates come from the Barbados Eye
Studies,
10-13
a 9-year follow-up of adults of African descent aged 40 years or older, showing incidence
rates of 33.8% for any cortical opacities and 42.0% for any nuclear opacities and indicating these
rates were higher in participants of African descent than those of white or Caucasian race/ethnicity
(risk ratio [RR], 1.8; 95% CI,1.2-2.8).
13
Incidence data are urgently needed for Africa to ensure
appropriate planning and allocation of scarce human resources and equipment.
In this present secondary analysis, we aimed to estimate the 6-year cumulative incidence of
visually impairing and blinding cataract among participants in the Nakuru Eye Disease Cohort Study.
This cohort comprised people of East African ethnicity aged 50 years or older who lived in
Nakuru, Kenya.
Methods
The methods of the Nakuru Eye Disease Cohort Study have been reported in detail previously,
14
are
summarized here, and appear in the eMethods of the Supplement. The present study, conducted
from February 2016 to April 2016, followed the Strengthening the Reporting of Observational
Studies in Epidemiology (STROBE) reporting guideline. It adhered to the tenets of the Declaration of
Helsinki
15
and was approved by the Ethics Committee of London School of Hygiene & Tropical Medicine
at both baseline and follow-up surveys.
The objectives of the Nakuru Eye Disease Cohort Study survey and the examination process
were explained in the local dialect to eligible participants in the presence of a witness. A participant
underwent examination only after written (or thumbprint) informed consent was obtained.
Participants identified with eye or other medical conditions were referred to local health care
services.
The initial or baseline population-based survey was conducted from January 2007 to November
2008. The sample size of 5000 participants aged 50 years or older was calculated according to an
expected prevalence of visual acuity (VA) less than 6/12 (Snellen equivalent) in the better eye owing
to posterior segment eye diseases (the primary outcome for the baseline survey) of 3.0% in this age
group, precision of 0.5%, design effect of 1.5%, and a response rate of 90%.
JAMA Network Open | Ophthalmology Incidence of Visually Impairing Cataracts Among Older Adults in Kenya
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One hundred clusters of 50 participants each were selected, with a probability proportional to
the size of the population across Nakuru district. Households were selected within clusters, using a
modified compact segment sampling method.
16
An eligible individual was someone aged 50 years or
older living in the household for at least 3 months in the previous year. All participants were invited
to undergo a comprehensive ophthalmic examination at a screening clinic. The follow-up survey of
the cohort was conducted from January 2013 to March 2014.
Baseline and Follow-up Examination Clinics
The following procedures were undertaken for all participants who attended the examination clinic
at baseline and follow-up surveys, and further details are available elsewhere.
14
Additional
procedures were undertaken that are not included here because they are not relevant to the
outcomes (eg, visual field assessment) being reported.
On examination day, the advance team confirmed the identity of participants against baseline
data (ie, age, date of birth, name, and identity cards). In cases of uncertain identity, confirmation was
made by retinal examination verified by comparison with the baseline photo.
A clinical officer assessed whether study participants wore distance correction glasses, owned
distance correction glasses but failed to bring them, did not own any distance correction glasses,
routinely used reading glasses, or wore aphakic glasses. Visual acuity was measured using a back-
illuminated modified logMAR reduced tumbling E chart (Sussex Vision Inc),
17,18
which has been used
in previous population-based studies.
19,20
The following vision categories were used to define eye-level and person-level (based on the
better-seeing eye) VA: normal (ⱕ6/12 Snellen; logMAR ⱕ0.3), mild visual impairment (VI; <6/12 to
6/18 Snellen; <0.3 to 0.48 logMAR), moderate VI (<6/18 to 6/60 Snellen; <0.48 to 1.0 logMAR),
severe VI (<6/60 to 3/60 Snellen; <1.0 to 1.3 logMAR), or blind (<3/60 Snellen; <1.3 logMAR). The
term visually impaired was used to describe participants with a VA less than 6/18 to no perception of
light and therefore included moderate VI, severe VI, and blind.
Pharmacologic dilation of the participant pupils was achieved by using tropicamide, 1%
(Mydriacyl; Alcon Laboratories Inc), with phenylephrine hydrochloride, 2.5%, if needed. The anterior
segment was examined by the study ophthalmologist (W.M. at baseline; A.B. at follow-up) using
slitlamp biomicroscopy. The World Health Organization Simplified Cataract Grading System was
used
21
following standard protocols. The lens was also examined for position, the presence of
hypermature (morgagnian) cataract, and previous lens (aphakic or pseudophakic) surgical
procedure. A red reflex lens image was taken when each participant took their retinal photographs.
Participants who were pseudophakic were assessed for the presence or absence of posterior
capsular opacification and, if present, whether it entered the visual axis.
Visually impairing cataract was defined as VA in the better-seeing eye of less than 6/18 and the
presence of a gradable cataract (nuclear, cortical, posterior capsular, or mixed, according to the
Simplified Cataract Grading System
21
), mature cataract, or hypermature cataract. Definitions of
incidence are found in eTable 1 in the Supplement.
All participants who had complete examinations at baseline and were not classified as having a
visually impairing cataract were considered to be at risk for incident visually impairing cataract.
Follow-up status at 6 years was categorized as (1) found and examined, (2) found and not examined,
(3) deceased, (4) moved away, or (5) unknown.
