Sample size for ophthalmology studies.

Knowledge and the usage of actual sample size formulae are a necessity as validity of the inferences from research studies is often dependent on this. This paper explains how sample sizes are calculated. The concept of sampling variation is explained to emphasize the need for its proper calculation. Sample size formulae are explained with examples to provide researchers with a means of calculating the sample sizes for the commonly used study designs. Ophthalmic data are used as examples. It is perceived that this will improve the quality of inferences drawn from ophthalmic research studies.

Optic disc size in ocular hypertension.

PURPOSE: To study the optic disc size in eyes with ocular hypertension (OHT) in comparison to primary open-angle glaucoma (POAG) and normals. METHODS: Optic disc photographs obtained with the Nidek 3dx NM camera were digitized (Nikon coolscan) and disc area calculated using Littmann correction in a randomly chosen eye of 28 OHT, 42 POAG and 30 normal subjects. OHT was defined as increased intraocular pressure with no disc or field changes suggestive of glaucoma with open angles. RESULTS: The optic disc area in OHT was 9.47 +/- 1.09 mm2; 12.27 +/- 2.87 mm2 in POAG; and 12.11 +/- 2.83 mm2 in normal individuals. CONCLUSION: Using magnification corrected morphometry and the criteria for OHT diagnosis, the optic disc area in OHT was significantly smaller (p < 0.0001) in POAG and normals.

Developing a model to reduce blindness in India: The International Centre for Advancement of Rural Eye Care.

With the continuing high magnitude of blindness in India, fresh approaches are needed to effectively deal with this burden on society. The International Centre for Advancement of Rural Eye Care (ICARE) has been established at the L.V. Prasad Eye Institute in Hyderabad to develop such an approach. This paper describes how ICARE functions to meet its objective. The three major functions of ICARE are design and implementation of rural eye-care centres, human resource development for eye care, and community eye-health planning. ICARE works with existing eye-care centres, as well as those being planned, in underserved areas of India and other parts of the developing world. The approach being developed by ICARE, along with its partners, to reduce blindness is that of comprehensive eye care with due emphasis on preventive, curative and rehabilitative aspects. This approach involves the community in which blindness is sought to be reduced by understanding how the people perceive eye health and the barriers to eye care, thereby enabling development of strategies to prevent blindness. Emphasis is placed on providing good-quality eye care with attention to reasonable infrastructure and equipment, developing a resource of adequately trained eye-care professionals of all cadres, developing a professional environment satisfactory for patients as well as eye-care providers, and the concept of good management and financial self-sustainability. Community-based rehabilitation of those with incurable blindness is also part of this approach. ICARE plans to work intensively with its partners and develop these concepts further, thereby effectively bringing into practice the concept of comprehensive eye care for the community in underserved parts of India, and later in other parts of the developing world. In addition, ICARE is involved in assessing the current situation regarding the various aspects of blindness through well-designed epidemiologic studies, and projecting the eye-care needs for the future with the help of reliable information. With balanced attention to infrastructure, manpower, financial self-sustenance, and future planning, ICARE intends to develop a practical model to effectively reduce blindness in India on a long-term basis.

Causes of corneal graft failure in India.

