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Biomedical subjects

K Zadnik

Publications and source records attributed to K Zadnik.

At least 73 records · Page 4Linked to original sources
PubMed1996

Crystalline lens parameters in infancy.

Despite the importance of lens power to ocular development, few data are available regarding infant crystalline lens parameters. Lens and corneal radii of curvature were measured in the horizontal meridian using a video-based keratophakometer, and refractive error was measured by cycloplegic retinoscopy in 19 out of 27 infants ranging in age from 3 to 18 months. The median refractive error was +1.50 D, and the median corneal power was 43.5 D. Using previously reported values for axial ocular dimensions, the median anterior and posterior lens radii of curvature were 8.7 and 5.6 mm, respectively, both substantially flatter than infant schematic eye values. The median equivalent refractive index of the lens was 1.49, considerably higher than previous reported schematic values for infants or children. There was a significant reduction in hyperopia with age (r = -0.47, P = 0.043), but no age-related trends in lens or corneal radii of curvature, suggesting that calculated values for lens power and equivalent index may undergo substantial decline with age during early childhood development as axial length increases. Most of the decrease in lens power (75%) may be due to decreases in equivalent index rather than to flattening of the surface radii of curvature. Videophakometry appears to be a feasible and useful technique for documenting the role of the crystalline lens in infant ocular development.

Anthropometry↗
PubMed1996

Is computer use a risk factor for myopia?

BACKGROUND: Many patients who become myopic or who undergo increases in myopia as adults have concerns about whether the use of video display terminals (VDTs) contributes to these changes in refractive error. This paper is an overview of the current literature on topics concerning VDTs and factors related to proposed etiologies for myopia. METHODS: Selected literature is reviewed on the relationship between VDTs and asthenopia, fatigue, accommodation, and vergence. Clinical studies of myopic progression and transient myopia among VDT users are considered, as is television viewing as a risk factor for juvenile myopia. RESULTS/CONCLUSIONS: Reports of asthenopia are common with VDT use by a factor of 1.4 to 1.5, compared to conventional office work. Questions of comparability remain between VDT users and nonusers with respect to confounding variables such as the number of work hours. Proofreading on a VDT appears to be less efficient than using printed copy. Despite screen flicker and reflections, the accommodative response appears to be accurate to a VDT. Transient, fatigue-induced changes in accommodation and vergence may occur after work with VDTs. Despite these near point changes, there is no compelling evidence in the literature that suggests there is a significant increase in the risk of myopia onset or progression from the use of VDTs by adults compared to other forms of near work.

Accommodation, Ocular↗
PubMed1996

Factors influencing graft clarity.

Successful penetrating keratoplasty depends on both host- and donor-related factors. We compared the results of keratoplasty in a group with a wide range of donor ages to gauge the effect of donor age on graft success. We conducted a retrospective review of donor data, recipient age, preoperative diagnosis, and postoperative complications with respect to graft clarity at 24 months after surgery in 99 consecutive patients in an effort to determine the role of donor age in graft clarity. The only factors that we isolated that appeared to influence graft clarity at 24 months postoperatively were a preoperative diagnosis classified as inflammatory/traumatic and the occurrence of postoperative complications. There was no association between graft clarity and recipient age, donor age, death-to-preservation time, or preservation-to-surgery time.

Age Factors↗
PubMed1995

The utility of three predictors of childhood myopia: a Bayesian analysis.

Any treatment to prevent the onset of juvenile myopia will require predictive tests in order to determine which children should receive treatment. Three risk factors for myopia were evaluated for their ability to predict myopia: (a) refraction at school entry; (b) refraction in infancy; and (c) parental history of myopia. Bayes' theorem was used to estimate these conditional probabilities. Refraction at school entry had twice the power to predict myopia (probability of juvenile myopia given the child is near emmetropia at school entry = 0.53) compared to either infant refraction (0.21-0.28) or parental myopia (0.20-0.25). While a history of any parent having myopia had the highest test sensitivity (probability of a positive family history of myopia given juvenile myopia in the child = 0.90) and refraction at school entry the highest test specificity (probability of more hyperopia than +0.50 D at school entry given no juvenile myopia = 0.91), none of these three factors had high values for both sensitivity and specificity. Further work is required to develop a battery of tests which could predict the onset of juvenile myopia with both adequate sensitivity and specificity.

Age Factors↗
PubMed1995

How applicable are animal myopia models to human juvenile onset myopia?

