Epidemiology of Eye Disease, Part 2 Review

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Section 2 Methodology was useful for the novice, such as myself, in epidemiology ---- what are cross - sectional studies, bias, cluster sampling, rapid assessment of avoidable blindness ( RAAB ), sample size, cohort studies, etc. The Baltimore Eye Survey is an example of cluster sampling in eye surveys. This was a population-based prevalence survey, the primary aim being the comparison of the black and white populations with respect to the prevalence of eye disease. Other clinical trails in ophthalmology were discussed.

Chapter 8 on screening in ophthalmology was of interest to me since in Belize we have begun a national screening for diabetic retinopathy program using retinal cameras and techs as screeners. Stationing the retinal camera not in the eye clinic but elsewhere ( offices for family medicine, pharmacy, nutritionist, etc. ) will be better in finding asymptomatic diabetics who have never had an eye exam previously.
Sensitivity ( true positive rate ) is the proportion of persons with a disease that the test correctly identifies. Specificity ( true -negative rate ) is the proportion of persons who are disease - free whom the test identifies as normal. Thus the specificity of a test is a measure of its ability to identify correctly those who are disease-free. Therefore sensitivity is the ability of a test to correctly identify those with the disease ( true positive rate ) whereas test specificity is the ability of a test to correctly identify those without the disease ( true negative rate ). The relationship between sensitivity and specificity is important. Increasing the sensitivity of a test will usually be at the expense of specificity, and vice versa. There usually is a trade – off between the measures. Positive predicative value ( PPV ) is the probability of being disease - positive if the test is positive.

Section 3 Specific Entities. This is probably the section of most interest for ophthalmic clinicians / diagnosticians. Chapter 10 ( Age - Related Cataract ) stated that cataract surgery is the most commonly performed surgical procedure in the USA with more than two million procedures performed annually. As with most ageing populations, estimates suggest that the prevalence of cataract will increase substantially in the coming decades. Nuclear cataract is typically the most common form of cataract. The prevalence and incidence of lens opacities differ between the sexes, with women having consistently higher rates of cortical opacities compared to men. Cigarette smoking is one of the most consistently reported risk factors for age-related cataract development. Population based studies have identified an excess risk of cortical and PSC lens opacities in persons with diabetes and the risk appears to increase with increasing duration of diabetes. As we all know, vitrectomies are associated with rapid development of nuclear opacities, and posterior lens exposure to higher than normal levels of oxygen is felt to be the mechanism that leads to the cataract development. Obviously all patients undergoing a vitrectomy should be told they will develop a cataract post-op. Topical beta-blockers may result in an increased incidence of nuclear opacity.

Chapter 13 Glaucoma. Drs. P. Foster and H. Quigley discussed glaucoma ---- definition, classification, the importance of gonioscopy, and prevalence. Secondary glaucoma is more often unilateral, rapidly progressive, and visually destructive, causing a high rate of uniocular blindness. The relative prevalence rates of POAG and PACG vary markedly between populations. PACG is much more common in Chinese people than in Europeans. The Japanese for their population have the highest rates of glaucoma occurring within the statistically normal range of IOP ( previously referred to as normal - tension glaucoma ). The prevalence rates of POAG in white Europeans, Americans, and Australians are similar when the figures are age-adjusted. Prevalence for open-angle glaucoma is highest among African and Latin American groups. Black populations in the Caribbean, East Africa, and the USA have a higher prevalence of POAG than those of European or Asian origin. It was calculated that by 2010, 4.5 million people would be blind from OAG, and 3.9 million blind from ACG. By 2020 these numbers are set to rise to 5.9 and 5.3 respectively. Data from Asia show that PACG and secondary glaucoma cause a greater proportion of blindness than POAG. So worldwide there are more cases of OAG than ACG but ACG is more likely to lead to blindness. In addition the risk factors for primary glaucoma ( open-angle and angle-closure ) and the genetics of glaucoma were discussed. My experience in the developing world, is that patients often present quite late with bilateral glaucoma. Patients should be told and retold that no drops, surgery procedures anywhere will restore their vision. Who wants to hear that?
Chapter 14 Visual Impairment And Blindness In Children. The subsections included : magnitude and causes, Vitamin A deficiency disorders, ophthalmia neonatorum, corneal disease in children, cataract in children, glaucoma in children, and retinopathy of prematurity. The control of blindness in children remains one of the main priorities of the WHO's Vision 2020 The Right To Sight. Vitamin A deficiency disorders ( VADD ) refers to all the health problems that can be associated with a Vitamin A deficiency : ocular manifestations but also depressed growth, disturbed haematopoiesis, disturbed epithelial barrier function, impaired immunity, and increased severity of infection and consequent mortality. Many children have mild VADD without frank xerophthalmia. VADD is concentrated in sub-Saharan Africa , SE Asia, and northern Latin America. The 1982 WHO system for classifying the clinical stages of xerophthalmia was presented and discussed.

Povidone-iodine 2 1/2% has been shown to be effective in preventing ophthalmia neonatorum ( ON ). An ophthalmic solution of 2.5% povidone-iodine provides effective antibacterial prevention in both chlamydial and gonococcal ON and is relatively non-toxic when placed on the conjunctiva of the newborn. There was an interesting discussion by Claire Gilbert on animism, which is still practiced in much of Sub-Saharan Africa, and the use of harmful traditional eye medicines which I have certainly seen in Sub-Saharan Africa. Many traditional medicines placed in the eye can cause severe damage and delay proper treatment. Training / educating traditional medical providers on eye care and treatment is often time well spent. Drs Edward Wilson and Rupal Trivedi presented an excellent overview of the state of the art concerning cataracts in children. Glaucoma in children and retinopathy of prematurity ( RoP ) were presented. ROP is a main concern in middle – income countries due to inadequate neonatal care and screening. A world map showed the current distribution of RoP. The section of color clinical photos / distribution maps was useful and I thought quite good.

I have not discussed nor mentioned many useful chapters due to a lack of space.  I have included this discussion attached in case someone would like to download it.

To be continued. Baxter



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