PubMed Health⌕ Search

Biomedical subjects

T T McMahon

Publications and source records attributed to T T McMahon.

At least 19 recordsLinked to original sources
PubMed2001

Precision of three topography instruments in keratoconus subjects.

PURPOSE: To determine the test-retest reliability of three popular and commercially available Placido-ring videokeratography instruments in subjects with keratoconus. METHODS: Nine subjects (16 eyes) with keratoconus of varying degrees of severity had up to four images per eye generated, in random order, from the EyeSys Model II, Dicon CT 200 and the Keratron Corneal Analyzer. Test-retest analyses for the images were sampled at four locations: 1.5 mm nasal, inferior, temporal, and superior from center. The average standard deviation of all points was used to determine the short-term variability of the measurements. RESULTS: The short-term variability (in diopters) of the Dicon, EyeSys, and Keratron was 0.61 to 3.31 D, 0.94 to 1.51 D, and 0.58 to 2.85 D, respectively, for axial distance maps and 1.07 to 6.82 D, 0.79 to 1.77 D, and 1.23 to 3.03 D for tangential curvature maps. CONCLUSION: Results support the notion of a loss in repeatability for all three instruments when corneal irregularity is present, which reduces test-retest reliability.

Cornea↗
PubMed2000

Twenty-five years of contact lenses: the impact on the cornea and ophthalmic practice.

PURPOSE: The history of contact lenses has occurred in the latter half of the 20th century. In particular, events in the 1970s through the 1980s related to the invention of soft, hydrogel contact lenses have revolutionized the contact lens industry and the eye care attached to it. This article recounts that history from the perspective of market forces, inventions, and discoveries about the physiologic functioning of the cornea. METHODS: The relevant literature is critically reviewed. RESULTS: Discoveries about the oxygen needs of the cornea and consumer pressure for clear, comfortable, around-the-clock vision have resulted in a history of rigid gas permeable and soft lenses that leads to today's contact lens picture. The short-term and long-term effects of chronic hypoxia and the levels of lens oxygen transmissibility necessary to avoid them have been well-described. The advent of the soft lens, followed by the "human experiment" with initial extended-wear modalities, led to the advent of the disposable soft contact lens. CONCLUSIONS: In the past 25 years, the development and wide acceptance of soft contact lenses have revolutionized the management of refractive error and corneal diseases.

Contact Lenses↗
PubMed1999

Discordance for keratoconus in two pairs of monozygotic twins.

PURPOSE: We present two pairs of monozygotic twins discordant for keratoconus. METHODS: Two pairs of twins, each with one twin with keratoconus, and available family members were examined clinically and with computer-assisted videokeratography. Polymerase chain reaction-based zygosity assays using between nine and 11 unique, anonymous DNA markers were performed on blood obtained from the twins and surviving parents to assess the probability of genetic monozygosity. RESULTS: DNA probes showed a >99% probability that each of the two sets of twins was monozygotic. One twin from each pair had clinically diagnosed keratoconus. The remaining twins were normal by clinical examination and corneal topography. Clinical results for all family members examined were normal except that five of 13 from one family and one of six from the other family demonstrated "suspicious" corneal topography. CONCLUSION: Recent advances in knowledge and understanding of the twinning process suggest that monozygotic twins discordant for keratoconus does not preclude the possibility of a significant genetic component.

Adult↗
PubMed1998

Nonulcerating bacterial keratitis associated with soft and rigid contact lens wear.

OBJECTIVE: An unusual presentation of contact lens-related bacterial keratitis is that of epithelial nodular infiltrates and stromal inflammation without epithelial ulceration. The authors study the initial diagnosis, clinical features, causative organisms, and outcomes of corneal infections presenting in this manner. DESIGN: The study design was a 20-month retrospective chart review. PARTICIPANTS: Five patients with culture-proven bacterial keratitis who had predominantly nodular epithelial lesions were studied. RESULTS: Four infections were associated with soft contact lens wear and one with rigid lens wear. All patients had largely intact epithelium; typical gray-colored epithelial nodules, some with underlying anterior stromal haze; and diffuse, fine, cellular stromal inflammation. Two patients were referred with the tentative diagnosis of Acanthamoeba infection and two as contact lens-related sterile keratitis. Epithelial cultures from three cases yielded Serratia sp., one yielded Corynebacterium, and one Streptococcus pneumoniae. All responded to antibacterial medication; final corrected visual acuity in all cases was 20/30 or better. CONCLUSIONS: Bacterial infection associated with contact lens wear can be established within the corneal epithelium without initially producing an ulcer. A wide range of both gram-positive and gram-negative organisms can be involved. Early recognition and treatment appear to result in a favorable outcome.