Statistical Analysis
Statistical analysis was performed with Stata, version 13 (StataCorp LLC), from January 2015 through
July 2015. All analyses accounted for the cluster survey design using Taylor linearized variance
estimation to calculate SEs. Pearson χ
2
tests corrected for the survey design were used to calculate
2-sided Pvalues to assess differences between participants seen and participants lost to follow-up as
well as between those known to have died and those with unknown outcome status.
JAMA Network Open | Ophthalmology Incidence of Visually Impairing Cataracts Among Older Adults in Kenya
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Participants who died and therefore did not have outcome data were excluded, as they were
not eligible for follow-up. Participants who were followed up but had no complete records for all
covariates at baseline were also removed from the cohort at this stage. An inverse probability
weighting model
22
was developed to allow estimation of cumulative incidence while accounting for
participants lost to follow-up. Multivariable logistic regression was used to identify independent
baseline covariates associated with lost to follow-up. Covariates with evidence of univariable
association with the outcome (P< .10 across all categories of the variable) were kept in a
multivariable model, whereas those with P> .10 were excluded from the model. From this final
model, the probability of being followed up was estimated on the basis of the presence or absence of
each of these baseline covariates. The inverse of this probability formed the weighting to be applied
to account for those lost to follow-up.
The final step was to remove those individuals lost to follow-up from the cohort so that all
subsequent analysis would be performed on only those with complete outcome records, with
inverse probability weighting applied to account for those lost to follow-up. A sensitivity analysis for
this approach involved a complete records analysis (ie, only including people who had complete
records for outcome and all variables).
The 6-year cumulative incidence of each outcome was calculated by dividing the number of
events identified at the 6-year follow-up by the number of people at risk at the beginning of
follow-up. We estimated 95% CIs, assuming a Poisson distribution of events. This step was done for
the population overall, which was stratified by each covariate.
To estimate age-adjusted associations between each outcome (VI and blindness), with baseline
covariates, we calculated age-adjusted RRs for each covariate using a Poisson regression model with
robust error variance to allow for the clustered design and including inverse probability weighting.
For multivariable analysis, an initial model was fitted that included those variables shown to be
associated with outcome in age-adjusted analysis (using a Wald test threshold P< .05 to indicate
association). A backward approach was then applied to obtain a final multivariable model, removing
one by one the variables with P> .05.
World Health Organization definitions of VI and blindness were used throughout
23
: monocular
VI was VA less than 6/18 (20/60) in either eye, VI was VA less than 6/18 in the better-seeing eye, and
monocular blindness was VA less than 3/60 (20/400) in either eye. A person was considered blind if
the VA in the better-seeing eye was less than 3/60. The definition of VI also included those who
were blind.
Diabetes was defined as (1) self-reported in the history, (2) random glucose level of 198.2 mg/dL
or higher (to convert to millimoles per liter, multiply by 0.0555), or (3) HbA
1c
percentage of total
hemoglobin level of 7.0 or higher (to convert to proportion of total hemoglobin, multiply by 0.01).
Estimates of cumulative incidence were extrapolated to estimate the number of adults older
than 50 years with incident VI or blindness in Kenya. The 2015 Census Bureau of Kenya population
estimates were identified by age category and sex and then multiplied by the age- and sex-specific
estimates of annual cumulative incidence.
The number of cataract surgical rate (CSR) per million of population (all ages) was estimated at
different surgical thresholds on the basis of 3 levels of VA (blind, severe VI, or moderate VI) and
whether for person or for individual eye. The estimated annual CSR per million of population was
calculated by multiplying the annual incidence rate for all aged 50 years or older by 1000 and by the
proportion of the population aged 50 years or older in Kenya (4.3 million of 45 million in 2015). The
CSR calculation assumed no cases of blinding or visually impairing cataract existed among people
younger than 50 years and was therefore likely to underestimate the true incidence by a
small amount.
JAMA Network Open | Ophthalmology Incidence of Visually Impairing Cataracts Among Older Adults in Kenya
JAMA Network Open. 2019;2(6):e196354. doi:10.1001/jamanetworkopen.2019.6354 (Reprinted) June 28, 2019 4/15
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Results
Estimates of Prevalence
In total, 4414 participants were recruited at the baseline survey in 2007 to 2008. Of these
participants, 4364 (98.9%; with a mean [SD] age of 63.4 [10.5] years and with 2275 women [52.1%])
had an examination of the lens and were given a lens status. Among the 4364 individuals who had
complete eye examinations, 669 (15.3%) had VA less than 6/12 in the better-seeing eye. Of these
669 participants, 180 (26.9%) were visually impaired (VA <6/18) from cataract, with 32 of them
blind, 11 with severe VI, and 137 with moderate VI.
Cataract was the most commonly associated with blindness, affecting 1968 participants
(45.1%), and severe VI, affecting 2666 participants (61.1%). Overall, 3591 participants (82.3%) did not
have VI or visually significant cataract; that is, they had no cataract and VA of 6/18 or better, had the
presence of cataract but VA of 6/18 or better, or had VA of worse than 6/18 but no evidence of
cataract (Table 1).
The types of lens opacities associated with the level of VI were examined (eTable 2 in the
Supplement). The most common findings were mixed opacities followed by nuclear opacities only,
cortical opacities only, and posterior subcapsular opacities only in all vision categories.
Estimates of Incidence
A total of 2159 participants (49.5%; with a mean [SD] age of 62.5 [9.3] years and with 1140 men
[52.8%]) were followed up in 2013 to 2014. Of these participants, 2129 (98.6%) had a complete
examination, including lens status.