The success of corneal grafting in visual rehabilitation of the corneal blind in India depends on survival of the grafts. Understanding the causes of graft failure may help reduce the risk of failure. We studied these causes in a series of 638 graft failures at our institution. Multivariate logistic regression analysis was used to evaluate the association of particular causes of graft failure with indications for grafting, socioeconomic status, age, sex, host corneal vascularization, donor corneal quality, and experience of surgeon. The major causes of graft failure were allograft rejection (29.2%), increased intraocular pressure (16.9%), infection excluding endophthalmitis (15.4%), and surface problems (12.7%). The odds of infection causing graft failure were significantly higher in patients of lower socioeconomic status (odds ratio 2.45, 95% CI 1.45-4.15). Surface problems as a cause of graft failure was significantly associated with grafts done for corneal scarring or for regrafts (odds ratio 3.36, 95% CI 1.80-6.30). Increased intraocular pressure as a cause of graft failure had significant association with grafts done for aphakic or pseudophakic bullous keratopathy, congenital conditions or glaucoma, or regrafts (odds ratio 2.19, 95% CI 1.25-3.84). Corneal dystrophy was the indication for grafting in 12 of the 13 cases of graft failure due to recurrence of host disease. Surface problems, increased intraocular pressure, and infection are modifiable risk factors that are more likely to cause graft failure in certain categories of patients in India. Knowledge about these associations can be helpful in looking for and aggressively treating these modifiable risk factors in the at-risk categories of corneal graft patients. This can possibly reduce the chance of graft failure.

Early diagnosis of mycotic keratitis: predictive value of potassium hydroxide preparation.

Potassium hydroxide (KOH) preparation is an underutilized modality in the diagnosis of mycotic keratitis. We have earlier shown its utility in the diagnosis of Nocardia and Acanthamoeba keratitis. The aim of this study was (i) to evaluate the sensitivity, specificity and predictive value of KOH preparation, and (ii) to compare its efficacy with other methods of corneal scraping examination, for the diagnosis of mycotic keratitis. The study was conducted in two phases. In phase I, randomized corneal scrapings were examined by KOH, Gram's stain, and lactophenol cotton blue (LPCB) in 91 infectious keratitis subjects. In phase II, 53 corneal scrapings were stained with KOH and calcofluor white (CFW), and viewed with bright field (KOH) and fluorescence (CFW) microscopy. The KOH and CFW readings were recorded by an observer masked to the clinical findings and culture results. Nineteen scrapings were examined by two masked observers. In 22 culture positive fungal keratitis patients in phase I, the sensitivity of KOH, Gram's stain, and LPCB methods was 100%, 86.4%, and 77.3%, respectively. In phase II, the specificities of KOH and CFW were identical (83.8%), while the sensitivities were 81.2% and 93.7%, respectively (p = 0.59), in 16 culture positive mycotic keratitis patients. There was no significant difference between the negative and positive predictive values of KOH and CFW. Furthermore, no significant interobserver variability was found in the specificity and sensitivity. The KOH method compares well with other microscopy methods in the diagnosis of keratomycosis and has a definite place in the armamentarium of diagnostic techniques.

Statistical analysis: the need, the concept, and the usage.

In general, better understanding of the need and usage of statistics would benefit the medical community in India. This paper explains why statistical analysis is needed, and what is the conceptual basis for it. Ophthalmic data are used as examples. The concept of sampling variation is explained to further corroborate the need for statistical analysis in medical research. Statistical estimation and testing of hypothesis which form the major components of statistical inference are construed. Commonly reported univariate and multivariate statistical tests are explained in order to equip the ophthalmologist with basic knowledge of statistics for better understanding of research data. It is felt that this understanding would facilitate well designed investigations ultimately leading to higher quality practice of ophthalmology in our country.

Design of a population-based study of visual impairment in India: The Andhra Pradesh Eye Disease Study.

