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Case report

Anterior chamber optical coherence tomography study of human natural accommodation in a 19-year-old albino

Baikoff, Georges MD*,a; Lutun, Erica; Wei, Jayb; Ferraz, Caroline MDa

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Journal of Cataract & Refractive Surgery: March 2004 - Volume 30 - Issue 3 - p 696-701
doi: 10.1016/j.jcrs.2003.12.043
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Abstract

Modifications of the anterior segment during accommodation elicit controversy. Until 1992, the theory of accommodation as described by Helmholtz1 in 1855 was unanimously accepted. In 1970, Coleman2 suggested that contraction of the ciliary body triggered a rise in vitreous pressure that had a predominantly hydraulic effect on crystalline lens deformation. Schachar3,4 has repeatedly questioned Helmholtz's theory and put forward a new one based on part of the zonules being stretched during accommodation.

Modifications of the crystalline lens and the ciliary body in primates5,6 and in humans have been explored with different devices: A-scan,7,8 ultrasound biomicroscopy (UBM),9–11 3-dimensional UBM,12 Scheimpflug photographs,13 and infrared cameras.14 Strenk et al.15 also studied crystalline lens modifications with magnetic resonance imaging, but this technique cannot be routinely used. Most of the studies coincide with Helmholtz's theory and confirm an increase in crystalline lens thickness during accommodation, reduction of the crystalline lens equatorial diameter, forward movement of the crystalline lens anterior pole, and contraction of the ciliary body with reduction of the ciliary ring diameter. We present a 19-year-old albino patient in whom an anterior segment exploration procedure was performed with an optical coherence tomography (OCT) device.

Materials and Methods

Anterior Chamber OCT

The OCT with its 820 nm wavelength is a well-known posterior segment imaging device.16,17 By 1994, Izatt18 had suggested using it for anterior segment imaging. It was only in 2001, with the introduction of high-speed anterior chamber (AC) OCT using a 1310 nm wavelength, that good quality and easy to interpret images became available. Once the image is taken, the device's software restores the image to its real dimensions, avoiding the errors induced by differences in ray transmission through the cornea. A measurement of the distance between 2 points, the curvature radius and the angles, is possible with software.

The analysis of an eye is a noncontact procedure, and the patient fixates on a target. The target's focus is adjustable with positive or negative lenses, which allows compensation of the patient's spherical ametropia and acquisition of images in an unaccommodated situation. It is also possible to defocus the target with negative lenses to induce physiological accommodation in the examined eye.

This technique is noncontact, there is no undue pressure on the anterior segment, image acquisition takes a few seconds, and only the eye under observation is stimulated in physiological conditions. This examination is different from an ultrasonic exploration, in which the fellow eye has to be stimulated, and the Scheimpflug technique, in which the pupil must be dilated to obtain images of the entire surface of the crystalline lens.

During all examinations, the patients were asked to focus on a central target. The tomographical cross sections of this report only looked at the horizontal meridian. The visual axis on the images is shown by a control beam. Reproductibility of the cross-sectional plane is excellent, as shown in Figures 1 and 2, in which the iris nodule is observed in the left part of the iris in both images.

Figure 1.
Figure 1.:
(Baikoff) The unaccommodated anterior segment. The anterior chamber depth is 2.668 mm.
Figure 2.
Figure 2.:
(Baikoff) An OCT image of the anterior segment of the same eye. The accommodation is 10.00 D, and the anterior chamber depth is 2.261 mm.

Study of the subject's accommodation was done with an OCT device derived from the OCT 3 used and modified specifically for the anterior segment. The light source's wavelength is stopped by the pigment epithelium. Thus, in studies carried out in normal subjects, it is not possible to visualize the ciliary body because of the screen formed by the iris. Because of the absence of pigment in albino patients, the wavelength theoretically passes through the iris pigment epithelium and it is possible to observe the anterior and posterior faces of the crystalline lens and the ciliary body during natural accommodation.

With an albino patient, it is likely that the modifications observed during accommodation are identical to those observed with a normal subject. Wilson14 assumed this during a study of the modifications of the crystalline lens diameter. The equipment has a target that can be defocused with positive or negative lenses. Using negative lenses stimulates the subject's natural accommodation without using miotic eyedrops. Photographs are taken at different degrees of accommodation. This equipment has made it easy to study in vivo natural accommodation in humans without resorting to chemical means. Quantitative and qualitative modifications of the anterior surface of the crystalline lens, lens thickness, pupil diameter variations, qualitative and quantitative modifications of the iris, and the ciliary body could be analyzed.