At baseline, 3591 participants were without visually significant cataract and 1821 (50.7%) were
followed up, with 1799 (98.8%) receiving a complete lens examination and therefore at risk of
developing incident visually impairing cataract. In the 6-year follow-up period, 449 (24.9%) of the
1799 participants who were at risk developed a visually significant cataract (VA <6/18 with the
presence of a cataract), and 7 (1.5%) of these 449 had become cataract blind.
Overall, 1944 participants had a cataract on clinical examination at baseline, of whom 773
(39.8%) had a visually significant cataract at baseline, with proportionally fewer (330 [42.7%])
available for follow-up examination. Most of these individuals (302 [91.5%]) had a visually significant
cataract at follow-up, whereas 28 (8.5%) no longer had a visually significant cataract at follow-up
despite no report of an operation (Figure 1). Eighteen (6.3%) of 284 participants at baseline who
were referred for cataract surgical procedure had undergone an operation at follow-up.
Because of the high percentage (50.5%) of people lost to follow-up, we compared baseline
features between participants who were followed up and those who were not (eTable 3 in the
Supplement). Notable differences were found between these 2 groups and those not known to be
deceased (n = 1524 [42.4%]), including proportionally fewer Kikuyus and Kalenjins (the 2 major
ethnic tribes in those not followed up) and proportionally more rural than urban dwellers among
those who were followed up. Notable differences between those followed up and those known to be
deceased included younger mean age (60.9 years vs 67.1 years), lower systolic blood pressure (139.1
mm Hg vs 145.1 mm Hg), lower random blood glucose (93.7 mg/dL vs 100.9 mg/Dl [to convert to
millimoles per liter, multiply by 0.0555]), higher body mass index (10.4% vs 23.4% underweight at
baseline), and lower alcohol consumption.
The 6-year cumulative incidence of visually significant cataract in either eye, after adjusting for
those lost to follow-up using the inverse probability weighting model, was 251.9 (95% CI,
228.5-276.8) per 1000 for all ages, with an increase with age from 128.9 (95% CI, 107.9-153.2) per
1000 for the group aged 50 to 59 years, 290.5 (95% CI, 249.6-335.2) per 1000 for the group aged
60 to 69 years, 565.3 (95% CI, 489.3-638.3) per 1000 for the group aged 70 to 79 years, and 624.5
(95% CI, 493.1-739.9) per 1000 for the group aged 80 years or older (Table 1). This cumulative
incidence equates to an annual incidence of visually significant cataract (<6/18 in either eye) of 45.0
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per 1000 people aged 50 years or older, along with 2.5 per 1000 people per year in this age group
developing cataract blindness (VA <3/60 in both eyes).
The 6-year incidence of persons (with better-seeing eye) becoming visually impaired was 134.9
(95% CI, 117.1-154.9) per 1000, severely visually impaired was 66.6 (95% CI, 54.9-80.6) per 1000,
or blind was 13.6 (95% CI, 9.4-19.5) per 1000 from cataract (Table 1).
When the cumulative incidence was extrapolated to all people in Kenya aged 50 years or older,
the estimated number of individuals per year who might develop visually impairing cataract in either
eye was 148 280 (95% CI, 134510-162 950), become visually impaired from cataract in the better-
seeing eye was 86 690 (95% CI, 75 240-99 570), develop severely visually impairing cataract in
either eye was 88 630 (95% CI, 78 140-100 280), become severely visually impaired from cataract
in the better-seeing eye was 46690 (95% CI, 38 500-56480), develop cataract blindness in either
Table 1. CumulativeIncidence of Visually Signif icant Cataract AmongStudy Participants
Age Group, y
Male Female Overall
Cases/at Risk, No. Risk per 1000/6 y (95% CI) Cases/at Risk, No Risk per 1000/6 y (95% CI) Cases/at Risk, No. Risk per 1000/6 y (95% CI)
Either-Eye Cataract Visual Impairment (VA <6/18)
50-59 43/379 117.9 (91.1-151.4) 76/542 136.8 (108.9-170.5) 119/921 128.9 (107.9-153.2)
60-69 77/301 272.0 (215.4-336.9) 85/286 309.5 (263.6-359.4) 162/587 290.5 (249.6-335.2)
70-79 74/127 584.3 (491.9-671.1) 55/101 542.0 (431.3-648.7) 129/228 565.3 (489.3-638.3)
≥80 19/31 622.1 (447.2-770.1) 20/32 627.0 (421.1-795.3) 39/63 624.5 (493.1-739.9)
All ages 213/838 258.5 (226.1-293.7) 236/961 246.2 (216.6-278.4) 449/1799 251.9 (228.5-276.8)
Person Cataract Visual Impairment (VA <6/18)
50-59 22/399 53.5 (36.6-77.5) 29/552 51.5 (35.3-74.5) 51/951 52.4 (40.0-68.2)
