Clinical evaluation of infrared autorefractors for use in contact lens over refraction

Accommodative response to peripheral stimuli in myopes and emmetropes

PURPOSE

It has been suggested that peripheral refractive error may influence eye growth and the development of axial refractive error, implying that the peripheral retina is sensitive to defocus. This study aimed to evaluate the steady-state accommodative response to peripheral stimuli in 10 young, adult myopes (mean spherical equivalent error -2.10 ± 1.72 D, median -1.63 D, range -0.83 to -6.00 D) and 10 emmetropes (mean spherical equivalent error -0.02 ± 0.35 D, median +0.08 D, range -0.50 to +0.50 D).

METHODS

The subjects were asked to view monocularly the centre of a screen displaying each of a series of eccentric accommodative targets placed at 5, 10 and 15°. An axial target was viewed for comparison purposes. Accommodation was measured using an open-field autorefractor, each stimulus being varied between about 0 and 4 D with spherical trial lenses placed in front of the viewing eye.

RESULTS

The results confirm that the peripheral retina is sensitive to optical focus, up to field angles of at least 15°, with accommodative responses weakening as the peripheral angle increases. There is some evidence that peripheral accommodation may be less effective in myopes than emmetropes.

CONCLUSIONS

Although peripheral accommodation can be demonstrated in the absence of a central stimulus, the accommodation response is normally dominated by the central stimulus and it seems unlikely that peripheral accommodation effects play an important role in refractive development.

Technical note: effect of contact lenses on measurement of the accommodation microfluctuations

AIM

Dynamic measurement of accommodation in subjects with myopia usually involves recording through soft contact lenses (CLs) to correct the refractive error. Conversely, dynamic accommodation measurement in emmetropic control subjects is generally undertaken without any corrective lenses. The aim of this experiment was to determine whether CL correction affects the measurement of accommodation microfluctuations using infrared refractometry, and whether this needs to be considered in studies which attempt to compare accommodation responses between the two groups.

METHODS

Ten young emmetropic subjects viewed a high contrast Maltese cross target monocularly using the right eye at a target vergence of 0 D. The subjects viewed the target under two conditions: with CL condition and without CL condition, where the subjects viewed the target with the eye only. Accommodation responses of the right eye were recorded continuously for 2 min at a sampling rate of 52 Hz using the Shin-Nippon SRW-5000 autorefractor.

RESULTS

No significant difference (two-tailed paired t-test, t(9) = -1.499, p = 0.168) was found in mean accommodation response between the with CL (mean +/- S.D. = -0.02 +/- 0.24 D) and without CL conditions (mean +/- S.D. = +0.01 +/- 0.25 D). No significant (two-tailed paired t-test, t(9) = 0.151, p = 0.883) difference in the magnitude of the accommodation microfluctuations was found between the with CL (mean +/- S.D. = 0.162 +/- 0.04 D) and without CL condition (mean +/- S.D. = 0.169 +/- 0.04 D). Power spectrum analysis revealed no differences in the characteristics of the microfluctuations waveform between the two conditions. A control experiment carried out on a subgroup of five subjects using a negative (-3 D) CL demonstrated that there was no significant effect of the dioptric power of the CL on the magnitude of the accommodation microfluctuations (anova: F(3,15) = 0.254, p = 0.782).

CONCLUSION

Thin soft CLs do not affect the magnitude or frequency characteristics of accommodation microfluctuations when measured using the Shin-Nippon SRW-5000.

Wavefront aberration and its relationship to the accommodative stimulus-response function in myopic subjects

BACKGROUND

Autorefractors are increasingly used in myopia research because they are convenient tools to investigate aspects of the accommodation response. The degree to which the autorefractor measures are affected by ocular aberrations has been highlighted by studies that have shown changes in aberration levels through different parts of the pupil and with accommodation. We have compared accommodative accuracy as measured with a Shin-Nippon SRW 5000 autorefractor with wavefront error as measured with a Hartmann-Shack wavefront sensor to investigate how factors such as accommodation demand, ocular aberrations, and pupil size can influence autorefractor measures.

METHODS

Accommodation stimulus-response curves were determined (using negative lenses) for 30 young healthy subjects (20 myopic [-0.75 to -6.00 D] and 10 emmetropic). Accommodation levels ranged from 0 to 4 D in 1 D steps. Wavefront aberrations were also determined for the same accommodation levels using a Hartmann-Shack wavefront sensor for both the subjects' natural pupil sizes and for a 2.9-mm pupil.

RESULTS

For all subjects, there was a consistent increase in negative spherical aberration with increases in accommodative stimulus. However, there was no consistent change in paraxial spherocylindrical refractive correction with accommodation stimulus. For the emmetropic subjects, accommodation error as measured with the autorefractor was statistically similar to the total spherocylindrical correction for the eye as estimated by the Hartmann-Shack wavefront sensor, but only for a 2.9-mm pupil (the pupil size utilized by the autorefractor). For the myopic subjects, accommodation error as measured with the autorefractor was statistically similar to the higher-order aberrations, but only when measured for a natural pupil size.

CONCLUSIONS

The relationship between the accommodation accuracy as measured with the autorefractor and the total wavefront aberration as measured with a Hartmann-Shack wavefront sensor is largely influenced by the higher-order (fourth and above) aberration levels. For the emmetropic subjects, the errors measured by the two methods agree when adjusted to measure at similar pupil sizes. For the myopic subjects with similar pupil sizes, however, the Hartmann-Shack wavefront sensor underestimates the accommodation error at higher accommodation levels (2 to 4 D) compared with the autorefractor.