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Biomedical subjects

S D Klyce

Publications and source records attributed to S D Klyce.

At least 19 recordsLinked to original sources
PubMed1995

Comparison of methods for detecting keratoconus using videokeratography.

BACKGROUND: The detection of keratoconus patterns on videokeratography is important for screening candidates for refractive surgery and for studying the genetic basis of keratoconus. OBJECTIVE: We compared three quantitative approaches to identifying keratoconus from videokeratographic information to examine the limitations and capabilities of each test and to determine their suitability for use in the clinical setting. METHODS: Videokeratographs typical of clinically diagnosed keratoconus (n = 44) and of various non-keratoconus conditions (n = 132, including normal, with-the-rule astigmatism, contact lens-induced corneal warpage, photorefractive keratectomy, keratoplasty, and pellucid marginal degeneration) were selected. Three methods for detecting keratoconus were used: keratometry (average Simulated Keratometry [SimK] readings > 45.7 diopters [D]); the modified Rabinowitz-McDonnell test (central corneal power > 47.2 D and/or Inferosuperior Asymmetry [I-S] value > 1.4 D); and an expert system classifier (classification based on discriminant analysis and classification tree with eight topographic indexes). Sensitivity and specificity were calculated for each test. RESULTS: Sensitivities were 84% for keratometry, 96% for the modified Rabinowitz-McDonnell test, and 98% for the expert system classifier. Specificities for the three methods were 86%, 85%, and 99%, respectively. In terms of sensitivity, the expert system classifier was significantly better than keratometry (P = .04). In terms of specificity, the expert system classifier was significantly better than either of the other methods (P = .001). CONCLUSIONS: For screening candidates for refractive surgery, where high sensitivity is needed, either the modified Rabinowitz-McDonnell test or the expert system classifier is suitable. For diagnosing keratoconus, where high specificity is more useful, the expert system classifier is more appropriate than the other two methods.

Cornea↗
PubMed1995

Neural network classification of corneal topography. Preliminary demonstration.

PURPOSE: Videokeratography is a powerful tool for the diagnosis of corneal shape abnormalities. However, interpretation of the topographic map is sometimes difficult, especially when pathologies with similar topographic patterns are suspected. The neural networks model, an artificial intelligence approach, was applied for automated pattern interpretation in corneal topography, and its usefulness was assessed. METHODS: One hundred eighty-three topographic maps were selected and classified by human experts into seven categories: normal, with-the-rule astigmatism, keratoconus (mild, moderate, advanced), postphotorefractive keratectomy, and postkeratoplasty. The maps were divided into a training set (108 maps) and a test set (75 maps). For each map, 11 topography-characterizing indices calculated from the data provided by the TMS-1 videokeratoscope, plus the corresponding diagnosis category, were used to train a neural network. RESULTS: The correct classification was achieved by a trained neural network for all 108 maps in the training set. In the test set, the neural network correctly classified 60 of 75 maps (80%). For every category, accuracy and specificity were greater than 90%, whereas sensitivity ranged from 44% to 100%. CONCLUSIONS: With further testing and refinement, the neural networks paradigm for computer-assisted interpretation or objective classification of videokeratography may become a useful tool to aid the clinician in the diagnosis of corneal topographic abnormalities.

Cornea↗
PubMed1994

Screening for corneal topographic abnormalities before refractive surgery.

