Myopia In Asian Countries Health And Social Care Essay – UK Essays



Vision results from entrance of light into the eye and the interpretation of this stimulus by the brain. For a normal eye, light is focused to a spot on the retina. This message would then be sent to the brain to be interpreted as a message. Visual acuity is defined as the clarity or sharpness of vision, which is the ability of the eye to see and distinguish fine details. [1].

The cornea is the most powerful refracting surface of the optical system of the eye, accounting for two-thirds of the eye’s focusing power. Production of a sharp image at the retinal receptors requires corneal transparency and appropriate refractive power. The refractive power of the cornea depends on its curvature and the difference in refractive indexes between it and air [9]. Refractive errors occur when the curve of the cornea is irregularly shaped (too steep or too flat). When the cornea is of normal shape and curvature, it bends, or refracts, light on the retina with precision. However, when the curve of the cornea is irregularly shaped, the cornea bends light imperfectly on the retina. [15]

When the cornea is curved too much, or if the eye is too long, faraway objects will appear blurry because they are focused in front of the retina. This is called myopia, or nearsightedness. Hyperopia, or farsightedness, is the opposite of myopia. Distant objects are clear, and close-up objects appear blurry. With hyperopia, images focus on a point beyond the retina. Hyperopia results from an eye that is too short. Astigmatism is a condition in which the uneven curvature of the cornea blurs and distorts both distant and near objects. A normal cornea is round, with even curves from side to side and top to bottom. With astigmatism, the cornea is curved more in one direction than in another. This causes light rays to have more than one focal point and focus on two separate areas of the retina, distorting the visual image.[15]

Measurement of corneal curvature/power can be performed with a variety of instruments, most commonly a keratometer, IOLMaster, or corneal topography device. Corneal curvature is usually used for IOL calculations and corneal refractive surgery. It is also helpful for contact lens fitting and detecting irregular astigmatism. [5]

The primary aim of this study is to investigate the relationship between corneal curvature and degree of refractive error among emmetropic and myopic young population in Malaysia and to determine the standard value of corneal curvature of young population.

1.2 Research question

Question 1:

Does corneal curvature vary significantly with different state of refractive error?

Question 2:

What are the range of readings of corneal curvature of emmetropic and myopic young subjects?

Question 3:

Is there any different between horizontal and vertical curvature of the cornea?

1.3 Objectives of the study

1.3.1 General objective

The general objective of this study is to investigate the relationship between corneal curvature and refractive status among emmetropic and myopic young patients in IIUM Kuantan Campus.

1.3.2 Specific objectives

To determine whether corneal curvature varies significantly with refractive error.

To compare the readings of corneal curvature between emmetropic and myopic young subjects.

To identify the normal range of corneal curvature for emmetropic and myopic young subjects.

To identify whether there is any different between horizontal and vertical curvature of the cornea.

To investigate the role of corneal curvature in refractive state among Malaysian population.

Hypothesis of the study

There is no significant relationship between corneal curvature and refractive status.

CHAPTER 2

LITERATURE REVIEW

2.1 Prevalence of myopia in Asian countries

National Eye Survey done in Malaysia [2,3] found the prevalence of visual impairment was 2.7%. The prevalence was higher in rural areas (2.9%) as compared to urban areas (2.5%). The prevalence of visual impairment in rural population in Selangor has been reported to vary from 5.6% to 18.9% in which they found that among the patients attending the Eye Clinic at University Malaya Medical Centre, the most common cause of visual impairment in children was refractive errors, whereas in elderly patient visual impairment is due to cataract, glaucoma and diabetic retinopathy.[4]

Myopia is a common cause for visual impairment among young population in Asia. Estimates of the proportion of myopia in the young population in Asia ranged from 30% to 65% (Saw et al. 1996; Chow et al. 1990), and the prevalence was found to be greater among Chinese people (Wensor et al. 1999; Sperduto et al. 1983). In Malaysia, the prevalence of myopia found among Malay, Chinese and Indian schoolchildren was 47%, 20% and 19.4% respectively (Garner et al.1990; Chung et al. 1995; Saadah et al. 2002). With the increasing rates of myopia, orthokeratology and refractive surgery such as LASIK, has become quite popular in Asia. When undertaking such procedures to correct myopia, corneal curvature is an important consideration in order to prevent the cornea from becoming too flat after the treatment.[6]

2.2 The importance of corneal curvature

Corneal curvature is usually used for IOL calculations and corneal refractive surgery. It is also helpful for contact lens fitting and detecting irregular astigmatism (Friedman, 2009). Measurements of central and peripheral corneal curvature are useful for diagnosing and monitoring corneal conditions such as keratoconus and for monitoring corneal shape following ocular surgery6–10 or refractive procedures such as orthokeratology.[13]

Liu Z., Pflugfelder SC, (2000) studied on the effects of long-term contact lens wear on corneal thickness, curvature, and surface regularity. They found that the corneal curvature, maximum keratometry (Max K) and minimum keratometry (Min K) readings, were significantly steeper in eyes wearing contact lenses than normal eyes (P < 0.01 for Max K and Min K measured by both instruments).