Investigations into the plasticity of eye growth and refractive error development have significantly expanded our knowledge of animal models of myopia in the last 15 yr. The applicability of this information is as yet undetermined, but hopefully this information will be useful in learning more about human myopia. This paper presents a critical review of the animal myopia literature as those data relate to the human condition. Differences between the chicken, tree shrew, and primate animal models of myopia are outlined, and the various experimental paradigms used to investigate refractive error development and ocular growth in the chicken are compared. Specific arguments against the application of animal models of myopia to the etiology of human juvenile onset myopia include the following: (1) there is no deprivation of form vision in the environment of the school-aged child as severe as that required to induce myopia in animals; (2) the sensitive period for deprivation myopia in animals appears to be too early to account for human juvenile onset myopia; and (3) studies in the chicken using spectacle lenses to create dioptric blur involve a choroidal thickness modulation that has no human analog. Ultimately, the results of investigations into the cellular and biochemical modulation of eye growth in animals may be the most relevant to human myopia.

Animals↗
PubMed1995

The equivalent refractive index of the crystalline lens in childhood.

Despite the importance of crystalline lens power in ocular development, schematic refractive index values used to calculate lens power have been validated for children. We measured refractive error and ocular component dimensions in 519 schoolchildren, calculating lens power using phakometrically measured lens radii and three different refractive index profiles: (1) Gullstrand-Emsley schematic indices [Gullstrand-Emsley lens power (GELP)]; (2) a 10-shell gradient index model [gradient index lens power (GILP)]; and (3) the equivalent refractive index (IND) needed to bring calculated and measured refractive error into agreement [calculated lens power (CLP)]. GELP was significantly lower than either GILP or CLP, indicating the Gullstrand-Emsley refractive index of 1.416 is too low for use in children. Variation in IND cannot be explained by measurement error alone. GILP and CLP also differed as a function of lens shape, with GILP greater than CLP at steeper external curvatures and less than CLP at flatter external curvatures. Variation in equatorial gradient index profile as a function of lens shape is proposed as an explanation for this bias. Equivalent index appears to be a useful tool for encompassing individual variation in lens gradient profiles as well as for assessing the relative role of lens surface curvature and refractive index changes during lens power development in childhood.

Adolescent↗
PubMed1995

Longitudinal evidence of crystalline lens thinning in children.

PURPOSE: Most earlier studies indicated that the eye's crystalline lens grows continually throughout life, but cross-sectional results of crystalline lens thinning during childhood have been reported. The authors investigated crystalline lens thickness in childhood using cross-sectional and longitudinal data. METHODS: The Orinda Longitudinal Study of Myopia is a community-based study of normal eye growth and myopia development in school-age children. During a 1-to 3-year period, A-scan ultrasonographic lens thickness measurements of 869 children 6 through 14 years of age were analyzed. RESULTS: On average, between the ages of 6 and 10 years, the crystalline lens thins in its axial dimension by almost 0.2 mm. This thinning can be depicted by a cubic model. In this sample, the children with myopia had thinner crystalline lenses than the children with emmetropia of the same age. CONCLUSIONS: This article provides the first longitudinal evidence that the crystalline lens thins during the period of coordinated ocular growth between the ages of 6 and 10 years. Further, it shows that lens thickness is associated with refractive error. Thinner crystalline lenses in children with myopia may result from one of two underlying mechanisms: Either the crystalline lens exhausts its ability to compensate for axial elongation after undergoing accelerated lens thinning before the onset of myopia, or the crystalline lens in the myopic eye may be thinner throughout childhood, during which it thins at a rate consistent with other refractive errors. If mechanical forces link eye growth to crystalline lens compensation, more complex, visually guided feedback loops may not be needed to explain the normal eye growth that results in emmetropization.

Adolescent↗
PubMed1995

Keratoconus.

Keratoconus is characterized by the presence of corneal distortion (secondary to thinning of the apex) and either Fleischer's ring (found in 57% of patients) or Vogt's striae (found in 44% of patients). Often the only presenting symptom is decreased visual acuity. Steep keratometric readings are not usually diagnostic; 18% of patients with keratoconus have readings flatter than 45.00 D. Treatment requires correction of acuity with spectacles or contact lenses. More advanced cases are generally managed with rigid gas-permeable contact lenses. Corneal scarring occurs in approximately 43% of cases; 10% to 20% of patients require penetrating keratoplasty to restore acuity.

Contact Lenses↗
PubMed1994

The effect of parental history of myopia on children's eye size.

