HOAs increase in the supine position

There is a marked increase in higher-order aberrations when a patient is in the supine position, according to the results of a study published in the February issue of the American Journal of Ophthalmology.

Takushi Kawamorita and his colleagues from Kitasato University Graduate School of Medical Sciences, Sagamihara, Japan, carried out an observational case series on nine healthy volunteers (three men, six women) with no known abnormalities.

Corneal aberration and curvature measurements were taken with a Placido-ring videokeratoscope, Keratron (Optikon 2000, Rome, Italy), which had been altered to measure patients in the supine position. Corneal wavefront aberrations were analysed at a 6 mm pupil diameter for total higher-order, coma-like and spherical-like aberrations. Three measurements were taken on each volunteer's left eye; the first while the subject was sitting and the remaining two once the patient was in the supine position, the first immediately and the second half an hour later. Data was analysed by repeated measure analysis of variance (ANOVA) and the Scheffé test.

The total higher-order aberration for 6 mm pupil diameter significantly increased from 0.419 ± 0.120 µm in the sitting position to 0.500 ± 0.157 µm in the supine position. Spherical-like aberrations found immediately after adopting the supine position also showed a significant increase compared with the sitting position but decreased within the following 30 minutes. In the coma-like aberration, corneal curvature, vertical axis, pupil diameter and off-set, no significant differences were found.

Although the cyclotorsion (rotation of the eye around its visual axis) may be associated with changes in the aberration pattern, the authors found no significant change in the vertical axis. Another contributing factor may be intraocular pressure (IOP), as it is known that IOP in the supine position is greater than in the sitting position. This, in addition to avoiding corneal deformation caused by the measuring instrument, was the reason cited for not measuring IOP in the study.

The authors concluded that the increase in higher-order aberrations may act to limit the improvements in visual performance after customized refractive surgery, based on wavefront measurements.