|Year : 2017 | Volume
| Issue : 2 | Page : 123-128
B-mode ocular ultrasound findings in adults with refractive errors at Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Nigeria
Ibukun A Abidoye1, Christiana M Asaleye1, Bernice O Adegbehingbe2
1 Radiology Department, Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Osun State, Nigeria
2 Ophthalmology Department, Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Osun State, Nigeria
|Date of Web Publication||15-Nov-2017|
Ibukun A Abidoye
Radiology Department, Obafemi Awolowo University Teaching Hospitals Complex, PMB 5538 Ile-Ife, Osun State
Source of Support: None, Conflict of Interest: None
Objective: To ascertain the relationship between B-mode ocular ultrasound findings and standard autorefraction results of participants with refractive errors, with the aim of using B-mode ultrasound as a complementary technique to determine the refractive state of the eye.
Materials and Methods: The study population consisted of 255 adult patients with refractive errors, aged 18–40 years. The ocular dimensions were measured using MINDRAY DC-7.0 real-time ultrasound machine with frequency probe of 5 and 7–12 MHz. The ocular shapes and other biometric measurements were then correlated with the results of the standard autorefraction.
Results: The sensitivity for prolate ocular shape corresponding with myopia/astigmatism was 88.6%, whereas that of oblate ocular shape corresponding with hyperopia/astigmatism was 87.4%. The Pearson’s correlation coefficient between right axial length and right spherical equivalent was negatively strong at −0.79 (P < 0.001). In addition, the Pearson’s correlation coefficient was negatively strong at −0.76 (P < 0.001) between left axial length and left spherical equivalent.
Conclusion: Real-time B-mode ocular ultrasound has a high sensitivity in determining ocular shape, which corresponded well with the refractive state of the participants’ eyes. Axial length (AL) was found to correlate strongly with spherical equivalent, and, thus, a regression equation can be used to predict the spherical equivalent from the AL measurements.
Keywords: Ocular dimensions, ocular shapes, refractive errors
|How to cite this article:|
Abidoye IA, Asaleye CM, Adegbehingbe BO. B-mode ocular ultrasound findings in adults with refractive errors at Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Nigeria. J Med Trop 2017;19:123-8
|How to cite this URL:|
Abidoye IA, Asaleye CM, Adegbehingbe BO. B-mode ocular ultrasound findings in adults with refractive errors at Obafemi Awolowo University Teaching Hospitals Complex, Ile-Ife, Nigeria. J Med Trop [serial online] 2017 [cited 2022 Jan 20];19:123-8. Available from: https://www.jmedtropics.org/text.asp?2017/19/2/123/218406
| Introduction|| |
Refractive error is a very common eye disorder. It may be defined as a state in which the optical system of the nonaccommodating eye fails to bring parallel rays of light to focus on the fovea. Refractive error is probably a consequence of a mismatch of the biometric parameters of the eyes [i.e., axial length (AL), corneal curvature, anterior chamber, lens thickness (LT), and vitreous chamber depth] and the refractive power of these structures. The result of refractive errors is blurred vision, which is sometimes so severe that it causes visual impairment. Refractive errors cannot be prevented, but they can be diagnosed by an eye examination and treated with corrective glasses, contact lenses, or refractive surgery. If corrected in time and by eyecare professionals, they do not impede the full development of good visual function. Uncorrected refractive error is increasingly recognized as a significant cause of avoidable visual disability worldwide and has been included as one of the priority areas of vision 2020.,
The final refractive state of an eye is dependent on an intricate emmetropization process, involving the interaction between individual ocular biometric components (i.e., axial ocular dimensions, corneal curvature, and lenticular power). Ocular shape is associated with refractive error in adult eyes, with myopic eyes tending to be prolate [having a longer AL than equatorial diameter (ED)] and hyperopic eyes tending to be oblate (having a broader ED than AL).
