Langis Michaud

Predicting estimates of oxygen transmissibility for scleral lenses

BACKGROUND/PURPOSE Although scleral contact lenses are prescribed with increasing frequency, little is known about their long-term effects on ocular physiology. The main goal of this paper is to predict values of oxygen transmissibility of scleral lens systems by applying the concept of resistors in series to parameters characteristic of current scleral lenses. A second aim is to find the maximal lens and post-lens tear layer thickness combinations above which hypoxia-induced corneal swelling would be found. METHODS Theoretical calculations were used to predict the oxygen transmissibility of scleral lens systems, considering several material permeabilities (Dks 100-170), varying lens thicknesses (250-500 μm), the known tear permeability (Dk of 80) and expected post-lens tear layer thicknesses (100-400 μm). The Holden-Mertz Dk/t criteria of 24 Fatt units for the central cornea and the Harvitt-Bonanno criteria of 35 Fatt units for the limbal area were used as reference points. RESULTS Our calculations of oxygen transmissibility, with varying tear layer and lens thicknesses, ranged from 10 to 36.7 at the scleral lens centers and from 17.4 to 62.6 at the peripheries. Our calculations of maximum central lens thicknesses show a practical range of 250-495 μm, in conjunction with a post-lens tear layer thickness of 100-250 μm. CONCLUSION Our computations show that most modern scleral lenses, with recommended fitting techniques, should lead to some level of hypoxia-induced corneal swelling. Recommendations are made to minimize hypoxia-induced corneal swelling: highest Dk available (>150) lens with a maximal central thickness of 250 μm and fitted with a clearance that does not exceed 200 μm.

Overwear of contact lenses: increased severity of clinical signs as a function of protein adsorption.

This study compared the clinical behavior of disposable and frequent replacement Acuvue and 1-Day contact lenses. Each type of lens was worn on one eye according to the schedule recommended by the manufacturer, and on the other eye for a longer period of time, up to 30 days in length. Both type of lenses were prescribed on a daily-wear basis. The amount of protein collected from the lenses was measured using two spectrophotometric protein assays. Visual acuity and comfort, along with several other clinical signs, were classified according to Cornea and Contact Lens Research Unit (CCLRU) scales, and possible associations between each of these signs and the amount of protein extracted from the lenses was tested. A comparison between the lens worn on the compliant eye with the lens worn on the noncompliant eye allowed us to measure the impact of overwear on ocular health and subjective clinical findings. After four months of study, the overwear of Acuvue and 1-Day lenses significantly increased the amount of protein bound on the contact lenses, as well as the severity of upper conjunctival papillae, upper lid conjunctival hyperemia, and limbal congestion. Even if reduced values for visual acuity and noninvasive break-up time (NIBUT) were identified, these variations were not found to be statistically significant. The clinical implications of this study would allow a practitioner to identify, according to the variations in several clinical signs, a patient who overwears contact lenses, so that action may be taken to reduce possible deleterious effects on ocular health.

Comparing the extent of protein build-up on several disposable lenses by two spectrophotometric methods

This study aims to quantify levels of protein build-up on high-water ionic (Type IV) disposable contact lenses worn by patients, by two spectrophotometric methods, previously validated only with conventional lenses. Forty lenses were analyzed and the related results were compared. The Ninhydrine-Lysozyme method and the Coomassie Blue Albumin one agreed in identifying the most spoiled lenses. Etafilcon A attracted significantly more protein than the other materials tested, which were in the following decreasing order of protein spoilation: Ocuflcon D, Ocufilcon A and Vasurfilcon A. These similar findings suggest that it is possible to use the above mentioned analysis methods for an accurate evaluation of the contamination of several materials of disposable/frequent replacement lenses as for conventional lenses.

Oxygen Tension Beneath Scleral Lenses of Different Clearances.