Reliable population-based epidemiologic data regarding vision and ocular morbidity, as well as those about the perceptions of people regarding visual impairment and eye care, are lacking for the most part in the developing world including India. These data are the basis on which effective eye care services can be developed. To meet this need we designed the Andhra Pradesh Eye Disease Study, a population-based epidemiology study of 10,000 people in the Indian state of Andhra Pradesh. The design of this study is described in this paper. Various options for the sample size, study areas, sampling procedure, and recruitment of subjects were considered. A sample size of 10,000 people, 5,000 each in the < or = 30 and > 30 years age groups, was determined to obtain reasonable confidence in estimating the prevalence of diseases and odds ratios for risk factors of interest. A multistage sampling strategy was chosen for the study which was assumed to give a design effect of 1.5 for the estimates. One urban area, Hyderabad, and three rural areas, West Godavari, Adilabad and Mahbubnagar districts, were selected in Andhra Pradesh. Interview instruments were developed to obtain detailed information about demographic data, diet, ocular and systemic history, risk factors for eye diseases, visual function, quality of life, barriers to eye care, and knowledge about eye diseases. A detailed examination procedure was devised to obtain a broad range of normative and abnormal data related to eyes and vision. A protocol was developed for doing automated visual fields, slitlamp and fundus photography. Computer databases were made in FoxPro for data entry and subsequent analysis with SPSS. Pilot studies were done to test the instruments, procedures, and logistics of the study in urban and rural areas. Information from the Andhra Pradesh Eye Disease Study is expected to help in planning and implementation of effective long-term preventive, curative, and rehabilitative eye care services in Andhra Pradesh.

Indications for penetrating keratoplasty in India.

Indications for penetrating keratoplasty (PK) in the developing world from a large series are not well documented. This study was done to evaluate the indications for PK in a major eye care institution in India. The records for a consecutive series of 1,964 PKs were analysed and multiple logistic regression was used to study the effect of age, socioeconomic status and sex on the indications for PK. The indications for PK were corneal scarring in 551 (28.1%) including adherent leukoma in 147 (7.5%), regrafts in 336 (17.1%), active infectious keratitis in 239 (12.2%), aphakic bullous keratopathy in 231 (11.8%), pseudophakic bullous keratopathy in 209 (10.6%), corneal dystrophies in 165 (8.4%) including Fuchs' dystrophy in 23 (1.2%), keratoconus in 118 (6%), and miscellaneous in 115 (5.9%). The odds that the patient belonged to lower socioeconomic status were significantly higher if the PK was done for active infectious keratitis (odds ratio 2.73, p < 0.0001), corneal scarring (odds ratio 1.72, p = 0.0009) or regraft (odds ratio 1.44, p = 0.047). Corneal scarring, including adherent leukoma, and active infectious keratitis are relatively more common indications whereas keratoconus, pseudophakic bullous keratopathy and Fuchs' dystrophy are less common indications for PK in India than reported from the developed world. Indications for PK which carry a poorer prognosis for graft survival are relatively more common in India than in the developed world.

Visual impairment in school children in southern India.

This study was done to determine the prevalence of visual impairment due to refractive errors and ocular diseases in lower middle class school children of Hyderabad, India. A total of 4,029 children, which included 2,348 males and 1,681 females, in the age range of 3 to 18 years from 9 schools were screened with a detailed ocular examination protocol. Among 3,669 children in whom visual acuity could be recorded, on presentation 115 (3.1%) had visual acuity < 6/18 in the better eye (equivalent to visual impairment), while 41 (1.1%) had visual acuity < or = 6/60 [corrected] in the better eye (equivalent to legal blindness) out of which 18 (0.5%) had visual acuity < 6/60 in the better eye (equivalent to economic blindness). Of 115 children who presented with initial visual acuity < 6/18, vision improved to > or = 6/18 with refraction in 109 (94.8%). No child was legally or economically blind after refractive correction. Prevalence of hyperopia was 22.6%, myopia 8.6% and astigmatism 10.3%. The prevalence of myopia was significantly higher among children > or = 10 years of age (P < 0.001). The maximum, mean and median values for myopia were 10.00, 1.35 and 0.75 D in the better eye. For hyperopia these values were 8.50, 0.65 and 0.50 D. The major causes for best corrected visual acuity < 6/9 in the worse eye for 51 (1.4%) children included amblyopia in 40 (1.1%), corneal diseases in 5 (0.1%), cataract in 2 (0.05%) and others in 4 (0.1%). Out of the total, 30 (0.7%) children had strabismus. These data support the assumption that vision screening of school children in developing countries could be useful in detecting correctable causes of decreased vision, especially refractive errors, and in minimising long term permanent visual disability.