Unexplained unilateral vision loss during centrifugation.

Vision loss during a centrifuge run is an expected occurrence given the G-profile, physical fitness of the subject, expected visual endpoint [central light loss (CLL) or peripheral light loss (PLL)] of the experimental protocol, and the cyclic nature of the anti-G straining maneuver (AGSM). During a relatively low level G exposure, a subject experienced a unilateral loss of vision that did not resolve spontaneously upon removal of the G load. An extensive medical workup did not reveal any medical explanation for the vision loss.

Determination of corneal image-forming properties from corneal topography.

Keratometry provides useful information about the cornea's image-forming properties, such as corneal astigmatism, but is inaccurate on irregular corneas. Quantitative corneal topographic information is now obtainable on irregular corneas, but is difficult for the clinician to interpret. We developed a method to determine the spherical power, astigmatism, and topographic irregularity of a cornea by finding the best-fit spherocylinder that was closest to its measured topography. Keratometric measurements and two videokeratographs were gathered prospectively on 262 normal and abnormal corneas. The best-fit measurements of spherical power, astigmatism, and topographic irregularity were reproducible with one standard deviation of 0.75 diopter or better; agreement with keratometric measurements in normal eyes was good (0.60 diopter or better). Topographic irregularity averaged 0.1 diopter on precision spheres, 0.4 diopter on 146 normal eyes, 0.8 diopter on 29 eyes after radial keratotomy, 2.0 diopters on 58 eyes after penetrating keratoplasty, and 3.0 diopters on 29 eyes with advanced keratoconus. We conclude the following: basic corneal image-forming properties can be measured from videokeratographs; the properties can be determined, by our methods, on irregular corneas in which keratometry is unreliable; and topographic irregularity provides a measure of irregular astigmatism.

Computer-assisted videokeratography of corneal topography after radial keratotomy.

We used computer-assisted videokeratography to compare the topographies of 32 corneas from 23 subjects after radial keratotomy with those of 47 normal corneas from 47 subjects controlled for age and preoperative keratometric and refractive power. Three ophthalmologists independently classified color-coded videokeratographs based on the color-coded pattern of dioptric power distribution and the cross-sectional shape. Corneas that had radial keratotomy exhibited a polygonal pattern not seen in normal eyes; this occurred in 59% of corneas. All normal corneas demonstrated a cross-sectional shape configuration that was steeper centrally than peripherally; 79% of corneas after radial keratotomy had a shape that was flatter centrally than peripherally. After radial keratotomy, the dioptric power increased from the center to the periphery (radius of approximately 4.6 mm) by 2.8 +/- 2.2 diopters (mean +/- SD), with a sharp inflection zone ("paracentral knee") 2.7 mm from the center; normal corneas showed a smooth decrease in power from the center to the periphery of 1.9 +/- 0.5 diopters.

Classification of normal corneal topography based on computer-assisted videokeratography.

We evaluated the topography of 399 normal corneas in 212 subjects with computer-assisted videokeratography. The mean subject age was 37 years (range, 8 to 79 years). Mean spherical equivalent refraction was -1.00 diopters (range, +5.50 to -8.37 diopters). A qualitative classification system for corneal topography was derived based on patterns seen on color-coded topographic maps. Corneas were classified into groups by three independent masked ophthalmologists based on this system. Patterns included round (22.6%), oval (20.8%), symmetric bow tie (17.5%), asymmetric bow tie (32.1%), and irregular (7.1%). All corneas were steeper centrally and flatter peripherally. There was a statistically significant difference among patterns for keratometric astigmatism, but not for spherical equivalent refraction, mean keratometric power, or age of subject. Classification of normal corneal topography is an important step in the process of characterizing the shape of normal and pathologic corneas.