|ORIGINAL RESEARCH REPORT
|Year : 2021 | Volume
| Issue : 1 | Page : 24-31
Comparison of autorefractor with focometer in patients with refractive errors attending Lagos University Teaching Hospital, Lagos, Nigeria - A cross sectional survey
Olaoluwa Olabode Amusan1, Kareem Olatunbosun Musa2, Olufisayo Temitayo Aribaba2, Akinsola Sunday Aina3, Adeola Olukorede Onakoya2, Folasade Bolanle Akinsola2
1 Viewpoint Specialist Eye Centre, Lagos, Nigeria
2 Department of Ophthalmology (Guinness Eye Centre), Lagos University Teaching Hospital/College of Medicine of the University of Lagos, Idi-Araba, Lagos, Nigeria
3 Department of Surgery, Bowen University Teaching Hospital, Ogbomosho, Oyo State, Nigeria
|Date of Submission||28-Apr-2020|
|Date of Acceptance||02-Jun-2020|
|Date of Web Publication||2-Feb-2021|
Dr. Olaoluwa Olabode Amusan
Viewpoint Specialist Eye Centre, Lagos
Source of Support: None, Conflict of Interest: None
Background: Uncorrected refractive error is the most common cause of visual impairment globally. Yet, there is paucity of refractionists in rural areas of most developing countries. Thus, there is a need for a cost effective but accurate method of refraction that could be used by rural health workers with minimal training. To compare refractive error measurements of autorefractor with that of focometer with a view to determining the accuracy and reliability of focometer. Methods: This was a comparative cross-sectional study conducted among patients with refractive errors attending the Guinness Eye Centre Clinic, Lagos University Teaching Hospital, Lagos, Nigeria. Consecutively consenting patients who met the eligibility criteria were recruited until the sample size was attained. All participants had a standardized protocol examination including visual acuity assessment and ocular examination. Refractive error was measured using the autorefractor, focometer and subjective refraction in both eyes of each participant. Comparison was done based on the means of variables of autorefractor, subjective refraction and focometer measurements using the paired-sample t-tests, Pearson's correlation and linear regression. Agreement between the measurements was investigated using the Bland-Altman analysis and reliability of the repeated measurements tested with Cronbach's alpha. The analysis was considered statistically significant when the P < 0.05. Results: Four hundred eyes of 200 patients were analyzed in this study. The mean age of respondents was 45.1 ± 16.3yrs and the male:female ratio was 1: 2.1. There was a statistically significant difference between the mean spherical (P < 0.001) and cylindrical (P < 0.001) readings of the focometer and autorefractor. However, the mean difference between the spherical equivalent of focometer and that of the autorefractor was not statistically significant (P = 0.66). Pearson correlation coefficient was high for the compared methods of refraction as both the bivariate linear regression between the autorefractor and focometer, and that between the subjective refraction and focometer showed good linearity. Bland-Altman plot showed good agreement between the mean focometer measurements with both the autorefractor (mean difference = +0.02 ± 0.85 DS; mean difference ± 1.96 standard deviation [SD] = 1.69 to − 1.65 DS) and subjective refractive (mean difference = +0.06 ± 0.72 DS; mean difference ± 1.96 SD = 1.49 to − 1.36 DS) measurements. Cronbach's alpha showed good reliability of focometer and autorefractor repeated measurements. Conclusion: This study showed a good correlation and agreement between focometer and autorefractor. Hence, focometer could be used for refraction in low resource settings where locals could be trained in its use.
