- Email: [email protected]
Keratoconus in Contact Lens Wear: Reply
VOL. 86, NO. 3
CORRESPONDENCE
onset of a clinical condition (keratoco nus). The authors contrast these findings with their observation that 27% of known keratoconus patients had a prior history of hard contact lens wear. This is a retro spective anlaysis, since it begins instead with patients already having the clinical end point (keratoconus), and determines the proportion with the potential risk fac tor (hard contact lens wear). Retrospective and prospective analyses obviously mea sure entirely different things and mixing the two is like comparing "apples and pears." There is also a problem with the au thors' choice of controls. They assure us the refractive error of the two groups was comparable. Although this may have been true for their spherical equivalents, it seems unlikely that the proportion of cases with irregular or high degrees of astigmatism was similar, since these con ditions require the choice of hard contact lenses. Presumably early in keratoconus, before the diagnosis is clear, a significant proportion of cases will suffer irregular or high degrees of astigmatism. A popula tion of hard contact lens wearers will therefore automatically contain a larger proportion of future keratoconus patients than a population of soft lens users. A L F R E D SOMMER,
Bandung,
M.D.
Indonesia
Reply Editor: In his letter, Dr. Sommer deals with two factors: (1) the type of epidemiology study he thinks our study is, and (2) what he thinks keratoconus is. The special or particular contribution of epidemiology lies in the study of the determinants of disease. This involves an interpretation of the distribution of dis ease in terms of possible causal factors. The use of epidemiologic methods in this manner to identify causal association is
443
sometimes called analytic epidemiology. There are three main groups of epidemio logic studies: (1) descriptive studies, (2) analytic studies, and (3) experimental studies. Analytic studies specifically de signed to examine a particular hypothesis, that is, the role of hard contact lens wear in the development of keratoconus, were more suitable for our study. The analytic studies can also be divided into two main groups, the case history study and the cohort study. We used the case history investigation in our study for the follow ing reasons: (1) One begins with a fixed number of cases and there needs to be relatively little concern for low incidence of the disease. For example, if one consid ers an average incidence of keratoconus of 1 in 10,000, we would have had to study 1,620,000 cases to obtain a sample of the same size as the one we used in our paper. (2) Age of onset: It is generally accepted that the average age of onset of keratoconus is 18 years. Therefore, if our consultand, that is, if the person whose genotype we are evaluating is still unaf fected at 20, 25, or 30 years old, the greater the probability that he did not inherit the biologic genetic factor pre sumably necessary for the development of keratoconus. Dr. Sommer's dogmatism could be caused by his confusion about keratoconus. Although it is relatively easy to find a cause in an iatrogenic disease with a simple unitary etiology, keratoco nus results from the interplay between multiple biological (genetic) and environ mental factors. Even when the environ mental contribution is considered apart from the inheritance as it was in our paper, multiple interaction between host factors and environmental factors can be expected to be responsible for the devel opment of events leading to the inception and progression of the disease. Dr. Sommer's difficulties in diagnosing keratoconus are his own. Before the diag-
444
AMERICAN JOURNAL OF OPHTHAMOLOGY
nosis of keratoconus can be made, it has to be suspected. The progress of myopia in keratoconus is not axial but curvature myopia, which can be objectively evalu ated with the keratometer. We conducted our study by using the analytic investigation, case history study. However, the lack of a better control group prevented us from even attempting to use statistics to prove our hypothesis. Therefore, a causal relationship was never proposed; instead, we formulated a hypothesis of circumstantial association between hard contact lens wear and the development of keratoconus. A review of the published reports shows that similar epidemiologic studies have made enor mous contributions to medicine: aniline dye and carcinoma of the bladder, asbes tos and lung cancer, fluoridation and den tal cavities, oxygen and retrolental fibroplasia, to name a few examples. Finally, as Sir William Osier said: "The greater the ignorance, the greater the dogma tism." A N T O N I O R. G A S S E T ,
Gainesville,
M.D.