In early studies, investigators measured AL either indirectly or directly with radiography, but these methods have now been replaced by ultrasound biometry, because ultrasound has been found to be more accurate and better depicts the anatomy of the eye than radiography. A-mode ultrasound is widely used in measuring the ocular dimensions, particularly because of its superiority over B-mode ultrasound in accurate measurement of ocular dimensions. This study, however, used B-mode ultrasound, because it is more readily accessible to the radiologist in our study environment.
| Patients and methods|| |
This was a prospective cross-sectional study conducted at the Department of Radiology in collaboration with the Department of Ophthalmology, Obafemi Awolowo University Teaching Hospitals Complex (OAUTHC), Ile Ife, Osun state, Nigeria. The duration of study was 1 year, from October 2014 to September 2015. The study included 255 consecutive adults aged 18–40 years diagnosed with refractive error. The participants met the inclusion criteria following automated refraction performed for them by a senior optometrist in the Ophthalmology unit using Grand Seiko GR-3100K AutoREF/Keratometer. The automated refraction results of the recruited participants were used as the control for the study. The result of the automated refraction was not made available as at the time of scanning the participant to avoid bias. However, each participant was identified by a number tag. Excluded from this study were participants with ocular pathologies such as glaucoma, cataract, retinal detachment, and past history of ocular injury as well as those with systemic diseases such as diabetes mellitus and hypertension. Written informed consent was obtained from each participant, while ethical approval for the study was obtained from the institution’s ethic and research committee.
B-mode ultrasound assessment was performed on both eyes using MINDRAY DC-7.0 ultrasound machine (MINDRAY Real time Ultrasound model DC-7 with Doppler Ultrasound facility, Shenzhen Mindray Bio-Medical Electronics Co., LTD, Shenzhen, China) with a high resolution (7–10 MHz) linear probe, whereas a deep probe (5 MHz) was used for the purpose of measuring the ED, since the measurement was along the posterior segment. The high-frequency linear probe was gently placed over the participant’s slightly closed eyelids after applying copious amount of coupling gel. The eyes were scanned in both transverse and sagittal planes with depth and gain adjusted to achieve acceptable images. The image was frozen when the whole dimension of the globe was visualized, with the anterior chamber, anterior and posterior capsules of the lens, and the vitreous chamber demonstrated [Figure 1]. The AL was measured in the transverse plane from the anterior corneal vertex to the retinal pigment epithelium along the fixation line pole [Figure 2]. The anterior chamber depth (ACD) was measured from the posterior corneal surface to the anterior capsule of the lens [Figure 3]. The LT was measured from the anterior capsule to the posterior capsule of the lens [Figure 4]. The ED was measured at the widest transverse diameter of the globe, perpendicular to the AL. A deep probe (5 MHz) was used to get accurate measurement of the ED following the same approach discussed above because of the technical difficulty in getting the two sides of the globe (i.e., the temporal and nasal sides) using a superficial probe on MINDRAY DC-7.0 ultrasound machine. All measurements were taken three times by the same researcher and averaged to make the results more accurate and to eliminate intraobserver error.
|Figure 1: A transverse section B-mode ocular sonogram. The cornea is represented in (a) and the anterior chamber is depicted in (b). The anterior and posterior aspects of the lens are shown in (c and d), while (e) is the vitreous chamber|
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|Figure 2: A transverse section B-mode ocular sonogram showing the axial length (represented by the line between the calipers)|
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|Figure 3: A transverse section of B-mode ocular sonogram showing the anterior chamber depth (represented by the line between the calipers)|
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|Figure 4: A transverse section of B-mode ocular sonogram showing the lens thickness (represented by the line between the calipers)|
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The heights of the participants were measured by the radiologist using a stadiometer from the foot to the vertex of the scalp.
