Ben Chan

The effect of age on ocular blood supply determined by pulsatile ocular blood flow and color Doppler ultrasonography

Purpose. Pulsatile ocular blood flow (POBF) assessment measures the choroidal circulation and provides diagnostic value to certain ocular diseases such as glaucoma. This technique assumes a constant ocular rigidity and is influenced by axial length, diurnal variation, and age. This study investigated the effect of age on POBF, with consideration of the above factors. Ocular blood supply in the ophthalmic artery was also determined using color Doppler ultrasonography. Methods. A total of 118 healthy subjects aged 19 to 75 years were recruited. They were divided into five groups (below age 30, 30 to 39, 40 to 49, 50 to 59, and 60 or above) of at least 20 subjects each. Only one eye of each subject, with axial length <24.5 mm, was considered. The subject’s supine POBF was determined followed by a measurement of the blood flow velocity in the ophthalmic artery using color Doppler ultrasonography. All the measurements were at around the same time of day to eliminate any effect from diurnal variation. The scleral rigidity was measured using a Schiotz tonometer with 5.5- and 7.5-g weights. Results. Linear regression analysis demonstrated a significant increase of scleral rigidity with age (Pearson correlation coefficient r = 0.26, p < 0.01) and a significant decrease of POBF with age (r = −0.35, p < 0.01). The reduction in peak systolic velocity in the ophthalmic artery with age was significant (r = −0.28, p < 0.01). Both the systolic and diastolic brachial pressure showed significant increase with age (r = 0.55, p < 0.01; r = 0.40, p < 0.01, respectively). Using multiple regression analysis, POBF showed a significant correlation with age (partial correlation r = −0.36, p < 0.01), but not with scleral rigidity or systolic or diastolic brachial pressure. The peak systolic velocity in the ophthalmic artery also showed significant correlation with age (partial correlation r = −0.29, p < 0.01). Conclusions. The reduction in POBF with age was significant. Although aging affects scleral rigidity and systemic blood pressure, multiple regression analysis indicates that the most influential factor affecting POBF is aging. The peak systolic velocity in the ophthalmic artery also decreased with age, indicating reduced ocular blood supply.

A comparative study of biweekly disposable contact lenses: silicone hydrogel versus hydrogel.

Background:  Our aim was to compare the clinical performance of a biweekly (second generation) silicone hydrogel lens and a biweekly hydrogel lens worn for daily wear modality.

Orthokeratology practice in children in a university clinic in Hong Kong

Purposes:  The aim of this study was to analyse clinical data of children undergoing orthokeratology (ortho‐k) and to investigate patients’/parents’ perspective on ortho‐k via telephone interviews.

Repeatability and agreement of two A‐scan ultrasonic biometers and IOLMaster in non‐orthokeratology subjects and post‐orthokeratology children

Purpose:  Our aim was to determine the repeatability of measurements of axial length (AL) and anterior chamber depth (ACD) made with two ultrasonic biometers and the IOLMaster in a group of non‐orthokeratology (ortho‐k) adult subjects and to investigate the agreement among instruments in children undergoing ortho‐k therapy and in children wearing spectacles.

The effect of axial length on ocular blood flow assessment in anisometropes.

Background: Pulsatile ocular blood flow (POBF) assessment measures the choroidal circulation and therefore provides data with diagnostic value in certain ocular diseases, such as glaucoma. The technique assumes a constant pressure–volume relationship. The current study investigated the effect of axial length on POBF from subjects with axial anisometropia. Ocular blood supply in the ophthalmic artery was also determined using colour Doppler ultrasonography.

Astigmatic changes in orthokeratology.

Purpose. To investigate and compare changes in refractive astigmatism and corneal toricity in orthokeratology (ortho-k) using the Thibos vector analysis. Methods. Consecutive records of 74 patients (aged 7 to 16 years) who were fitted with spherical ortho-k lenses at the Optometry Clinic of The Hong Kong Polytechnic University between April 2000 and November 2003 and who had been on the treatment for at least 6 months were reviewed. Manifest refractive error and topographic data were retrieved from the right eye for data analysis. Changes in refractive astigmatism and corneal toricity were evaluated using the Thibos vectors (refractive vectors, RJ0 and RJ45; corneal vectors, CJ0 and CJ45) among the astigmats (refractive astigmatism 0.75 diopters cylinder (DC) or above) and the non-astigmats (refractive astigmatism 0.50 DC or less). Results. The mean changes in refractive astigmatism, RJ0 and RJ45 were 0.51 ± 0.51 diopter (D), −0.31 ± 0.35 D, and 0.09 ± 0.16 D, respectively, among the astigmats; and 0.01 ± 0.36 D, 0.00 ± 0.17 D, and 0.03 ± 0.13 D, respectively, among the non-astigmats. RJ0 was significantly reduced, and RJ45 was significantly increased only in the astigmats (one sample t tests, astigmats: p < 0.01; non-astigmats: p > 0.10). Although there were significant correlations between the initial RJ0 and RJ45 with their changes in all subjects and in the astigmats, only 21 to 55% of the variability could be explained by the regression equations (Pearson r: all subjects −0.63 < r < −0.62; astigmats −0.74 < r < −0.46, p < 0.001). No significant changes in corneal toricity and no correlations between the changes in refractive astigmatism and changes in corneal toricity were found (p > 0.13). Conclusions. Spherical ortho-k leads to significant changes in RJ0 and RJ45, but only 38% of the variance of the changes in all subjects could be explained by relationship between changes and the initial RJ0 or RJ45. No correlation between changes in refractive astigmatism and changes in corneal toricity was found, and ortho-k treatment did not induce astigmatism in our non-astigmatic subjects.

