Biomedical subjects

Timothy T McMahon

Publications and source records attributed to Timothy T McMahon.

16 recordsLinked to original sources

PubMed2006

Epidemiological characteristics of a Chicago-area Acanthamoeba keratitis outbreak.

PURPOSE: To characterize Acanthamoeba keratitis (AK) cases and analyze the geographical distribution within the Chicago-Gary-Kenosha metropolitan area, Chicago, Illinois, USA. DESIGN: Retrospective, population-based cohort study. METHODS: All AK cases diagnosed at the University of Illinois at Chicago Cornea Service from June 1, 2003, to November 30, 2005, were included in analysis. Patients with keratitis were defined as cases through confocal microscopy, histology, and/or positive cultures. Exploratory analyses were performed to evaluate whether AK cases were unequally distributed geographically. County population data were extracted from US Census 2000 data, and rates were age-standardized to Cook County. Poisson regression analysis was used to estimate the age-standardized rate ratio (RR) between AK cases and county of residence. Current cases (June 1, 2003 to November 30, 2005) were compared with historical cases (June 1, 2000 to November 30, 2002) to determine if the current rate of AK diagnosis differed from historical rates. RESULTS: Forty AK cases were diagnosed between June 1, 2003 and November 30, 2005. The average (+/-SD) age of patients with AK was 28.0 +/- 15.0 years (range, 13 to 70 years), 52.5% were men, and 95.0% wore contact lenses. Estimated RR measures demonstrated increased rates for all counties relative to Cook, and were significant for both DuPage County (RR 3.59; 95% confidence interval [95% CI] 1.44, 8.39) and Will County (RR 3.66; 95% CI 1.18 to 9.56). Current AK diagnosis rates were significantly higher than historical rates (RR 6.67; 95% CI 3.05 to 17.52). CONCLUSIONS: AK cases are increasing in frequency. The increased rates are unevenly distributed in the study area. Further research is warranted to better understand the increase and unusual geographical distribution.

Acanthamoeba Keratitis↗

PubMed2006

Longitudinal changes in corneal curvature in keratoconus.

PURPOSE: This study was designed to identify factors that predict longitudinal increases in corneal curvature as measured by the First Definite Apical Clearance Lens (FDACL) and flatter keratometric reading (Flat K) in keratoconus. METHODS: The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a long-te rm evaluation of the natural history of keratoconus involving 1209 patients. This report uses 8 years of follow-up data from 1032 patients who had penetrating keratoplasty in neither eye at baseline and who provided enough data to compute the slope of the change with time in the FDACL or the Flat K. Outcome measures included the aforementioned slopes and whether the FDACL or the Flat K increased by 3.00 or more diopters (D) in at least 1 eye. RESULTS: At CLEK Study entry, patients were aged 38.9 +/- 10.8 years. Overall, 44.3% of them were women, and 69.3% of them were white. The slope of the change in FDACL (0.18 +/- 0.60 D/y) and Flat K (0.20 +/- 0.80 D) during 8 years translates into expected 8-year increases of 1.44 D in FDACL and 1.60 D in Flat K. Increases of > or = 3.00 D in either eye had an 8-year incidence of 24.8% for FDACL and 24.1% for Flat K. Independent predictors of increased FDACL included younger age, poorer baseline high-contrast manifest refraction visual acuity, and non-white race. Younger age and poorer high-contrast manifest refraction visual acuity were independent predictors of a >3.00-D increase for both FDACL and Flat K. CONCLUSION: CLEK patients exhibited a slow but clear increase in corneal curvature. Younger age and poorer high-contrast manifest refraction visual acuity at baseline predicted the rate of change in corneal curvature.

Adult↗

PubMed2006

Filamentous fungal invasion of a methafilcon contact lens.