Keywords: Autorefractor, focometer, Lagos, Nigeria, refractive errors
|How to cite this article:|
Amusan OO, Musa KO, Aribaba OT, Aina AS, Onakoya AO, Akinsola FB. Comparison of autorefractor with focometer in patients with refractive errors attending Lagos University Teaching Hospital, Lagos, Nigeria - A cross sectional survey. J Clin Sci 2021;18:24-31
|How to cite this URL:|
Amusan OO, Musa KO, Aribaba OT, Aina AS, Onakoya AO, Akinsola FB. Comparison of autorefractor with focometer in patients with refractive errors attending Lagos University Teaching Hospital, Lagos, Nigeria - A cross sectional survey. J Clin Sci [serial online] 2021 [cited 2021 May 16];18:24-31. Available from: https://www.jcsjournal.org/text.asp?2021/18/1/24/308596
| Introduction|| |
Refractive errors occur when the image of an object of regard cannot be brought to a point focus on the sentinel layer of the retina, with the eye in a position of rest. If uncorrected, refractive errors could result in visual impairment and blindness with concomitant short- and long-term consequences for the individual. These include lost educational and employment opportunities, lost economic gain for individuals, families and societies, and impaired quality of life.,, Yet, uncorrected refractive error is the most common cause of visual impairment around the world and the second leading cause of treatable blindness after cataract.,
It is estimated that about 285 million people are visually impaired worldwide, with approximately 90% of them living in developing countries such as Nigeria., Refractive error accounts for 42% of people with visual impairment and 18.5% of blindness worldwide., In Sub-Saharan Africa, refractive error has been found to be a significant cause of visual impairment., According to the Nigeria National Blindness and Visual Impairment survey, uncorrected refractive error accounted for 77.9%, 57.1% and 11.3% of mild, moderate and severe visual impairment respectively. In fact, refractive error was regarded as one of the eye diseases whose prevention or cure could provide huge savings while enhancing societal development., Refractive errors have also been found to be the commonest cause of visual impairment in rural settings in Nigeria, accounting for 54.6% of all cases of bilateral visual impairment.
Correction of these refractive errors can be achieved with the aid of eye-glasses, contact lenses and refractive surgery., However, prescription of eye-glasses is often the preferred means of correction as it is among the most cost-effective interventions in eye health care. Refraction (objective and subjective) is done to get the accurate power and axis of the spectacles. The gold standard of performing objective refraction is retinoscopy, however this has a high learning curve and the accuracy of the result depends on the skill of the refractionist, but with the invention of the autorefractor, the procedure became easier and less dependent on the skill of the refractionist., However, an autorefractor is relatively expensive and dependent on electricity, thus may not be readily affordable and appropriate for use in rural areas of developing countries, where electricity is either unavailable or epileptic in supply., Moreover, studies have documented dearth of qualified ophthalmologists and optometrists in many parts of the world including Nigeria especially in rural areas.,, Therefore, there is a need for a relatively inexpensive instrument, which is able to determine refractive errors accurately yet would not rely on electricity or on the skill of the ophthalmologist or optometrist.
Focometer seems to meet these criteria-provide refraction without the need for electricity or complicated protocols. This portable, hand-held instrument may be appropriate for use in remote, poor-resource settings with few trained eye care staff such as many rural areas in Nigeria. The advantages of a focometer over other methods of refraction for use in developing countries are that it is lightweight, compact, relatively inexpensive, fairly quick, and easy to use with minimal training of the performer.,,
However, despite its potential usefulness in estimation of refractive errors, there have been just few studies globally and only one locally to the best knowledge of these researchers that compare the focometer with the standard methods of determining refractive errors. Therefore, this study aims to compare the refractive error measurements using the autorefractor with that of the focometer, with a view to ascertaining the reliability and accuracy of the focometer in estimating refractive errors.
| Materials and Methods|| |
This was a comparative cross sectional study conducted among adult patients with refractive errors attending Guinness Eye Centre, Lagos University Teaching Hospital (LUTH) from December 2016 to April 2017. The study was approved by the Health Research Ethics Committee of LUTH (ADM/DCST/HREC/APP/1220) and it adhered strictly to the tenets of Helsinki declaration. Written informed consent was obtained from each participant after a detailed explanation of the study.
A minimum sample size of 163 was calculated using the formula for pooled variance and difference between means. This was increased to 196 to accommodate 20% attrition and eventually approximated to 200. Therefore, 400 eyes of 200 patients were analyzed. The inclusion criteria were adults aged 18 years and above, clear ocular media and presenting distance visual acuity between 6/9 and 6/60 in each eye with improvement of at least one line when tested with a pinhole. The following patients were excluded from the study: patients below 18 years, patients with Type II or III pterygium, and/or corneal leucoma in visual axes; individuals with obvious or known ocular pathology, or with history of past ocular surgery; patients with deranged blood sugar test (fasting blood sugar >7.0 mmol/L or Random Blood Sugar >11.1 mmol/L); patients who have undergone refractive surgery and patients with visual acuity worse than 6/60 or better than 6/9.