Adolescent↗
PubMed1998

Visual acuity repeatability in keratoconus: impact on sample size. Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group.

PURPOSE: The purpose of this paper is to determine the repeatability of visual acuity measurement in keratoconus and to describe the impact of measurement repeatability on sample size. METHODS: Approximately 10% of a 1209 patient sample in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study were selected randomly for a Repeat CLEK Study Visit. Patients were tested at the 15 CLEK Participating Clinics. The test-retest sample consisted of 1 34 keratoconus patients who met the entry criteria of the CLEK Study. High and low contrast Bailey-Lovie visual acuity was measured with the patient's habitual visual correction (entrance visual acuity monocularly and binocularly), and with the best correction monocularly (either the patient's rigid contact lens and over-refraction, or with a CLEK Study trial lens and appropriate over-refraction) at two visits separated by a median of 90 days (range 22 to 268 days). RESULTS: The mean absolute differences between the number of letters correct at the two visits ranged from a low of 3.24 +/- 3.1 for entrance high contrast binocular acuity to a high of 5.48 +/- 5.1 for best corrected low contrast monocular acuity. Intraclass correlation coefficients ranged from 0.757 to 0.853. The visual acuity score was somewhat higher at the Repeat Visit than at the Baseline Visit when the examiners were different between visits. CONCLUSIONS: Given the variable vision reported by keratoconus patients, visual acuity in this sample was very repeatable. Repeatability was slightly poorer when different examiners tested visual acuity at the Baseline and Repeat Visits.

Adolescent↗
PubMed1998

Baseline findings in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study.

PURPOSE: To describe the baseline findings in patients enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. METHODS: This is a longitudinal observational study of 1209 patients with keratoconus enrolled at 16 clinical centers. Its main outcome measures are corneal scarring, visual acuity, keratometry, and quality of life. RESULTS: The CLEK Study patients had a mean age of 39.29+/-10.90 years with moderate to severe disease, assessed by a keratometric-based criterion (95.4% of patients had steep keratometric readings of at least 45 D) and relatively good visual acuity (77.9% had best corrected visual acuity of at least 20/40 in both eyes). Sixty-five percent of the patients wore rigid gas-permeable contact lens, and most of those (73%) reported that their lenses were comfortable. Only 13.5% of patients reported a family history of keratoconus. None reported serious systemic diseases that had been previously reported to be associated with keratoconus. Many (53%) reported a history of atopy. Fifty-three percent had corneal scarring in one or both eyes. CONCLUSIONS: Baseline findings suggest that keratoconus is not associated with increased risk of connective tissue disease and that most patients in the CLEK Study sample represent mild to moderate keratoconus. Additional follow-up of at least 3 years will provide new information about the progression of keratoconus, identify factors associated with progression, and assess its impact on quality of life.

Adult↗
PubMed1997

Contact lens use after corneal trauma.

BACKGROUND: This study was performed to explore the epidemiology and visual outcomes of major anterior segment trauma cases referred to the Contact Lens Service (CLS) in a large, urban, eye trauma center at a university hospital. METHODS: All eye trauma patients seen on the CLS at The University of Illinois in Chicago for a 2-year period were identified (n = 122). A search of all eye trauma cases seen at UIC (n = 2279) during the same period yielded 757 cases with the same diagnostic codes. RESULTS: CLS cases represent 16.1% of cases with similar diagnoses and 5.35% of all cases seen during the period of study. Cases were predominantly young men (mean age, 28 years). The mean age for CLS cases (mean age, 17 years) was significantly younger. Best-corrected visual acuity was 20/100 or better in 75% of cases with a contact lens and in 68% of cases with a spectacle lens; 82% of CLS cases had iris damage versus 14.5% of non-CLS cases; 73% of CLS cases were aphakic; 105 CLS cases were given contact lenses. Mean follow-up time was 24 months. Mean wearing time was 11 hours/day. On interview, 35 of 79 cases (44%) continued to wear lenses. CONCLUSIONS: Cases referred to the CLS tended to be young men with unilateral injuries-predominantly open globes-with resultant aphakia and iris damage. Good visual acuity was achieved in most cases. Long-term success wearing contact lenses for this population was poor, suggesting that alternatives to contact lens wear should be more actively considered.

Adolescent↗
PubMed1996

Recovery from induced corneal edema and endothelial morphology after long-term PMMA contact lens wear.