In highlighting the importance of corneal curvature in IOL calculations, Schena LB (2008) cited from Dr. Majmudar, in patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters (based on keratometer measurements). However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if examiners just rely on the topography, they may be off, and for every 1 diopter off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.[11]

Besides that, keratometry plays a critical role in the accuracy of IOL power calculation by detecting and measuring astigmatism by determining the steepest and flattest meridians of the corneal surface. [16]

Keratometry plays a critical role in the accuracy of IOL power calculation, detecting and measuring astigmatism by determining the steepest and flattest meridians of the corneal surface. Keratometry plays a critical role in the accuracy of IOL power calculation, detecting and measuring astigmatism by determining the steepest and flattest meridians of the corneal surface. Keratometry plays a critical role in the accuracy of IOL power calculation, detecting and measuring astigmatism by determining the steepest and flattest meridians of the corneal surface. Keratometry plays a critical role in the accuracy of IOL power calculation, detecting and measuring astigmatism by determining the steepest and flattest meridians of the corneal surface. Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”Dr. Majmudar explained that keratometers measure the curvature of the anterior surface about 3.2 mm from the center of the cornea. “In patients who have not undergone previous surgery, the value at the center of the cornea is roughly the same as the value at 3.2 millimeters,” he said. “However, patients who have undergone LASIK or PRK can have altered corneas, and the value at the central cornea, which is the goal of measurement, may be very different from that at 3.2 millimeters. Consequently, if you just rely on the topography, you may be off, and for every 1 diopter you are off in measuring the corneal curvature, a roughly 1 diopter miscalculation will result for the patient’s refractive outcome.”Top of Form

Bariah et al. (2009) suggested that every demographic has a different average and range of corneal curvature values and its relationship to the degree of myopia among normal population.

2.3 Corneal curvature in hyperope, emmetrope and myope

In research by Bariah et al. (2009), results from keratometer measurement indicated that the mean of corneal curvature for the all myopic young subjects was 7.74 ± 0.52 mm, with 7.87 ± 0.28 mm for Chinese subjects and 7.60 ± 0.66 mm for Malays.

In study by Mainstone JC et al (2010), the correlation between corneal radius of curvature and spherical equivalent refractive error was not statistically significant (r2 = 0.038, p =0.2609). A previously study by Carney LG et al. showed that there is a tendency for the cornea to flatten less rapidly in the periphery with increasing myopia. This suggests that there are corneal shape changes that occur in myopic eyes, perhaps as a result of abnormal ocular growth, that are not seen in hyperopic eyes because a different mechanism of refractive error development operates in this latter group. However, a research by Bariah et al (2009) with mean refractive error in spherical equivalent was –3.50 ± 2.10 DS, they found that there is poor correlations between myopic refractive error and corneal curvature (r = 0.246, p > 0.05).

On the other hand, a recent study by Iyamu E et al. (2010) on a Nigerian population found that there was a significant positive correlation between corneal curvature and spherical equivalent refraction. ANOVA showed that the difference in mean corneal curvature across the refractive status groups was statistically significant (F = 27.9, df = 2, 67, p < 0.0001). Post hoc test with Fisher’s LSD showed that myopes had steeper corneas than the other two groups (steeper by 0.27 mm than that of hyperopes, and 0.28 mm than that of emmetropes). The linear regression model is represented by: CRC = 7.853 + 0.069SER. From the equation it can be predicted that for every 1.00D increase in myopia, the cornea is steepened by approximately 0.07 mm.

2.4 Measuring corneal curvature using Pentacam®

The Pentacam has become a popular clinical device for evaluating the anterior segment of the eye since its launch in 2004. It contains a Scheimpflug camera that rotates through 360 degrees and captures 25 or 50 Scheimpflug slit images within two seconds. Each image shows a cross-section of the cornea and the anterior segment. The images contain 500 data points and up to 25,000 points (50 slits ¥ 500 data points) are measured to various parameters and to construct a three-dimensional cornea. The Pentacam is capable of measuring topographic corneal thickness, anterior and posterior corneal topography, anterior chamber depth and angle and corneal aberrations. It also provides parameters such as corneal volume and keratometric power difference, which offer new and technical analyses of the cornea, respectively.[13]

For historical reasons, most Placido topographers and keratometers use the refractive index of 1.3375 for the refractive power of the cornea. This refractive index is actually incorrect even for the untreated eye (n~1,332) and assumes a constant ratio between the anterior and posterior curvature of the cornea. As many IOL power calculation formulas use the incorrect ‘K-reading’ directly an empirical correction is needed in the end to adjust the correct IOL power in these formulas even in normal cases. [12]

In a study by Chen D. et al, the Pentacam system was shown to be highly reliable in the Sim K measurements, at both the anterior and posterior corneal surfaces (Cronbach’s alpha test, α ≥ 0.990; intraclass correlation coefficient, ICC ≥ 0.972).