PURPOSE: The purpose of this prospective study is to evaluate the corneal topography of patients who sought an opinion regarding refractive surgery for the correction of myopia. METHODS: Both eyes of 53 patients were evaluated with a topographic modeling system. Forty-two patients wore contact lenses (84 eyes: 36 rigid contact lenses and 48 soft contact lenses). Ten patients (20 eyes) wore glasses alone and one patient (2 eyes) wore neither glasses nor contact lenses for correction of myopia. RESULTS: Thirty-five (33%) of 106 eyes were found to have abnormal corneal topography. Of the 42 patients (84 eyes) who wore contact lenses, 32 eyes (38%) had irregular astigmatism, loss of radial symmetry, or absence of the normal progressive flattening from the center to the periphery of the cornea, consistent with contact lens-induced corneal warpage. Alterations were more frequent and severe in rigid contact lens wearers. Three patients (5.7%) received a diagnosis of definite keratoconus, a higher incidence than has been reported in the general population. Topographic abnormalities in most, if not all, of the eyes would not have been detected by visual inspection of the photokeratoscopic images alone. CONCLUSIONS: Appropriate preoperative detection and management of corneal topographic abnormalities are essential steps in every refractive surgical procedure. The overall efficacy and safety of procedures such as radial keratotomy and photorefractive keratectomy likely will be improved once the unpredictable variables of contact lens-induced warpage and occult ectatic disease are eliminated by topographic screening before surgery. Patients with keratoconus may be over-represented in the refractive surgery population due to self-selection.

Contact Lenses↗
PubMed1994

Keratoconus and contact lens-induced corneal warpage analysis using the keratomorphic diagram.

PURPOSE: Videokeratography of early keratoconus may be difficult to distinguish from contact lens-induced corneal warpage, even by experienced examiners. Furthermore, topographic irregularity may be judged inconsistently if quantitative standards are not applied. Quantitative measures based on videokeratographic data were developed and evaluated to determine if improved corneal topographic classification can be achieved. METHODS: The Corneal Irregularity Coefficient (CIC) and Corneal Power Coefficient (CPC) were derived from multiple measures of mean corneal power and its variance for 207 videokeratographs of normal, warped, keratoconus, and keratoconus-suspect corneas. CIC was plotted against CPC, creating a distribution of points representing all maps that tended to be grouped according to surface conditions (the Keratomorphic Diagram). Normal, steep, abnormal, and warped zones were defined by CIC and CPC cutoff values chosen to distinguish normal from keratoconus corneas graphically. RESULTS: Seventy of 76 normal corneas were grouped in the normal zone and 6 in the steep zone; 84 of 84 keratoconus corneas were grouped in the abnormal zone; 35 of 35 contact lens-induced warpage cases were grouped in the warped zone; and 10 of 12 keratoconus-suspect corneas were grouped in the warped zone, with 2 in the abnormal zone. Serially plotted data of keratoconus progression and warpage regression demonstrated that the vector displacement of CIC and CPC values may provide a potentially useful means of distinguishing contact lens-induced warpage from keratoconus-suspect corneas. CONCLUSION: The Keratomorphic Diagram aids in classifying and comparing corneal shape by plotting indices along axes with easily recalled scales. The diagram may become a useful tool to assess presurgical corneal surface instability and postoperative progression of corneal shape change due to healing.

Contact Lenses↗
PubMed1994

Alteration of corneal asphericity in rigid gas permeable contact lens induced warpage.

We developed the corneal asphericity index (CAI), which indicates the asphericity of the central cornea using the TMS-1 videokeratoscope, and used the CAI to evaluate both normal corneas and corneas with rigid gas permeable (RGP) lens induced warpage. The CAI (mean +/- standard deviation) for the 22 control corneas was 0.33 +/- 0.26, which indicates that the normal central cornea has a prolate shape. The average CAI for the 24 corneas with RGP lens induced warpage was significantly lower (-0.15 +/- 0.36, P = 0.0001). These data suggest that some corneas have abnormal asphericity in the central cornea when warpage occurs with RGP lenses. CAI is useful for the quantitative assessment of asphericity and topographic abnormalities in the central cornea caused by contact lens induced warpage.

Adult↗
PubMed1994

Automated keratoconus screening with corneal topography analysis.