Consecutive consenting patients who met the inclusion critera were enrolled into the study. A pilot study involving 20 patients was conducted at the LUTH Annex, Pakoto, Ogun State to test the feasibility of the study procedures and data collection process as well as standardization of the filling of the questionnaire and test methods. Interviewer-administered questionnaires were used to obtain information on socio-demographic characteristics, medical and past ocular history All participants had either a fasting blood sugar (FBS) test or random blood sugar (RBS) test done using glucometer and strips (Accu check). Respondents with deranged blood sugar test (FBS >7.0 mmol/L or RBS > 11.1 mmol/L) were excluded and referred to diabetic and endocrinology clinic LUTH.
The distance visual acuity in each eye was tested uniocularly using an illuminated Snellens' chart at six metres. The visual acuity was retested with pinhole if worse than 6/9. In patients who wear spectacles, their visual acuity was also checked with their spectacles. The last completely read line was recorded as the visual acuity.
All participants had pen-torch and slit lamp examination of the adnexae and anterior segments of both eyes. A direct ophthalmoscopy was performed to examine the posterior segment using the Heine direct Ophthalmoscope with pupils not dilated. Participants with ocular abnormality in any of the ocular structures were referred appropriately.
All the patients included in the study had their refraction done with the autorefractor (Humphrey Zeiss Model 530). The patient was instructed to sit with his/her neck tilted to rest on the chin rest of the autorefractor while focusing on the viewing aperture of the autorefractor. The reading displayed on the screen after a sharp focus identified was noted and recorded. The refractive error measurements were recorded in sphere, negative cylinder, and cylinder axis format. The spherical equivalent was then derived (geometric sum of sphere and half of cylinder) and also noted. Each eye was tested three times and the average of the results calculated, thereafter; the visual acuity of each eye was re-checked with lenses corresponding to the average value of the autorefraction results.
Similarly, all the participants had refraction with focometer (InFOCUS, Texas, USA). The patient was then positioned six meters away from the manufacturer's clock target chart (240 mm in diameter, with each of the 12 arms being 95 mm in length) in a well-lit consulting room. Each eye was tested one at a time while the other eye was occluded with a patch. Before each measurement, the focus collar was set at the highest plus position. The participants were instructed to look at the chart through the focometer aperture, he/she then continued to rotate the collar of the focometer until either all or one of the radials on the clock target was very clear, he/she then stops turning the collar. If all the radials on the clock target entered focus at the same time then the dioptre scale on the focometer was read off giving the spherical error of the participant, but if one or two radials enter focus before others then the participant required astigmatic correction.
In identifying the magnitude of the astigmatic correction the patient was instructed continue looking through the aperture of the focometer focusing on the clock target chart positioned six meters away, he/she continued rotating the collar and stopped as soon as the first radial becomes very clear. This is the first reading (diopters and axis) and it is recorded as such. The subject then continued to look through the aperture of the focometer and resume rotation of the collar until the second radial perpendicular to the first radial came into view, the second reading (diopters and axis) was then recorded. The final prescription was the first reading's dioptric value giving the sphere directly while the cylinder was obtained from the difference of dioptric values of the first and the second reading (second reading – first reading). The axis of the second reading was used directly as the axis of the final prescription. The same steps will then be repeated for the second eye. Each eye was tested three times each and the average of the results calculated, thereafter; the visual acuity of each eye was re-checked with lenses corresponding to the average value of the focometer results.
Thereafter, subjective refraction was done for each participant. This was done using the readings obtained from the autorefractor as a guide. Autorefractor readings were used because it is an objective refraction. The subjective refraction was done by placing the readings from the autorefractor in the right eye trial frame first while occluding the left eye. These readings were then refined (sphere refined before the cylinder). Then the left eye readings from the autorefractor placed in the trial frame were refined while occluding the right eye. In refining the sphere, the strongest plus or weakest minus lens that gives the visual acuity of 6/6 or better was prescribed. The cylinder was refined using Jackson cross. Final readings from each eye were used to check the visual acuity monocularly then binocularly. The spherical equivalent of the subjective refraction was also noted. All patients were then prescribed spectacles based on the subjective refraction results and it was ensured each one received his/her pair of glasses.
