Manuel A. Parafita

Central and peripheral corneal thickness measurement with Orbscan II and topographical ultrasound pachymetry

Purpose: To compare thickness measurements of the central 6.0 mm of the cornea obtained with the Orbscan® II topography system and topographical ultrasound pachymetry. Setting: School of Optometry, University of Santiago de Compostela, Galicia, Spain. Methods: In 24 right eyes, pachymetric measurements were taken at the center and 1.2 mm and 3.0 mm on the superior and inferior hemimeridians. A 1‐sample t test was applied to assess the significance of the relationship between Orbscan II and ultrasound methods. The relationship between the 2 was assessed by analyzing regression and plotting the differences against the mean corneal thickness. Orbscan II data were analyzed in 3 ways: (1) without the application of an acoustic equivalent correction factor; (2) with a correction factor of 0.92, as recommended by the manufacturer; (3) with correction using the equations derived in this study. The data were systematically compared with those of ultrasound pachymetry. Results: Before the correction factor was applied, the Orbscan II overestimated the corneal thickness at all locations, with the mean difference (48.15 &mgr;m ± 33.74 [SD]) significantly different from zero (P < .001). Differences increased toward the periphery, and the reliability of Orbscan II readings seemed to decrease with thicker corneas. After the acoustic equivalent was applied, the differences were significantly less; however, this effect did not seem clinically significant as large differences remained. When specific corrective equations were applied for each corneal location, the level of agreement between Orbscan II and ultrasound pachymetry improved substantially; the mean (–0.11 ± 15.22 &mgr;m) was not statistically different from zero (P > .05). Conclusions: The acoustic equivalent correction factor proposed by the manufacturer to obtain corneal thickness measurements with the Orbscan II compared to those from ultrasound pachymetry was not valid for all corneal topography positions. Orbscan II measurements agreed better with those of ultrasound pachymetry when equations for the central and each peripheral location across the topography were applied.

Clinical performance of the Reichert AT550: a new non-contact tonometer

The aim of the present study was to assess the level of accuracy for measurements of intra‐ocular pressure (IOP) obtained with a new non‐contact tonometer (NCT) the Reichert AT550®. Measurements were compared against those obtained with the Reichert Xpert Plus, Goldmann applanation tonometer and Perkins tonometer. Thirty‐five university students were assessed with the four tonometers in a randomised order, with non‐contact tonometry performed first. Each of the four measurement devices had its own trained clinical observer. Plots of differences of IOP as a function of the mean for each pair of instruments were obtained. No statistically significant differences were found when comparing the AT550® NCT with contact applanation tonometry (AT) (p > 0.05), displaying the closest level of agreement (as represented by the lowest mean difference and the narrowest confidence interval) with the Goldmann tonometer (limits of agreement, 0.12 ± 2.17). In conclusion, readings of IOP with the AT550® NCT are clinically comparable with those obtained with Goldmann tonometry in a population with IOP within the normal range.

Accuracy and precision of EyeSys and Orbscan systems on calibrated spherical test surfaces.

Background: It is important to know how well the surface topography can be measured with current devices for corneal topographic analysis. There are several applications that need an accurate and precise method to measure corneal shape and variations, such as the effect of contact lens wear and the different refractive surgery techniques. Purpose: The aim of this study is to compare the accuracy and reproducibility of the measurement of the central curvature on calibrated steel balls using the EyeSys videokeratoscope and the Orbscan corneal topography system. Methods: The videokeratoscope (EyeSys Corneal Analysis System 2000, version 3.1) and the Orbscan corneal topography system (Orbscan II version 3.0) were used by four trained investigators to measure a series of five uniform and calibrated test surfaces with known radius of curvature ranging from 6.13 to 9.00 mm. Result: No statistically significant difference was found between the videokeratoscope and Orbscan systems in relation to accuracy or precision. The 95% confidence limits showed a close agreement between both instruments. The mean bias was less than ± 0.05 mm for both devices. The precision of the instruments was found to be similar. Conclusion: The EyeSys seems to measure more accurately, but the accuracy of the Orbscan was also acceptable, suggesting that both instruments are accurate and precise enough for research and clinical purposes. However, further studies of accuracy and repeatability of topographical measurements on human eyes generated by different topographers are necessary.

Correlations between corneal biomechanical properties measured with the ocular response analyzer and ICare rebound tonometry.