Two different pro formas were used for data gathering, one for the radiologist and the other one for the optometrist for the autorefraction results. At the end of the research, the data collected by the optometrist were matched number for number with the ones with the radiologist. The data were analyzed using the Statistical Package for the Social Sciences version 20.0 software for Windows (SPSS Inc., USA). The relationships were determined using Pearson’s correlation, because all the data obtained were normally distributed. Regression analysis was used for predicting the refractive state of the participants’ eyes from the measurements of ocular biometry. Student’s t test was used to determine differences in mean parameters with P value of <0.05 considered statistically significant.
| Results|| |
In this study, 255 participants within the age range of 18 and 40 years with refractive errors were investigated. Each participant had both eyes investigated. The control used in this study was the standard autorefraction results performed for the participant. Out of the 255 participants, 199 (78.0%) were females and 56 (22.0%) were males. The mean age of the participants was 26.96 ± 8.48 years. The mean values for AL, ACD, ED, and LT in the right and left eyes are shown in [Table 1]. There were significant statistical difference in the means of the ocular biometric parameters of both eyes except for the right lens thickness (RLT) and left lens thickness (LLT), which were not statistically significant (P = 0.12). For the right eye, the mean AL for myopia/astigmatism was 23.96 ± 1.27 mm, whereas hyperopia/astigmatism was 21.94 ± 0.99 mm (P < 0.001). The left eye had mean AL for myopia/astigmatism to be 23.68 ± 1.28 mm and mean AL for hyperopia/astigmatism to be 21.85 ± 1.16 mm (P < 0.001).
|Table 1: Comparison between ocular ultrasound biometric parameters of both eyes|
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In this study, out of the 255 participants analyzed, about two-thirds (n = 167) had hyperopia/astigmatism of both eyes, whereas only about a third (n = 88) had myopia/astigmatism of both eyes, as established by the standard autorefraction performed for them.
For both eyes, out of the 88 participants that had myopia/astigmatism as established by autorefraction, 78 (88.6%) truly had prolate ocular shape, while the remaining 10 (11.4%) had an oblate ocular shape as deduced from the B-mode ocular ultrasound biometry. This puts the sensitivity of the B-mode ocular ultrasound measurement at 88.6% for myopia/astigmatism corresponding to the prolate ocular shape. In addition, out of the 167 who had hyperopia/astigmatism as established by autorefraction, 146 (87.4%) had an oblate shape as deduced from the B-mode ocular ultrasound biometry. Thus, the sensitivity of using ocular ultrasound measurement in determining the oblate ocular shape, which corresponds to hyperopia/astigmatism, is, therefore, 87.4%.
The Pearson’s correlation coefficient (R) between right axial length (RAL) and right spherical equivalent (RSE) was negatively strong at −0.79 and statistically significant (P < 0.001). A linear regression [Table 2] established that RAL in mm could statistically significantly predict RSE [F (1, 248) = 414.569, P < 0.001], and RAL accounted for 62.0% of the explained variability (R2) in RSE.
|Table 2: A linear regression table showing the relationship between RAL and RSE|
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The regression equation was as follows: Predicted RSE = 27.47−(1.23 × RAL). The coefficient indicates that for every additional mm increase in RAL, it can be expected that RSE will decrease by an average of 1.23 dioptre sphere (DS). Left axial length (LAL) and left spherical equivalent (LSE) also had a negatively strong Pearson’s correlation coefficient (R) of −0.76, which was statistically significant (P < 0.001). LAL in mm could statistically significantly predict LSE [F (1, 248) = 340.043, P < 0.001], and LAL accounted for 58.0% of the explained variability (R2) in LSE. This was established by a linear regression [Table 3].
|Table 3: A linear regression table showing the relationship between LAL and LSE|
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The regression equation was as follows: Predicted LSE = 28.61−(1.27 × LAL). The coefficient indicates that for every additional mm increase in LAL, it can be expected that LSE will decrease by an average of 1.27 DS. A weak Pearson’s correlation coefficient (R) was noticed between RLT and LLT and standard error (SE), which was 0.29 and 0.27, respectively (P < 0.001). The Pearson’s correlation coefficient between the participant’s height and RAL was very weak, which was 0.10. A very weak correlation was also noted between the participant’s height and Right Anterior Chamber Depth (RACD), which was 0.14. The same trend was also noted for the left eye.