PURPOSE: Although visual inspection of corneal topography maps by trained experts can be powerful, this method is inherently subjective. Quantitative classification methods that can detect and classify abnormal topographic patterns would be useful. An automated system was developed to differentiate keratoconus patterns from other conditions using computer-assisted videokeratoscopy. METHODS: This system combined a classification tree with a linear discriminant function derived from discriminant analysis of eight indices obtained from TMS-1 videokeratoscope data. One hundred corneas with a variety of diagnoses (keratoconus, normal, keratoplasty, epikeratophakia, excimer laser photorefractive keratectomy, radical keratotomy, contact lens-induced warpage, and others) were used for training, and a validation set of 100 additional corneas was used to evaluate the results. RESULTS: In the training set, all 22 cases of clinically diagnosed keratoconus were detected with three-false-positive cases (sensitivity 100%, specificity 96%, and accuracy 97%). With the validation set, 25 out of 28 keratoconus cases were detected with one false-positive case, which was a transplanted cornea (sensitivity 89%, specificity 99%, and accuracy 96%). CONCLUSIONS: This system can be used as a screening procedure to distinguish clinical keratoconus from other corneal topographies. This quantitative classification method may also aid in refining the clinical interpretation of topographic maps.

Cornea↗
PubMed1993

Standardized color-coded maps for corneal topography.

PURPOSE: Modern videokeratoscopy is useful in assessing corneal shape. The purpose of this study is to compare color-coded topographic maps using standardized scales with 1.0- and 1.5-diopter (D) intervals. The authors assessed the use of the two scales for detecting clinically relevant features of corneal topography such as regular astigmatism, irregular astigmatism, early keratoconus, and contact lens-induced corneal warpage. METHODS: A total of 50 normal corneas, 50 corneas with contact lenses, 50 that had keratoconus (25 early to moderate and 25 advanced), 50 that had penetrating keratoplasty, 20 that had extracapsular cataract surgery, 17 that had excimer laser photorefractive keratectomy for myopia, 10 that had radial keratotomy, 3 that had aphakic epikeratophakia, and 2 that had myopic epikeratophakia were analyzed with a corneal topographic analysis system. Color-coded maps with 1.0-D intervals (Maguire/Waring scale) and 1.5-D intervals (Klyce/Wilson scale) were compared. RESULTS: There were no topographic characteristics that were not appreciated with either scale for corneas with dioptric powers that fell within their ranges. Conversely, for corneas that had powers outside the range of the 1.0-diopter scale, but within the range of the 1.5-diopter scale, the former produced a map in which the flattest or steepest areas were artifactually smoothed. CONCLUSIONS: This study suggests that the Klyce/Wilson scale (constant, 1.5-D intervals) provides the best combination of sensitivity for detection of clinically significant topographic features and the widest range of coverage of powers that are found on a variety of normal, pathologic, and surgically altered corneas. The adoption of the Klyce/Wilson scale by all manufacturers of corneal topographic instruments as a primary standard will facilitate communication and will make the interpretation of corneal topography easier for both the expert and the novice.

Cataract Extraction↗
PubMed1993

Corneal topographic alterations in normal contact lens wearers.

PURPOSE: The purpose of this study is to investigate the corneal topography of visually normal asymptomatic eyes that wore rigid and soft contact lenses compared with visually normal eyes that had never worn contact lenses. METHODS: Thirty-seven normal corneas and 74 corneas in asymptomatic eyes that wore rigid (12 polymethylmethacrylate and 23 gas-permeable) and soft (26 daily-wear and 13 extended-wear) contact lenses for refractive correction underwent slit-lamp examination, keratometry, computer-assisted topographic analysis, refraction, and rigid contact lens over-refraction. RESULTS: Topographic abnormalities tended to be more common and more severe in corneas that wore rigid contact lenses, but significant changes were noted in some eyes that wore daily-wear or extended-wear soft contact lenses. A number of eyes in the rigid polymethylmethacrylate (9 of 12) and rigid gas-permeable (6 of 23) contact lens groups had a correlation between the most frequent resting position of the contact lens and the corneal topography, with relative flattening of the corneal contour beneath a decentered lens. A total of 10 eyes in the rigid contact lens groups had a 1-line decrease in best spectacle-corrected visual acuity attributable to contact lens-induced topographic abnormalities. CONCLUSIONS: Corneal topographic alterations are common in asymptomatic contact lens wearers and are frequently detectable only with computer-assisted topographic analysis. It is important that topographic abnormalities be excluded in contact lens wearing eyes before refractive surgical procedures.

Contact Lenses↗
PubMed1992

Contact lens manipulation of corneal topography after penetrating keratoplasty: a preliminary study.