PURPOSE: To report a case of Fusarium species invasion of a methafilcon matrix soft contact lens in a first-time lens wearer. RESULTS: The authors describe the history and ocular examination of a 19-year-old woman with large fungal deposits on her contact lens. Culture of the contact lens grew Fusarium species. On histologic examination, fungal filaments were seen on both surfaces of the contact lens as well as invading the lens matrix. The fungal morphology changed as the fungus penetrated the contact lens. CONCLUSION: To our knowledge, this is the first documented case of fungal invasion of a methafilcon matrix lens and the second histologically documented case of Fusarium invasion of a contact lens.

Adult↗

PubMed2006

A new method for grading the severity of keratoconus: the Keratoconus Severity Score (KSS).

PURPOSE: To define a new method for grading severity of keratoconus, the Keratoconus Severity Score (KSS). METHODS: A rationale for grading keratoconus severity was developed using common clinical markers plus 2 corneal topographic indices, creating a 0 to 5 severity score. An initial test set of 1012 eyes, including normal eyes, eyes with abnormal corneal and topographic findings but not keratoconus, and eyes with keratoconus having a wide range of severity, was used to determine cutpoints for the KSS. Validation set 1, comprising data from 128 eyes, was assigned a KSS and compared with a clinician's ranking of severity termed the "gold standard" to determine if the scale fairly represented how a clinician would grade disease severity. kappa statistics, sensitivity, and specificity were calculated. A program was developed to automate the determination of the score. This was tested against a manual assignment of KSS in 2121 (validation set 2) eyes from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study, as well as normal eyes and abnormal eyes without keratoconus. Ten percent of eyes underwent repeat manual assignment of KSS to determine the variability of manual assignment of a score. RESULTS: From initial assessments, the KSS used 2 corneal topography indices: average corneal power and root mean square (RMS) error for higher-order Zernike terms derived from the first corneal surface wavefront. Clinical signs including Vogt striae, Fleischer rings, and corneal scarring were also included. Last, a manual interpretation of the map pattern was included. Validation set 1 yielded a kappa statistic of 0.904, with sensitivities ranging from 0.64 to 1.00 and specificities ranging from 0.93 to 0.98. The sensitivity and specificity for determining nonkeratoconus from keratoconus were both 1.00. Validation set 2 showed kappa statistics of 0.94 and 0.95 for right and left eyes, respectively. Test-retest analysis yielded kappa statistics of 0.84 and 0.83 for right and left eyes, respectively. CONCLUSION: A simple and reliable grading system for keratoconus was developed that can be largely automated. Such a grading scheme could be useful in genetic studies for a complex trait such as keratoconus requiring a quantitative measure of disease presence and severity.

Cornea↗

PubMed2006

Simulation of machine-specific topographic indices for use across platforms.

PURPOSE: The objective of this project is to simulate the current published topographic indices used for the detection and evaluation of keratoconus to allow their application to maps acquired from multiple topographic machines. METHODS: A retrospective analysis was performed on 21 eyes of 14 previously diagnosed keratoconus patients from a single practice using a Tomey TMS-1, an Alcon EyeMap, and a Keratron Topographer. Maps that could not be processed or that contained processing errors were excluded from analysis. Topographic indices native to each of the three devices were recorded from each map. Software was written in ANSI standard C to simulate the indices based on the published formulas and/or descriptions to extend the functionality of The Ohio State University Corneal Topography Tool (OSUCTT), a software package designed to accept the input from many corneal topographic devices and provide consistent display and analysis. Twenty indices were simulated. Linear regression analysis was performed between each simulated index and the corresponding native index. A cross-platform comparison using regression analysis was also performed. RESULTS: All simulated indices were significantly correlated with the corresponding native indices (p < 0.01), with a mean R of 0.84, ranging from 0.42 to 0.99. Cross-platform comparisons were nonsignificant for specific indices and devices. CONCLUSION: Topographic indices native to three devices were successfully simulated. Cross-platform comparisons may be limited for specific indices.

Computer Simulation↗

PubMed2005

Differences in keratoconus as a function of gender.

