The National Eye Survey of Trinidad and Tobago (NESTT): Rationale, Objectives and Methodology

Figures & data

Table 1. Previous population-based surveys of vision impairment in the Caribbean.

Location	Population	Year	Age, years	Sample size, n (response rate, %)	Prevalence outcomes
Barbados	National	1987	40–84	4631 (82.1)	Visual impairment,^Citation4 blindness, glaucoma,^Citation5 cataract,^Citation6 DR,^Citation7 AMD,^Citation9 refractive error^Citation8
Cuba	Local (urban)	2005	50–99	2716 (98.4)	Blindness, low vision^Citation10
Dominican Republic	National	2008	50–99	3873	Blindness, low vision^Citation11

DR, diabetic retinopathy; AMD, age-related macular degeneration.

Table 2. Sample size required to give precise estimates of the prevalence of different ocular diseases, in the National Eye Survey of Trinidad and Tobago (NESTT).

Condition	Prevalence, %	Precision, 95% CI	Required sample, n
Blindness	1.70^4	1.19–2.21	3455
Vision impairment	5.90^4	4.90–6.90	2986
Myopia	21.9^8	19.9–23.9	2300
Hyperopia	46.9^8	44.4–49.4	2143
Cataract	41.0^6	38.5–43.5	2082
Glaucoma	7.0^5	5.8–8.2	2431
Diabetic retinopathy	1.0^7	0.5–1.5	2130
Exudative AMD	0.50^9	0.25 – 0.75	4281

Based on a 2-sided type 1 error, α, of 0.05 for different prevalence rates, and adjusted for the design effect due to clustering (1.4), but not including anticipated non-response (for which the sample was increased by 20%).

CI, confidence interval; AMD, age-related macular degeneration.

Figure 1. Map of Trinidad and Tobago showing distribution of the 120 National Eye Survey of Trinidad and Tobago (NESTT) clusters.

Table 3. Variables included in structured questionnaires, National Eye Survey of Trinidad and Tobago (NESTT).

Questionnaire	Variables	Source of questions
Individual enumeration	Sex, age, date and place of birth, ethnicity, position in household, employment status, number of years resident in Trinidad and Tobago, basic medical and ophthalmic history, self-reported vision status, disabilities, and reason if not able to attend clinic for full assessment	Trinidad and Tobago Population and Housing Census,^Citation13 RAAB instruction manual^Citation27
Household enumeration	Wall and roof material, main fuel used for cooking, household ownership status, and ownership of a set of preselected goods	Trinidad and Tobago Population and Housing Census^Citation13
Demographic	Place of birth, marital status, main language, religion, education, employment, household income, driving history, communication access, and health insurance status	Trinidad and Tobago Population and Housing Census,^Citation13 International Standard Classification of Occupations^Citation28
Socioeconomic	Usage and out-of-pocket expenditure on health care over past 12 months, usual transportation mode, informal care required on account of vision loss, number of eye care-related sick days, and lost income in the past 12 months	UKPDS Study Healthcare Costs^Citation29
Medical and ophthalmic	Past medical and ocular history, medication history and compliance, family history, and exposure history (alcohol, tobacco, and illicit drugs)	The INTERHEART study,^Citation30 RAAB instruction manual^Citation27
Patient-reported outcomes	Three standardized instruments: VisQoL, the IVI and the 5-level EuroQol questionnaires. These instruments were tested and validated in the pilot survey. The IVI was only administered to those with best-corrected vision worse than 6/18 in the better seeing eye, and to a randomly selected control group of people with normal vision	VisQoL instrument,^Citation31 IVI,^Citation32,Citation33 EuroQol^Citation34,Citation35 questionnaires
Low vision	Age at onset, duration and rate of vision loss, eye care service use history, functional adaptations and use of low vision aids, access to low vision services and barriers, feedback on experience using eye care services, and potential to improve quality of life of visually impaired people	Developed through consultation with the Blind Welfare Association, Trinidad and Tobago
Cardiovascular	History of passive smoke exposure, activity level at work and during leisure time, dietary intake of fruit and vegetables, sleep and snoring history	The INTERHEART study^Citation30

RAAB, rapid assessment of avoidable blindness; UKPDS, United Kingdom Prospective Diabetes Study; VisQoL, Vision Quality of Life Index; IVI, Impact of Visual Impairment.