PurposeTo investigate the biomechanical properties of the normal cornea, and correlate them with central and peripheral corneal thickness and age. MethodsSeventy-six right eyes of volunteers were measured with Ocular Response Analyzer (ORA), ICare rebound tonometry and an ultrasound pachymeter at corneal center and at 4 mm from corneal center in the nasal and temporal directions. ResultsICare readings were significantly correlated with central and peripheral corneal thickness and corneal biomechanical properties. Corneal resistance factor was the biomechanical parameter with the higher correlation with ICare intraocular pressure (IOP) values. ICare tonometry at center and Goldmann equivalent IOP obtained with ORA were significantly higher for thicker than thinner corneas (P<0.05). IOP compensated for corneal properties with the ORA was lower than the remaining IOP values measured in the study. Higher correlation was found between Goldmann equivalent IOP with ORA and ICare IOP values. ConclusionsIOP values obtained with the rebound tonometer are higher in thicker corneas and are positively correlated with biomechanical corneal parameters, namely corneal resistance factor. Although corneal thickness plays a significant role in rebound tonometry, elastic and viscous properties of the cornea seem to play a significant role in the interaction of the tonometer probe with the ocular surface. However, the mechanism behind this process is presently unknown.

Clinical performance of non-contact tonometry by Reichert AT550 in glaucomatous patients.

Measuring intraocular pressure (IOP) by non‐contact tonometry (NCT) has been demonstrated to be a valid and reliable technique to be used in primary eye care; it is easier to use, it does not transmit infectious diseases, and it is not necessary to use anaesthetic or staining eye drops. Recently, a new NCT device has showed an excellent level of agreement with Goldmann tonometry, but there are no records of its performance in glaucomatous eyes. To rectify this, IOP was measured in twenty‐two patients (44 eyes) receiving medical treatment to control elevated IOP, with AT550® and Goldmann tonometry. Mean values of IOP were 18.98 ± 2.77 and 19.08 ± 3.02 mmHg using Goldmann and AT550®, respectively. Plots of differences against means displayed good agreement (mean difference ± limits of agreement, −0.09 ± 3.30); this value was not significantly different from zero (t‐test for dependent samples, p = 0.709). In conclusion, IOP values as measured with the AT550® NCT are clinically comparable with those obtained with Goldmann tonometry in glaucomatous patients. This validates this NCT not only for screening of IOP but to follow‐up glaucomatous patients with a rapid, non‐invasive method.

Changes in Corneal Structure with Continuous Wear of High-Dk Soft Contact Lenses: A Pilot Study

Purpose. Despite numerous studies that have considered the effects of extended wear of high-Dk soft contact lenses on ocular physiology, little attention has been paid to the impact of such lenses on central or peripheral corneal thickness and curvature. The present study aims to report the time course of changes in corneal thickness and curvature that accompanies the 30-night continuous wear of new silicone hydrogel soft contact lenses in a neophyte population in a longitudinal study. Methods. Six subjects wore high-Dk lotrafilcon (Dk = 140) on a 30-night replacement schedule for 12 months. Only measurements from the right eye were considered for analysis. Topographical measurements of corneal thickness and curvature were taken. The same parameters were monitored for an additional period of 3 months after lens removal. Results. An almost homogenous increase in corneal radius of curvature was detected for all the locations studied, being statistically significant for the 4-mm cord diameter area. This effect was associated with a progressive thinning effect for the central cornea, whereas midperipheral and peripheral areas did not display such a thinning effect during continuous wear. These effects were still evident for the central cornea 3 months after contact lens wear discontinuation. Conclusions. Continuous wear of high-Dk silicone hydrogel contact lenses is associated with clinically appreciable changes in topographical corneal curvature, whereas only a reduction in corneal thickness is appreciated in the central area. This effect seems to be a result of mechanical pressure induced by these hybrid hyperpermeable materials, characterized by a higher modulus of elasticity. The small sample size compromises the conclusions addressed from this study, and further work will be necessary to confirm the present results.

Symptoms in a population of contact lens and noncontact lens wearers under different environmental conditions

