Measurement of Cylinder Error in a Native American Population With a Shack–Hartmann Sensor

Abstract

Abstract: : Purpose: The viability of using a Shack–Hartmann sensor for measuring astigmatism was assessed in a sample of Native American children from a tribe in which there is a high prevalence of large levels of astigmatism (>2 diopters), often not corrected. The Shack–Hartmann system is being evaluated as a rapid screening tool to detect astigmatism so early correction can be provided. Methods: A table–top version of the Shack–Hartmann sensor was used to record right eye non–cyclopleged aberration patterns in 21 Tohono O’odham children in kindergarten through 2^nd grade. The astigmatism terms from the wavefront expansion were compared to cycloplegic cylinder error measured with Retinomax autorefractor confirmed by retinoscopy. Results: We were able to obtain repeated measurements in 13 of 21 children. In six of the remaining children, a strong back–reflection from the cornea corrupted the measurement. The final two children who went unmeasured could not reliably look into the device during the measurement process. Results from the 13 children showed a strong correlation (r² = 0.90) between the autorefractor–measured cylinder error and the Shack–Hartmann sensor measured cylinder error. The mean difference in cylinder error between the devices was –0.05 D (range: –0.46 D to 0.39 D). Conclusions: The Shack–Hartmann sensor gives reliable measures of astigmatism and may be useful as a screening tool in young children. The primary limitation to the technology is the retro–reflection from the cornea that occurs in some measurements. Recording of video clips is being explored to alleviate this problem. In addition, development of a hand–held child–friendly version of the Shack–Hartmann sensor may also be valuable in measuring astigmatism in infants and young children, who often can not cooperate for measurement with table–top instruments.

Keywords: astigmatism • refractive error development • optical properties