Florida
BOOK REVIEWS Neuro-ophthalmology. By Joel S. Glaser. Hagerstown, Maryland, Harper and Row, Inc., 1978. Hardcover, 364 pages, table of contents, index, 220 black and white figures, 3 color plates. $35 This textbook was published previous ly as a section in "Clinical Ophthalmolo gy," the superb five-volume loose-leaf series edited by Thomas D. Duane. In revising the original section from "Clini cal Ophthalmology," a separate index was added and each chapter now begins with a short outline. Joel Glaser wrote the first eight of the 15 chapters of the text. In Chapters 1 to 3,
SEPTEMBER, 1978
he discusses neuro-ophthalmologic histo ry and physical examination. In Chapter 4, he studies the anatomy of the visual sensory system. In Chapters 5 through 7, Glaser discusses the prechiasmal, chiasmal, and retrochiasmal visual pathways, respectively. In Chapter 8, he reviews the pupil and accommodation. Robert Daroff, Louis Dell'Osso, and Todd Troost were responsible for Chapters 9 to 11, which comprise a section on the ocular motor system. These chapters contain many bioengineering terms and the authors have provided a separate glossary for this section. In Chapters 9 to 11, the au thors discuss eye movement characteris tics and recording systems, supranuclear disorders of eye movements, and nys tagmus and related ocular oscillations. Joel Glaser wrote Chapters 12 and 13 concerning infranuclear disorders of eye movements and congenital anomalies of innervation. Todd Troost discusses mi graines and related conditions. Glaser and Troost are co-authors of Chapter 15, which reviews aneurysms, arteriovenous communications, and related vascular malformation. The text provides a concise survey of neuro-ophthalmologic topics. The refer ences at the end of each chapter are exten sive and up-to-date. The index is 12 pages long and is well organized. The photo graphs are generally of high quality. One disappointing feature of the text is the cursory discussion of computed tomogra phy. This noninvasive radiologic tech nique has become an important diagnos tic tool and could have been more fully covered. No illustrations of computed to mography scans appear in the text. Although this book is not as compre hensive as "Clinical Neuro-ophthalmo logy" by Walsh and Hoyt, it is current, concise, well written, and can be highly recommended to the resident in training as well as the practicing ophthalmologist. WALTER JAY
CORRESPONDENCE
onset of a clinical condition (keratoco nus). The authors contrast these findings with their observation that 27% of known keratoconus patients had a prior history of hard contact lens wear. This is a retro spective anlaysis, since it begins instead with patients already having the clinical end point (keratoconus), and determines the proportion with the potential risk fac tor (hard contact lens wear). Retrospective and prospective analyses obviously mea sure entirely different things and mixing the two is like comparing "apples and pears." There is also a problem with the au thors' choice of controls. They assure us the refractive error of the two groups was comparable. Although this may have been true for their spherical equivalents, it seems unlikely that the proportion of cases with irregular or high degrees of astigmatism was similar, since these con ditions require the choice of hard contact lenses. Presumably early in keratoconus, before the diagnosis is clear, a significant proportion of cases will suffer irregular or high degrees of astigmatism. A popula tion of hard contact lens wearers will therefore automatically contain a larger proportion of future keratoconus patients than a population of soft lens users. A L F R E D SOMMER,
Bandung,
M.D.