PURPOSE To evaluate the relative partial pressure in oxygen (pO2) at the corneal surface under Boston XO2 scleral lenses (SL) fitted with targeted clearances of 200 and 400 μm (SL200 and SL400). METHODS During this prospective study, the right eyes of eight normal subjects were fitted with SL200 and SL400. Clearance, validated after 5 minutes of wear with an optical coherence tomograph, was used with lens thicknesses to calculate transmissibility and estimate pO2. Corneal pO2s were measured with an oxygen electrode after 5 minutes of (1) corneal exposure to calibrating gases with various pO2 or of (2) SL wear. Decays in pO2 were modeled to an exponential. Linear regression between exponent k of these decays and calibrating gas pO2s allowed for the calculation of corneal pO2 under SL. Differences between pO2s beneath SL200 and SL400 were tested with a mixed ANOVA. RESULTS The estimated transmissibility based on thicknesses and clearances (239.7 ± 34.7; 434.5 ± 33.2 μm) predicted a corneal pO2 of 8.52 ± 0.51 and 6.37 ± 0.28% for SL200 and SL400. These values were close to measured pO2: 9.07 ± 0.86 and 6.19 ± 0.87% (mean ± SEM) (P < .05) for SL200 and SL400, respectively. Both pO2 measurements fall short of the theoretical values needed to prevent hypoxia during lens wear (9.9% and above). CONCLUSIONS As shown in vivo for the first time, an 18-mm scleral lens fitted with a 400-μm clearance reduces the oxygen tension available to the cornea by 30% compared to a similar lens fitted with a 200-μm clearance after 5 minutes of wear.

Vascular Tortuosities of the Upper Eyelid: A New Clinical Finding in Fabry Patient Screening

Purpose. To report a new clinical finding related to Fabry disease. Methods. Fabry subjects were enrolled in the study, matched for age and sex with healthy individuals as a control group. This is a prospective review of all upper lid pictures taken for every subject at their last visit. A 4-step grading scale is proposed to classify this new entity. Results. Group A (Fabry) comprised 16 males and 22 females, aged 40 (±14) years on average. Group B (control) comprised 7 males and 8 females, aged 37 (±12) Vessels tortuosity was identified on the external superior lid in 36 of the Fabry patients (94.7%), while none of the subjects in group B showed similar vessels tortuosity. In addition, microaneurysms (MAs) were found in 10/38 group A subjects while none in group B presented a similar finding. The differences are highly significant. Conclusion. This paper proposes that blood vessels tortuosity on the upper eyelid be recognized as a new clinical entity for inclusion among the classic ocular manifestations of Fabrys disease. Without evidence of any negative impact, it should be considered a benign sign contributing to evidence of suspected Fabry disease.

Empirical fitting of soft or rigid gas-permeable contact lenses for the correction of moderate to severe refractive astigmatism: A comparative study

BACKGROUND This study aims to validate the fitting of contact lenses derived from refractive and keratometric values in a group of subjects with moderate (2.50 diopters [D] to 5.00 D) to severe (>5.00 D) astigmatism. It aims also to show whether soft or rigid gas-permeable (GP) contact lenses offer the best correction and to determine which modality is preferred by subjects. METHODS Twenty subjects were randomly fitted with both soft and GP lenses. Group A was first fitted with soft lenses then switched to GPs and vice versa for group B. For each type of lens worn, low- and high-contrast visual acuity (VA) and stereoscopy were evaluated at both near and far. Each subject was asked to select the lens type of choice and to rate quality of vision in day-to-day activities through a questionnaire. RESULTS There was no significant difference in objective binocular VA between current spectacles and empirically calculated soft toric lenses and GP toric/bi-toric contact lenses at all distances. That was also true for stereoscopy. Subjectively, most of the subjects preferred rigid contact lenses because of the quality of their vision in day-to-day activities, and a majority of them decided to continue with that modality. CONCLUSION Empirical fitting of toric soft or GP contact lenses leads to very good results, which suggests that each type of lens might constitute a viable option.

Contribution of soft lenses of various powers to the optics of a piggy-back system on regular corneas.

PURPOSE This study aims to report on the measured in vivo contribution of soft lenses of various powers to the optics of a piggyback system (PBS). METHODS This prospective, non-dispensing clinical study was conducted on regular wearers of contact lenses who showed regular corneal profiles. Subjects were masked to the products used. The study involved the use of a spherical soft lens of three different powers in a PBS, used as a carrier for a rigid gas permeable lens. Baseline data were collected and soft lenses were then fitted on both eyes of each subject. Both lenses were assessed for position and movement. Over-refraction was obtained. Soft lens power contribution to the optics (SLPC) of a PBS system was estimated by computing initial ametropia, lacrymal lens, rigid lens powers and over-refraction. A set of data on one eye was kept, for each subject, for statistical analysis. RESULTS Thirty subjects (12 males, 18 females), aged 24.4 (±4.5) years, were enrolled. The use of plus powered soft lenses enhanced initial RGP lens centration. Once optimal fit was achieved, all lenses showed normal movement. SLPC represented 21.3% of the initial soft lens power when using a -6.00 carrier, and 20.6% for a +6.00. A +0.50 did not contribute to any power induced in the system. These results are generally in accordance with theoretical model developed in the past. CONCLUSION On average, except for the low-powered carrier, the use of a spherical soft lens provided 20.9% of its marked power. To achieve better results, the use of a plus-powered carrier is recommended.

