Simple method of determining the axial length of the eye

Positions of the horizontal and vertical centre of rotation in eyes with different refractive errors

Purpose

To determine how the position of the centre of rotation of the eyeball is related to axial length and refractive error when horizontal and vertical eye movements are performed.

Methods

A custom‐built eye tracker was used that determined the centre of rotation of the eye (COR) from lateral displacements of the pupil centre. Horizontal and vertical eye movements were studied in the right eyes, and each measurement performed five times in 59 subjects (32 females) with an average age of 36.6 ± 9.1 years. Spherical equivalent refractive errors ranged from −9.7 to +6.8 D with an average error of −1.5 ± 2.9 D. Axial lengths were measured with the ZEISS IOL Master 500.

Results

The mean horizontal centre of rotation (COR) of the right eye for a saccade from 0° to ±11.9° was 15.3 ± 1.5 mm behind the corneal apex, while the average vertical COR for the same angle of eccentricity was 12.5 ± 1.4 mm, indicating that the horizontal COR was 2.8 ± 1.7 mm behind the vertical COR. In right eyes, horizontal COR was significantly correlated with axial length (r = 0.28, p = 0.02) but not with the spherical equivalent refractive error (r = 0.39, p = 0.90). Similarly, vertical COR was significantly correlated with axial length (r = 0.25, p = 0.03) but not with the spherical equivalent refractive error (r = 0.17, p = 0.90).

Conclusions

While it might be expected that the COR is dependent on axial length, the correlation was not strong. Interestingly, the location of the COR was substantially different for horizontal and vertical eye movements which may relate to the flatter curvature of the eyeball in the vertical meridian, compared to the horizontal, as described in previous studies.

Reproducibility of a long-range swept-source optical coherence tomography ocular biometry system and comparison with clinical biometers

OBJECTIVE

To demonstrate a novel swept source optical coherence tomography (SS-OCT) imaging device using a vertical cavity surface-emitting laser (VCSEL) capable of imaging the full eye length and to introduce a method using this device for noncontact ocular biometry. To compare the measurements of intraocular distances using this SS-OCT instrument with commercially available optical and ultrasound biometers. To evaluate the intersession reproducibility of measurements of intraocular distances using SS-OCT.

DESIGN

Evaluation of technology.

PARTICIPANTS

Twenty eyes of 10 healthy subjects imaged at the New England Eye Center at Tufts Medical Center and Massachusetts Institute of Technology between May and September 2012.

METHODS

Averaged central depth profiles were extracted from volumetric SS-OCT datasets. The intraocular distances, such as central corneal thickness (CCT), aqueous depth (AD), anterior chamber depth (ACD), crystalline lens thickness (LT), vitreous depth (VD), and axial length (AL), were measured and compared with a partial coherence interferometry device (IOLMaster; Carl Zeiss Meditec, Inc., Dublin, CA) and an immersion ultrasound (IUS) A-scan biometer (Axis-II PR; Quantel Medical, Inc., Cournon d'Auvergne Cedex, France).

MAIN OUTCOME MEASURES

Reproducibility of the measurements of intraocular distances, correlation coefficients, and intraclass correlation coefficients.

RESULTS

The standard deviations of the repeated measurements of intraocular distances using SS-OCT were 6 μm (CCT), 16 μm (ACD), 14 μm (AD), 13 μm (LT), 14 μm (VD), and 16 μm (AL). Strong correlations among all 3 biometric instruments were found for AL (r > 0.98). The AL measurement using SS-OCT correlates better with the IOLMaster (r=0.998) than with IUS (r=0.984). The SS-OCT and IOLMaster measured higher AL values than ultrasound (175 and 139 μm, respectively). No statistically significant difference in ACD between the optical (SS-OCT or IOLMaster) and ultrasound methods was detected. High intersession reproducibility of SS-OCT measurements of all intraocular distances was observed with intraclass correlation coefficients >0.99.

CONCLUSIONS

The SS-OCT using VCSEL technology enables full eye length imaging and high-precision, noncontact ocular biometry. The measurements with the prototype SS-OCT instrument correlate well with commercial biometers. The SS-OCT biometry has the potential to provide clinically useful comprehensive biometric parameters for pre- and postoperative eye evaluation.