PURPOSE: To characterize gender differences in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. DESIGN: Observational, longitudinal study. METHODS: A total of 1209 subjects at 16 clinics. For eye-specific categorical variables, the number of eyes per subject with the characteristic was counted. For eye-specific continuous variables, the mean of both eyes was calculated. Multivariate linear (for continuous outcomes) and logistic (for categorical outcomes) regression models were created for each baseline characteristic with statistically significant (P < or = .05) differences between men and women. Age, race, education, and corneal curvature were covariates. RESULTS: The women were older, more likely to report a family history of keratoconus, more likely to be nonwhite, and less likely to complete college than men. Vogt's striae and monocular and binocular high-contrast entrance acuity were the only visual characteristics that varied between men and women in the multivariate model. Women were more likely than men to report ocular symptoms of dryness and complaints based upon a composite score of ocular symptoms. Women reported more hours per day of near work and were less likely to report the ability to wear contact lenses for enough hours to permit reading at home in the evening. Women reported more visits to their eye care practitioner during the previous 12 months. NEI-VFQ results revealed differences in self-reported difficulty with distance activities and driving. CONCLUSIONS: Gender differences exist in patient history, vision, and ocular symptoms in keratoconus patients.

Adult↗

PubMed2005

Repeatability of autorefraction and axial length measurements after laser in situ keratomileusis.

PURPOSE: To assess the repeatability and agreement of refractive error measurements and the repeatability of axial length (AL) measurements in patients after laser in situ keratomileusis (LASIK). SETTING: The Ohio State University College of Optometry, Columbus, Ohio, USA. METHODS: Subjective refraction, autorefraction measurements with the Grand Seiko and Humphrey autorefractors, and AL measurements with the IOLMaster were completed for 40 previously myopic LASIK patients under noncycloplegic and cycloplegic conditions on 2 separate occasions. RESULTS: The mean difference between visits for axial length measurements was 0.008 mm +/- 0.04 (SD). The between visits repeatability for all refractive error measurements were <0.75 diopter (D). The mean difference between the subjective refraction and the Humphrey autorefractor for spherical equivalent was statistically significant under noncycloplegic conditions (-0.90 D, P<.0001) and cycloplegic conditions (-2.05 D, P<.0001). The mean difference between subjective refraction and Grand Seiko autorefraction measurements was not significant under noncycloplegic conditions (+0.05 D, 95% limits of agreement [LoA]=-0.99, 1.09; P=.52) conditions but was statistically significant, but not clinically relevant, under cycloplegic conditions (+0.17 D, 95% LoA=-0.73, 1.07; P=.03). CONCLUSIONS: Refractive error measurements after LASIK using the Grand Seiko autorefractor are reliable and agree well with subjective refraction measurements.

Adult↗

PubMed2005

Gas-permeable contact lenses can protect against ocular trauma: two cases.

PURPOSE: To report two cases of rigid gas-permeable contact lens wearers who were protected from serious foreign body-related injury through the wearing of their contact lenses. METHODS: Two case reports. RESULTS: Rigid gas-permeable contact lenses appear to have protected the eyes of two patients from probable serious injury. CONCLUSIONS: Rigid gas-permeable contact lenses can have a protective effect against some types of trauma, although they are not recommended to be used for protective purposes.

Accidents, Occupational↗

PubMed2005

Repeatability of corneal topography measurement in keratoconus with the TMS-1.

PURPOSE: The purpose of this study was to report the test-retest variability of simulated indices derived from the TMS-1 topography instrument (Tomey Technology, Waltham, MA) in keratoconus subjects enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. METHODS: Four images were taken at an initial visit and at a repeat visit several weeks later. From these images, 17 indices were simulated from published formulas. Mixed-model analysis was used on test-retest data from the TMS-1 videokeratography instrument during the baseline year. This analysis yields estimates of within- and between-visit variability. RESULTS: Repeatability analysis revealed that within-visit standard errors were 1.0 to 5.9 times greater in keratoconus eyes than in normal controls when two images were analyzed from each visit. These values changed only slightly when more images were used. The ratio of between-visit standard errors of the indices were nearly equally greater than normal controls for (0.9-4.6 and 0.9-4.3) two images per eye and all images per eye, respectively. CONCLUSIONS: These results suggest that the repeatability of simulated indices derived from TMS-1 topography in keratoconus subjects is poorer than in normal controls.