PURPOSE: To study the effect of long-term polymethyl methacrylate (PMMA) contact lens wear (CLW) on corneal endothelial morphology and corneal hydration control (CHC). METHODS: Sixteen long-term PMMA CL wearers (mean duration of wear 20.8 +/- 8.39 SD years; mean age 40.37 years +/- 10.47 SD) served as subjects and 14 age-matched, non-lens wearing subjects (mean age 41.29 years +/- 10.15 SD) served as controls. Corneal function was assessed by measuring the corneal thickness recovery rate (PRPH) after induced edema from a hypoxic stress load. Endothelial morphology was assessed by taking specular microscopy photographs, digitizing the images, and calculating the cell density, size, and coefficient of variation of cell size (CV). RESULTS: Morphologic analysis showed a significant decrease in mean endothelial cell density (2147 vs. 2865, p < 0.01), an increase in mean cell size (582 vs. 381, p = 0.001), and an increase in polymegethism (CV, 0.32 vs. 0.26, p < 0.01) for PMMA lens wearers compared to agematched controls. Corneal function as measured by PRPH was not significantly different for PMMA lens wearers compared to controls when taken as a whole. However, a subgroup of PMMA lens wearers (seven eyes, four subjects) did show a substantial reduction in PRPH. These subjects also showed a strong relation between reduced PRPH, morphology, and years of lens wear (r2 = 0.99). CONCLUSIONS: Longterm PMMA CLW leads to increased polymegethism in most, if not all, cases and results in a substantial decrease in corneal function in some cases.

Adult↗
PubMed1996

Standardized rigid contact lens fitting protocol for keratoconus.

Keratoconus is typically managed by a variety of rigid contact lens fitting techniques and lens designs. The two most fundamental fitting techniques are apical corneal touch (including divided or three-point touch) and apical clearance. In the course of designing a multi-center study of keratoconus patients, a standardized keratoconus fitting protocol was developed. All contact lens parameter options are uniform except for base curve and secondary curve radii, which are determined by interpretation of fluorescein patterns using the CLEK Study trial lens set and protocol. The initial trial lens's base curve is the average keratometric reading; sequentially steeper lenses are applied until definite apical clearance is observed. We have evaluated the feasibility of this standardized fitting protocol on 30 keratoconus patients. Our results suggest that we have developed a standardized contact lens fitting set and fitting protocol to simplify contact lens management in patients with mild to moderate keratoconus.

Contact Lenses↗
PubMed1996

Feasibility of fitting contact lenses with apical clearance in keratoconus.

Despite the wide variety of rigid contact lens fitting philosophies for the visual correction of keratoconus, questions remain, including which approach-flat, divided support, or steep-contributes the most toward the preservation of a clear cornea. One goal of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Pilot Study was to determine the feasibility of managing early keratoconus patients with apical clearance rigid contact lenses. Of 30 keratoconus patients identified with at least 1 nonscarred cornea, 17 patients (30 eyes) were randomly assigned to a steep lens fitting protocol. After trial fitting with a standardized lens design demonstrating minimum apical clearance, lenses were dispensed whose base curve was 0.2 mm steeper than the minimum apical clearance lens. Patients were re-evaluated on a quarterly schedule concluding at 12 months. Changes in keratometry between baseline and 12 months identified unequal steepening of the flat and steep corneal curvatures, suggestive of corneal molding. Best corrected rigid lens visual acuity measures illustrated no significant changes over the course of the study. Clinically significant corneal compromise was transiently observed in some patients. Only 1 of 22 eyes completing the pilot study and fitted with apical clearance developed mild corneal scarring.

Adolescent↗
PubMed1994

Dynamic shifts in corneal topography after radial and transverse keratotomy.

PURPOSE: The authors aimed to quantitate the dynamic patterns of change in corneal topography after multistaged radial and transverse keratotomy using digitized video-keratography. METHODS: Single and paired radial and transverse keratotomies, with videokeratoscopy between each stage and at the end of the procedure, were performed on fresh animal cadaver eyes using an artificial orbit system. RESULTS: All incisions led to central flattening. A single radial keratotomy caused flattening adjacent to the incision, and steepening 180 degrees away. A paired radial keratotomy caused increased flattening in the meridian of the incisions, and less flattening 90 degrees away. A single transverse incision caused steepening adjacent to the incision and diffuse flattening elsewhere. A paired transverse incision caused flattening near the optical center along the meridian bisecting the incisions and steepening 90 degrees away. CONCLUSION: The authors have demonstrated that computerized videokeratography can be used successfully to systematically quantitate dioptric shifts in multiple hemimeridians and measurement zone diameters after refractive surgery.

Animals↗
PubMed1994

Effects of long term extended wear of PMMA lenses on corneal function: a case report.