OBJECTIVE: To evaluate whether eye size and shape are different in children based on their parental history of myopia. DESIGN: A community-based cohort study of schoolchildren (aged 6 to 14 years), the Orinda (Calif) Longitudinal Study of Myopia. SETTING: Four campuses of the Orinda Union School District, a predominantly white, high socioeconomic status community. PARTICIPANTS: A cross-sectional volunteer sample of 716 children (662 non-myopic) in the first, third, and sixth grades in 1989, 1990, and 1991. All children in those grades were eligible for inclusion in the study. INTERVENTION(S): None. MAIN OUTCOME MEASURES: Refractive error (measured by autorefraction), corneal curvature (measured by photokeratoscopy), crystalline lens power (measured by video phakometry), and axial ocular dimensions (measured by ultrasonography). RESULTS: With prevalent cases of myopia excluded and grade in school and "near work" controlled for, children with two myopic parents had longer eyes and less hyperopic refractive error (analysis of covariance, P < or = .01) than children with only one myopic parent or no myopic parents. A model incorporating parental history is only improved by the addition of near work for the prediction of refractive error. CONCLUSIONS: Even before the onset of juvenile myopia, children of myopic parents have longer eyes. These results suggest that the premyopic eye in children with a family history of myopia already resembles the elongated eye present in myopia.

Adolescent↗
PubMed1994

Critically reviewing the ophthalmic literature.

In the current era, where optometry's scope and very definition are evolving, the practitioner's ability to evaluate critically the profession's clinical and basic science literature is crucial. This paper reviews the components of a scientific paper both in theory and by specific example. Emphasis is placed on reading the literature without accepting it at face value, including the description of the study designs frequently encountered in clinical research, the features of a study design that make it appropriate or inappropriate to answer the central hypothesis, and a brief introduction to the common pitfalls of routinely used statistical analysis.

Humans↗
PubMed1994

The effect of cycloplegia on measurement of the ocular components.

PURPOSE: The purpose of this study was to examine the effect of cycloplegic agent on the measurement of refractive error and the ocular components. METHODS: We compared two commonly used topical cycloplegic agents, 1% tropicamide and 1% cyclopentolate, for their effect on the measurement of refractive error (by Canon R-1 autorefraction), accommodative response (by Canon R-1 autorefraction and by the conventional, subjective "pushup" method), crystalline lens power (by video phakometry and by calculation), and axial ocular dimensions (by A-scan ultrasonography) in 20 emmetropic to moderately hyperopic children. RESULTS: Comparison of refractive error at each drug's reported time of maximum cycloplegia (30 minutes for tropicamide and 60 minutes for cyclopentolate) showed that distance autorefraction in the vertical meridian differed by +0.20 +/- 0.30 diopters (D) (P = 0.008). The average difference was +0.07 +/- 0.10 mm for anterior chamber depth (P = 0.004), -0.03 +/- 0.05 mm for crystalline lens thickness (P = 0.025), -0.65 +/- 0.69 D for phakometrically measured crystalline lens power (P < 0.001), +0.03 +/- 1.55 D for calculated crystalline lens power (P = 0.94), and -0.09 +/- 0.19 mm for vitreous chamber depth (P = 0.062, all paired t tests; positive signs denote greater values with cyclopentolate). Residual accommodation was 0.47 and 0.67 D greater with tropicamide when measured by autorefraction and the pushup method (P = 0.013 and 0.08 respectively, paired t test). All significant differences were consistently in the direction of poorer cycloplegia with tropicamide. CONCLUSIONS: Although tropicamide, as expected, showed poorer cycloplegia compared to cyclopentolate, the degree of difference appeared to be small, with minimal effect on the measurement of distance refractive error and the ocular optical components.

Accommodation, Ocular↗
PubMed1994

Contact lenses in the geriatric patient.

Contact lenses in the elderly patient are often prescribed for visual correction and/or corneal rehabilitation. The success of contact lenses in the geriatric patient is often limited by such factors as abnormal adnexal physiology, decreased manual dexterity, high degrees of refractive error, and low vision. Assessment of these factors prior to contact lens fitting can greatly increase the success of these unique contact lens applications. Indications and contraindications for contact lenses in the elderly, the need for special lens handling and care by family members, and available specialty lenses will be discussed.

Aged↗
PubMed1993

Initial cross-sectional results from the Orinda Longitudinal Study of Myopia.

BACKGROUND: Although investigations of human refractive error development and normal ocular growth have been conducted for the last 50 years, no previous study of refractive error and the ocular components has measured all the ocular components. METHODS: The Orinda Longitudinal Study of Myopia was initiated to characterize the development of refractive error and normal eye growth in a sample of predominantly Caucasian children ages 6 to 14 years. RESULTS: Cross-sectional results from 530 children ages 5 to 12 years in the 1st, 3rd, and 6th grades are presented. CONCLUSIONS: This sample's refractive error decreased toward emmetropia with age from an average of +0.73 D at age 6 years to an average of +0.50 D by age 12 years. Between the ages of 6 and 12 years, the vitreous chamber elongated (by 0.52 mm) and the crystalline lens power decreased (by 1.35 D); surprisingly, the crystalline lens thinned by 0.14 mm during this same time period.

California↗