Algorithms↗

PubMed2005

Comparison of flat and steep rigid contact lens fitting methods in keratoconus.

PURPOSE: The purpose of this article is to compare the safety and efficacy of flat- and steep-fitting rigid contact lenses in keratoconus. METHODS: The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a 16-center observational study. Cross-sectional results at baseline were generated for 1091 subjects with longitudinal results from the 871 subjects who completed 8 years of follow up. RESULTS: Of the 761 rigid contact lens-wearing patients at baseline, 41% had a scar at baseline compared with 24% of the nonrigid contact lens wearers (odds ratio [OR], 2.15; 95% confidence interval [CI], 1.35-3.43; p = 0.001). Eighty-seven percent were fitted with flat-fitting lenses, whereas 13% were fitted with steep-fitting lenses. Rigid lens fitting method was also associated with incident corneal scarring. A greater proportion of the corneas wearing flat-fitting contact lenses were scarred (43% compared with 26% for the steep-fitted eyes; OR,= 2.19; 95% CI, 1.37-3.51; p = 0.001). After controlling for corneal curvature, the association of rigid contact lens fit and corneal scarring at baseline did not persist (adjusted OR, 1.20; 95% CI, 0.70-2.06; p = 0.52). Thirty-two percent of unscarred eyes at baseline fitted flat had developed an incident corneal scar by the eighth year follow-up visit compared with 14% of eyes fitted steep (OR, 2.93; 95% CI, 1.34-6.42; p = 0.007). CONCLUSIONS: The data reported here indicate that, after controlling for disease severity in the form of corneal curvature, keratoconic eyes fitted with a rigid contact lens resulting in an apical touch fluorescein pattern did not have an increased risk of being scarred centrally at baseline. This "natural history" sample cannot determine causal proof that one method of fitting lenses is safer than another. To achieve this, a randomized clinical trial is needed.

Contact Lenses↗

PubMed2004

Late-stage progressive corneal astigmatism after penetrating keratoplasty for keratoconus.

PURPOSE: Progressive corneal astigmatism occurring at least 10 years after penetrating keratoplasty for keratoconus is a late-phase complication of surgery. This report characterizes this condition in a series of patients from three corneal referral centers in the United States. METHODS: Charts were retrospectively reviewed which met the following criteria: penetrating keratoplasty performed for keratoconus at least 10 years ago, keratometry or simulated keratometry from topography as well as manifest refraction recorded at least 6 months after the last suture removal ("baseline"), and an increase in corneal astigmatism of at least three D over baseline recorded at least 5 years later. Patients who had any other corneal or intraocular surgery performed were excluded. RESULTS: Data from 15 patients (11 males and 4 females) who had penetrating keratoplasties performed by 8 different surgeons are included in this descriptive series. Postoperative follow-up averaged 17.27 years (range 11-24 years). The average donor button size was 7.83 mm (range 7.25-8.5 mm). Baseline corneal astigmatism was obtained an average of 5.2 years after penetrating keratoplasty (range 1.5-16 years) and was on average 3.57 +/- 1.8 D (10 with-the-rule [WTR], 3 against-the-rule [ATR], 2 oblique). Corneal astigmatism significantly increased to an average of 11.23 +/- 3.56 D (range 8.00-19.37 D, P < 0.0001) and most astigmatism was regular and WTR (9 WTR, 3 ATR, 3 oblique) 15.3 years (range 10-22 years) after surgery. Inferior steepening on topography was often noted, even those with oblique and ATR axes. CONCLUSIONS: High, late-stage, regular astigmatism after penetrating keratoplasty for keratoconus is described in a series of patients occurring at least 10 years after surgery. Possible mechanisms of this progressive astigmatism are recurrence of keratoconus in the graft, progressive corneal thinning of the host cornea, or progressive misalignment of the graft-host interface over time.