We conducted a preliminary retrospective study of five eyes that had undergone penetrating keratoplasty (PK) to determine if rigid gas permeable (RGP) contact lenses could be used as splints or molds to improve postoperative corneal topography. RGP lenses were fit 3-6 months (mean: 4.1 months) after PK and 3-4 months (mean: 1.8 months) after removal of the 10-0 nylon suture. Lenses were fit to attain a contact lens resting position centered on the corneal graft. After 4 to 8 months (mean: 6.2 months) of contact lens wear, analysis of corneal topography showed a decrease in simulated keratometry cylinder from 4.7 +/- 0.6 D (mean +/- 1 standard deviation) to 1.8 +/- 0.8 D (P = 0.02) and an improvement in anterior corneal surface regularity (surface regularity index: initial = 1.40 +/- 0.2; final = 1.0 +/- 0.2; P = 0.04). Three eyes were fit with a well-centered RGP lens; two eyes were fit with lenses that rode off-center. Increased irregular astigmatism in the two eyes with decentered contact lenses suggests the importance of centering RGP lenses with respect to corneal grafts. The best corrected spectacle visual acuity either improved or remained unchanged in four of five eyes after contact lens wear. One eye demonstrated a decrease in best corrected spectacle acuity by one Snellen line after 3 months of lens wear. Further clinical studies are needed to determine the potential of postoperative use of RGP lenses to improve corneal surface regularity, to decrease astigmatism, and to optimize visual results.

Astigmatism↗
PubMed1992

Physiological effects of tert-butyl hydroperoxide on the rabbit corneal epithelium.

Rabbit corneas were mounted atraumatically into an automatic voltage clamp apparatus for the determination of open circuit potential difference and short circuit current. The corneal tissue was bathed on both sides with a Ringer solution, continuously stirred and aerated. After a minimum 1 hour equilibration, a single dose of tert-butyl hydroperoxide (t-BHP) (0.1 mM to 2 mM) was administered to the tear-side solution. This led to an immediate fall in epithelial potential difference and short circuit current. The electrical conductance tended to be subnormal during the peak response. The initial decreases were partially reversed, but even after several hours the majority of corneas sustained abnormal levels of potential difference and conductance. Administration of 2 mM t-BHP to the tearside solution led to an immediate fall in epithelial thickness to -25% of baseline. This was followed by large-amplitude oscillations in corneal epithelial thickness which were sustained over several hours. We conclude that prolonged exposure of the perfused rabbit cornea to small tearside amounts of t-BHP are capable of modulating the electrophysiologic integrity of the corneal epithelium. T-BHP exceeding 1 mM in the tears could compromise the non-catalase dependent antioxidant defense mechanisms in rabbit corneal epithelium.

Animals↗
PubMed1991

Quantitative descriptors of corneal topography. A clinical study.

Quantitative descriptors of corneal topography determined by computerized algorithm and designed to augment the information derived from topographic maps were evaluated in a clinical study. The surface regularity index, a measure of central corneal optical quality, was highly correlated with best spectacle-corrected visual acuity (rho = 0.80, P less than .001). The relatively low correlation between the surface asymmetry index, a measure of central corneal asymmetry, and best spectacle-corrected visual acuity (rho = 0.62, P less than .005) suggests that corneal symmetry is a lesser but still important determinant of the optical performance of the anterior corneal surface. There was also a high correlation between factors determined from computer-generated orthogonal simulated keratometer values and clinical keratometry values (mean corneal power, total corneal cylinder, and the location of the steepest corneal meridian). The information derived from these algorithms has both clinical and research applications and can be incorporated into computer-assisted topographic analysis systems.

Algorithms↗
PubMed1991

Changes in corneal topography after excimer laser photorefractive keratectomy for myopia.

