Is an irregular curvature of the cornea causing light to focus incorrectly on the retina?

Emmetropia, myopia and hyperopia

Briefly, a normal-sized eye, anteroposterior dimension 22–24 mm, is termed an emmetropic eye and is one in which the image is focused sharply on the retina by the combined refracting properties of the cornea and the lens (eFig. 22). Myopia or short-sightedness occurs in eyes with above average length and the image therefore is focused in front of the retina; conversely, long-sighted eyes have a shorter than normal length and the image is focused behind the retina. Myopia can be corrected by placing a convex lens in front of the cornea, which thus causes some divergence of the light rays and with the correct lens, the image can be formed on the retina correctly. Myopia can also be corrected by ‘reshaping’ the cornea (see pp. 218-219) to cause some degree of ‘flattening’ of its curvature and thus reduce its refractive power. Conversely, hyperopia can be corrected using a convex lens and similarly corneal surgery can be performed to correct hyperopia, although it is generally less successful.

Further information on refraction with links to optometry literature is given inChapter 5, p. 285

eFig. 4.22. Emmetropia is a state of refraction where a point at an infinite distance from the eye is conjugate to the retina. Ametropia is a state where a refractive error is present or when distant points are no longer focused properly to the retina. Myopia or near-sightedness (short-sightedness) is one form of ametropia where the eye is effectively too long or has too high a power. Consequently, a point at infinity focuses in front of the retina. In myopia, a point lying between infinity and the eye is conjugate to the retina. Hyperopia or far-sightedness is a form of ametropia where the eye’s power is too weak or the eyeball too short. In this case, a point at infinity focuses behind the retina. A point behind the eye is therefore conjugate to the retina. Astigmatism is a form of ametropia in which refractive error changes with the optical meridian.

Emmetropia, Myopia, Hyperopia, Astigmatism, and Pinhole Effect

Emmetropia is the refractive state of an eye in which parallel rays of light entering the eye are focused on the retina, creating an image that is perceived as crisp and in focus. Myopia, hyperopia, and astigmatism are abnormalities of this desired condition (Fig. 1-4). In myopia, or nearsightedness, the refractive power of the eye exceeds the refraction necessary for the axial length of the eye. As a result, the image is focused in front of the retina. Most commonly, this abnormality takes the form of axial myopia, in which the eye is abnormally long, but other causes are an abnormally steep cornea and lens abnormalities (e.g., cataract), or a combination of these factors. Correction is obtained by placement of a “minus,” or concave, lens in front of the eye, thus adding divergence to the incoming light rays and moving the focused image onto the retina.

Hyperopia, or farsightedness, is a refractive condition of the eye in which the axial length is too short, the cornea is too flat, or the lens has too little refractive power to focus the image on the retina. The image is therefore focused posterior to the retina. This condition is corrected by addition of a “plus,” or convex, lens to the optical system, which provides additional convergence to the light rays entering the eye, thereby moving the image forward onto the retina.

In astigmatism, the refractive power of the eye in one plane (or meridian) is different from the refractive power in a different meridian. This results in essentially two focal planes from these two meridians, causing a blurred, distorted image. The disparity is corrected by placement of a cylindrical lens in front of the eye.

If the visual acuity is poor because of a refractive error, the patient's vision should improve on looking through a pinhole aperture. If a pinhole aperture is not available, a 3-inch by 5-inch card with several tiny holes (1 to 2 mm across) created with a sharp pencil may be used. Pinhole testing generally demonstrates correction of any uncorrected refractive errors. If correction is not seen, the examiner must ascertain whether a pathologic cause of decreased visual acuity such as unclear ocular media (e.g., a cataract), optic nerve disease, or retinal disease is present.

Suppression in normal binocular vision

In complex three-dimensional scenes, there are several types of binocular information that must be combined to produce an integrated percept of depth and distance. First, some of the visual information is coherent and unambiguous, such as images formed within Panum's fusional areas and the coherent information contributes to normal fusion and stereoscopic depth perception. However, some forms of binocular information do not stimulate binocular fusion. For example, regions of space may be clearly visible to only one eye (i.e. partially occluded to other eye), and this fragmented binocular information also contributes to the perceptions of distance and depth. In addition, information from binocular images that are uncorrelated, either because the matches are ambiguous or would conflict with other cues, influences depth and distance perception. Visual confusion, in which there is superimposition of separate diplopic images arising from objects seen behind or in front of the plane of fixation, is an example of uncorrelation of binocular images that can be interpreted to suggest depth or distance. The binocular visual system attempts to preserve as much of the depth information as possible from all three sources to make inferences about objective space without introducing ambiguity or confusing space perception. At times, however, conflicts between the two eyes are so great that the conflicting information must be resolved by suppressing or suspending the perception arising from one of the retinal images.

