Kenneth A. Polse

Tear mixing under a soft contact lens: effects of lens diameter.

PURPOSE Tear exchange under a soft contact lens is modest, and higher exchange rates may be necessary to reduce extended-wear complications; what is not known is the optimal soft lens design to increase tear mixing. We explored the effect of lens diameter on tear mixing. METHODS Twenty-three subjects wore four different soft contact lenses with diameters of 12.0, 12.5, 13.0, and 13.5 mm. Tear mixing was quantified by placing fluorescein isothiocyanate-dextran on the posterior lens surface, inserting the lens, and monitoring the changes in fluorescence intensity in the postlens tear film. Tear mixing, expressed as the percentage decrease in fluorescence intensity per blink, was estimated using an exponential model. Lens movement was videotaped and lens comfort was graded on a 50-point scale (50 = excellent comfort). Subjects reporting a comfort level of less than 35 were excluded. RESULTS The mean +/- SE tear mixing rates were 1.82% +/- 0.17%, 1.61% +/- 0.16%, 1.34% +/- 0.17%, and 1.24% +/- 0.17% per blink for the 12.0-, 12.5-, 13.0-, and 13.5-mm diameter lenses, respectively. By regression analysis we found that, on average, mixing under the 12.0-mm lens was 0.59% per blink greater than with the 13.5-mm lens (P = .0024). Lens diameter was a significant predictor of lens comfort, and adjusting for the effects of comfort weakened the relationship between diameter and tear replenishment rate, although the mean rate under the 12.0-mm lens was still 0.43% per blink greater than with the 13.5-mm lens (P = .0468). CONCLUSIONS These data suggest that smaller-diameter soft lenses provide substantially better tear mixing than larger lenses; however, even small lenses provide modest tear mixing compared with rigid contact lenses.

The Berkeley Orthokeratology Study, Part II: Efficacy and duration.

ABSTRACT Relative efficacy of orthokeratology (OK) was evaluated by assessing changes in refractive error, visual acuity, and corneal curvature in 31 treated and 28 randomized control subjects who wore conventional rigid contact lenses. The duration of changes was studied by monitoring subjects after lens wear was discontinued. After an average of 444 days of contact lens wear the treatment group showed an overall mean reduction in spherical equivalent refractive error of 1.01 D compared with 0.54 D in the control group (p = 0.02). Both groups had considerable variation in refractive error change. Corresponding mean improvements in unaided visual acuity were −0.27 and −0.20 log of the minimum angle of resolution [log (MAR)]. Corneal curvature decreased in both comparison groups, but the actual diopter value was about one‐half that of the refractive change. The changes in these characteristics tended to occur during the first 132 days of wear, and additional aggressive lens therapy during the remaining 241 days of treatment produced little additional change. The refractive error fluctuated considerably during the period of follow‐up and these fluctuations tended to be larger in those subjects who had shown greater changes in refractive error. When the lenses were removed, ocular characteristics returned steadily toward baseline levels. Ninety‐five days after discontinuing lens wear, the refractive error had returned 75 and 69% of the way to baseline levels for the treatment and control groups, respectively. Visual acuity and corneal curvature showed similar rebound after 95 days. We conclude that it is possible to reduce myopia about 1 D; however, the change is not permanent. Results indicate that the level of vision during periods of nonlens wear would be unstable, making it difficult to predict what the quality of vision would be under a retainer lens wear program.

Changes in tear flow accompanying aging.

&NA; The tear‐turnover rate of 68 normal human subjects (age range 15‐63 yr) was determined using a slit lamp modified to serve as a fluorophotometer. Tear volume and tear secretion, computed from tear‐turnover rate, had mean values of 8.47 &mgr;l and 1.78 &mgr;l/min, respectively. Analysis of the results shows a decrease in tear production with age. The mean tear‐flow rate was 1.55 &mgr;l/min in subjects aged 15‐39 and 0.95 &mgr;l/min in subjects aged 40‐63. This fluorophotometer is adaptable to a clinical setting and may prove useful in diagnosis and treatment of anomalies of tear production.