BACKGROUND: Recent studies have shown that some long-term PMMA contact lens wearers who are presumably exposed to hypoxia can develop modest but permanent changes in corneal structure and function. What is not known is the corneal effects that may result from severe and long-term exposure to an hypoxic environment. This case report is presented to illustrate an extreme example of the effects of the cornea from prolonged exposure to hypoxia. METHODS: A 37-year-old male who had worn PMMA lenses on a 2-week extended wear basis for 20 years was examined in the laboratory to assess changes in corneal structure and function. Function was determined by monitoring the rate at which the cornea recovered from induced edema and is expressed as a percent recovery per hour (PRPH). Morphology was studied via specular microscopy. RESULTS: For this subject, the PRPH was 39.6 percent/hr and 40.5 percent/hr for right and left eyes respectively; these values are substantially reduced compared to the reported 55.3 percent/hr for an age-matched, non-contact lens wearing population. Specular microscopy demonstrated reduced cell counts of 1,052 cells/mm2 OD and 1,741 cells/mm2 OS compared to an age adjusted normal population with 2,853 +/- 320 cells/mm2. CONCLUSIONS: This case example appears to represent an extreme case of contact lens abuse and the effects of long-term hypoxic contact lens wear on corneal function.

Adult↗
PubMed1994

Optometric clinical abbreviations.

BACKGROUND: Standard clinical abbreviations useful to optometric practitioners are provided. This list contains terms that should be used in the examination, diagnosis and management of patients and serve in inter- and intra-professional communications.

Abbreviations as Topic↗
PubMed1993

Saccadic eye movements as a measure of the effect of low vision rehabilitation on reading rate.

We assessed saccadic eye movements and reading capabilities in individuals with macular disease by a masked clinical study to determine whether changes in saccadic frequency were correlated to changes in reading rates or number of errors for a reading task. We used a measure of eye movement characteristics in the form of a saccadic frequency score (SFS). Twelve subjects with macular disease were tested before and after an intense 6-week inpatient vision rehabilitation program. The mean age was 70.9 years (63 to 77 years). Mean visual acuity was 10/100 (10/40 to 10/200). Mean reading rates at entry and discharge were 8.5 words/min and 20.9 words/min, respectively (p = 0.02). SFS improved from 2.709 to 2.022 (p = 0.07). Reading errors improved from 5.60 to 4.00 (p = 0.03). Reading rates and SFS's were highly inversely correlated both pre- and post-treatment (Pearson r = -0.892 and -0.896, respectively). SFS and the number of reading errors were highly correlated with both pre- and post-treatment as well (Pearson r = 0.782 and 0.601, respectively). Interestingly the data appeared to cluster into two groups, one showing improvement and the other showing none. Cluster analysis revealed a strong association among reading rate, saccade scores, and reading errors. Membership in the two clusters is consistent pre- and post-rehabilitation in all but one case. Clustering is most evident for reading rate and reading errors and less so for saccade scores.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1993

Multivariate assessment of computer-analyzed corneal topographers.

Methodological aspects of multivariate statistical models to describe and to assess data from computer-analyzed corneal topographers (CACT) are considered. The data generated by repeated curvature mappings of calibrated steel balls are discussed in detail with the objective of formulating the basic questions and suggesting directions to assess data from clinical applications of CACT. The interpretation of seemingly straightforward concepts such as accuracy and precision is revisited with the objective of understanding its meaning in future clinical and experimental applications of CACT. Some of the statistical problems related to the analysis of corneal astigmatism based on CACT data are also discussed.

Astigmatism↗
PubMed1992

Contact lens failure in keratoconus management.

PURPOSE: Surgery is indicated for keratoconus when management with contact lenses fails. The authors sought to determine the relative contribution of various preoperative patient and ocular factors to the ultimate causes of contact lens failure. METHODS: The records of unoperated eyes of keratoconus patients whose contact lenses were managed intensively before undergoing penetrating keratoplasty (PK) at the authors' institution between 1981 and 1990 were selected for study. Univariate and multivariate analyses were performed to identify risk factors for early contact lens failure. RESULTS: The records of 99 keratoconic eyes of 75 patients with an average age of 34 years and average keratometry readings of 57.5 diopters at presentation were studied. Cases had been followed for an average of 27 months before PK. The primary reasons for PK were a best-corrected visual acuity of under 20/40 (despite good contact lens fit) causing disability for the patient (43%), contact lens intolerance (32%), frequent lens displacement (13%), and significant peripheral thinning of the cornea (12%). The referral source of the patient, sex, a history of PK in the fellow eye, or of contact lens wear in either eye did not alter the relative contributions of these parameters to surgery. CONCLUSION: Poor best-corrected visual acuity at presentation, higher keratometry readings (greater than or equal to 55 D), age (greater than or equal to 40 years), and duration of disease (greater than 5 years) were significantly associated with failure due to poor functional acuity and peripheral thinning, frequently leading to surgery within the first 12 months after presentation.

Adolescent↗