Four classes of stimuli have been identified that appear to evoke different interocular suppression mechanisms in normal binocular vision.

1. Interocular blur suppression occurs when the retinal images have significant differences in defocus or contrast.152 A wide variety of natural conditions produce unequal contrast of the two ocular images, including naturally occurring anisometropia, unequal amplitudes of accommodation, and asymmetric convergence on targets that are closer to one eye than to the other. The relative differences in defocus can be partially eliminated by differential accommodation of the two eyes105 and by interocular suppression of the blur.145 The latter mechanism is requisite for adaptation to a monovision correction of presbyopia. Monovision suppression allows clear, non-blurred, binocular percepts and retention of stereopsis albeit with the stereo-threshold elevated by approximately two-fold.38,145

2. Suspension is the mechanism that eliminates one of the two ocular percepts that arise during physiological diplopia, in which targets lying in front of or behind the singleness horopter would be seen as doubled.30 Binocular eye alignment optimizes depth information from binocular disparities near the horopter, but at the same time it produces large disparities for the objects that are at some distance in front of or behind the plane of fixation. Even with binocular disparities that are well outside the limits of Panum's fusional areas, the perception of diplopia under normal casual viewing conditions is rare because of the suppression of one image. The suppression of physiological diplopia is called suspension because this form of suppression does not alternate between the two images. Instead, only one image is continually suppressed, favoring visibility of the target imaged on the nasal hemi-retina.35,43,92 However, suspension is not obligatory, and calling attention to the disparate target can evoke physiological diplopia. It has also been suggested that suspension may be involved in the permanent suppression of pathological diplopia associated with strabismus.44,63,135,136

Postoperative target refraction

Most surgeons aim for emmetropia, but patients who are myopic by around −2.50 D may habitually read unaided and would not be happy if this ability is taken away. If there is a significant refractive error and only one eye is to have surgery, the consequences of unequal refractive error or anisometropia must be considered.

Aniseikonia (unequal image size) is rarely a problem until the right/left difference exceeds 3.00 D. However differential prismatic effect may be a problem for bifocal and varifocal wearers at just 1.50 D difference because, as shown in Figure 9.8, they read through a point at least 10 mm below the optical center of the lens.

Prentice's rule tells us that at 1 cm a power difference of 1.00 D gives 1 Δ vertically. The maximum vertical prism tolerated by most patients is 1.5 Δ, so the vertical power difference most be kept within 1.50 D, and less for varifocal wearers.

Emmetropia, Ametropias, and Presbyopia

The successful performance of refractive surgery demands a thorough understanding of the optics of the human eye. The refractive power of the eye is predominantly determined by 3 variables: the power of the cornea, the power of the lens, and the length of the eye. In emmetropia, these 3 components combine in such a way as to produce no refractive error. When an eye is emmetropic, a pencil of light parallel to the optical axis and limited by the pupil focuses at a point on the retina (i.e., the secondary focal point of an emmetropic eye is on the retina;Fig. 1.1). The “far point” in emmetropia (defined as the point conjugate to the retina in the nonaccommodating state) is optical infinity.

Eyes with refractive errors can have abnormalities in one or more of the above variables, or all variables can be in the normal range but incorrectly correlated, resulting in a refractive error. For example, an eye with an axial length in the upper range of normal may be myopic if the corneal variable is also in the steeper range of normal. In a myopic eye, a pencil of parallel rays is brought to focus at a point anterior to the retina. This point, the secondary focal point of the eye, is in the vitreous. Rays diverging from the far point of a myopic eye will be brought to focus on the retina without the aid of accommodation.

The hyperopic eye, on the other hand, brings a pencil of parallel rays of light to focus at a point behind the retina. Accommodation of the eye may produce enough additional plus power to allow the light rays to focus on the retina. Rays converging toward the far point farther behind the eye will be focused on the retina while accommodation is relaxed.

For full correction of myopia and hyperopia, a distance corrective lens placed in front of the eye must have its secondary focal point coinciding with the far point of the eye so that the newly created optical system focuses parallel rays onto the retina.