Table 4. Variables included in the examination, with brief outline of equipment and measurement protocol, National Eye Survey of Trinidad and Tobago (NESTT).

Examination variable	Equipment	Measurement protocol
Vision screening
Distance visual acuity	3 m 2000 Series Revised ETDRS Chart, or PV Numbers acuity vision test, Precision Vision, La Salle, IL, USA	If literate: ETDRS Fast Protocol;^Citation24,Citation25 Beginning with the top row the screener invited the participant to identify only one letter per line by briefly pointing. To guarantee the same degree of difficulty for each row, only Sloan letters of intermediate difficulty coefficient were chosen (D, K, V, R, H). At the first letter read incorrectly the subject was required to read the whole preceding row. This step was repeated upward if the subject made two or more errors. The participant then read all rows downward, letter by letter, until the screener determined that no further meaningful readings could be made despite urging the subject to read or guess. If not literate: Standard ETDRS Protocol:^Citation26 participants asked to identify all PV numbers from the top, using a matching card if needed, with the same stopping rules as the ETDRS-Fast protocol
Near visual acuity	Sloan 2-sided ETDRS Format Near Point Test or PV Numbers Near Vision Card, both with 40 cm measuring cord, Precision Vision,; Reading lamp	Standard ETDRS Protocol:^Citation26 participants asked to read all letters from the top, with the same stopping rules as the ETDRS-Fast protocol
Medical exam
Weight	Analogue weighing scale	Nurse measured to nearest kilogram with shoes removed
Height	Wall-mounted tape measure with horizontal measuring level	Nurse measured after removal of shoes to nearest centimeter with participant standing against wall, and stretching their back with their head level and feet together
Waist circumference	Non-stretch fiberglass tape measure	Nurse measured at the smallest circumference between the ribs and iliac crest, to the nearest 1 cm, while standing with the abdomen relaxed at the end of a normal expiration. Where there was no natural waistline, measurement was taken at the level of the umbilicus
Blood pressure and pulse rate	HEM 907XL IntelliSense Professional Digital Blood Pressure Monitor, Omron Corporation, Kyoto, Japan	Nurse measured blood pressure and pulse rate with participant seated after 5 minutes of rest, using an appropriate cuff size for the left arm circumference
Capillary blood glucose	Accu Check, Roche, Basel, Switzerland	Nurse swabbed finger with alcohol wipe and used safety lancet used to obtain drop of blood. Glucose level recorded (mg/dL). Fasting defined as having had no food and no drink except water for 8 hours. If not fasted, recorded as random level.
Optometry exam
Auto refraction, keratometry and corneal topography	KR8000-PA, Topcon, Tokyo, Japan	Auto refraction sphere, cylinder and axis, and corneal radius of curvature in the horizontal and vertical meridian. One measurement taken of each eye, and repeated if measurement error
Spectacle prescription	Model 11360 Manual Lens meter, American Optical, Southbridge, MA, USA	Manual focimetry
Habitual reading distance	Tape measure	Participant asked to hold the near chart at their usual preferred reading distance and this “habitual distance” was measured from the corneal surface to the chart with a tape measure
Optimal near add	Trial Lens Frame, Viewlight, Miami, FL, USA; Trial Lens Set 266BL, Viewlight	Trial frame fitted to the participant’s face with the distance prescription mounted (that required to achieve at least 6/9 with auto-refraction correction, or the lens achieving best correction). Bracketing used to identify the plus DS lens prescription, ranging from 0.25DS to 3.00DS, required to achieve best near visual acuity in each eye, with the other occluded.
Contrast sensitivity	Mars Letter Contrast Sensitivity Test, Precision Vision	Binocular presenting contrast sensitivity at 50 cm measured using the Mars chart, with participants in their habitual near optical state
Ophthalmic exam
Face, adnexa, ocular movements		Face, adnexa, globe, ocular alignment and ocular movements documented normal or abnormal with description if abnormal
Pupils	Pen torch	Appearance of the pupils, direct, indirect and relative pupil reactions documented as normal or abnormal with description if abnormal
Anterior segment	Slit lamp model BQ-900, Haag-Streit, Bern, Switzerland	Any abnormalities of the anterior segment documented. Van Herick anterior chamber depth graded: 4 (≥100%), 3 (>25–50%), 2 (25%) or 1 (<25%)^Citation36
Posterior segment (after dilation)	90D MaxField and 78D MaxField High Mag, Ocular Instruments, Bellevue, WA, USA; Slit lamp model BQ-900, Haag-Streit	Lens graded using the LOCS III grading system, with comparison to the standard photographic transparency.^Citation37 Nuclear color and opalescence grades were amalgamated into a single grade.^Citation38 Vitreous, macula, retina and optic disc^Citation39 were documented as normal or abnormal, with description if abnormal
Ocular imaging and measurement
Intraocular pressure and corneal hysteresis	Ocular Response Analyzer, Reichert Technologies, New York, NY, USA	Three measurements taken of each eye, aiming to optimize the signal to noise ratio, and the best-waveform values for two measures of IOP (corneal compensated IOP and Goldmann-correlated IOP) documented for each eye
Ocular biometry	Lenstar LS 900^R, Haag-Streit	Corneal thickness, axial length, white-to-white distance, lens thickness, anterior chamber depth, keratometry and pupillometry. Three measurements taken of each eye
Color photographs and optical coherence tomography	3DOCT2000, Topcon Corporation	B-scan of the temporal iridocorneal angle, radial B-scan of the cornea, and an external color photograph. After dilation, two 45° color photographs of ETDRS standard field 1 (centered on the optic disc) and ETDRS standard field 2 (centered on the fovea).^Citation40 Spectral domain optical coherence tomography images including the “macula wide,” “5-line cross” and “3D disc.”