Case Report

A 19-year-old albino patient had a right eye refraction of +3.75 −2.50 × 50 and a left eye refraction of +4.25 −4.25 × 160. Visual acuity in each eye was 0.3. The patient had a slight congenital nystagmus without blockage. The intraocular pressure was normal. The iris was without pigmentation, and the patient had high photophobia.

Figures 1 and 2 show the accommodative miosis and the change in the anterior curvature radius of the crystalline lens during 10.0 diopters (D) of accommodation. There was a forward thrust of the crystalline lens anterior pole with an approximate 400 μm reduction in the AC depth.

Figures 3 and 4 are centered on the iris, the area around the iridocorneal angle, and the ciliary body. Thinning of the iris stroma during contraction of the pupil, the inward movement of the ciliary sulcus, and the internal thrust of the ciliary body are visible. It is possible to observe that the iridocorneal angle is also subject to slightly more discreet modifications, as the iridocorneal angle/ciliary sulcus block is a soft tissue in close contact with the ciliary muscle. Movement of these structures to different degrees is inevitable during ciliary contraction. The modifications are insignificant in the iridocorneal angle, but more significant in the ciliary body. These images can be animated. Static images are not as eloquent as the dynamic images. In Figures 3 and 4, which are done in the same zone with the same magnification, the iridocorneal angle is different between the unaccommodated and accommodated states and the distance between the sinus and the scleral spur also appears to vary slightly.

Figure 3.
Figure 3.:
(Baikoff) A close-up of the unaccommodated iridociliary block.
Figure 4.
Figure 4.:
(Baikoff) A close-up the of accommodated ciliary body. The discrete modifications of the iridocorneal angle and an obvious inward thrust of the sulcus and the ciliary body towards the center of the eye are visible.

Figures 5 and 6 show that by using a fixed mark on the OCT images, the internal diameter of the ciliary body could be measured from crest to crest. Unaccommodated, the internal diameter measured 11.215 mm; with 10.00 D accommodation, it was reduced to 10.346 mm. There was approximately 1.00 mm contraction of the entire ciliary body. In reality, this contraction could be more significant because it is not easy to see the most internal inward movement of the ciliary crests. (The cross sections do not necessarily go through the summit of the ciliary processes.)

Figure 5.
Figure 5.:
(Baikoff) The internal diameter of the unaccommodated ciliary body is 11.215 mm.
Figure 6.
Figure 6.:
(Baikoff) The measurement of the accommodated ciliary ring of the same eye is 10.346 mm.

Figures 7 and 8 show that the anterior curvature of the crystalline lens is remarkable. Unaccommodated, almost the entire length of the crystalline lens anterior surface appears to form the arc of a circle. Thus, it is impossible to visualize the crystalline lens equator. This contradicts several theories describing the shape of the anterior surface of the crystalline lens as a parabolic curve. The anterior curvature radius of the unaccommodated crystalline lens measured 14.3 mm; under a 10.0 D accommodative stimulus, it decreased to 9.1 mm. The perfect circle arc shape appears to remain. This principle will have to be confirmed by a more extensive statistical study with a larger number of patients.

Figure 7.
Figure 7.:
(Baikoff) The anterior surface of the unaccommodated crystalline lens and its curvature radius, which is 14.3 mm.
Figure 8.
Figure 8.:
(Baikoff) The curvature radius is 9.0 mm in the eye in Figure 7 (accommodated) (10.00 D).

Figures 9 and 10 show the crystalline lens thickness. Two control indicators were placed in Figures 9, left and right, one according to the reflection axis of the light source and the other parallel to the pupil plane passing through the scleral spur. On the 2 images (unaccommodated versus accommodated), the 2 indicators overlapped perfectly. When the thickness of the crystalline lens increased, there was an almost symmetrical movement of the anterior and posterior pole. The thickness of the lens increased under the influence of its anterior and posterior face.

Figure 9.
Figure 9.:
(Baikoff) Left: Crystalline lens thickness (unaccommodated): 4.947mm. Right: Crystalline lens thickness in same eye with 10.00 D accommodation: 5.607 mm.
Figure 10.
Figure 10.:
(Baikoff) Left and right: Mark near the scleral spur showing movement of the crystalline lens anterior and posterior pole during accommodation.