Case report: the occurrence of fibrillary lines in overnight orthokeratology.

Background:  To report the occurrence of concentric fibrillary lines in the central corneas of a 13‐year‐old girl during overnight orthokeratology.

Relationship between corneal topographical changes and subjective myopic reduction in overnight orthokeratology: a retrospective study.

Background:  To investigate the relationship between the change in the manifest refractive error (ΔM), the change in apical corneal power (ΔACP) and initial corneal asphericity (Q) in overnight orthokeratology (ortho-K). Methods:  One hundred and twenty-eight clinical records of children undergoing ortho-K from a university optometry clinic were reviewed. The refractive and topographical data at baseline and at two-week visit of 58 patients who fulfilled the inclusion criteria were retrieved and analysed. Results:  Significant differences (p < 0.001) between the change in manifest refractive error and changes in the apical corneal power or the maximum change in corneal power (ΔMCP) within the treatment zone were found. Linear regression analysis was used to describe the change in manifest refractive error and the change in apical corneal power, and the change in manifest refractive error and the maximum change in corneal power, with the equations: ΔM = 0.91ΔACP + 0.57 (r = 0.78, p < 0.001) and ΔM = 0.93ΔMCP + 0.01 (r = 0.79, p < 0.001) respectively. On average, the change in apical corneal power underestimated the change in manifest refractive error by 0.34 ± 0.57 D; whereas on average, the maximum change in corneal power overestimated the change in manifest refractive error by 0.23 ± 0.57 D (paired-t-tests, p < 0.001). A low but significant correlation between initial corneal asphericity and the change in manifest refractive error (Spearman r = -0.33, p = 0.01) was observed. Conclusions:  The change in apical corneal power underestimates the change in manifest refractive error in ortho-K, whereas the maximum change in corneal power overestimates this parameter. Compared with retinoscopy and autorefraction, the change in apical corneal power is still useful for estimation of the change in manifest refractive error. Although the maximum change in corneal power appears to give a closer estimation of the change in manifest refractive error than the change in apical corneal power, there is no advantage in the use of maximum corneal power (manually located) instead of apical corneal power (a default given by the topographer) to estimate the change in manifest refractive error, as there is no significant difference in the estimations by either parameter. Initial corneal asphericity measured by the Medmont E300 corneal topographer has limited usage in predicting the change in manifest refractive error in overnight ortho-K.Background:  To investigate the relationship between the change in the manifest refractive error (ΔM), the change in apical corneal power (ΔACP) and initial corneal asphericity (Q) in overnight orthokeratology (ortho‐K).

Toric orthokeratology: a case report

Spherical reverse geometry lens (RGL) designs used in orthokeratology (ortho-k) have been shown to be effective in correcting low to moderate levels of myopia, however, they are ineffective for the correction of astigmatism, although there are reports of some effect. Lens decentration is the most common problem with spherical ortho-k lenses on patients with corneal astigmatism, as poor lens centration can lead to induced astigmatism, glare and poor vision. The number of ortho-k wearers, most of whom are children, has been increasing in Hong Kong in recent years due to the promising results in retarding myopic progression. Ortho-k is particularly popular in Hong Kong, where the prevalence of myopia is high in children. Most children undergo this treatment for control of myopia and this is mainly restricted to those with low to moderate corneal toricity (less than 1.50 D, with-the-rule). High corneal toricity (greater than 1.50 D, with-therule) is considered to be a contraindication for the treatment. Toric RGL designs (toric reverse and/or alignment zone) have been developed for patients with high corneal toricity and promising results have been reported. The current report presents the case of a young female subject whose parents were concerned about her myopic progression. She also had high corneal astigmatism and was successfully prescribed a pair of toric peripheral design RGL (toric alignment zone). The tenets of the Declaration of Helsinki as revised in 2002 were followed and ethics approval was obtained from the Departmental Research Committee of the School of Optometry of The Hong Kong Polytechnic University before commencing the treatment.

Do fenestrations affect the performance of orthokeratology lenses

Purpose. To investigate the effect of lens fenestrations on the performance of orthokeratology lenses. Methods. Twenty-two subjects (aged 11 to 31 years) were fitted with identical reverse geometry orthokeratology lenses in the two eyes. One eye was randomly designated to wear a lens with three 0.20 mm fenestrations at 120° intervals placed at the junction of the reverse and alignment curves. The lens for the other eye was not fenestrated. Subjects were reviewed at 1 week, 1 month, 3 months, 6 months, and 1 year after fitting. Data were collected on refraction, visual performance, incidences and severity of corneal staining, lens binding, and corneal pigmented arc. Results. Fifteen subjects achieved full correction in OU. There were no statistically significant differences in refractive and corneal changes, visual performance, incidences and severity of corneal staining and corneal pigmented arc formation between the two eyes. The incidence of lens binding was consistently higher in eyes wearing non-fenestrated lenses but was not statistically significant. Severity of lens binding, however, was statistically significant at the 1-, 3-, 6- and 12-month visits, with the non-fenestrated lenses causing more severe binding. Conclusions. The addition of fenestrations to orthokeratology lenses has no effect on the efficacy of the treatment or outcomes in low to moderate myopic subjects. However, the severity of lens binding is reduced. Fenestrating reverse geometry lenses may be of assistance in cases where lens binding is a problem.