ETDRS, Early Treatment Diabetic Retinopathy Study; DS, diopter sphere; LOCS, Lens Opacities Classification System; IOP, intraocular pressure.

Figure 2. National Eye Survey of Trinidad and Tobago (NESTT) clinical pathway.

Table 5. Examination procedures for a subset of participants according to specific indications, National Eye Survey of Trinidad and Tobago (NESTT).

Examination variable	Equipment	Measurement protocol	Indication
Glycosylated hemoglobin A1C	Rapid point-of-care assay machine (DCA Vantage Analyzer, Siemens, Berlin, Germany)	Droplet of capillary blood obtained with safety lancet. Rapid point-of-care assay performed according to manufacturer’s instructions	Previous diagnosis of diabetes or a fasting blood glucose ≥126 mg/dl (7.0 mmol/l) or a random blood glucose >200 mg/dL (11.1 mmol/L)^Citation41
Best-corrected visual acuity	Trial Lens Frame, Viewlight, Miami, FL, USA; Trial Lens Set 266BL, Viewlight	Subjective refraction performed by optometrist	Presenting distance visual acuity worse than LogMar 0.20 (approximately 6/9) in either eye
IOP (manual)	AT900 Applanation Tonometer, Haag-Streit, Bern, Switzerland; proxymetacaine hydrochloride 0.5%; dry strip of fluorescein; Slit lamp model BQ-900, Haag-Streit	Care taken to use just enough fluorescein to obtain mires of standard thickness. IOP measured once in each eye using Goldmann applanation tonometer	Ocular Response Analyzer measurement of IOP not possible
Cup-to-disc ratio (manual)	78D MaxField High Mag, Ocular Instruments, Bellevue, WA, USA; Slit lamp model BQ-900, Haag-Streit	Optic disc examined at x10 magnification and vertical dimensions of the disc and cup measured using the eyepiece light, in 0.1 mm units, excluding areas of peripapillary atrophy and Elschnig’s ring. The margins of the cup were defined by stereoscopic examination as the point of maximum inflexion of contour, and the height of the cup was measured as the vertical distance between the points of maximal centrifugal extension of the cup between 11 and 1 o’clock and 5 and 7 o’clock.^Citation42 For small discs with no visible cup, the measurement was taken as the diameter of the emerging retinal vessels^Citation39	Automated cup-to-disc ratio measurement not possible owing to OCT machine malfunction or the presence of significant media opacity
Gonioscopy	Magna View Gonio Lens without flange, Ocular Instruments; proxymetacaine 0.5%; Gel tears, Bausch & Lomb Incorporated, Rochester, NY, USA	The visibility of the four key structures was documented	Van Herick limbal chamber depth grade 2 or less^Citation36
Visual field test	Humphrey Visual Field Analyzer II (model 740i), Carl Zeiss, Meditec AG, Jena, Germany	24-2 SITA static, threshold-related visual field test performed with near refractive correction in place, prior to dilation, in both eyes. Test reliability determined by the instrument’s algorithm. Test repeated once if Glaucoma Hemifield Test abnormal, borderline, or reduced sensitivity	(1) Vertical cup-to-disc ratio ≥0.70 (2) IOP ≥21mmHg (3) Abnormal optic disc features suggestive of glaucoma (4) History of diagnosed glaucoma or ocular hypertension Not performed in eyes with a visual acuity worse than 0.48 Log MAR (6/18)
Low vision tests	Mars Letter Contrast Sensitivity Test, Precision Vision, La Salle, IL, USA; MN Read English Continuous text chart Black/White, Precision Vision	(1) Uniocular Mars contrast sensitivity in best-corrected state (2) Uniocular MN Read test in best-corrected state	Distance best-corrected visual acuity in the better seeing eye worse than 6/18 (Log MAR 0.48)

