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Biomedical subjects

K Zadnik

Publications and source records attributed to K Zadnik.

100 records · Page 6Linked to original sources
PubMed1984

An analysis of contrast sensitivity in identical twins with keratoconus.

We studied a pair of 36-year-old identical twins with keratoconus, one of whom had undergone bilateral penetrating keratoplasties. Both patients had final best corrected Snellen visual acuity of approximately 20/20. However, contrast sensitivity functions for the unoperated twin were abnormal when compared to the operated twin. In addition, contrast sensitivity data for the operated twin during and after a graft rejection indicate that corneal graft opacity and distortion can significantly alter the contrast sensitivity function. The unusual occurrence of identical twins with keratoconus afforded the unique opportunity to explore the visual and psychophysical aspects of keratoconus in a genetically controlled setting.

Adult↗
PubMedDate not supplied

Refracting the corneal graft.

Patients who have undergone penetrating keratoplasty have unique postoperative refractive requirements. These include irregular astigmatism, high corneal toricity, and anisometropia. We present a stepwise clinical approach for accurate, efficient refraction of these patients, utilizing visual acuity measurement, retinoscopy, subjective refraction, keratometry, photokeratoscopy, and diagnostic rigid contact lens application.

Humans↗
PubMedDate not supplied

Repeatability of corneal topography: the "corneal field".

BACKGROUND: The purpose of this study was to determine the interoccasion repeatability of keratometry, photokeratography, and videokeratography and to describe the "corneal field," a scheme for explaining videokeratography results. METHODS: A single examiner obtained corneal curvature measurements with a keratometer, a photokeratoscope, and the TMS-1 in 29 adult patients on two occasions. RESULTS: Estimates for the repeatability of keratometry were +/- 0.49 and +/- 0.65 diopters (D) for the horizontal and vertical meridians respectively. The repeatability of photokeratography was +/- 0.90 and +/- 1.21 D. We presented a rational method for presenting numeric videokeratographic data by temporally and spatially averaging corneal curvature values and grouping them into 24 regions. The repeatability of videokeratography with the TMS-1 was +/- 0.50 D centrally, +/- 0.65 D paracentrally, and +/- 0.80 to +/- 1.00 D in the midperiphery. CONCLUSIONS: Repeatability of the photokeratoscope for central measurements is considerably worse than the keratometer. The repeatability of videokeratography is worse toward the periphery. Refractive surgeons and contact lens investigators need to be aware of these limitations so that true change can be distinguished from measurement error.

Adult↗
PubMedDate not supplied

Quantifying corneal toricity from videokeratography with Fourier analysis.

BACKGROUND: Fourier analysis can be used to quantify corneal toricity from videokeratography data. This study measures the repeatability of Fourier-derived toricity values in normal adult corneas. METHODS: The Topographic Modeling System (TMS) was used to model the corneas of 29 subjects on two occasions, and Fourier analysis applied to the data. Repeatability of Fourier-derived toricity values between sessions was examined, and 95% limits of agreement were established. RESULTS: The 95% limits of agreement between sessions for the Fourier method were -0.02 +/- 0.16 diopters (D) and -0.5 +/- 5.7 degrees for toricity amount and axis, respectively. CONCLUSIONS: The Fourier method yields highly repeatable toricity values and thus provides a sensitive means of detecting longitudinal toricity change in normal corneas.

Adult↗
PubMedDate not supplied

Photodocumentation of corneal scarring. Collaborative Longitudinal Evaluation of Keratoconus Study Group.

PURPOSE: A standardized system for photodocumenting corneal scars and for evaluating these photographs does not exist and is essential for clinical research. To address this need, we developed a system for photographing and evaluating corneal scars. METHODS: In the "Development Phase," we tested several procedures in small samples totaling 40 eyes of 20 keratoconus patients. In the "Test Phase," we used an independent sample of 150 eyes of 82 keratoconus patients. Fifty-nine of these 150 eyes had corneal scars, and 91 of the eyes did not as determined by the clinician. RESULTS: The photography protocol requires four central parallelepiped and two whole cornea oblique photographs after pupil dilation. With the clinician as gold standard, this technique yielded sensitivity of 96% and specificity of 83%. Evaluation of the corneal photographs as to the presence or absence of corneal scarring was performed independently by two masked readers. Agreement between clinicians on the presence of corneal scarring was 0.99 (kappa); agreement between readers for the presence of scarring was 0.80 (kappa). CONCLUSIONS: The corneal photography protocol we describe is recommended for use in clinical investigations of cicatricizing corneal disease and appears robust enough to be used in multicenter studies.

Corneal Diseases↗
PubMedDate not supplied

Repeatability and validity of astigmatism measurements.

BACKGROUND: As more patients inquire about refractive surgical procedures, the measurement of astigmatism prior to surgery becomes more important in assessing refractive outcome. Knowledge of the repeatability of the astigmatism measurement allows one to distinguish a true change in cylinder power from measurement error. METHODS: Forty adults with structurally normal eyes and refractive errors were evaluated for the repeatability of astigmatic refractive error measures. Noncycloplegic and cycloplegic measurements of refractive astigmatism were made by retinoscopy, subjective refraction, and autorefraction. All measures were made at 2 visits within 2 weeks by the same examiner. Difference versus mean plots and the 95% limits of agreement of each technique determined the repeatability of a measurement and the agreement between the methods of measurement. RESULTS: The most reliable measure of astigmatic refractive error was cycloplegic autorefraction, with 95% limits of agreement of +/- 0.28 D, followed by noncycloplegic autorefraction (+/- 0.35 D) and cycloplegic subjective refraction (+/- 0.44 D). Noncycloplegic retinoscopy was the least reliable astigmatic refractive error measure, with interoccasion 95% limits of agreement of +/- 1.02 D. The most repeatable measurement of cylinder axis was cycloplegic autorefraction; none of the measurements differed by 10 degrees or more. The least repeatable measurement was noncycloplegic retinoscopy; 40% of the measurements differed by 10 degrees or more. CONCLUSION: For studies seeking to measure changes in astigmatism in normal eyes, cycloplegic autorefraction is the method of choice.

Adult↗
PubMedDate not supplied

Hydrophilic contact lenses for wound stabilization in keratoplasty.

Hydrophilic contact lenses may be used in selected cases in which penetrating keratoplasty is complicated by partial wound dehiscence and anterior slippage of the donor tissue. We report five cases and describe a technique in which, rather than resuturing, a soft contact lens is used as a corneal stent to realign the graft-host interface. This technique allows for non-surgical correction of a potentially serious complication, with preservation of good vision during the treatment period. In the present series, no serious complications were encountered, and all wounds were adequately reapposed. Successful application of this technique requires appropriate patient selection and careful monitoring during the course of treatment.

Adult↗