The Orbscan is another commonly used corneal topographer. It is based on a hybrid system incorporating both the Placido disc and scanning slit principles. Similar to the Pentacam, the Orbscan is non-invasive, simple to use and quick, however, previous studies found the Orbscan to have questionable performance in topographic pachymetry and in the evaluation of anterior21 and posterior corneal curvatures.[14]

CHAPTER 3

MATERIALS AND METHODS

Study area

Location of study is at Clinic of Optometry International Islamic University Malaysia, Kuantan.

3.2 Study design

The study is an experimental case-control study. The parameters for the collected data will be performed on both eyes. Data will be collected from each subject on a single visit.

3.3 Subject of the study

3.3.1 Sampling of the study

Sampling of the study is convenient sampling. Subjects who fulfilled the inclusion criteria for the study are invited to participate in the study.

Sample size

The sample size will be based on the study by Bariah et al (2009) “Corneal Thickness and Curvature of One Sample of Young Myopic Population in Malaysia” where the subjects consisted of 84 myopic university. Should cite the latest paper with groups comparison “Ocular Dimensions of Young Malays in Malaysia” (Bariah Mohd-Ali, and Muhammad Afzam Shah Abdul-Rahim, and Zainora Mohammed, and Norhani Mohidin, (2011) Ocular dimensions of young Malays in Malaysia. Jurnal Sains Kesihatan Malaysia, 9 (1). pp. 35-39. ISSN 1675-8161.

On the sample size calculation, refer to relevant previous paper and enter the respective values (mean, standard deviation etc.) in this website:

http://www.stat.ubc.ca/~rollin/stats/ssize/n2.html

Sample of the study

Sample of the study comprised of groups of emmetropic and myopic young subjects.

Subject’s inclusion criteria

Age between 20 to 26 years old.

VA at distance 6/6 or better.

VA at near N5 at 33 cm or better.

Refractive power between +0.50 DS to -0.50 DS and ≥ – 2.00 DS

Non-contact lens wearer. Soft contact lens wearer less than 5 years are permitted provided lenses were not worn on the day of testing

Good ocular health tested with slit lamp and fundus examination.

Good general health taken from self-report.

Subject’s exclusion criteria

Rigid contact lens wearer

History of corneal abnormality, refractive surgery or orthokeratology treatment.

3.4 Study tools and procedure

Within study sites, clinical examinations are generally performed at one location (IIUM Optometry clinic). Before clinical eyes examination is performed on the participant, they will be asked for a complete personal and family general health and eye health history. Later, participants underwent an extensive and standardized examination procedure, which included visual acuity (VA) testing, refraction and a detailed ocular slit lamp and fundus examination

The visual Acuity (VA) will be measured using a Snellen chart at 6m. Measurement of refractive status will be conducted using subjective refraction with cross cylinder. Subjects were categorized into emmetropia, hyperopia and myopia using the spherical equivalent refraction. Categorization was done based on: Emmetropia ≤± 0.50DS and Myopia ≥ 2.00DS. Next, measurement of corneal curvature will be taken using Pentacam (Oculus, Inc., Germany). Average corneal curvature (AVK) was obtained by the average of the horizontal and vertical corneal curvature. Finally, the external eye and anterior segment (eyelid, conjunctiva, cornea, iris, and pupil) will be examined with slit lamp, and followed by examination of media and fundus using indirect opthmoscopy and fundus camera.

3.5 Data analysis

The raw data are keyed in and organized using statistical software SPSS (Statistical Package for Social Science) version 17.0. The data will be analyzed using the same software. Statistical analyses being employed are:

Normality test on each data set using Histogram.

significant level: 0.05 for 95% CI

Descriptive analysis (mean, standard deviation, median and frequency).

One-way analysis of variance (ANOVA) will be used to compare the mean corneal radius of curvature across the refractive status groups

The relationships among parameters will be tested using Pearson’s correlation and linear regression analysis.

3.6 Ethical consideration

Ethical approval will be acquired from the ethical committee of Kulliyyah of Medicine, International Islamic University Malaysia (IIUM). Participation in this study is on a voluntary basis and participants will be asked to sign the consent form if they agree to participate in this study.

A written consent will be acquired prior to data collection (Appendix B). In this consent form, candidates will be informed that they will act as ‘subject’ in this study. Participants will be given explanation on the procedures involved in this study before data collection is performed.The procedure is considered safe because it is non-invasive. Moreover, the participants’ data is confidential to public. Any question from the subject will be answered accordingly. Participants may also withdraw from this study at any time.




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