The etiology of refractive changes at high altitude after radial keratotomy: Hypoxia versus hypobaria

OBJECTIVE Refractive changes at high altitude that occur after radial keratotomy (RK) may be caused by hypoxia or hypobaria. DESIGN A prospective study was performed to evaluate the effects of hypoxia on RK and non-RK corneas. PARTICIPANTS There were 20 RK and 20 control eyes. INTERVENTION These eyes were subjected to ocular surface hypoxia using an air-tight goggle system at sea level for 2 hours. MAIN OUTCOME MEASURES Keratometry, cycloplegic refraction, and pachymetry were evaluated using repeated measures analysis of variance. RESULTS A significant hyperopic shift (P < 0.0001) and corneal flattening (P < 0.0013) occurred in all subjects with RK compared with those of control subjects. Corneal thickening occurred symmetrically in both groups. CONCLUSIONS These results suggest that refractive changes in subjects with RK occur at high altitude as a direct result of corneal hypoxia.

Central and peripheral corneal swelling accompanying soft lens extended wear.

ABSTRACT Central and peripheral corneal thickness changes were measured after 3 hr of eye closure with and without contact lens wear. Spun cast and lathe cut hydrogel extended wear lenses of −1.25, −6.00, and −9.00 D were used. Each of these conditions produced 25 to 50% less swelling in the peripheral than in the central cornea. Central swelling was greater with higher minus spun cast lenses, but not related to the power of the lathe cut lenses. Higher minus lens powers of both lens types produced greater peripheral swelling. Differences between central and peripheral swelling were smaller with the higher minus lenses, indicating little, if any, averaging of tear oxygen tension under the lens when the eyes are closed. The mechanism resulting in less peripheral corneal swelling is likely to be caused by an anatomical “clamping” at the corneal limbus.

Soft lens extended wear affects epithelial barrier function.

OBJECTIVE To explore the impact of eye closure and soft contact lens extended wear (SCLEW) on epithelial permeability to fluorescein (Pdc). DESIGN A prospective cohort study. PARTICIPANTS Thirty-one noncontact lens (CL) wearers participated. INTERVENTION The effects of eye closure on Pdc were evaluated by comparing morning (AM) and afternoon (PM) measurements on non-CL subjects. The effects of SCLEW on Pdc were determined by measuring Pdc before beginning SCLEW and then after 2, 4, and 12 weeks of SCLEW. MAIN OUTCOME MEASURES Pdc measured in the morning versus the afternoon and before versus after SCLEW was examined. RESULTS Analyses of Pdc were done using the natural logarithm (ln). The mean +/- standard error (SE) ln(Pdc) measured in the AM versus PM on 16 non-CL wearers did not differ significantly (-2.56+/-0.16 vs. -2.69+/-0.15, respectively; P = 0.46). In contrast, the mean +/- SE ln(Pdc) in 15 subjects after 2 (-1.73+/-0.183, P < 0.001), 4 (- 1.59+/-0.188, P < 0.001), and 12 weeks (-1.99+/-0.206, P = 0.02) of SCLEW was substantially greater than that measured before lens wear (-2.42+/-0.159 ln(nm/sec)). Interestingly, the mean+/-SE ln(Pdc) measured in the afternoon on seven subjects after 13 weeks of SCLEW was similar to their average baseline ln(Pdc) (-2.62+/-0.27 vs. -2.52+/-0.243, respectively; P = 0.54). Further analysis showed that Pdc was highest in the morning and decreased approximately 12.5%/hour (P < 0.001) during the day. CONCLUSIONS Sleeping without CLs does not appear to alter Pdc; however, 2 weeks of SCLEW caused a 99% increase in permeability without observable changes by slit-lamp examination. Increases in Pdc appear greatest in the morning after SCLEW and then decrease exponentially during the day. Whether changes in Pdc will predict CL-associated keratopathy needs further study.