| Discussion|| |
This study was conducted on adult Nigerians aged between 18 and 40 years. About 67.1% of the participants investigated had tertiary education. This may be due to the fact that the research was conducted in a teaching hospital. Majority (78%) of the investigated participants who had refractive error were females. This is similar to a study conducted by Adegbehingbe et al. at OAUTHC, Ile-Ife, in which refractive errors were twice more common among the females than the males. This is probably so because both studies were conducted in the same locality. In addition, females seek medical attention more than their male counterparts in our study environment. Sex differences in refractive error have been reported previously, but findings are not consistent. Studies conducted in Baltimore, Singapore, Sydney, and Barbados did not identify significant sex differences. This study revealed that women are more hyperopic, similar to studies conducted in Shihpai and Beijing, where they also found women to be more hyperopic. One hundred and sixty-seven (65.5%) of the participants in this study had hyperopia/astigmatism. This is contrary to the study of Adegbehingbe et al. on the pattern of refractive errors at OAUTHC, Ile-Ife, in which they found that myopia was the most common refractive error. This difference may be due to the large disparity in the number of participants investigated in both studies, as well as the fact that this study focused on participants aged between 18 and 40 years, while the previous study had no age limit.
The sensitivity of the B-mode ocular ultrasound measurement for prolate shape corresponding with myopia/astigmatism was 88.6%, whereas for oblate shape corresponding with hyperopia/astigmatism, it was 87.4%. Ocular shape is associated with refractive error in adult eyes, with myopic being more prolate (having a longer AL than ED) and hyperopic eyes tending to be oblate (having a broader ED than AL). This was found be true in this study. These findings were in concordance with those of Mutti et al. in their work on peripheral refraction and ocular shape in children. They found that the eyes of myopic children were both elongated and distorted into a prolate shape. According to Atchison et al., although there are considerable individual variations, in general, myopic eyes are elongated relative to emmetropic eyes, more in length than in height and even less in width (prolate ocular shape). This study also observed that not all who were myopic had a prolate ocular shape. This could be explained by the presence of curvature myopia in such patients. Furthermore, a strong correlation was found between AL and the refractive power of the eye (using the spherical equivalent). As there was an additional increase in AL, the SE decreased by a unit value, thereby moving the participant to the myopic range [Table 2] and [Figure 5]. Ojaimi et al. also found that AL and ACD correlated negatively with refractive error. These correlations indicated that longer eyes and those in which axial elongation had outpaced changes in corneal curvature were more likely to be myopic, or as was in their sample, less hyperopic. Both Gwiazda et al. (COMET Study) and Fulk et al. showed increases in vitreous chamber depth and AL in myopes.
|Figure 5: a: A scattergram showing the correlation between RAL and RSE. b: A scattergram showing the correlation between LAL and LSE|
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A weak positive correlation coefficient (R) at 0.29 was noticed between LT and SE in this study. This indicates that as the LT increases, the participants will be expected to move away from the myopic range to the hyperopic range. However, this was not so reliable because of the weak correlation. A similar finding was reported by Shufelt et al., who discovered that the thicker the lens, the more hyperopic the participants investigated were. They also alluded to the fact that the contribution of LT to refractive error was small but significant with a Pearson’s correlation coefficient of 0.4. This was very similar to the Pearson’s correlation coefficient of 0.29 in this study. It was found that not so much work has been performed with regard to LT and refractive errors in Africa.The correlation between participant’s height and AL in this study was found to be very weak. In addition, the correlation between the participant’s height and ACD was weak. Data on the association between refraction and stature are inconsistent. In Labrador, Johnson et al. observed a positive correlation between AL and height. However, this study found no association between height and refractive state of the eye. According to Wong et al., the relationship between ocular dimensions and adult stature is not clear. However in general, they agreed that taller persons were more likely to have longer ALs, deeper ACDs, longer Vitreous Chamber Depth (VCDs), flatter corneas, and thinner lenses than shorter persons of similar age, sex, education level, socioeconomic status, and weight.
| Conclusion|| |
AL using real-time B-mode ocular ultrasound was found to correlate strongly with spherical equivalent in majority of the participants; hence, a regression equation can be used in predicting the spherical equivalent from the AL measurements (present study). The presence of non-axial curvature or index myopia may be responsible in those with no correlation. Thus, real-time B-mode ocular ultrasound can be a very good alternative technique in determining the refractive state of the eyes in areas where standard autorefraction keratometer is not available.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3]