Noel A. Brennan

Anatomically accurate, finite model eye for optical modeling

There is a need for a schematic eye that models vision accurately under various conditions such as refractive surgical procedures, contact lens and spectacle wear, and near vision. Here we propose a new model eye close to anatomical, biometric, and optical realities. This is a finite model with four aspheric refracting surfaces and a gradient-index lens. It has an equivalent power of 60.35 D and an axial length of 23.95 mm. The new model eye provides spherical aberration values within the limits of empirical results and predicts chromatic aberration for wavelengths between 380 and 750 nm. It provides a model for calculating optical transfer functions and predicting optical performance of the eye.

Visual decrement with deposit accumulation on HEMA contact lenses

ABSTRACT The vision decrement associated with deposit accumulation on hydroxyethyl methacrylate (HEMA) contact lenses was assessed in 51 patients presenting consecutively to a large clinic. Both high contrast visual acuity (HCA) and low contrast visual acuity (LCA) (logMAR) decreased with increased deposition (Rudko classification). Apart from being consistently lower than HCA, LCA offered no additional useful information. Both HCA and LCA worsened with lens age, whilst lens deposition increased with lens age (p < 0.001). No associations between any of the above measures and patient symptoms were revealed. In general, unacceptable vision decrement and deposit formation occurred after 12 months or 4,000 h of daily lens wear. Clinicians can use the data presented in this paper to reconcile vision loss with deposit formation in patients wearing HEMA lenses.

A 1-year prospective clinical trial of balafilcon a (purevision) silicone-hydrogel contact lenses used on a 30-day continuous wear schedule

PURPOSE To evaluate silicone-hydrogel balafilcon A (PureVision, Bausch & Lomb Inc., Rochester, NY) contact lenses worn on a 30-day continuous wear basis when compared with a traditional hydrogel (Acuvue, Vistakon, Johnson and Johnson Vision Products, Inc., Jacksonville, FL) worn on a 7-day extended wear schedule. DESIGN Prospective, comparative, paired-eye, interventional, multicenter clinical trial. PARTICIPANTS Two hundred twelve ametropes with no significant ocular pathology. METHODS Subjects with normal ocular health who conformed to a set of standard criteria were monitored at seven different sites around the globe. Subjects wore both study lenses simultaneously, one on each eye. Lenses were assigned to right and left eyes according to a randomized schedule. The silicone hydrogel was removed and replaced every month and the traditional hydrogel was removed every 7 days and replaced at 14-day intervals. Subjects were followed for up to 1 year with discontinuation on indication divided between lens-related causes, unrelated factors, and loss to follow-up. MAIN OUTCOME MEASURES Discontinuations, symptoms, vision, lens fit, lens surface assessment, slit-lamp assessment, and patient preferences. RESULTS One hundred twenty three subjects completed the 12-month wearing period. There were no vision-threatening events with either lens type. The silicone-hydrogel lenses showed statistically superior performance in physiologic terms with regard to epithelial microcysts, striae, corneal staining, limbal injection, and bulbar injection. Inflammatory reactions, such as corneal infiltrates and tarsal conjunctival abnormalities occurred at similar rates between the lens types. Visible deposition and postlens debris with associated transient corneal indentation were significantly lower with the traditional hydrogel material at specific time points in the study. Visual acuity was comparable with the two lens types. Subjects rated the silicone-hydrogel material as preferable on subjective scales of dryness, comfort, and lens handling. Overall subjective performance was rated superior with the silicone-hydrogel material. CONCLUSIONS The high oxygen performance of the silicone-hydrogel lenses provides a superior physiologic environment compared with a traditional hydrogel material during continuous wear. Despite the slightly higher levels of visible deposit on the silicone-hydrogels, superior subjective responses suggest that the plasma coating technology on these lenses is effective in providing comfortable lens wear. The results look promising for the success of silicone-hydrogels as the material of choice for continuous wear contact lenses.

Symptomatology of HEMA contact lens wear.

Identification of the determinants of patient symptoms is a requisite for successful patient management in contact lens practice. Symptoms reported by 104 patients wearing hydroxyethyl methacrylate (HEMA) contact lenses, who presented consecutively to a large contact lens clinic, were analyzed. Dryness was reported more frequently than the symptoms scratchy and watery (p<0.001). Females using oral contraceptives were more likely to experience the symptoms of scratchiness (p<0.01) and dryness (p<0.05) than females who were not using oral contraceptives. The symptom dryness was reported more frequently by patients whose lenses were older than 6 months (p<0.05), and those wearing toric lenses (p<0.01). These findings will assist practitioners to anticipate those patients who are likely to develop contact lens-related symptoms and lay the foundations for a model of symptomatology during contact lens wear.

Corneal pathophysiology with contact lens wear

Contact lens wear induces a wide spectrum of changes in the appearance and function of the cornea. The most salient effect of lens wear is the hypoxically induced reduction in the rate of metabolic activity of the corneal epithelium and its sequellae. Other important alterations to corneal health associated with contact lens wear may be caused by antigenic and toxic stimuli, mechanical forces, osmotic effects and carbon dioxide retention. Perhaps the most important task facing the contact lens clinician is to distinguish between an acceptable state of physiological modification and an anomalous or pathological state of hypofunction. In this article, we review the assortment of corneal changes primarily on the basis of the causative agents and time scale with reference to the physical and chemical processes leading to the observed signs or symptoms. This procedure allows a strong foundation for understanding the etiology and management principles for the variety of effects that contact lenses may have on the cornea.