60-69 42/325 131.6 (96.9-176.1) 50/310 160.1 (120.2-210.2) 92/635 145.7 (118.5-177.9)
70-79 42/155 276.9 (203.9-364.0) 43/135 319.1 (237.3-413.8) 85/290 296.8 (247.0-351.8)
≥80 19/43 432.3 (294.9-580.9) 20/45 457.4 (315.3-606.8) 39/88 445.2 (353.9-540.3)
All ages 125/922 134.2 (110.3-162.4) 142/1042 135.5 (112.1-162.9) 267/1964 134.9 (117.1-154.9)
Either-Eye Severe Visual Impairment (VA <6/60)
50-59 24/391 63.8 (43.7-92.3) 26/555 46.4 (30.2-70.7) 50/946 53.8 (40.2-71.7)
60-69 46/324 142.4 (110.6-181.6) 45/315 149.6 (112.8-195.9) 91/639 146.0 (121.2-175.0)
70-79 40/157 260.3 (196.8-335.7) 39/134 297.2 (218.2-390.4) 79/291 277.5 (222.9-339.6)
≥80 25/55 461.0 (327.0-600.8) 19/55 362.0 (232.9-514.6) 44/110 410.7 (317.2-511.0)
All ages 135/927 146.4 (124.5-171.5) 129/1059 125.3 (104.5-149.5) 264/1986 135.1 (119.1-152.9)
Person Severe Visual Impairment (VA <6/60)
50-59 5/404 13.3 (5.7-30.6) 7/563 13.0 (5.6-29.9) 12/967 13.1 (7.3-23.6)
60-69 21/348 57.6 (37.6-87.2) 26/332 82.4 (52.7-126.7) 47/680 69.9 (50.0-96.8)
70-79 26/181 154.6 (106.4-219.4) 27/155 169.0 (112.4-246.0) 53/336 161.3 (122.3-209.8)
≥80 17/63 260.8 (172.3-374.1) 11/64 171.4 (96.7-285.5) 28/127 215.2 (153.0-293.9)
All ages 69/996 69.2 (54.9-86.9 71/1114 64.3 (49.1-83.8) 140/2110 66.6 (54.9-80.6)
Either-Eye Cataract Blindness (VA <3/60)
50-59 10/393 24.4 (11.6-50.6) 10/555 17.4 (9.3-32.3) 20/948 20.4 (12.8-32.4)
60-69 27/329 81.4 (56.1-116.6) 15/317 50.0 (30.0-82.0) 42/646 65.7 (49.4-86.9)
70-79 23/159 156.5 (97.0-242.7) 24/137 174.1 (119.8-246.2) 47/296 164.8 (122.6-217.8)
≥80 13/56 218.3 (127.6-347.7) 11/55 219.3 (116.2-375.1) 24/111 218.8 (141.4-322.6)
All ages 73/937 77.2 (58.4-101.4) 60/1064 58.1 (45.4-74.2) 133/2001 67.0 (55.6-80.6)
Person Cataract Blindness (VA <3/60)
50-59 1/404 2.3 (0.3-16.8) 0/563 1/967 1.0 (0.1-7.2)
60-69 5/350 14.9 (6.1-35.7) 3/332 9.9 (2.9-32.9) 8/682 12.4 (6.0-25.4)
70-79 4/182 29.7 (10.7-79.6) 4/155 24.8 (9.8-61.4) 8/337 27.4 (13.7-54.1)
≥80 2/67 38.0 (8.4-155.7) 8/64 116.7 (58.4-219.7) 10/131 76.7 (40.7-140.1)
All ages 12/1003 13.8 (7.9-24.2) 15/1114 13.3 (8.1-21.9) 27/2117 13.6 (9.4-19.5)
Abbreviation: VA, visual acuity.
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eye was 44 260 (95% CI, 36 700-53 240), and develop cataract blindness in the better-seeing eye
was 9540 (95% CI, 6610-13 750) (Table 2).
These rates indicate that a CSR of 232 is required to match the annual new cases of persons who
are cataract blind. This CSR goes up as the threshold for surgical procedures goes down (eTable 4 in
the Supplement;Figure 2).
Multivariable analysis showed alcohol consumption, diabetes, educational level, and increasing
age to be associated with incident visually impairing cataract. With an RR of 1.4 (95% CI, 1.1-1.8) in
current alcohol drinkers, compared with never drinkers, former drinkers were not at an increased risk
(RR, 1.1; 95% CI, 0.9-1.3). Those with diabetes had an RR of 1.7 (95% CI, 1.3-2.3) compared with those
without diabetes, and those with higher educational level tended to have less incident cataract
(primary education only, 341.2 [95% CI, 299.8-385.3] vs more than secondary education, 91.2 [95%
CI, 55.7-145.7]). Compared with those aged 50 to 59 years, the RR was 2.0 (95% CI, 1.6-2.6) in those
aged 60 to 69 years, 3.7 (95% CI, 2.9-4.7) in those aged 70 to 79 years, and 3.8 (95% CI, 2.6-5.5) in
those aged 80 years or older (Table 3).
Discussion
To our knowledge, this study is the first long-term population-based survey on eye disease in Africa.
The annual incidence of visually impairing cataract (VA <6/18 in either eye) in those aged 50 years
or older was 45.0 per 1000 people per year and 2.5 per 1000 per year were cataract blind (VA <3/60
in both eyes).
Increasing age, diabetes, alcohol consumption, and low educational level were associated with
incident visually impairing cataract. Aging has been a well-described risk factor for incident cataract
throughout the world.
12,13,24,25
Diabetes has also been associated with incident cataract,
26,27
although most cohort studies have not found an association with alcohol consumption; however, a
U-shaped association was found in an Australian cohort, with moderate consumption being
seemingly protective compared with abstinence or heavy consumption.
28
Some evidence of an
inverse association exists between educational level and incident cataract,
29,30
as demonstrated in
this population: notably, educational level affects incidence of cataract operation more commonly
than do cataract formation.