Data analysis was done using IBM Statistical Package for Social Sciences (IBM-SPSS version 21, IBM Corps, Armonk, NY, USA). Bivariate analysis was conducted to evaluate association between variables. Comparison was done based on the means of variables of autorefractor and subjective refraction measurements with that of the focometer using the t-tests, Pearson's correlation and linear regression. Furthermore, agreement between measures was investigated using the Bland-Altman analysis.,, The analysis was considered statistically significant when the P < 0.05. Reliability of the focometer repeated readings was determined by Cronbach's alpha.
| Results|| |
Four hundred eyes of 200 participants were analyzed in this study. The age range of all participants was between 18 and 86 years with an overall mean age of 45.1 ± 16.3 years. One hundred and three (51.5%) participants were between the ages of 40–59 years, 41 (20.5%) were 60 years and above and 56 (28.0%) participants were below the age of 40 years. There were 136 (68.0%) female with a male to female ratio of 1: 2.1. One hundred and twenty-two (61.0%) participants had tertiary education as the highest level of education while 14 (7.0%) and 61 (30.5%) participants were educated up to primary and secondary school level respectively. Three (1.5%) participants had no formal education.
Two hundred and fifty (62.5%) eyes had unaided visual acuity (VA) of 6/9 to 6/12, 115 (28.7%) eyes had VA of 6/18 to 6/24 while 35 (8.8%) eyes had VA of 6/36 to 6/60. For objective refraction using autorefractor, 131 (32.8%) eyes had hypermetropia, 95 (23.7%) were myopic while 174 (43.5%) had astigmatism whereas 211 (52.8%) eyes were hypermetropic, 185 (46.2%) were myopic while only 4 (1.0%) eyes had astigmatism using focometer. On subjective refraction, 183 (45.8%) eyes had hypermetropia, 101 (25.2%) were myopic while 116 (29.0%) had astigmatism. However, spherical equivalent of objective refraction using autorefractor revealed that 227 (56.8%) eyes had hypermetropia while 173 (43.2%) were myopic whereas 211 (52.8%) eyes were hypermetropic while 189 (47.2%) were myopic using focometer. Spherical equivalent of subjective refraction revealed that 224 (56.0%) eyes had hypermetropia while 176 (44.0%) were myopic.
[Table 1] shows the comparison of magnitude of spherical error using different refraction methods with the aid of paired samples t-test. There was a statistically significant difference between the spherical errors measured by the autorefractor and focometer (P < 0.001), as well as the spherical errors measured by the subjective refraction and focometer. Similarly, there was a statistically significant difference between the cylindrical errors measured by the autorefractor and focometer (P < 0.001), as well as the cylindrical errors measured by the subjective refraction and focometer [Table 2]. However, comparison of magnitude of the spherical equivalence of refractive errors using different refraction methods shows no statistically significant difference between the spherical equivalent errors measured by the autorefractor and focometer (P < 0.05) as shown in [Table 3]. Also, the spherical equivalence of refractive errors measured by the subjective refraction and focometer were not statistically different.
|Table 1: Comparison of degree of spherical errors using the different refraction methods|
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|Table 2: Comparison of degree of cylindrical errors using the different refraction methods|
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|Table 3: Comparing of degree of spherical equivalent errors using the different refraction methods|
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The mean focometer readings were within and outside ± 0.50 DS of the mean autorefractor readings in 326 (81.6%) eyes and 74 (18.4%) eyes respectively. Also, the mean focometer readings were within and outside ± 0.5 0DS of subjective refraction in 323 (80.8%) eyes and 77 (19.2%) eyes respectively.
With focometer reading, the final visual acuity was 6/6 in 311 (77.8%) eyes, 6/9 in 40 (10.0%) eyes, 6/12 in 38 (9.5%) eyes and ≤6/18 in 11 (2.7%) eyes while with autorefractor reading, the final visual acuity was 6/6 in 323 (80.8%) eyes, 6/9 in 44 (11.0%) eyes, 6/12 in 28 (7.0%) eyes and ≤6/18 in 5 (1.2%) eyes. After subjective refraction, the final visual acuity was 6/6 in 345 (86.2%) eyes, 6/9 in 35 (8.8%) eyes, 6/12 in 16 (4.0%) eyes and ≤6/18 in 4 (1.0%) eyes.