Astigmatism↗

PubMed2004

Is "whole eye" wavefront analysis helpful to corneal refractive therapy?

PURPOSE: To examine how corneal refractive therapy (CRT) affects the balance between corneal surface and whole eye spherical aberration (Z4) and to identify whether corneal aberrations alone can accurately predict optical outcomes of CRT. METHODS: Whole eye and corneal spherical aberrations were measured at baseline and 1 month after CRT in 12 dilated eyes (baseline myopia range, -2.25 to -6.00 diopters [D]; mean, -3.23 +/- 1.06D). Whole eye aberrations were measured under cycloplegia using a Shack-Hartmann aberrometer, and corneal aberrations were measured using Keratron corneal topography elevation maps converted to wavefront Zernike polynomials using VOL-CT. Whole eye and corneal surface aberrations were scaled for 6-mm pupil diameters. RESULTS: No statistically significant correlation existed between corneal and whole eye spherical aberration at baseline. Corneal and whole eye spherical aberration increased in all eyes after CRT. A statistically significant and moderately positive correlation was found between corneal and whole eye spherical aberration after CRT (R = 0.632, P = 0.004). The linear regression slope was positive (m = +1.449) and indicated that whole eye spherical aberration exceeds corneal spherical aberration after CRT. CONCLUSIONS: Whole eye wavefront analysis is essential to methods designed to optimize visual performance with CRT; results indicate that CRT affects the entire optical system and not just the anterior corneal surface. Moreover, because CRT-induced whole eye spherical aberration cannot be explained by corneal aberrations alone, commonly accepted beliefs that the mechanism of action is limited to corneal tissue redistribution may need to be revisited.

Adult↗

PubMed2003

Higher-order wavefront aberrations in corneal refractive therapy.

PURPOSE: To assess the changes in higher-order (third through sixth) ocular wavefront aberrations produced by Corneal Refractive Therapy (CRT; Paragon Vision Sciences, Mesa, AZ). METHODS: Eighteen eyes of nine myopic subjects were fit with CRT contact lenses. Baseline subjective spherical refraction ranged from -2.25 to -6.00 D (mean +/- SD, -3.33 +/- 1.26 D), and baseline logarithm of the minimum angle of resolution (logMAR) best-corrected visual acuity was -0.13 +/- 0.06 (20/15 Snellen equivalent). Whole-eye ocular wavefront aberrations were measured using a previously validated Shack-Hartmann aberrometer. Measurements were taken at baseline and 1 month after treatment initiation. Nine measurements per dilated subject were taken and averaged. Zernike coefficients were used to calculate the third-, fourth-, fifth-, and sixth-order root-mean-square values for each eye for both 3- and 6-mm pupil sizes, and aberrations were averaged and compared with prior baseline readings. RESULTS: The mean (+/-SD) myopia reduction was 3.08 +/- 0.93 D, resulting in a subjective refraction of -0.22 +/- 0.38 D after 1 month of lens wear. Both logMAR uncorrected visual acuity (-0.07 +/- 0.18; 20/15- Snellen equivalent) and best-corrected visual acuity (-0.14 +/- 0.09) after CRT wear were not significantly different from baseline logMAR best-corrected visual acuity (paired two-tailed t-test; p = 0.41 and 0.65, respectively). Whole-eye aberrations showed a statistically significant increase in higher-order aberrations for both 3-mm (factor of 2.66; p = 0.01) and 6-mm pupils (factor of 2.50; p = 0.005). Each individual higher-order aberration also increased, ranging from a factor of 2.01 to 3.20 for 3-mm pupil sizes and 2.52 to 2.98 for 6-mm pupil sizes. Spherical-like aberrations (S4 and S6) increased by a factor of 1.79 for 3-mm pupil sizes and 2.42 for 6-mm pupil sizes. The Zernike coefficient most affected by CRT was spherical aberration (Z40), which increased from 0.084 +/- 0.16 to 0.39 +/- 0.16 microm (p = 0.0002) for 6-mm pupils. CONCLUSIONS: Use of current CRT lenses for the reduction of myopia increased higher-order wavefront aberrations and spherical aberration (Z40) in particular.