Critique of Claims of Improved Visual Acuity after Hypnotic Suggestion

Psychological approaches to improving vision present an enticing alternative to invasive procedures and corrective lenses; hypnotic suggestion is one such technique. During the past 60 years, multiple studies have documented improvements in the vision of myopic individuals after hypnotic interventions. Given the increasing interest in behavioral and alternative approaches, we have reviewed the pertinent studies to evaluate their validity. We delineate various shortcomings in these reports, including potential methodological caveats, problems with experimental controls, and controversial data interpretation. Overall, the data do not seem to support hypnosis as a viable option for significant long-term improvement of myopia. However, hypnosis can increase one's subjective feeling of enhanced visual acuity by affecting higher cognitive functions, such as attention, memorization, and perceptual learning, which could influence performance on visual tasks.

Behavior modification and the treatment of myopia

Myopia is a highly prevalent disorder of vision that has traditionally been managed by means of corrective lenses. In recent years, however, there has been an upsurge of interest in the use of vision training programs as an alternative form of treatment, and particularly in the application of behavior modification techniques for the improvement of visual acuity. The most impressive experimental work to date has been in the area of operant conditioning methods (principally feedback and fading) on the modification of acuity, and this research is reviewed in depth. Also considered here are a number of relevant studies on the use of hypnosis and biofeedback training of accommodation. Research in this area is particularly timely in view of the increasing interest in the role of learning factors in both the development and maintenance of myopia. Despite a number of methodological problems identified in this review, there appears to be considerable promise in the application of behavior modification to disorders of vision.

Ocular pneumoplethysmography in carotid-cavernous sinus fistulas

Pulsatile exophthalmos in association with carotid-cavernous sinus fistulas has been well defined anatomically, by angiography. This paper presents the physiological assessment of this entity, as measured with ocular pneumoplethysmography (OPG-Gee). The abnormal arteriovenous communication lowers resistance to arterial flow. This is characterized by a lowered ophthalmic systolic pressure and an increased ocular blood flow. The OPG readily documents the physiological result of therapeutic intervention.

Laser Doppler technique for measurement of eye movement

The spectrum of Doppler-shifted laser light scattered from the cornea was used to measure the distribution of eye speeds in human subjects while the eye followed a moving target. Speed distributions obtained by the laser Doppler technique were compared to those recorded with a double-Purkinje-image eyetracker.

Survey of ophthalmic conditions in a Labrador community. I. Refractive errors

Of the 745 available members of the population of Nain in Labrador 650 (87%) were screened for refractive errors and ocular disease. Refraction by retinoscopy was done in 553 and axial length measured by an optical method in 514. The results showed that the incidence of low degrees of myopia was higher in Inuit (Eskimos) and those of Mixed Inuit-Caucasian blood in the age groups 10 to 40 than in those over 40. 75% of the myopes came from 20 families in which myopia was present in 2 or more generations. Although there was no significant correlation between the refraction of parents and offspring, there were significant correlations between them for axial length. The axial lenths of the myopic eyes of the Inuit and Mixed populations were significantly longer than emmetropic and hypermetropic eyes. The younger memebers of the population were taller than their parents, and except in female Caucasians axial length showed a significant positive correlation with height. More myopes than emmetropes and hypermetropes achieved grade 8 or more in school. It is suggested that the increased incidence of myopia in the younger age groups might be due to environmental factors interfering with the process of emmetropisation in eyes with a genetic predisposition to myopia by virtue of inheriting a slightlt longer eye. Better nutrition resulting in an increase in stature may also have had some influence.

Comparison of ultrasonic and photographic methods of axial length measurements of the eye

The axial length of 25 eyes was measured by the photographic method and by ultrasonography. The photographic measurements were consistently larger than the ultrasonic, but analysis gave a correlation coefficient of r = 0-8483. The results suggest that the centre of rotation of the eye lies posterior to the anatomical centre of the globe. Using an empirical method of calculation and allowing for an error of +/-0-3 mm in the ultrasonic measurement, the photographic results were within +/-0-5 mm in 13 eyes and +/-1-0 mm in 7 eyes. The photographic method may be helpful as a guide to the power of intraocular lens implants and for comparing axial lengths in population studies.