Influence of Wearing Schedule on Extended-wear Complications

The effect of wearing schedule on extended-wear (EW) success was evaluated over a nine-month period by assigning 36 subjects to 5 different wearing schedules (removal every 4, 7, 14, or 28 days, or daily wear [DW]). All but one EW patient required interruption of extended wear at least once; half of the DW patients completed the study without interruption. The most prevalent (100%) complication in the EW group was epithelial microcysts; 54% had to discontinue EW for 67 +/- 30 days while severe microcysts subsided. Evidence is given for a hypoxic etiology of microcysts. Other EW complications included: persistent punctate keratitis, infiltrative keratitis, infection, and idiopathic red eye. Neither the level of overnight corneal swelling nor the period between removals influenced the incidence or severity of complications. Data is given on the recovery of corneal parameters after EW is discontinued.

The Berkeley Orthokeratology Study, part III: safety.

ABSTRACT The safety of orthokeratological (OK) procedures was assessed by monitoring several ocular characteristics including corneal thickness, refractive astigmatism, corneal astigmatism, correctable spectacle acuity, corneal edema, corneal staining, and endothelial cell density. Safety was also assessed by reviewing the number and causes of extra clinical visits that occurred because of ocular complications and by determining whether safety factors were related to patients lost to followup. Over the 1.2‐year period from the baseline examination to the end of the lens‐wearing phase of the study, the Treatment (T) and Control (C) groups had, respectively, only small changes in their mean levels of corneal thickness (0.2 vs. 3.2 &mgr;m), refractive astigmatism (0.07 vs 0.01 D), and corrected spectacle acuity (0.02 vs 0.018 logarithm of the minimum angle of resolution). Analysis of time trend data obtained from scheduled monthly examinations also showed that changes in mean corneal thickness and mean corneal astigmatism were small and not clinically important in either comparison group during the course of the treatment. As a result of complications from lens wear, the T group required 1.25 times as many extra visits as the C group (p = 0.14). However, none of these visits provided sufficient clinical indication for the discontinuance of lens wear. Loss to follow‐up typically occurred because of either poor compliance with the study protocol or loss of motivation caused by minor contact lens‐related symptoms such as blurred vision, slight discomfort, or frequently lost lenses. These reasons were similar in both comparison groups. It appears that OK treatment is safe for the types of patients who participated in this study, but it may require more patient monitoring than would be needed to achieve and maintain a physiologically acceptable fit with conventional hard contact lens prescriptions.

Effect of Rigid Contact Lens Oxygen Transmissibility on Stromal PH in the Living Human Eye

Corneal stromal pH was measured fluorometrically in nine human subjects after 1.5 hours of eye closure while wearing hard gas permeable contact lenses. Six lens types providing a wide range in oxygen transmissibilities (Dk/L)O2 from 0.15 to 55.0 X 10(-9) (cm ml O2/seconds of ml mmHg) were used. Stromal pH upon opening the eyes was directly related to (Dk/L)O2 and ranged from 7.01 to 7.26. These pH values were all significantly lower than the control (no lens), which had a pH of 7.38 (P less than 0.005). After eye opening (lens still on) stromal pH increased and reached a new steady-state (range, 7.11-7.45) in approximately 30 minutes; however, these pH levels were still below the control (no lens) (pH 7.54; P less than 0.005). The rate of pH change after eye opening also increased directly with (Dk/L)O2. The authors conclude that the lenses presently available provide insufficient gas exchange to prevent alteration of corneal pH and suggest that the measurement of stromal pH may provide a sensitive index for evaluating the metabolic effects of contract lens wear.

Hypoxia, overnight wear, and tear stagnation effects on the corneal epithelium: data and proposed model.

Purpose. To explore the possible mechanisms that may lead to overnight contact lens–associated corneal morbidity by examining data from several corneal epithelial permeability experiments obtained under different environmental and lens fitting paradigms. Methods. Epithelial permeability was assessed by using fluorometry to determine the fluorescein penetration rate from the tear film into the corneal stroma. Changes in this rate provide an index of the corneal epithelial status; increased permeability leads to decreased barrier function. Results. Hypoxia and tear stagnation during overnight lens wear play a significant role in altering the corneal epithelial barrier function (P<0.05). However, eliminating lens-induced hypoxia alone does not ameliorate changes in epithelial status (P<0.05). Conclusions. Based on data from these experiments, it is suggested that hypoxia and tear stagnation should be eliminated to minimize alteration of the corneal epithelium associated with overnight contact lens wear.