31
Figure 1. Study Participants
28 Had no visually significant
cataract at follow-up
302 Had visually significant
cataract at follow-up
1350 Had no visually significant
cataract at follow-up
449 Had visually significant
cataract at follow-up
3591 Had no visually significant
cataract at baseline
1821 Followed up
773 Had visually significant
cataract at baseline
338 Followed up
435 Lost to follow-up
8Excluded for missing lens data
1770 Lost to follow-up
22 Excluded for missing lens data
5010 Participants enumerated
4414 Recruited
4364 Completed baseline lens status
596 Excluded
50 Excluded for missing lens status
Visually significant cataract indicates visual acuity less than 6/18 and proven cataract.
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Previous studies have reported that, at baseline, 63 of the 71 who were blind were known to
have cataract.
5
At the 6-year follow-up, 2164 participants were seen with complete follow-up data,
of whom 24 were blind at baseline and therefore were excluded from the analysis because they were
not considered at risk of becoming blind; in total, 29 new cases of bilateral blindness were confirmed,
which equated to a 6-year cumulative incidence of VI of 11.9% (95% CI, 10.3%-13.8%) and blindness
of 1.51% (95% CI, 1.0%-2.2%).
5
In this analysis, we found that most incident VI and blindness cases
were associated with cataract.
Of the 29 blind persons at 6-year follow-up, 27 had bilateral cataracts. However, blindness could
not be associated with a single condition when a participant had comorbidity, and this finding should
be kept in mind when interpreting the data. Six-year cumulative incidence of cataract-associated
blindness was 13.6% (95% CI, 9.4%-19.5%), which took into account the definition used in this cohort
that a person was deemed an incident case if found to be pseudophakic at the 6-year follow-up,
assuming the person had visually impairing cataract between assessments that had warranted
Table 2. ExtrapolatedNumber of New Adults With Visually Significant or Blinding Cataract in 2015
Age Group, y
Extrapolated No. (95% CI)
Male Female Overall
Either-Eye Cataract Visual Impairment (VA <6/18)
50-59 19 750 (15 250-25 350) 25 640 (20 410-31 950) 45 760 (38 330-54 390)
60-69 21 970 (17 400-27 210) 31 380 (26 730-36 450) 52 950 (45 490-61 080)
70-79 16 730 (14 080-19 210) 20 760 (16 520-24 840) 37 700 (32 630-42 570)
≥80 2910 (2090-3600) 4510 (3030-5720) 7360 (5810-8720)
All ages 68340 (59 780-77 650) 79 940 (70 320-90 390) 148 280 (134 510-162 950)
Person Cataract Visual Impairment (VA <6/18)
50-59 9230 (6320-13 370) 9800 (6720-14 170) 18 990 (14 510-24 750)
60-69 11 570 (8520-15 490) 17 420 (13 070-22 860) 28 700 (23 340-35 030)
70-79 9760 (7190-12 830) 15 370 (11 430-19 930) 24 610 (20 480-29 180)
≥80 3080 (2100-4130 4970 (3430-6590) 7950 (6320-96,50)
All ages 39 110 (32130-47 320) 47 620 (39 410-57 240) 86 690 (75 240-99 570)
Either-Eye Cataract and Severe Visual Impairment (VA <6/60)
50-59 10 950 (7500-15 830 8870 (5780-13 510) 19 500 (14 560-26 000)
60-69 12 380 (9610-15 780) 16 810 (12 670-22 010) 29 060 (24 120-34 820)
70-79 9560 (7230-12 330) 14 600 (10 720-19 190) 23 750 (19 080-29 060)
≥80 3840 (2720-5010) 4870 (3130-6920) 8860 (6840-11020)
All ages 43 140 (36 670-50 520) 45 330 (37 800-54 110) 88 630 (78 140-100 280)
Person Cataract Severe Visual Impairment (VA<6/60)
50-59 2330 (1000-5380) 2530 (1090-5790) 4850 (2690-8730)
60-69 5350 (3490-8100) 9670 (6180-14870) 14 690 (10 520-20 350)
70-79 6500 (4470-9220) 9420 (6270-13720) 15 730 (11 930-20 470)
≥80 2890 (1910-4150) 2860 (1610-4760) 5940 (4230-8120)
All ages 22 060 (17500-27700) 24 590 (18 780-32 030) 46 690 (38 500-56 480)
Either-Eye Cataract Blindness (VA <3/60)
50-59 4190 (1990-8700) 3330 (1790-6170) 7400 (4640-11 750)
60-69 7150 (4930-10 250) 5650 (3400-9270) 13 190 (9920-17 450)
70-79 5800 (3590-8990) 8670 (5970-12 260) 14 260 (10 610-18 850)
≥80 1880 (1100-2990) 2990 (1580-5110) 4820 (3110-7100)
All ages 22 940 (17 350-30 140) 21 120 (16 490-26 960) 44 260 (36 700-53 240)
Person Cataract Blindness (VA <3/60)
50-59 410 (60-2950) 0 370 (50-2670)
60-69 1390 (570-3330) 1170 (350-3880) 2620 (1280-5360)
70-79 1250 (450-3350) 1380 (540-3430) 2680 (1340-5290)
≥80 440 (100-1810) 1970 (980-3700) 2180 (1150-3970)
All ages 4430 (2520-7760) 5110 (3090-8390) 9540 (6610-13 750) Abbreviation: VA, visual acuity.
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surgical intervention. The exact proportion of overall incident blindness that can be associated with
cataract was not possible, but cataract was the primary risk factor of incident blindness.