Astigmatism may be caused by a toric cornea or, less frequently, by astigmatic effects of the native lens of the eye. Astigmatism is regular when it is correctable with cylindrical or spherocylindrical lenses so that pencils of light from distant objects can be focused on the retina. Otherwise, the astigmatism is irregular. Visual acuity is expected to decline for the different degrees of astigmatism. Astigmatism of 0.50 to 1.00 diopters (D) usually requires some form of optical correction. An astigmatic refractive error of 1.00 to 2.00 D decreases uncorrected vision to the 20/30 to 20/50 level, whereas 2.00 to 3.00 D may decrease UCVA to the 20/70 to 20/100 range.5

Presbyopia is the age-related loss of accommodation. Onset of presbyopia will vary with the refractive error and its method of correction. For example, myopes corrected with spectacles can simply remove their glasses for improved reading vision. Latent hyperopes, on the other hand, use their accommodative reserve for clear distance vision; as the amplitude of accommodation wanes with age, reading difficulties emerge.

IOL calculations after PIOL implantation

Calculation of the correct IOL power for emmetropia will be difficult in patients with a history of PIOL.90 The presence of the posterior chamber IOL leads to a decrease in ultrasound velocity. This results in a longer axis measurement of the globe than normal. Wiechens et al90 suggested that subsequent correction for velocity effects would be needed. Intraoperative retinoscopy of aphakic refraction and IOL calculation would be recommended. In addition, selection of a stronger power obtained in the IOL calculations (not SRK II) may be needed. Hoffer suggested a method of calculating the axial lengths in biphakic eyes91: If the eye is measured at an average velocity of 1555 meters/second, the following calculation can be used, depending on the material of the phakic IOL: AL(corrected) = AL1555 + X*T, where T is the central thickness of the phakic IOL and X = + 0.42 (0.41–0.42) for PMMA, −0.59 (0.56–0.59) for silicone, +0.11 (0.10–0.12) for Collamer, and +0.23 (0.23–0.24) for acrylic. After the AL has been approximated by the above formula, the author recommended the Hoffer Q formula should be used for eyes <22.0 mm, Holladay 1 formula (not Holladay 2) for eyes between 24.5 and 26.0 mm, and the SRK/T formula for those longer than 26.0 mm.

Compensation for negative lenses

Form deprivation myopia demonstrates that the visual environment plays a role in establishing and maintaining emmetropia. Recognition that the emmetropization mechanism uses visual feedback to match the axial length to the focal plane emanates from studies that used negative-power (and positive-power) lenses to shift the focal plane of the eye.96–98 As shown in Figure 55.2B, a monocular negative lens shifts the focal plane posteriorly, away from the cornea. This consistently produces a compensatory increase in the axial elongation rate of the growing eye, such that the retinal location is shifted to match the shifted focal plane (Figure 55.2C.) When measured with the lens in place, the refractive state matches the untreated fellow control eye.96,99,100 Thus, in compensating for the negative lens the eye is, in fact, restoring optical emmetropia.

With the lens removed, the eye is myopic (Figure 55.2D). Compensation can be quite accurate101–103 and negative lenses of different powers produce different axial elongation appropriate to move the retina to compensate for the lens power. Some strains of mice can develop negative lens-induced myopia, even though mice are not strongly dependent on vision.104 Fish (Tilapia), which are more vision-dependent, display this modulation characteristic.105

Interestingly, for negative lens compensation to occur in animals, the lens must be worn almost constantly. Removing the negative lens and allowing normal unrestricted vision (or plano-lens wear) for as little as 2 hours per day is sufficient to block negative lens compensation in monkeys,106 tree shrews,107 and chicks.108

PLANNED STAGING OF RADIAL KERATOTOMY PROCEDURES

With greater experience, many surgeons modified their approach to the RK patient. Although emmetropia was attractive to patients, especially younger myopes, experience and research suggested that the refractive goal should be slight undercorrection. PERK study data revealed a 5-year hyperopic shift in 22% of patients and a 10-year shift in 43%.19,21 Deitz also reported a hyperopic shift in 38% of patients following RK using metal-blade knives.60 Because it is not possible to identify these patients preoperatively, one approach was to operate with the intention of leaving patients slightly undercorrected. A small amount of residual myopia permitted useful vision at all ages, whereas a hyperopic final correction would be less desirable. As younger patients became presbyopic, the benefits of additional useful near-vision gained from a diopter of myopia, and the delayed need for reading glasses was appreciated. Presbyopic candidates for refractive surgery realized the difficulties with near-vision and often opted for more or less undercorrection or monovision (one eye distance, one eye near).

From a practical standpoint, the adjustment of surgical goals influenced surgical planning and method. If the goal was to undercorrect and never go beyond emmetropia, then a staged approach to surgery allowed the surgeon to aim for undercorrection on an initial procedure and to perform secondary (enhancement) procedures after the initial outcome was known, so as to bring the refractive result into an acceptable range for both the surgeon and the patient (Fig. 22-14).61 The major difficulty with this approach was selection of a secondary procedure that would achieve the desired final correction.