Variation of clearance considering viscosity of the solution used in the reservoir and following scleral lens wear over time

PURPOSE This study aims to evaluate the settling of a scleral lens and if this process is influenced by the nature of the fluid layer. METHODS A prospective, non-randomized control study was performed using an 18mm scleral lens. They were fitted with a central clearance of 400 um at insertion. One eye was randomly assigned to be fitted with a non-preserved gel solution of carboxymethylcellulose, while the other was inserted with non-preserved saline. Measurements of clearance in 3 locations were taken (OCT) at baseline, every 30min up to 1h30 post insertion and every 2h thereafter up to 6h00 of wear. A two-way repeated measure analysis of variance (liquids×times) was used to test central, nasal and temporal fluid thickness. RESULTS Following 6h of wear, the 18mm lens had a mean central settling of 70.0±9.8μm, 36.7±9.8μm of which occurred within the first 30min of wear. There was no significant difference between lenses filled with non-preserved saline to those with non-preserved gel. However, a paired comparison concluded to a significant difference between mean nasal settling (41.4μm) and temporal settling (20.4μm). CONCLUSION With respect to the lens studied, current results suggest that practitioners can evaluate the lens 30min post insertion and can estimate the amount of fluid that will remain after lens stabilization by doubling the value obtained initially. The use of non-preserved saline or non-preserved more viscous solution to fill the lens does not influence its settling.

Comparing two different daily disposable lenses for improving discomfort related to contact lens wear.

PURPOSE This study aimed to compare two daily disposable lenses to reduce lens-induced discomfort (CLD) among different populations of symptomatic wearers. METHODS This was a multicenter, crossover, study. Subjects were contact lens wearers reporting CLD. Subjects (76) were randomly assigned to wear Nelfilcon A (NF) then Delefilcon A (DF)- group A- or vice-versa (group B). Tear break-up time (TBUT), corneal (CS) and conjunctival staining (CJS) were graded at every visit. Subjectssymptoms were evaluated with Contact Lens Dry Eye Questionnaire (CLDEQ-8) and a research questionnaire (Université de Montréal). RESULTS Compared to baseline (BL), TBUT did not vary (BL) 6.7+2.1s vs. 6.7+1.8s (NF) vs. 6.4+1.9 (DF) (F=1.69; p=0.201) NF wearers were more at risk of developing moderate CJS (odds ratio 14.324; 95% CI 3.103-66.126) and more likely to show increased CS (odds ratio 8.754; 95% CI 2.181-35.146). Comfortable hours of wear reported at BL (7.6+2.3h) did not improve significantly with NF lenses (8.5+ 2.6h; F=1.095; p=0.394) but increased by 22% with DF lenses (10.7+3.0h; F=2.434; p=0.031). The total CLDEQ-8 score was improved with DF lenses (BL 17.2 vs. NF 16.1, p=0.397; vs. DF 8.3, p<0.001), especially on more symptomatic subjects. In the end, 92% of patients preferred to switch over DD lenses, 78% selecting DF material for the future. CONCLUSION Outcome from switching CLD wearers to DD lenses is material related. It seems that initial severity of the symptoms could be also a factor to consider.

Prevalence of asymptomatic ocular conditions in subjects with refractive-based symptoms

BACKGROUND This study aims to determine the overall prevalence of ocular conditions in a population from 19 to 64 years old, presenting with refractive-based symptoms only. Results could impact clinical standard of eye care on a similar population. METHODS This is a retrospective study on patients seen for an eye examination at the Clinique Universitaire de la Vision (CUV), between January 2007 and 2009. Files of individuals who presented with refractive symptoms were only selected and classified by file number. Then, every third file from the beginning was kept and reviewed by a reader. A second reader did the same with every third file from the end. Both readers were trained to use the same analysis grid to classify the diagnosed ocular conditions. In the case of multiple findings, the most severe condition was considered. The overall prevalence of ocular conditions was determined by calculating their occurrence divided by the number of files analyzed. RESULTS A total of 860 charts were analyzed. In 26.1% of the cases an ocular condition was diagnosed. This work establishes a higher prevalence of ocular conditions compared to another study conducted in Canada in the past. This difference can be explained by a different analytical methodology and by the fact that all examinations, in this study, were made under pupillary dilation. CONCLUSION The presence of ocular conditions in 26% of asymptomatic patients supports the need to assess ocular health under pupil dilation as part of any eye examination. However, further cost-to-benefit analysis is required before establishing such a recommendation.