[Figure 1] shows the scatter plot of the focometer and autorefraction readings. Pearson correlation coefficient (r = 0.88) is large. Also, the coefficient of determination (r2) which is a measure of the how close the data are to the fitted line is, 0.77, while the slope and intercept are 0.85 and-0.02 respectively (95% confidence interval [CI] of the slope: 0.80–0.89; 95% CI of the intercept: −0.096–0.062). This suggests a good linear relationship between the focometer and autorefractor. [Figure 2] shows the scatter plot of the mean focometer and subjective refraction readings. Pearson correlation coefficient (r = 0.90) is large. The coefficient of determination (r2) which is a measure of how close the data are to the fitted line is 0.815, while the slope and intercept are 0.97 and −0.06 respectively (95% CI of the slope: 0.92–1.01; 95% CI of the intercept: −0.13–0.011). This suggests a good linear relationship between the focometer and subjective refraction.
|Figure 1: Bivariate linear regression graph for focometer and autorefractor readings|
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|Figure 2: Bivariate linear regression graph for focometer and subjective refraction readings|
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[Figure 3] shows Bland-Altman analysis to determine agreement between autorefractor and focometer. The mean difference between mean autorefractor and mean focometer readings was +0.02 ± 0.85 DS. The mean difference ±1.96 standard deviation (SD) was 1.69 to −1.65 DS. The Bland-Altman plot shows fairly good agreement between the two test instruments, with some exceptional outliers.
|Figure 3: Comparison between autorefractor and focometer using the Bland-Altman plot|
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[Figure 4] shows Bland-Altman analysis to determine agreement between subjective refraction and focometer. The mean difference between subjective refraction and mean focometer readings was +0.06 ± 0.72 DS. The mean difference ±1.96 SD was 1.49 to −1.36 DS. The Bland-Altman plot shows fair agreement between the two methods of refraction, with some exceptional outliers.
|Figure 4: Comparison between subjective refraction and focometer using the Bland-Altman Plot|
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[Table 4] shows Cronbach's-alpha analysis to determine the reliability or internal consistency for repeated spherical equivalent measurements for focometer and autorefractor. The focometer and autorefractor shows good repeatability with a Cronbach's-alpha of 0.98 and 0.97 respectively.
|Table 4: Reliability of repeated spherical equivalent measurements of focometer and autorefractor (Cronbach's-alpha)|
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| Discussion|| |
In this study, mean age of 45.1 ± 16.3 years more than half of the participants were between the ages of 40–59 years. Previous studies have reported a higher prevalence of uncorrected refractive errors in middle aged individuals., Majority of the respondents had completed their education up to the tertiary level. This may be due to the fact that this study centre is situated in an urban environment where many people desire higher education in the pursuit of white collar jobs.
Hypermetropia and astigmatism were the most common refractive errors in this study using focometer and autorefractor respectively for objective refraction. However, hypermetropia predominated for both focometer and autorefractor using the spherical equivalence of objective refraction. These findings are contrary to the observations of Aina et al. and du Toit et al. where majority of participants were myopic. These variations could be related to the differences in study population as the study by Aina et al. was conducted among school children, while the participants in the study by du Toit et al. were majorly Caucasians. However, this is in tandem with the higher prevalence of hypermetropia in black adult population (similar age group to most participants in this study) found by Wu et al. and Ezelum and Razavi.,
The poor ability of the focometer to detect astigmatism as highlighted in this study is in agreement with the findings of Aina et al. and du Toit et al. This wide variation is also evident by the statistically significant difference when comparing degree of cylindrical errors using the two refraction methods (P < 0.001). This could be due to the fact that the focometer chart clock target has radial at 30° interval, thus, the cylinder axis can only be measured to the nearest 15° as pointed out by du Toit et al. In addition, badal optometer principle has been shown to be poor at detecting irregular astigmatism.,
There was no statistically significant difference between the mean autorefractor spherical equivalent reading and the mean focometer spherical equivalent reading (P = 0.66) on one hand and on the other hand there was also no statistically significant difference between the subjective refraction reading and the mean focometer reading (P = 0.09). Moreover, the focometer readings were within ± 0.50 DS of the autorefractor readings and subjective refraction readings in 326 (81.6%) and 323 (80.8%) participants' eyes respectively. Comparatively, Aina et al. found focometer readings to be within ± 0.50 DS of the autorefractor readings and subjective refraction readings in 56.8% and 80% of participants respectively. Also, du Toit et al. found that 62% of the focometer spherical equivalent measurements were within ± 0.50 DS of the autorefractor. The agreement of the autorefractor and focometer in this study seemed to be better than that in previous similar studies., This could be related to the fact that both instruments used in this study are based on the optometer principle as opposed to autorefractors designed based on the Scheiner principle.
