Halo size under distance and near conditions in refractive multifocal intraocular lenses

Abstract

Aims: To calculate the diameter of halos perceived by patients with multifocal intraocular lenses (IOLs) and to stimulate halos in patients with refractive multifocal IOLs in a clinical experiment.

Methods: Calculations were done to show the diameter of halos in the case of the bifocal intraocular lens. 24 patients with a refractive multifocal IOLs and five patients with a monofocal IOL were asked about their subjective observation of halos and were included in a clinical experiment using a computer program (Glare & Halo, FW Fitzke and C Lohmann, Tomey AG) which simulates a light source of 0.15 square degrees (sq deg) in order to stimulate and measure halos. Halo testing took place monoculary, under mesopic conditions through the distance and the near focus of the multifocal lens and through the focus of the monofocal lens.

Results: The halo diameter depends on the pupil diameter, the refractive power of the cornea, and distance focus of the multifocal IOL as well as the additional lens power for the near focus. 23 out of 24 patients with a refractive multifocal IOL described halos at night when looking at a bright light source. Only one patient was disturbed by the appearance of halos. Under test conditions, halos were detected in all patients with a refractive multifocal IOL. The halo area testing through the distance focus was 1.05 sq deg +/- 0.41, through the near focus 1.07 sq deg +/- 0.49 and in the monofocal lens 0.26 sq deg +/- 0.13.

Conclusions: Under high contrast conditions halos can be stimulated in all patients with multifocal intraocular lenses. The halo size using the distance or the near focus is identical.

Figures

Figure 1

(A) A light source nearly at infinity (1) emits parallel light rays (2) that are bent at the cornea (3) and the bifocal lens (4). The distance focus of the bifocal lens produces a sharp image on the retina (5). In the axial presentation this image is shown as a white spot (5). The near focal point in this constellation is in front of the retina (6) producing an out of focus image on the retina (7). The greater diameter of the out of focus image is represented by the dark grey shading in the axial presentation (7) around the focused image (5). (B) A light source is positioned at the reading distance (1) and the light rays reaching the cornea are drawn in (2). These rays are bent at the cornea (3) and the bifocal lens (4). The near focus of the bifocal lens produces a sharp image on the retina (5) also seen in the axial presentation (5). The far focus of the lens would produce an image behind the retina, producing an out of focus image on the retina (6). The greater diameter of this out of focus image is responsible for the halo (6). g = object distance; bnear = image distance produced by the near focus; bdistance = image distance produced by the distance focus; dH = halo diameter; ΔD = power difference between the distance and near focus.

Figure 2

(A) The patient is sitting 2 metres away from a video monitor with the best correction. A white, round light source 15 mm in diameter with an illumination of 56.6 cd/m2 is simulated in the centre of the monitor. The background illumination is 0.01 cd/m2. The test takes place under mesopic conditions. A small red spot can be moved with the computer mouse along lines from the periphery to the centre in 30° degree steps (a). The task for the patient is to report when the red spot touches the edge of a possible halo. The halo margin is then determined along the next line 30° away (b). Finally, the extent of the halo is indicated in 30° steps. The computer program automatically computes the area of the halo. (B-D) The circle in the middle represents the extent of the light source. Monocular testing with best distance refraction plus 0.5 D was performed at a distance of 2 metres under mesopic conditions in order to force the patient to use the distance focus (SA 40 N), and with the best distance refraction plus minus 3.0 D in order to force the patient to use the near focus (SA 40 N). In the monofocal group the correction used was the best distance refraction plus 0.5 D (SI 40 NB). The halo is visible as a grey area; the dotted lines represent the standard deviation. ns = not significant.