Hiroshi Uozato

Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery

Purpose: To calculate the apparent posterior corneal changes after keratorefractive surgery and reevaluate corneal ectasia displayed by Orbscan (Orbtek). Setting: Department of Ophthalmology, Nara Medical University, Nara, Japan. Methods: Postoperative:preoperative magnification ratio of the posterior surface of the cornea was calculated in a theoretical eye model. Results: Assuming the preoperative corneal thickness is 600.00 μm, the preoperative refractive power of the anterior corneal surface is 48.0 diopters (D), the refractive power of the cornea is 1.376, the ablation diameter is 6.0 mm, the postoperative corneal thickness is 480.00 μm, the postoperative refractive power of the anterior corneal surface is 38.0 D, and the posterior surface of the cornea does not change postoperatively, the apparent image of the posterior surface of the cornea becomes 0.778% smaller postoperatively. If the posterior radius of curvature of the cornea is 6.2 mm, it becomes smaller by 48.24 μm. If this change directly affects the difference map, the posterior surface of the cornea moves forward by 48.24 μm. Conclusion: The results correspond to the amount of ectasia in previous reports. This artifact may explain the apparent ectasia detected by Orbscan.

[The influence of corneal thickness and curvature on the difference between intraocular pressure measurements obtained with a non-contact tonometer and those with a Goldmann applanation tonometer].

Purpose: The influence of corneal thickness and curvature on the difference between intraocular pressure (IOP) measurements obtained with a non-contact tonometer (NCT) and those with a Goldmann applanation tonometer (GAT) was studied.Methods: The corneal thickness and curvature were obtained in 230 eyes of 115 subjects. The correlation between them and ratios of measurement with NCT to that with GAT ([NCT/GAT]) were examined.Results: [NCT/GAT] and corneal thickness showed a significant positive correlation (r = 0.556, P <.01), but, the correlation between [NCT/GAT] and the radius of corneal curvature was not statistically significant (r = -0.035, P =.30).Conclusion: The thick cornea has more influence on the measurement with NCT than GAT, because IOP is measured with NCT over a wider applanation area. The corneas with steeper curvature also cause higher corneal rigidity and produce more overestimation of NCT measurement, while they have stronger capillary attraction of the precorneal tear film for the GAT tip and also produce overestimation of GAT measurement. As a result, [NCT/GAT] was believed to be not influenced by the corneal curvature.

Accommodation causes with-the-rule astigmatism in emmetropes

Purpose To measure the changes in astigmatism when bilateral emmetropes accommodate. Methods Bilateral emmetropes accommodative responses were measured with an improved photorefractometer PR-1100, which measured binocular refraction in all meridians simultaneously as a fixation target was shown in natural space. Results The accommodative responses in the vertical meridian are greater than those in the horizontal meridian. In the horizontal meridian, the accommodative responses are less than the accommodative stimulus. Conclusions When accommodating the majority of bilateral emmetropes show with-the-rule astigmatism and lag of accommodation in the horizontal meridian.

Posterior corneal surface changes after hyperopic laser in situ keratomileusis

PURPOSE: To evaluate posterior corneal surface topographic changes after hyperopic laser in situ keratomileusis (H‐LASIK) using Orbscan I (Orbtek, Inc.). SETTING: Department of Ophthalmology, Nara Medical University, Nara, Japan. METHODS: In 25 eyes of 15 patients who had H‐LASIK, the posterior corneal surface was measured with slit‐scanning corneal topography (Orbscan I) preoperatively and 1 year postoperatively. The center as a fit zone and calculated posterior corneal surface changes were taken at 4 points: nasal, temporal, superior, and inferior sides in the 5.0 mm diameter. The posterior corneal topographic changes were analyzed using an analysis of variance. The postoperative:preoperative magnification ratio of the posterior corneal surface was calculated in a theoretical eye model. RESULTS: When a “+” reading was defined as the forward displacement and “−” was defined as the backward displacement, the mean posterior corneal topographic changes were −2.8 μm ± 27.9 (SD) at the nasal side, −4.5 ± 27.8 μm at the temporal side, −3.9 ± 20.1 μm at the superior side, and −2.3 ± 20.1 μm at the inferior side. The posterior corneal surface between any 2 examined points showed no significant difference after H‐LASIK. In addition, the hypothetical change in the posterior cornea was −8.3 μm after +3.0 diopter H‐LASIK, which was approximately closer to the study results. In each side, the amount of the attempted correction was significantly correlated with the posterior corneal topographic change. CONCLUSIONS: Clinical measurement of the posterior corneal displacement after H‐LASIK with Orbscan revealed a backward shift. This change corresponded to the hypothetical artifactual changes with Orbscan; that is, changes in the magnification ratio.