The duration of normal visual exposure necessary to prevent form deprivation myopia in chicks

The aim of this study was to determine the minimum daily period of exposure to normal visual stimulation required to prevent occlusion induced myopia in chicks. Chicks were treated with monocular translucent occlusion in a 12 hr light/12 hr dark cycle. Occluders were removed for 0 (constant occlusion), 15, 20, 30, 40, 60, 75, 90, 120, 150, 240 or 720 (no occlusion) minutes each day for either 2 or 3 weeks. Fellow eyes and the eyes of normal chicks (bilaterally unoccluded) were used as controls. Occlusion-induced myopia and axial elongation were found to decrease significantly (P < 0.01) with increasing daily exposure to normal visual stimulation. Application of a time series equation to the data estimates that 30 and 130 min of normal visual exposure per day reduces myopia by 50 and 95% respectively. This study demonstrated that the regulation of ocular growth is affected strongly by short periods of normal visual stimulation in the presence of long periods of abnormal stimulation.

Oxygen permeability and water content of silicone hydrogel contact lens materials.

Purpose. To measure the oxygen permeability (Dk) and water content (WC) of silicone hydrogel (Si-Hy) contact lens materials. Methods. Randomized and masked determinations of the Dk of 5 Si-Hy and two hydrogel materials were made using a modified version of the polarographic measurement method described in ISO 9913-1. Stacks of one to six parallel-sided contact lenses (all –1.00 DS) were evaluated, with each stack measured at least twice. The resulting value for t/Dk was plotted against the thickness (t) of each stack, with Dk calculated as the inverse of the gradient of this relationship. This methodology corrects for boundary effects. A mathematical calculation was used to correct for edge effects. Gravimetric determination of lens WC was conducted at room temperature and 35°C. Results. Measured values (±95% CI) of Dk, and WC at room temperature, with manufacturer-claimed values in parentheses, were Focus Night & Day: Dk 162.0 ± 9.8 (140), WC 23.0 ± 3.2 (24); Acuvue Oasys: Dk 107.4 ± 7.4 (103), WC 36.9 ± 1.0 (38); O2 Optix: Dk 80.5 ± 4.9 (110), WC 32.1 ± 1.2 (33); PureVision: Dk 75.9 ± 6.6 (91), WC 35.8 ± 1.3 (36); Acuvue Advance: Dk 75.2 ± 9.8 (60), WC 46.5 ± 1.1 (47); 1.Day Acuvue: Dk 21.0 ± 1.0 (21.4), WC [not measured] (58); and Seequence: Dk 8.2 ± 0.7 (8.5), WC 36.6 ± 2.7 (38). Conclusions. Claimed Dk values for Acuvue Oasys and the two reference hydrogel materials fell within the 95% confidence interval of our measured values. Our measurements of Dk for the other four Si-Hy lenses were not in agreement with claimed values. There is a general inverse relation between Dk and WC (both at 35°C) for Si-Hy lenses. Our modified polarographic methodology can be successfully employed for measuring the Dk of Si-Hy materials.

Effect of Hydrogel Lens Wear on Tear Film Stability

Noninvasive break-up time (NIBUT) of the tears was measured in a controlled, randomized, double-masked study to assess: (1) the stability of the prelens tear film during wear of new high and low water content lenses and (2) the stability of the precorneal tear film following lens removal after 1 h of wear. The prelens tear film NIBUT of 6 subjects was found to be relatively constant over a 1-h wearing period, averaging 6.1 ± 1.1 s (mean ± SEM). These values were significantly (Scheffes S test, p<0.05) lower than those recorded for the precorneal tear film before lens insertion (33.5 ± 10.6; mean ± SEM), although 85% of prelens tear film Nl- BUTs were greater than the 3-s average interblink period reported previously for soft lens wearers. After lens removal, precorneal tear film NIBUT was reduced significantly compared to prewear levels (Scheffes S test, p<0.05) for up to 15 min. Application of the monomolecular growth model to the NIBUT recovery data revealed a half-time for recovery of 6.0 min, with recovery 95% complete 25.8 min after lens removal. Lens type was not a significant factor in tear film stability, either during wear or after lens removal. The basis for reduced precorneal tear film NIBUT after lens removal is unknown; however, a disruption of the mucin layer coating the corneal epithelium is the most likely mechanism. Indeed, the technique of measuring precorneal tear film NIBUT after lens removal may be a useful determinant of the extent to which contact lens wear disrupts the precorneal mucin layer, providing an indication of the susceptibility of the cornea to a variety of complications.

Decreased uncorrected vision after a period of distance fixation with spectacle wear

Myopes of low degree commonly report that their vision seems poorer upon removal of their spectacles compared to that after a period without spectacle wear. Notably, this difference in vision can be appreciated after distance fixation. In this paper, we propose and test several alternative hypotheses to explain the phenomenon: an accommodative response to spectacles, sensory adaptation, or altered criteria for blur of psychological origin. We measured visual acuity (VA), refractive error, and lens thickness on 10 subjects with less than 2.00 D of myopia. Testing was performed after two 90-min sessions viewing at distance. At one session, the subjects wore their current spectacle correction and, at the other session, no correction was worn. VA underwent a slight but significant decrease (0.4 of a line) after the session in which spectacles were worn, but no difference in refractive error or lens thickness was found. The change in acuity in the absence of a refractive shift suggests sensory adaptation to blur. However, the demonstrated change in VA appears to be less than that which is subjectively reported; accordingly, psychological input cannot be ruled out.

The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Materials, Design, and Care Subcommittee

Jones, L., Brennan, N. A., Gonzalez-Meijome, J., Lally, J., Maldonado-Codina, C., Schmidt, T. A., … Nichols, J. J. (2013). The TFOS International Workshop on Contact Lens Discomfort: Report of the Contact Lens Materials, Design, and Care Subcommittee. Investigative Opthalmology & Visual Science, 54(11), TFOS37. https://doi.org/10.1167/iovs.13-13215