Past analysis of this cohort with regard to the incidence of diabetes and diabetic retinopathy
7
showed cataract as a growing public health concern and diabetes as a risk factor for cataract. In this
cohort, given the high prevalence and incidence of cataract, cataract, not diabetic retinopathy, was
the leading risk factor in VI in diabetes.
Blindness and VI from cataract are associated with reduced quality of life
32
and visual function,
which can be reversed by low-cost surgical management.
33
Considerable social and economic
disadvantages are associated with cataract, especially in low-income communities, which may
perpetuate the cycle of poverty.
34
Conversely, poverty can be alleviated with the provision of
cataract surgery.
35
Management of cataract is recognized as a priority for the VISION 2020: The Right
to Sight global initiative,
36
which targets avoidable blindness. However, to our knowledge, incidence
data including risk factors for visually impairing cataract were not previously available from the
African continent, limiting the ability to effectively plan and to resource services for the continent.
On the basis of presenting VA less than 6/18 in either eye with a cataract verified by dilated
slitlamp examination or the participant being newly pseudophakic, we found the incidence of
cataract in Kenya to be high. As expected, the incidence of visually impairing cataract increased
substantially with age. Comparison with other cohorts is limited, in part because of the lack of other
data from the region and variations on the definition of visually impairing cataract; however, the
estimates from the Nakuru Eye Disease Cohort Study show a higher incidence than in most other
cohort studies outside of Kenya.
30,31,37-39
This higher incidence may be associated with the study
population’s high exposure to UV light as well as genetic and nutrition factors. The high prevalence
of untreated cataract in the Nakuru Eye Disease Cohort Study may reflect a combination of limited
access to ophthalmic services and high incidence of new cataract.
40
This study also highlighted the low uptake of services by those needing cataract operation. At
baseline, all participants identified by the lead ophthalmologist as having an operable cataract were
offered a referral to the regional eye unit. However, few participants accessed the service, with only
18 (6.3%) of 284 individuals at follow-up reporting to have had surgical intervention. Barriers to
cataract surgical procedures have been previously described in this population and included lack of
Figure 2. Estimated Minimal Cataract Surgical Rate (CSR) for Annual Incidence of Visually Impairing Cataract in Kenya
4260 9540
88
630 46
690
148
280
4298
Required
CSR
VA
<6/18 86
690
Annual new cases of
unilateral cataract
Annual new cases of
bilateral cataract
2305
Required
CSR
VA
<6/60
1143
Required
CSR
VA
<3/60
2000
Required
CSR
VA
<6/60
1000
Required
CSR
VA
<3/60
4000
Required
CSR
VA
<6/18
4260 9540
88
630 46
690
148
280 86
690 2000
Required
CSR
Annual new cases of
unilateral cataract
Annual new cases of
bilateral cataract
2302
Required
CSR
1136
Required
CSR
232
Required
CSR
1000
Required
CSR
250
Required
CSR
CSR needed according to new cases of cataract
A
Simplified CSR pyramid
B
The different surgical thresholds shown are based on presenting visual acuity (VA) in either the better or worse seeing eye.
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Table 3. Age-Adjusted and Multivariable Analysis in the Nakuru EyeDisease Cohort Study
Variable
Study Sample (n = 1799)
At Risk of Cataract, No. (%) Incident Cataract, No. (%) Risk per 1000/6 y (95% CI)
Risk Ratio (95% CI)
Age Adjusted Multivariable Adjustment
Age, y
50-59 921 (51.2) 110 (6.1) 119.0 (98.0-143.8) 1 [Reference] 1 [Reference]
60-69 587 (32.6) 152 (8.4) 270.3 (231.6-312.8) 2.3 (1.8-2.9) 2.0 (1.6-2.6)
70-79 228 (12.7) 122 (6.8) 534.1 (458.5-608.2) 4.5 (3.5-5.7) 3.7 (2.9-4.7)
≥80 63 (3.5) 38 (2.1) 601.3 (459.2-728.2) 5.1 (3.6-7.1) 3.8 (2.6-5.5)
Sex
Male 838 (46.6.) 204 (11.3) 246.7 (215.7-280.6) 1 [Reference] NA
Female 961 (53.4) 218 (12.1) 226.0 (198.1-256.6) 1.0 (0.9-1.2) NA
BMI classification
a
Underweight 187 (10.4) 61 (3.4) 329.6 (262.3-404.6) 1 [Reference] NA
Normal 890 (49.5) 231 (12.8) 260.7 (228.6-295.7) 0.9 (0.7-1.1) NA
Overweight 444 (24.7) 86 (4.8) 193.7 (157.5-235.9) 0.8 (0.6-1.0) NA
Obese 272 (15.1) 42 (2.3) 155.0 (111.1-212.0) 0.6 (0.4-0.9) NA
Location
Rural 1332 (74.0) 333 (18.5) 259.0 (232.0-288.0) 1 [Reference] NA
Urban 467 (26.0) 89 (4.9) 192.3 (158.6-231.3) 0.9 (0.8-1.2) NA
SES quartile
a
Lower 378 (21.0) 123 (6.8) 329.7 (282.0-381.2) 1 [Reference] NA
Lower middle 491 (27.6) 124 (6.9) 253.8 (215.5-296.2) 0.8 (0.7-1.0) NA
Upper middle 476 (26.5) 107 (5.9) 224.8 (187.1-267.7) 0.8 (0.7-1.0) NA
Upper 446 (24.8) 68 (3.8) 155.6 (129.2-186.3) 0.6 (0.5-0.8) NA
Smoking status
Never smoked 1255 (69.8) 284 (15.8) 228.6 (203.1-256.2) 1 [Reference] NA