Although the term enhancement procedure was popularized through RK courses, it should be emphasized that this was really a reoperation. Patients perceived an enhancement as a less significant procedure with smaller risks, designed solely to improve the previous refractive result.

The staged approach to refractive surgery had several potential disadvantages as well. Multiple procedures entail extra cost and effort to patients, doctor, and assistants. This included direct financial costs, time off from work for the patient and escort, and additional time in the office for follow-up examinations. Furthermore, the staged approach may have raised patients’ expectations regarding the final result because of the presumed possibility of refining the refractive outcome through multiple ‘enhancements.’

Effects of contact lens wear on corneal physiology.

Contact lenses are common optical devices which correct the refractive errors of the eye to achieve emmetropia. The tear film bathes both surfaces of the contact lens (Fig. 4-38), but corneal epithelial function is still relatively compromised. The corneal epithelium layer receives its oxygen from the tears and its glucose from the circulation via the aqueous and the limbal vessels (see above). Contact lenses reduce the direct availability of oxygen to the epithelium, thus shifting the balance from aerobic to anaerobic metabolism. The already high lactate levels in the cornea are doubled with contact lens wear and carbon dioxide production is increased. The induced acidosis has a direct effect on stromal hydration by impairing deturgescence mechanisms (see above).

Hard (rigid) contact lenses are usually made from polymethylmethacrylate (PMMA) and have the greatest effect on corneal function; in addition to restricting oxygen availability, hard lenses deplete glycogen stores, even though the level of glucose availability is not reduced. It has been suggested that hard lens-induced inhibition of aerobic enzymes such as hexokinase reduces direct glucose utilization by the cornea. Prolonged wear of hard contact lenses is therefore not possible, owing to the damaging effect on corneal transparency induced by the disturbed metabolism. Soft contact lenses are made from polymers of hydroxyethyl methacrylate (HEMA), poly(HEMA/vinylpyrrolidones), silicone or other similar materials, and permit extended wear of the lens owing to their permeability to oxygen and carbon dioxide. However, there is still some degree of lactate accumulation with soft lenses and prolonged use appears to affect the function of the endothelium. HEMA-based lenses are hydrophilic, while silicone-based lenses are hydrophobic: accordingly, there is less protein deposition on the latter but greater levels of denatured proteins. However, corneal inflammatory episodes are if anything more frequent with silicone lenses. Manufacturers of contact lenses continually produce new ‘biomimetic’-type lenses with increasing water content (up to 59%) in attempts to support normal corneal physiology (hydrogel lenses). In addition, incorporating other material into contact lenses such as polyethylene glycol and cross-linked hyaluronan are other possibilities under consideration to improve biocompatibility.

A popular compromise in contact lens type is the gas-permeable rigid lens, which combines the reduced toxicity of PMMA with high gas-transfer capability. The wide variety of lens types and materials has led to their being characterized on the basis of their oxygen flux, defined as the DK value:

RH→R•+O2→ROO •

where D is the diffusion coefficient, K is the solubility, and L is the thickness of the lens material. ΔP is the change in the partial pressure of oxygen across the material. HEMA and PMMA have a low oxygen flux, while hydrogels and silicones have a high flux. Both the thickness of the lens and the DK value determine its suitability for use in terms of its gas permeability. The actual amount of oxygen that reaches the cornea is the most important factor in the design of a contact lens, and most practitioners describe contact lenses in terms of their equivalent oxygen performance (EOP).

Contact lenses may have deleterious effects on the epithelium, causing thinning, reduction in the hemidesmosome density and the number of anchoring fibrils, and reduced adhesion of the epithelium to the basement membrane. This may be a direct effect of low O2-transmitting lenses on basal epithelial cell proliferation. This is especially true of extended-wear hydrogel lenses. In severe cases, excessive use of contact lenses produces epithelial oedema and keratopathy in the form of punctate epithelial erosions. Rigid contact lenses also produce tear film instability by causing damage to the epithelium and the mucin layer in particular.

Contact lens wear may also induce changes in the corneal stroma (thickening) and the endothelium (polymegathism).

Near-Sightedness and Far-Sightedness Are Problems in Focusing the Image on the Retina

In the normal resting eye, parallel light rays are focused on the retina. This condition is called emmetropia, from the Greek en meaning in and metron meaning measure. In some cases, the resting eye does not focus light on the retina. In myopia, or nearsightedness, the image is formed in front of the retina. Usually an abnormally long eyeball causes nearsightedness but sometimes abnormally powerful refractive power causes myopia. Lenses that diverge light (indicated by a − sign) correct myopia. In hypermetropia, or farsightedness, the image forms behind the retina because the eyeball is too short. Converging lenses (+ lenses) correct hypermetropia.