Marwan Suheimat

Refractive indices used by the Haag-Streit Lenstar to calculate axial biometric dimensions

To estimate refractive indices used by the Lenstar biometer to translate measured optical path lengths into geometrical path lengths within the eye.

Amplitude of accommodation in type 1 diabetes.

PURPOSE People with diabetes have accelerated age-related biometric ocular changes compared with people without diabetes. We determined the effect of type 1 diabetes on amplitude of accommodation. METHODS There were 43 participants (33 ± 8 years) with type 1 diabetes and 32 (34 ± 8 years) age-balanced controls. There was no significant difference in mean equivalent refractive error and visual acuity between the groups. Amplitude of accommodation was measured using two techniques: objective by determining the accommodative response to a stimulus in a COAS-HD wavefront aberrometer and subjective with a Badal hand optometer. Influences of age and diabetes duration on amplitude of accommodation were analyzed using multiple regression analysis. RESULTS People with diabetes had lower objective (2.7 ± 1.6 diopters [D]) and subjective (4.0 ± 1.7 D) amplitudes than controls (objective 4.1 ± 2.1 D, subjective 5.6 ± 2.1 D). Across both groups, objective amplitude was less than subjective amplitude by 1.4 ± 1.2 D. For objective amplitude and the whole group, the duration of diabetes contributed 57% variation to the loss of amplitude relative to that provided by age. For the objective amplitude and only the diabetes group, this was 78%. For subjective amplitude, the corresponding proportions were 68% and 103%. CONCLUSIONS Lowered amplitude of accommodation exists in individuals with type 1 diabetes when compared with age-matched controls. The loss correlated strongly with duration of diabetes. The results suggest that individuals with diabetes will experience presbyopia earlier in life than people without diabetes, mainly due to changes in the lens.

Biometry of eyes in type 1 diabetes

This is a comprehensive study of a large range of biometric and optical parameters in people with type 1 diabetes. The parameters of 74 people with type 1 diabetes and an age matched control group were assessed. Most of the people with diabetes had low levels of neuropathy, retinopathy and nephropathy. Marginal or no significant differences were found between groups for corneal shape, corneal thickness, pupil size, and pupil decentrations. Relative to the control group, the diabetes group demonstrated smaller anterior chamber depths, more curved lenses, greater lens thickness and lower lens equivalent refractive index. While the optics of diabetic eyes make them appear as older eyes than those of people of the same age without diabetes, the differences did not increase significantly with age. Age-related changes in the optics of the eyes of people with diabetes need not be accelerated if the diabetes is well controlled.

Peripheral Refraction, Peripheral Eye Length, and Retinal Shape in Myopia.

Purpose To investigate how peripheral refraction and peripheral eye length are related to retinal shape. Methods Relative peripheral refraction (RPR) and relative peripheral eye length (RPEL) were determined in 36 young adults (M +0.75D to −5.25D) along horizontal and vertical visual field meridians out to ±35° and ±30°, respectively. Retinal shape was determined in terms of vertex radius of curvature Rv, asphericity Q, and equivalent radius of curvature REq using a partial coherence interferometry method involving peripheral eye lengths and model eye raytracing. Second-order polynomial fits were applied to RPR and RPEL as functions of visual field position. Linear regressions were determined for the fits’ second order coefficients and for retinal shape estimates as functions of central spherical refraction. Linear regressions investigated relationships of RPR and RPEL with retinal shape estimates. Results Peripheral refraction, peripheral eye lengths, and retinal shapes were significantly affected by meridian and refraction. More positive (hyperopic) relative peripheral refraction, more negative RPELs, and steeper retinas were found along the horizontal than along the vertical meridian and in myopes than in emmetropes. RPR and RPEL, as represented by their second-order fit coefficients, correlated significantly with retinal shape represented by REq. Conclusions Effects of meridian and refraction on RPR and RPEL patterns are consistent with effects on retinal shape. Patterns derived from one of these predict the others: more positive (hyperopic) RPR predicts more negative RPEL and steeper retinas, more negative RPEL predicts more positive relative peripheral refraction and steeper retinas, and steeper retinas derived from peripheral eye lengths predict more positive RPR.

Influence of gravity on ocular lens position.

PURPOSE We determined whether human ocular lens position is influenced by gravity. METHODS Anterior chamber depth (ACD) and lens thickness (LT) were determined with a Haag-Streit Lenstar LS900 for right eyes of participants in two age groups, with a young group of 13 participants aged 18 to 21 years (mean, 21 years; SD, 1 year) and an older group of 10 participants aged 50 to 63 years (mean, 58 years; SD, 4 years). There were two sessions for each participant separated by at least 48 hours, with one session for the usual upright head position and one session for a downwards head position. In a session, testing was done for minimum accommodation followed by testing at maximum accommodation. A drop of 2% pilocarpine nitrate was instilled, and testing was repeated after 30 minutes under minimum and maximum accommodation conditions. RESULTS Gravity, manipulated through head posture, affected ACD for young adult and older adult groups but mean effects were only small, ranging from 0.04 to 0.12 mm, and for the older group required the instillation of an accommodation-stimulating drug. Gravity had a weakly significant effect on LT for the young group without accommodation or a drug, but the effect was small at 0.04 ± 0.06 mm (mean ± SD, P = 0.04). CONCLUSIONS There is a small but real effect of gravity on crystalline lens position, manifested as reduction in ACD at high levels of accommodative effort with the head in a downwards position. This provides evidence of the ability of zonules to slacken during strong accommodation.