Former smoker 138 (7.7) 37 (2.1) 259.3 (190.2-342.8) 1.0 (0.8-1.4) NA
Current smoker 406 (22.6) 101 (5.6) 250.6 (209.9-296.2) 1.0 (0.8-1.1) NA
Hypertensive
a
No 917 (51.0) 204 (11.3) 226.3 (199.9-255.1) 1 [Reference] NA
Yes 875 (48.6) 216 (12.0) 244.2 (214.0-277.2) 0.9 (0.8-1.1) NA
Diabetes
a
No 1710 (95.0) 388 (21.6) 227.3 (204.8-251.6) 1 [Reference] 1 [Reference]
Yes 88 (4.9) 34 (1.9) 388.8 (302.2-483.1) 1.6 (1.2-2.0) 1.7 (1.3-2.3)
Alcohol use status
a
Never drank 774 (43.0) 155 (8.6) 197.6 (168.9-229.8) 1 [Reference] 1 [Reference]
Former drinker 753 (41.9) 190 (10.6) 256.1 (224.8-290.1) 1.1 (0.9-1.3) 1.1 (0.9-1.3)
Current drinker 269 (15.0) 77 (4.3) 279.9 (220.0-348.8) 1.3 (1.0-1.7) 1.4 (1.1-1.8)
Ethnic group
Kikuyu 1172 (65.1) 278 (15.5) 238.9 (212.0-268.0) 1 [Reference] NA
Kalenjin 419 (23.3) 108 (6.0) 266.1 (226.1-310.3) 1.2 (1.0-1.5) NA
Other 208 (11.6) 36 (2.0) 182.8 (137.8-238.4) 1.1 (0.8-1.4) NA
Educational level
a
No education 180 (10.0) 19 (1.1) 105.3 (68.5-158.3) 1 [Reference] 1 [Reference]
Primary 494 (27.5) 167 (9.3) 341.2 (299.8-385.3) 2.0 (1.3-3.1) 2.2 (1.4-3.4)
Secondary 922 (51.3) 219 (12.2) 241.2 (211.7-273.4) 1.7 (1.1-2.7) 1.9 (1.2-2.9)
College/university 202 (11.2) 17 (0.9) 91.2 (55.7-145.7) 0.9 (0.5-1.7) 0.9 (0.5-1.8)
Abbreviations: BMI, body mass index; NA, not applicable; SES, socioeconomic status.
a
These variables were missing data: BMI had 6 missing values; SES, 8; hypertensive (yes
or no), 7; diabetes (yes or no), 1; alcohol status, 3; and educational level, 1.
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awareness, high cost, distance from services, fear, and feeling that treatment was unnecessary.
41,42
Ultimately, these barriers meant that visually significant cataract remained untreated.
43
The results suggested that 148 280 new cases of eyes with VI (VA <6/18) per year existed,owing
to cataract in people aged 50 years or older, of whom 9540 were blind. Extrapolating these
estimates suggested that either 232 (only 1 eye of people who had VA <3/60 in both eyes), 2305 (all
eyes with VA <6/60 with cataract), or 4298 (all eyes with VA <6/18 with cataract) cataract operations
needed to be conducted per million population per year (CSR) to manage the new cataract cases,
depending on which vision threshold for surgical intervention was used.
Strengths and Limitations
Strengths of the Nakuru Eye Disease Cohort Study included being a representative population-based
sample in an area of ethnic, socioeconomic, and educational diversity; having a large sample size;
undertaking a comprehensive assessment of risk factors; providing high-quality assessment of
vision, and using the same tools at baseline and follow-up. The methods it used to assess ophthalmic
disease were consistent with those in studies of well-developed health systems in high-income
countries, such as the United States
44
and Australia,
45
that used the latest available equipment.
14
A limitation of this study was the low-participation rate at follow-up (50%); however, having the
baseline characteristics of nonparticipants was a strength that enabled weighting, which ensured
better estimates of cumulative incidence. This information on those lost to follow-up may have led to
an underestimation or overestimation of incident cataract VI and blindness, depending on the
general characteristics of the nonrespondents. The predominant risk factor for incident VI or
blindness was age, and given that age was closely matched between participants and
nonparticipants (mean [SD] age, 62.7 [9.4] years vs 62.5 [10.4] years), the estimates were likely to be
an acceptable reflection. This is further supported by minimal changes being apparent after adjusting
estimates for missing data.
Reasons for the low participation included ethnic violence, which displaced large numbers of
people in the study sample area, and postelection violence in 2007 and 2008, which led to the
internal displacement of up to 600 000 people and to 1300 fatalities.
46
In numerous study clusters,
entire ethnic groups present at baseline were no longer available or traceable at follow-up. Great
efforts were made to locate individuals on 2 or 3 preexamination visits. We promoted attendance by
providing transportation support and notification of alternative dates to attend clinics in the same
location.
Another study limitation included the restriction of the inclusion criteria at baseline to those 50
years or older, which reduced the generalizability of the results to the entire population. This
restriction is, however, comparable with most population-based studies of eye disease, which limit
inclusion to 40- or 50-year-old participants. Sampling people aged 50 years or older was appropriate
for the outcomes of interest in this study, given that the highest prevalence and incidence of cataract
were in this age group, making this sample appropriate both for epidemiologic (sample size
considerations) as well as for public health and policy planning purposes.