IOP, intraocular pressure; OCT, optical coherence tomography; SITA, Swedish Interactive Threshold Algorithm; LogMAR, logarithm of the minimum angle of resolution.

Table 6. Kappa coefficient for inter-observer agreement in binary and categorical examination variables, National Eye Survey of Trinidad and Tobago (NESTT).

Observers	Examination variable	Kappa (p-value)
All vision screeners	Monocular distance visual acuity Visual acuity ≥6/6 Visual acuity <6/6 and ≥6/18 Visual acuity <6/18	0.81 (<0.0001) 0.76 (<0.0001) 0.85 (<0.0001)
Two survey ophthalmologists	Lens opacity LOCS III^a Nuclear Cortical Posterior subcapsular	0.70 (<0.0001)^a 0.75 (<0.0001)^a 0.86 (<0.0001)^a
Two survey ophthalmologists	Van Herick limbal chamber depth	0.79 (<0.0002)
Two survey ophthalmologists	Pupil normal or abnormal	1.00 (<0.0001)
Two survey ophthalmologists	Macula normal or abnormal	1.00 (<0.0001)
Two survey ophthalmologists	Retina normal or abnormal	1.00 (<0.0001)
Two survey ophthalmologists	Optic disc normal or abnormal	0.87 (<0.0001)

^aKappa weighting: 1, 0.6, 0.3, 0, 0, 0, 0.

20 eyes of 20 volunteers included in analysis.

LOCS, Lens Opacities Classification System.

Table 7. Bland-Altman limits of agreement in the measurement of continuous examination variables, National Eye Survey of Trinidad and Tobago (NESTT).

Variable (unit)	Observer	N	Mean (SD) and difference in mean (SD)	Bland-Altman upper and lower limits of agreement (95% CI)
Distance visual acuity (number of letters correctly identified)	Trainer (ophthalmologist) versus Each vision screener	20 left eyes	Trainer: 56.4 letters (9.0), range 33–66 Most dissimilar screener: 55.4 letters (9.5), range 32–66 Mean difference: 1.1 letters (3.1), range −6–9	Upper limit: 7 letters (95% CI 5, 10) Lower limit: −5 letters (95% CI −3, −8) 100% within 10 letters (2 lines) of the trainer’s measure; 85% within 5 letters (1 line)
IOP (mmHg)	Manual GAT versus Automated Goldmann-correlated IOP measured by the Ocular Response Analyzer (g-IOP)	101 left eyes	GAT: 15.8 mmHg (4.1 mmHg), range 9–36 mmHg g-IOP: 16.0 mmHg (4.8 mmHg), range 7–39 mmHg Mean difference: 0.26 mmHg (2.2 mmHg) (p = 0.25)	Upper limit: 4.2 mmHg (95% CI 3.4, 4.9 mmHg) Lower limit: −4.7 mm Hg (95% CI −5.4, −3.9 mmHg) 83.2% of GAT IOP within 2 mmHg of g-IOP

SD, standard deviation; CI, confidence interval; IOP, intraocular pressure; GAT, Goldman applanation tonometry.

Figure 3. Multilevel selection of a nationally representative population-based sample for the National Eye Survey of Trinidad and Tobago (NESTT).

Figure 4. Geographic distribution of National Eye Survey of Trinidad and Tobago (NESTT) sample in comparison to the 2011 Population and Household Census population.

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