Indonesia
Reply Editor: In his letter, Dr. Sommer deals with two factors: (1) the type of epidemiology study he thinks our study is, and (2) what he thinks keratoconus is. The special or particular contribution of epidemiology lies in the study of the determinants of disease. This involves an interpretation of the distribution of dis ease in terms of possible causal factors. The use of epidemiologic methods in this manner to identify causal association is
443
sometimes called analytic epidemiology. There are three main groups of epidemio logic studies: (1) descriptive studies, (2) analytic studies, and (3) experimental studies. Analytic studies specifically de signed to examine a particular hypothesis, that is, the role of hard contact lens wear in the development of keratoconus, were more suitable for our study. The analytic studies can also be divided into two main groups, the case history study and the cohort study. We used the case history investigation in our study for the follow ing reasons: (1) One begins with a fixed number of cases and there needs to be relatively little concern for low incidence of the disease. For example, if one consid ers an average incidence of keratoconus of 1 in 10,000, we would have had to study 1,620,000 cases to obtain a sample of the same size as the one we used in our paper. (2) Age of onset: It is generally accepted that the average age of onset of keratoconus is 18 years. Therefore, if our consultand, that is, if the person whose genotype we are evaluating is still unaf fected at 20, 25, or 30 years old, the greater the probability that he did not inherit the biologic genetic factor pre sumably necessary for the development of keratoconus. Dr. Sommer's dogmatism could be caused by his confusion about keratoconus. Although it is relatively easy to find a cause in an iatrogenic disease with a simple unitary etiology, keratoco nus results from the interplay between multiple biological (genetic) and environ mental factors. Even when the environ mental contribution is considered apart from the inheritance as it was in our paper, multiple interaction between host factors and environmental factors can be expected to be responsible for the devel opment of events leading to the inception and progression of the disease. Dr. Sommer's difficulties in diagnosing keratoconus are his own. Before the diag-
444
AMERICAN JOURNAL OF OPHTHAMOLOGY
nosis of keratoconus can be made, it has to be suspected. The progress of myopia in keratoconus is not axial but curvature myopia, which can be objectively evalu ated with the keratometer. We conducted our study by using the analytic investigation, case history study. However, the lack of a better control group prevented us from even attempting to use statistics to prove our hypothesis. Therefore, a causal relationship was never proposed; instead, we formulated a hypothesis of circumstantial association between hard contact lens wear and the development of keratoconus. A review of the published reports shows that similar epidemiologic studies have made enor mous contributions to medicine: aniline dye and carcinoma of the bladder, asbes tos and lung cancer, fluoridation and den tal cavities, oxygen and retrolental fibroplasia, to name a few examples. Finally, as Sir William Osier said: "The greater the ignorance, the greater the dogma tism." A N T O N I O R. G A S S E T ,
Gainesville,
M.D.
Florida
BOOK REVIEWS Neuro-ophthalmology. By Joel S. Glaser. Hagerstown, Maryland, Harper and Row, Inc., 1978. Hardcover, 364 pages, table of contents, index, 220 black and white figures, 3 color plates. $35 This textbook was published previous ly as a section in "Clinical Ophthalmolo gy," the superb five-volume loose-leaf series edited by Thomas D. Duane. In revising the original section from "Clini cal Ophthalmology," a separate index was added and each chapter now begins with a short outline. Joel Glaser wrote the first eight of the 15 chapters of the text. In Chapters 1 to 3,
SEPTEMBER, 1978
he discusses neuro-ophthalmologic histo ry and physical examination. In Chapter 4, he studies the anatomy of the visual sensory system. In Chapters 5 through 7, Glaser discusses the prechiasmal, chiasmal, and retrochiasmal visual pathways, respectively. In Chapter 8, he reviews the pupil and accommodation. Robert Daroff, Louis Dell'Osso, and Todd Troost were responsible for Chapters 9 to 11, which comprise a section on the ocular motor system. These chapters contain many bioengineering terms and the authors have provided a separate glossary for this section. In Chapters 9 to 11, the au thors discuss eye movement characteris tics and recording systems, supranuclear disorders of eye movements, and nys tagmus and related ocular oscillations. Joel Glaser wrote Chapters 12 and 13 concerning infranuclear disorders of eye movements and congenital anomalies of innervation. Todd Troost discusses mi graines and related conditions. Glaser and Troost are co-authors of Chapter 15, which reviews aneurysms, arteriovenous communications, and related vascular malformation. The text provides a concise survey of neuro-ophthalmologic topics. The refer ences at the end of each chapter are exten sive and up-to-date. The index is 12 pages long and is well organized. The photo graphs are generally of high quality. One disappointing feature of the text is the cursory discussion of computed tomogra phy. This noninvasive radiologic tech nique has become an important diagnos tic tool and could have been more fully covered. No illustrations of computed to mography scans appear in the text. Although this book is not as compre hensive as "Clinical Neuro-ophthalmo logy" by Walsh and Hoyt, it is current, concise, well written, and can be highly recommended to the resident in training as well as the practicing ophthalmologist. WALTER JAY