47
The definitions of
blindness and VI in this study were based solely on presenting central logMAR VA and did not include
peripheral vision loss. The definitions potentially underestimated incident VI and blindness when
compared with studies that included these criteria, although this was of less concern given that the
focus of the study was cataract.
The current estimate of CSR in Kenya is 550 per 1 million population. Recent estimates indicate
that 100 ophthalmologists work in a country of approximately 45 million, and 50% of these
ophthalmologists are based in the capital city of Nairobi. This lack of eye care practitioners leaves
92% of the population (approximately 40 million people) under the care of 50 ophthalmologists.
Overall, Kenya is better than many other African countries in terms of human and other resources,
despite still being well below recommended targets.
48
Continued efforts to strengthen the eye
health system in Kenya are necessary to support the growing unmet need of this aging and growing
JAMA Network Open | Ophthalmology Incidence of Visually Impairing Cataracts Among Older Adults in Kenya
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population. High-quality, high-volume surgical treatment for cataract and greater awareness of and
demand for eye health services at the community level are also required.
Conclusions
The incidence of visually impairing cataract in this population of Kenyan adults was considerably
higher than in comparable studies worldwide. Cataract remains the priority condition for the
prevention of avoidable blindness and VI. High-quality, high-volume cataract operations and an
increased awareness and demand for services at the community level are required to lower the
burden of VI and blindness.
ARTICLE INFORMATION
Accepted for Publication: May 11, 2019.
Published: June 28, 2019. doi:10.1001/jamanetworkopen.2019.6354
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2019 Bastawrous
Aetal.JAMA Network Open.
Corresponding Author: Andrew Bastawrous, BSc (Hons), MRCOphth, PhD, International Centre for Eye Health,
Clinical Research Department, London School of Hygiene & Tropical Medicine, Keppel St, London WC1E 7HT,
United Kingdom (Andrew.bastawrous@Lshtm.ac.uk).
Author Affiliations: International Centre for Eye Health, Clinical Research Department, London School of Hygiene
& Tropical Medicine, London, United Kingdom (Bastawrous, Rono, Foster, Burton, Kuper); Rwanda International
Institute of Ophthalmology and Dr Agarwal’s Eye Hospital, Kigali, Rwanda (Mathenge, Nkurikiye); Department of
Non-Communicable Disease Epidemiology, London School of Hygiene & Tropical Medicine, London, United
Kingdom (Wing); Kitale Eye Unit and Trans Nzoia County, Kitale,Kenya (Rono); Ministry of Health, Nairobi, Kenya
(Rono); MRC Tropical Epidemiology Group, Department of Infectious Disease Epidemiology, London School of
Hygiene & Tropical Medicine, London, United Kingdom (Gichangi); Moorfields Eye Hospital, London, United
Kingdom (Weiss, Macleod).
Author Contributions: Dr Bastawrous had full access to all of the data in the study and takes responsibility for the
integrity of the data and the accuracy of the data analysis.
Concept and design: Bastawrous, Mathenge, Foster, Burton, Kuper.
Acquisition, analysis, or interpretation of data: Bastawrous, Nkurikiye, Wing, Rono, Gichangi, Weiss,
Macleod, Kuper.
Drafting of the manuscript: Bastawrous, Macleod.
Critical revision of the manuscript for important intellectual content:Bastawrous, Mathenge, Nkurikiye, Wing, Rono,
Gichangi, Weiss, Foster, Burton, Kuper.
Statistical analysis: Bastawrous, Wing, Weiss, Macleod.
Obtained funding: Bastawrous, Burton, Kuper.
Administrative, technical, or material support: Bastawrous, Mathenge, Nkurikiye, Rono, Gichangi.
Supervision: Foster, Burton, Kuper.
Conflict of Interest Disclosures: Dr Bastawrous reported other financial support from Peek Vision outside of the
submitted work and grants from the Medical Research Council (MRC), Department for International Development
(DFID), Fight for Sight, Fred Hollows Foundation, and International Glaucoma Association during the conduct of
the study. No other disclosures were reported.
Funding/Support: This study was funded in part by the MRC and the DFID under the MRC/DFID Concordat
agreement, Fight for Sight, the International Glaucoma Association (equipment and field staff), the British Council
for the Prevention of Blindness and the Fred Hollows Foundation(baseline study), the Queen Elizabeth Diamond
Jubilee Trust (salary support for Dr Bastawrous), grant 098481/Z/12/Zfrom the Wellcome Trust (salary support for
Dr Burton), and grant G0700837 from the MRC and the DFID (salary support for Dr Weiss).
Role of the Funder/Sponsor:The funders had no role in the design and conduc t of the study; collection,
management, analysis, and interpretation of the data; preparation, review, or approvalof the manuscript; and
decision to submit the manuscript for publication.
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SUPPLEMENT.
eMethods. Method Details
eTable 1. Definitions of Incidence
eTable 2. Types of Lens Opacity at Baseline Among Those Where Main Cause of VI Is Cataract
eTable 3. Baseline Characteristics of All Individuals With Non-visuallySignif icant Cataract at Baseline According to
Availability of Visually Significant Cataract Status at Six-year Follow Up
eTable 4. Cataract Surgical Rates (CSR) Required to Match Incident Cataract Vision Lossat Different Thresholds
JAMA Network Open | Ophthalmology Incidence of Visually Impairing Cataracts Among Older Adults in Kenya
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