Purpose. To investigate ocular symptoms related to dryness in an adult population of contact lens (CL) and non contact lens wearers (n-CL) using video display terminals (VDT) for different periods of time under different indoor conditions related to air conditioning (AC) and heating units (HU) exposure. Methods. A questionnaire was distributed to 334 people within a university population of which 258 were part of the n-CL group and 76 of the CL wearers to assess symptoms of ocular discomfort potentially related to dryness. Only soft contact lens (SCL) wearers (n = 71) were included for further statistical analysis because of the reduced number of people wearing other lens types. A 2:1 match by gender group of 142 subjects in the n-CL group was used as a control sample. Results. There was a marked difference between the prevalence of symptoms and the way they are reported by CL and n-CL wearers. Red eye, itching, and scratchiness are more common among CL wearers, but the difference is statistically significant only for scratchiness (p < 0.01, &khgr;2). The vast majority of subjects who reported symptoms often and at the end of the day are significantly more prevalent among CL wearers (p < 0.01, &khgr;2). Gender differences were also encountered. Female CL wearers reported more scratchiness than males in the n-CL wearing group (p = 0.029, &khgr;2) and in the CL group (p < 0.008, &khgr;2). Females wearing CL reported symptoms of red eye (p = 0.043, &khgr;2) and scratchiness (p < 0.001, &khgr;2) more significantly than those in the n-CL group. Within the CL group, the prevalence of symptoms occurring sometimes or often and at the end of the day was higher among females (p < 0.001, &khgr;2). The use of VDT was associated with a higher level of scratchiness among CL wearers (p < 0.05, &khgr;2). The number of hours working with VDTs seemed to be associated with an increase in the prevalence of burning sensation in the CL group (p < 0.01, &khgr;2), whereas symptoms like red eye and scratchiness also increased significantly among n-CL wearers. Compared to n-CL wearers, all symptoms increase in CL wearers in environments with AC and HU, except excessive tearing. However, these differences are only statistically significant for scratchiness. Conclusions. Our results show that people who wear soft CL and work with VDTs for longer periods of time are more likely to develop symptoms like eye burning and scratchiness than n-CL wearers. This risk could be higher for women than men. Scratchiness and the appearance of symptoms near the end of the day are typically associated with ocular discomfort during CL wear in this sample, and clinicians should question their patients about these symptoms to anticipate serious discomfort.

Surface AFM microscopy of unworn and worn samples of silicone hydrogel contact lenses

PURPOSE To evaluate the qualitative and quantitative topographic changes in the surface of worn contact lenses (CLs) of different materials using atomic force microscopy (AFM). METHODS The topography of five different CL materials was evaluated with AFM over a surface of 25 microm(2) according to previously published experimental setup. Average roughness (R(a)) and root mean square (Rms) values were obtained for unworn and worn samples. RESULTS The R(a) value increased for balafilcon A (11.62-13.68 nm for unworn and worn samples, respectively), lotrafilcon A (3.67-15.01 nm for unworn and worn samples, respectively), lotrafilcon B (4.08-8.42 nm for unworn and worn samples, respectively), galyfilcon A (2.81-14.6 nm for unworn and worn samples, respectively), and comfilcon A (2.87-4.63 nm for unworn and worn samples, respectively). Differences were statistically significant for all lenses except Rms and R(a) for comfilcon A, and R(a) parameter for balafilcon A (p > 0.05). The least relative increase was observed for some balafilcon A samples and for some of these samples the roughness decreased after the lenses had been worn. CONCLUSION The changes in surface roughness between unworn and worn lenses are different for different silicone-hydrogel materials. Overall all CLs increased the degree of surface roughness after being worn, even for very short periods of time. However, for samples of balafilcon A, roughness increases at a lower extent or even can decrease as compared to unworn samples of the same material due to filling of the macropores.

Refractive, biometric and topographic changes among Portuguese university science students: a 3-year longitudinal study

Purpose:  The aim of this study was to investigate the changes in refractive, biometric and topographic ocular parameters among university students in Portugal during a 3‐year period.

Age differences in central and peripheral intraocular pressure using a rebound tonometer.

Aim: To evaluate the influence of age on the measurements and relationships among central and peripheral intraocular pressure (IOP) readings taken with a rebound tonometer. Methods: The IOPs were measured using the ICare rebound tonometer on the right eyes of 217 patients (88 men and 129 women) aged 18–85 years (mean 45.9 (SD 19.8) years), at the centre and at 2 mm from the nasal and temporal limbus along the horizontal meridian. Three age groups were established: young (⩽30 years old; n = 75), middle aged (31–60 years old; n = 77) and old patients (>60 years old; n = 65). Results: A high correlation was found between the central and peripheral IOP readings, with the central readings being higher than the peripheral ones. Higher IOP values for the central location were found in the younger patients. Older patients had significantly lower temporal IOP readings than those for the remaining two groups (p<0.001), whereas no significant differences were found among groups when IOP was measured at the central and nasal locations. A significant decrease was observed in the nasal and temporal IOP readings as the age increased (p = 0.011 and 0.006, respectively). Conclusion: Older patients had lower IOP values than the middle-aged and younger patients in the temporal peripheral location. A negative correlation was found between age and IOP by rebound tonometry in the corneal periphery but not in its centre.