The mean difference between the spherical equivalent readings of the autorefractor and focometer in this study was 0.02 ± 0.85 (95% CI: −0.07–0.10; P = 0.66), while the mean difference between the spherical equivalent measurements of subjective refraction and that of the focometer reading was 0.06 ± 0.73 (95% CI: −0.01–0.13; P = 0.09). These findings suggest that generally, the focometer gave slightly more negative readings than the autorefractor and subjective refraction measurements. This could be due to the psychological pseudomyopia (the myopic shift that results from a zero accommodative stimulus) and convergence accommodation associated with a monocular instrument. However, this may not necessarily have a negative consequence as negative defocus prescription errors tend to be tolerated more than positive errors., Similarly, Berger et al. found a mean difference between the spherical equivalent readings of subjective refraction and focometer of 0.028DS (P = 0.5). In contrast, however, Aina et al. and du Toit et al. found mean differences of − 0.24 DS (P = 0.006) and − 0.25 DS (P = 0.01) respectively between the autorefractor and focometer.
Seventy-eight percent of the eyes of the participants were improved to ≥6/6 visual acuity by the focometer prescription. Smith et al. and du Toit et al. reported that 84% and 67% of their study participants respectively achieved a visual acuity of at least 6/6 with their focometer prescription. However, Aina et al. reported that 100% of participants with refractive error achieved a visual acuity of 6/6 with the focometer prescription. The much higher percentage in the study by Aina et al. could be due to the younger age group in that study compared to this study who were not likely to have other factors such as cataract that could reduce the corrected visual acuity.
There was good linear relationship between the autorefractor and focometer on one hand (r = 0.88; r2 = 0.77; intercept = −0.02; slope = 0.85), and subjective refraction and focometer on the other hand (r = 0.90; r2 = 0.82; intercept = −0.06; slope = 0.97). These findings also indicate no systematic disagreement between the methods of refraction being compared, as the intercept is not significantly different from zero. However Berger et al. reported a coefficient of determination (r2) of 0.09 between autorefractor and focometer in children. The difference in age group of participants could be a reason for these findings.
The Bland-Altman analysis revealed that the difference in the spherical equivalent of the mean autorefractor and mean focometer showed a 95% limit between − 1.7 DS and +1.7 DS with a mean of 0.02DS and P = 0.66, while that between subjective refraction and mean focometer showed 95% limit between − 1.36 and 1.49 with a mean of 0.06DS and P = 0.09. Most of the Bland-Altman plots fall within the 95% CI with few outliers. These findings suggest the differences between the measurements of the compared methods of refraction are not statistically significant.
The focometer and autorefractor showed good repeatability as evidenced by the values of Cronbach's alpha of 0.98 and 0.97 respectively. Similar to this finding, Aina et al. found a coefficient of repeatability of 0.8 and 0.75 for the focometer and autorefractor respectively, which suggests the focometer, is highly repeatable. Du Toit et al. found that the coefficient of repeatability of the mean differences were similar for the autorefractor and focometer (0.36 and 0.31, respectively) for participants who had formal training in measurements with optical instruments. In contrast, for participants who had no formal training in measurements with optical instruments, the coefficient of repeatability for the mean differences of the focometer were double those of the autorefractor (0.72 and 0.38, respectively). These differences were more apparent between the first and the third measurements (0.45 and 1.02), and the first and second measurement (0.57 and 1.08) than the second and third measurements (0.45 and 0. 64), suggesting that the more the focometer measurement is repeated by an individual the higher the likelihood of getting a reliable result. However, this current study suggests that multiple readings of the focometer may not be required as the first reading alone may be reliable if the patient understands and properly follows instruction on the use of the focometer.
This study is limited by the restricted range of the focometer (+10.75 to −7.75) limits its use in patients with very high refractive errors. Also, because the focometer has to be held close to the eye by the patient during the period of the test, its use may not be very practicable in elderly ones who easily get tired and those with challenges with manual dexterity.
| Conclusion|| |
This study showed a good correlation between the focometer (InFOCUS, Texas, USA) and autorefractor (Humphrey Zeiss Model 530), and also focometer and subjective refraction. Also, there was good agreement between the compared instruments, although, it has a very poor ability to detect astigmatism. The study also showed that the focometer prescription improved the visual acuity of most participants to at least 6/6 using the spherical equivalent readings. Thus, the focometer may be useful in rural environments where there is a high magnitude of refractive errors and spectacles based on spherical equivalent readings could be prescribed to improve visual acuity of these patients with refractive errors.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3], [Table 4]