It was difficult to precisely measure the movement of the posterior pole. However, knowing that with an accommodation of 10.00 D, the lens thickness increase is 660 μm (since thickness increased from 4.947 to 5.607 mm during accommodation) and that the movement of the anterior pole is 400 μm, one can deduce that there is a backward movement of the posterior pole of about 250 μm.

To summarize the observations, during natural accommodation and in our study conditions, miosis associated with a reduction in the anterior curvature radius of the crystalline lens and an increase in lens thickness was observed (Figures 11 and 12). One can reasonably think that the anterior and posterior poles of the crystalline lens moved symmetrically toward the center of the crystalline lens during accommodation. It was not possible to visualize the zonules and the crystalline lens equator. Modifications of the ciliary body were obvious, and the reduction of the ciliary ring during accommodation was confirmed. A reduction in the diameter of the ciliary sulcus and slight modifications to the iridocorneal angle were also observed.

Figure 11.
Figure 11.:
(Baikoff) Diameter of ciliary ring according to accommodation. The darker line shows a decreasing linear curve and the lighter line, a decreasing polynomial curve. A reduction in the ciliary diameter during accommodation is observed.
Figure 12.
Figure 12.:
(Baikoff) The crystalline lens thickness increases in a linear way with accommodation up to 10.00 D.

Discussion

The modifications observed in our albino subject's anterior segment are in accordance with Helmholtz's1 theory. Most studies that followed his confirmed a reduction in the AC during accommodation, a decrease in the crystalline lens central curvature radius, an increase in crystalline lens thickness, and contraction of the ciliary body. However, Helmholtz1 estimated that the anterior movement of the posterior pole of the crystalline lens was slight. This has never been established, and several studies give contradictory results.21–23 In our study, there appeared to be a backward movement of the crystalline lens posterior pole, but to reduce the risk of error in our measuring, this must be confirmed with other observations performed under similar conditions.

If the posterior movement of the crystalline lens is confirmed, it would bring Coleman's24 hydraulic theory into question. According to Coleman, most of the modifications to the crystalline lens are secondary to a hydraulic push of the vitreous on the posterior surface of the crystalline lens during ciliary contraction. Thus, there would be no modification or movement of the posterior pole but a unique movement of the anterior pole.

Studies by Koretz and coauthors25 using the Scheimpflug technique and dilating the pupil with epinephrine could be altered by the instillation of eyedrops, which alter accommodation to make visualization of the crystalline lens easier. Moreover, accommodation of the eye under observation is always stimulated indirectly by stimulating the fellow eye.

The exact shape of the anterior crystalloid in vivo is also uncertain. Our observation contradicts Schachar's3,4 theory of accommodation, particularly concerning our patient, in whom the distortion of the anterior curvature of the crystalline lens was close to the perfect arc of a circle, with no flattening of the periphery and with steepening in the central part of the anterior side of the crystalline lens. Brown21 and Koretz and coauthors22 describe the anterior face of the crystalline lens as a parabolic curve, which our case does not confirm.

We observed internal movement of the ciliary processes without a forward thrust; in particular, without a space being created in the ciliary sulcus. To the contrary, a centripetal movement was observed. In this observation, none of the images observed was compatible with traction on the central zonules3,4 because the ciliary processes and the sulcus moved inward as a block.

Our study confirms those by Glasser and Kaufman5 and Glasser and Campbell,8 which showed on the iridectomized primate overlapping images of the contraction of the ciliary body. With UBM, Ludwig and coauthors,9 Kano and Kuwayama,10 and Bacskulin and coauthors11 also showed the contraction of the ciliary body and its internal push. Our images do not show evidence of forward movement of the ciliary body.

This case report was motivated by Wilson's14 observations using an infrared light to study the modifications of the crystalline lens diameter. We were able to confirm the modifications of the anterior segment during natural accommodation by using optical visualization techniques of the anterior segment with an OCT device in a human patient. All the modifications observed confirm the merits of Helmholtz's1 theory and coincide with what Beauchamp and Mitchell23 had described: the backward movement of the crystalline lens posterior pole during accommodation. This observation contradicts theories that suggest the zonules have a traction effect on the anterior surface of the crystalline lens or that the vitreous has a predominant hydraulic effect. Additional information on the anatomical state of the anterior segment and the modifications observed during accommodation is available in a more documented statistical study carried out on normal subjects with the same equipment.20 It has not been possible to study the ciliary body with a normal subject because of the epithelium pigment barrier.19

References

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