Adult↗

PubMed2003

The case for rigid lenses.

The case is made for rigid lenses in the face of increasing market share for soft lenses. Safety, visual function, long-term comfort and durability are key positive features for these devices.

Contact Lenses↗

PubMed2002

Lysosomal hydrolase staining of conjunctival impression cytology specimens in keratoconus.

PURPOSE: This study sought to assess the feasibility of impression cytology for the determination of conjunctival intracellular lysosomal hydrolase (acid esterase) levels in patients with keratoconus. METHODS: Twenty-two patients with keratoconus currently enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study and 22 age-and sex-similar controls underwent impression cytology. Samples were collected from each subject and control pair on the same day. The cells of the respective specimens were fixed immediately and were stained for acid esterase with use of identical batches of fixatives and stains. After staining, the specimens were cleared in xylene for mounting in synthetic resin on glass slides. The acid esterase staining intensity of each specimen was quantified as the percentage of light transmitted with use of an image analysis system (Zeiss). Multiple cells from each specimen were analyzed for each sample collected. RESULTS: Mixed model analysis was used to account for the subject-control pairings and for the multiple cells from each sample. With this method, the mean light transmission for normal controls (mean = 63.0; standard error [SE] = 3.0) was highly statistically significantly different from that for the keratoconus subjects (mean = 52.4; SE = 3.0) (two-tailed p= 0.0032). CONCLUSIONS: This study establishes the feasibility of adapting an acid esterase staining technique to conjunctival cells collected via impression cytology. Higher levels of lysosomal enzyme staining in patients with keratoconus have been previously reported by other investigators using full-thickness conjunctival specimens. We also demonstrate the value of using objective microspectrophotometry in measuring lysosomal enzyme staining with impression cytology specimens.

Acetylesterase↗

PubMed2002

The effectiveness of occluder contact lenses in improving occlusion compliance in patients that have failed traditional occlusion therapy.

PURPOSE: Management of infants and young children with dense amblyopia, including that which results from unilateral congenital cataracts, is challenging because of noncompliance with occlusion therapy. Occluder contact lenses (OCLs) have been described to successfully improve visual acuity in patients with amblyopia. The purpose of this study was to evaluate the effectiveness of OCLs in improving occlusion compliance in infants and young children with dense amblyopia who had failed traditional occlusion therapy. METHODS: Thirteen patients were fit in their nonamblyopic eye with OCLs provided by Wesley Jessen (Des Plaines, IL). All patients had dense amblyopia as diagnosed by a pediatric ophthalmologist. Nine of the 13 patients had deprivational amblyopia associated with unilateral aphakia, 2 patients had strabismic amblyopia, and the other 2 had mixed mechanism amblyopia. All patients had failed traditional occlusion methods. Occlusion was prescribed on a part-time basis to prevent occlusion amblyopia to the better-seeing eye. Compliance with the OCLs was determined by questioning the parents as to whether the prescribed occlusion time was accomplished. RESULTS: Five of the 13 patients (38.4%) did not succeed in the initial 1-mo trial period because of parental difficulties with lens handling. Four patients (30.7%) achieved partial success, as they were able to wear the lens between 4 to 14 mo, at which time they learned to remove the occluder lens. The four remaining patients (30.7%) successfully wore the occluder lenses until the conclusion of the study, a time period ranging from 26 to 60 mo. No complications resulted from OCLs. CONCLUSIONS: OCLs can provide an alternative to achieving occlusion compliance after the failure of traditional occlusion therapy in infants and young children with dense amblyopia.

Amblyopia↗