Pilot study: effect of age on visual acuity with defocus and astigmatism.

Purpose To investigate how distance visual acuity in the presence of defocus and astigmatism is affected by age and whether aberration properties of young and older eyes can explain any differences. Methods Participants were 12 young adults (mean [±SD] age, 23 [±2] years) and 10 older adults (mean [±SD] age, 57 [±4] years). Cyclopleged right eyes were used with 4-mm effective pupil sizes. Thirteen blur conditions were used by adding five spherical lens conditions (−1.00 diopters [D], −0.50 D, plano/0.00 D, +0.50 D, and +1.00 D) and adding two cross-cylindrical lenses (+0.50 DS/−1.00 DC and +1.00 D/−2.00 DC, or 0.50 D and 1.00 D astigmatism) at four negative cylinder axes (45, 90, 135, and 180 degrees). Targets were single lines of high-contrast letters based on the Bailey-Lovie chart. Successively smaller lines were read until a participant could no longer read any of the letters correctly. Aberrations were measured with a COAS-HD Hartmann-Shack aberrometer. Results There were no significant differences between the two age groups. We estimated that 70 to 80 participants per group would be needed to show significant effects of the trend of greater visual acuity loss for the young group. Visual acuity loss for astigmatism was twice that for defocus of the same magnitude of blur strength (0.33 logMAR [logarithm of the minimum angle of resolution]/D compared with 0.18 logMAR/D), contrary to the geometric prediction of similar loss. Conclusions Any age-related differences in visual acuity in the presence of defocus and astigmatism were swamped by interparticipant variation.

Lens shape and refractive index distribution in type 1 diabetes

PURPOSE To compare lens dimensions and refractive index distributions in type 1 diabetes and age-matched control groups. METHODS There were 17 participants with type 1 diabetes, consisting of two subgroups (7 young [23 ± 4 years] and 10 older [54 ± 4 years] participants), with 23 controls (13 young, 24 ± 4 years; 10 older, 55 ± 4 years). For each participant, one eye was tested with relaxed accommodation. A 3T clinical magnetic resonance imaging scanner was used to image the eye, employing a multiple spin echo (MSE) sequence to determine lens dimensions and refractive index profiles along the equatorial and axial directions. RESULTS The diabetes group had significantly smaller lens equatorial diameters and larger lens axial thicknesses than the control group (diameter mean ± 95% confidence interval [CI]: diabetes group 8.65 ± 0.26 mm, control group 9.42 ± 0.18 mm; axial thickness: diabetes group 4.33 ± 0.30 mm, control group 3.80 ± 0.14 mm). These differences were also significant within each age group. The older group had significantly greater axial thickness than the young group (older group 4.35 ± 0.26 mm, young group 3.70 ± 0.25 mm). Center refractive indices of diabetes and control groups were not significantly different. There were some statistically significant differences between the refractive index fitting parameters of young and older groups, but not between diabetes and control groups of the same age. CONCLUSIONS Smaller lens diameters occurred in the diabetes groups than in the age-matched control groups. Differences in refractive index distribution between persons with and without diabetes are too small to have important effects on instruments measuring axial thickness.

Influence of eye rotation on peripheral eye length measurement obtained with a partial coherence interferometry instrument.

The eye rotation approach for measuring peripheral eye length leads to concern about whether the rotation influences results, such as through pressure exerted by eyelids or extra‐ocular muscles. This study investigated whether this approach is valid.

Relationship between retinal distance and object field angles for finite schematic eyes.

Retinal anatomical studies have used the Drasdo & Fowler three‐refracting surface schematic eye to convert between retinal distances and object field angles. We compared its performance at this task with those of more sophisticated four‐refracting surface schematic eyes.

Anterior Corneal, Posterior Corneal, and Lenticular Contributions to Ocular Aberrations.

Purpose To determine the corneal surfaces and lens contributions to ocular aberrations. Methods There were 61 healthy participants with ages ranging from 20 to 55 years and refractions -8.25 diopters (D) to +3.25 D. Anterior and posterior corneal topographies were obtained with an Oculus Pentacam, and ocular aberrations were obtained with an iTrace aberrometer. Raytracing through models of corneas provided total corneal and surface component aberrations for 5-mm-diameter pupils. Lenticular contributions were given as differences between ocular and corneal aberrations. Theoretical raytracing investigated influence of object distance on aberrations. Results Apart from defocus, the highest aberration coefficients were horizontal astigmatism, horizontal coma, and spherical aberration. Most correlations between lenticular and ocular parameters were positive and significant, with compensation of total corneal aberrations by lenticular aberrations for 5/12 coefficients. Anterior corneal aberrations were approximately three times higher than posterior corneal aberrations and usually had opposite signs. Corneal topographic centers were displaced from aberrometer pupil centers by 0.32 ± 0.19 mm nasally and 0.02 ± 0.16 mm inferiorly; disregarding corneal decentration relative to pupil center was significant for oblique astigmatism, horizontal coma, and horizontal trefoil. An object at infinity, rather than at the image in the anterior cornea, gave incorrect aberration estimates of the posterior cornea. Conclusions Corneal and lenticular aberration magnitudes are similar, and aberrations of the anterior corneal surface are approximately three times those of the posterior surface. Corneal decentration relative to pupil center has significant effects on oblique astigmatism, horizontal coma, and horizontal trefoil. When estimating component aberrations, it is important to use correct object/image conjugates and heights at surfaces.