Computer-assisted analysis of corneal topography was performed in 17 normally sighted human eyes during the first year after excimer laser photorefractive keratectomy (PRK) for myopia. Laser ablation of the central cornea produced an optical zone with a smooth power transition to the peripheral cornea. Decentration of the ablation was noted in some eyes (less than 0.5 mm in 3 eyes, 0.5 to 1.0 mm in 10 eyes, 1 to 1.5 mm in 3 eyes, and 2.1 mm in 1 eye), suggesting that careful alignment of the laser beam is critical. Improved methods to align the ablation within the center of the entrance pupil are needed. In 12 of 17 eyes, the topographic pattern appeared to stabilize between 3 and 7 months after PRK. In the remaining five eyes, central ablation power changed by more than 0.5 diopters (D) between the 6- and 12-month examinations. Regression was more common and more pronounced in eyes with intended corrections more than 5 D, whereas the majority of eyes with intended corrections of 5 D or less showed good correspondence between the final change in central ablation power and the attempted correction. Two eyes had a loss of at least two lines of best spectacle-corrected visual acuity that was attributable to irregular astigmatism, decentration of the ablation, and/or corneal opacification.

Adult↗
PubMed1991

Central photorefractive keratectomy for myopia. Partially sighted and normally sighted eyes.

Ten partially sighted and 19 normally sighted eyes underwent excimer laser photorefractive keratectomy for the correction of myopia. Nine of the partially sighted and 17 of the normally sighted eyes had 12 months of follow-up. Epithelial healing was complete in all eyes by day 6. None of the eyes had recurrent erosions, infections, or other medical complications. An increase in corneal haze after surgery was followed by a slow trend toward clearing. Average uncorrected visual acuity in the 7 normally sighted eyes with attempted corrections of 5 diopters (D) or less was 20/40 from month 2 on; the eyes with greater than 5 D attempted corrections had an average of 20/80--at month 2, which declined to 20/200--by month 6. Best spectacle-corrected visual acuity was within +/- 1 Snellen line of preoperative values in 14 of the normally sighted eyes, improved 2 or more lines in 2 eyes, and worsened two or more lines in two eyes. Hard contact lens overcorrection restored all of the two-line loss in 1 eye and 1 line of the 3-line loss in the other. Refraction and keratometry indicated corneal flattening without induced astigmatism.

Adult↗
PubMed1991

Corneal topography of keratoconus.

Sixty-three eyes of 49 patients with keratoconus were studied by means of computer-assisted corneal topographic analysis. Two patients with keratoconus in one eye had no topographic evidence of keratoconus in the opposite eye. Keratoconic topographic alterations were classified into two groups. The majority of the patients (72%) had peripheral cones, with steepening extending to the limbus. In this group, the steepening was commonly restricted to one or two quadrants. In some cases, however, the changes involved nearly the entire corneal surface. The remaining group (about one quarter of the keratoconus patients) had steepening that was restricted to the central cornea. In either group, the topographic alterations in the two eyes of any single patient were characterized by a high degree of nonsuperimposable mirror-image symmetry (enantiomorphism). In the majority of patients, however, there was a large and statistically significant disparity between the two eyes in the power at the apex of the cone and the total cylinder [mean differences 8.2 +/- 6.0 D (p less than 0.0001) and 4.3 +/- 2.7 D (p less than 0.0001), respectively].

Humans↗
PubMed1990

One-year refractive results of central photorefractive keratectomy for myopia in the nonhuman primate cornea.

Photorefractive keratectomy for the correction of myopia was performed in 32 eyes of 16 green monkeys. The corneas healed satisfactorily, with normal formation of basal lamina and hemidesmosomal attachments visible in 14-week histologic specimens. No recurrent erosions were observed clinically. After a transient period of faint haze, all corneas were clear at 17 weeks and remained clear through the 1-year follow-up. In terms of accuracy, all corneas demonstrated a significant flattening compared with preoperative values, but no significant difference was seen between the groups with different intended corrections (1.5 and 3 diopters). The changes in corneal shape stabilized by 17 weeks, as measured by keratometry. The clinical results suggest that mechanical removal of the epithelium is preferable to laser ablation of the epithelium. Overall, the results demonstrate that excimer laser ablation of the corneal stroma can produce a stable diptric change in the primate cornea with good healing and long-term corneal clarity.

Animals↗