The Binocular Accommodative Response in Uncorrected Ametropia

Purpose. To measure the changes in astigmatism and in the degree of anisometropia when ametropes respond to accommodative stimulation. Methods. The accommodative responses of ametropes were binocularly measured with an improved photorefractometer (PR-1100), which objectively measured binocular refraction in all meridians simultaneously, when a visual fixation target was shown in natural space at 5 and 0.5 m. Results. (1) Changes of astigmatism and anisometropia occur during binocular accommodation. (2) Changes of astigmatism mainly result from lag of accommodation in the horizontal meridian. (3) Changes of anisometropia result from the effort to focus. (4) The interaction of both eyes is suggested as the cause of the changes of astigmatism and anisometropia with accommodation. (5) In the majority of strong anisometropes (nonoverlapping group in this paper), the magnitude of anisometropia decreases (approaching isometropia) with accommodation. Conclusion. Changes in astigmatism and anisometropia can occur during accommodation, and these interactions of both eyes with accommodation may contribute to the development and maintenance of binocular function.

Lag of Accommodation

There are many concepts of lag of accommodation [1,2]. In this paper it is defined as follows: regardless of how clear an image an accommodating eye can maintain of a nearer object, its retinal conjugate point does not reach that object (Fig. 1).

Measurement of Visual Axis Using a Laser Beam

A number of axes are recognized in the eye, such as optic axis, visual axis, pupillary axis, fixation axis, and line of sight [1,2], as shown in Fig. 1. It is well known that the visual axis is the most important for ophthalmic prescription, measurement of refraction and corneal configuration and so on [3] (Fig. 2). The visual axis is defined as the two lines, one from the fixation point to the first nodal point in object space, and other from the fovea to the second nodal point in image space [1]. However, objective measurement of the visual axis is extremely difficult because it is imaginary. In clinical viewpoint, the pupillary axis and line of sight are well in use because of the simplicity of measurement. Strictly speaking, these axes differ fairly from the visual axis, and do not pass through the fovea.

Application of Laser Speckle Patterns to Visual Evoked Potentials

Visual evoked potentials (VEPs) are gross electric signals generated in the occipital cortex in response to visual stimulation such as light or pattern. It is well known that the VEP is becoming a common clinical test for objective diagnosis of the function of the visual sense [1]. Recently versatile methods for the VEPs are commonly utilized, especially pattern VEPs which have been developed remarkably with the aid of microcomputer systems and commercial TV monitors [2]. However, these conventional pattern VEPs are strongly affected by refractive, accommodative, and fixation states of the subject’s eyes. Although these methods are applicable to the objective determination of refraction [3], it is necessary to correct fully these factors for the assessment of visual function. In infants and patients with refractive or accommodative anomalies which are extremely difficult to correct, the conventional pattern VEPs are of little clinical value.

Speckle Pattern Stimulator for Visual Evoked Potentials

A new method to stimulate visual evoked potentials (VEPs) using laser speckle patterns is described. The principle of producing the speckle patterns is theoretically analyzed and an optimal optical system for stimulation of the VEPs is considered. The feasibility of this method is demonstrated by simple experiments carried out on a group of normal subjects. Application of this method to the assessment of the visual function of cataractous eyes is investigated.