C H A P T E R 1 Refractive Error and Its Correction: The Cornea and Its Physical Characteristics Chapter 1 1.1 Refractive Error and Its Correction The cornea supplies most of the refracting power of the eye by virtue of its anterior curvature at the interface between different media (air and tissue) with significant differences in indices of refraction. When light rays travel through a transparent medium (such as air) and pass into a second transparent medium with a different density (such as water), they bend at the surface of the two media. This is known as refraction (see Fig. 1-1). Fig. 1-1. Refraction of light rays passing from 2 air into water (Tortora & Anagnostakos). The eye has four such media of refraction: the cornea, aqueous humor, lens and vitreous humor. In order for vision to occur, light must pass through all these media to reach the rods and cones of the retina, and then nerve impulses must conduct to the visual areas of the cerebral cortex to form a retinal image. Light rays entering the eye from the air are refracted at the following points: (1) The anterior surface of the cornea as they pass from the lighter air into the denser cornea (2) The posterior surface of the cornea as they pass into the less dense aqueous humor (3) The anterior surface of the lens as they pass from the aqueous humor into the denser lens (4) The posterior surface of the lens as they pass from the lens into the less dense vitreous humor. The degree of refraction that takes place at each surface of the eye is very precise. When an object is 6 metres or more away from the viewer, the light rays reflected from the object are nearly parallel to one another. The parallel rays must be bent sufficiently to fall exactly on the 4 central fovea, where vision is sharpest. This is known as emmetropia . Emmetropia is an optical condition in which there is no refractive error, so that rays of light parallel to the visual axis upon entering the eye are brought to a focus on the fovea centralis when no accommodation is exerted (see Fig. 1-2). There is an exact correlation between the refractive power of the anterior segment and the axial length of the eye. Clinically, this rarely occurs, because the correlation is not exact. Fig. 1-2. In a normal or emmetropic eye, light rays from an object are bent sufficiently by the four refracting media and converged on the central fovea. A clear image is formed (Tortora & Anagnostakos). 2 Light rays that are reflected from near objects are divergent rather than parallel. As a result, they must be refracted toward each other to a greater extent. This change in refraction is brought about by the lens of the eye through the process known as accommodation . Accommodation is an ocular function in which optical power is increased by the eye in order to maintain a clear image as it moves closer (see Fig. 1-3). It occurs through a process of ciliary muscle contraction, which pulls the ciliary process and choroid forward toward the lens, releases the tension on the lens and suspensory ligament by zonular relaxation that causes the elastic-like lens to shorten. The increase in curvature and optical power bends the rays toward the central fovea to form a retinal image. Fig. 1-3. Accommodation. (a) For objects at 6 meters or more. (b) For objects less than 6 meters (Tortora & Anagnostakos). 2 5 Conversely, in far vision, the ciliary muscle is relaxed and the lens is flatter. With aging, the lens loses elasticity and, therefore, its ability to accommodate. This condition is known as presbyopia . Myopia is an optical condition in which rays of light entering the eye parallel to the visual axis come to a focus in front of the retina. The condition occurs because (1) the refractive power of the anterior segment is too great for the length of the eye or (2) the eye is too long for the refractive power present. Patients with uncorrected myopia do not accommodate to improve vision, because accommodation shifts the focal point even further anterior to the retina and blurs vision. Patients generally display certain degrees of difficulty in seeing distant objects, depending on the severity of the refractive error. Myopia is neutralized by concave lenses, which cause the parallel rays of light incident at the concave lens to focus at the far point of the eye, to be imaged by the optical system of the eye onto the retina (see Fig. 1-4). Thus, an infinite point would be conjugate with the retina after having passed through the combination of concave lens and the un-accommodated eye. Fig. 1-4. In the myopic eye, the image is focused in front of the retina. The condition is corrected by a concave lens which diverges entering light rays so that they have to travel further through the eyeball and are focused exactly on the retina (Tortora and Anagnostakos). 2 Hyperopia is a refractive condition of the eye in which, with accommodation suspended, parallel rays of light are intercepted by the retina before coming into focus. The condition occurs because (1) the anterior segment is inadequate for the length of the globe or (2) the globe is too short for the amount of refractive power present (see Fig. 1-5). Accommodation increases the refractive power of the anterior segment and may compensate for mild degree of 6 hyperopia and provide normal vision. However, patients with a moderate or greater degree of hyperopia would have some difficulty in seeing near to intermediate distance objects. Hyperopia can be either latent or manifest. Latent hyperopia is that portion of the hyperopic error that is completely corrected by the eye’s own accommodation. The younger the patient, the greater the latent component. Manifest hyperopia may be subdivided into facultative and absolute types. Facultative hyperopia is the portion of the refractive error that may be measured and corrected by convex lenses but is also corrected by accommodation in the absence of lenses. A patient with facultative hyperopia will have normal visual acuity both with and without the convex lenses that correct this portion of the refractive error. The patient’s own accommodation can relax and permit the lenses to correct the error. Absolute hyperopia is the portion of the refractive error that cannot be compensated by the patient’s own accommodation. Both near and distance vision are blurred and the patient readily accepts convex lenses. Fig. 1-5. In the hyperopic eye the image is focused behind the retina. The condition is corrected by a convex lens which converges entering light rays so that they focus exactly on the retina (Tortora and Anagnostakos). 2 As stated earlier, the amplitude of accommodation decreases as the patient ages. There is a tendency to go from latent to manifest hyperopia and need a reading correction (or bifocal if there is also distant refractive error). This is known as presbyopia. Presbyopia is an optical condition of decreased accommodation which occurs normally in middle-aged and older individuals. As described earlier, the process of accommodation brings about change in the shape of the elastic-like lens when focused on a near object. With each year of life the lens loses some of its elasticity, decreasing the amount of accommodation. Generally, there are about 14 diopters of accommodation at 10 years of age and only 2 7 diopters of accommodation by 50 years of age. The decrease occurs gradually, in all individuals, irrespective of their refractive error. However, mildly myopic people have an advantage, as they are able to read comfortably without additional lenses due to the nature of myopic eye described previously. A moderately myopic individual may compensate for presbyopia by removing the lens that corrects distance vision. Presbyopia, however, is aggravated in a hyperopic individual if the lens that corrects the hyperopia is removed. The main symptom of presbyopia is inability to see near work distinctly, which is aggravated in dim illumination and with small print. Other symptoms can be ocular discomfort, headaches, and tired eyes, especially if the person attempts to persist with reading or doing close work, and annoyance at having to place reading material further away from the eyes than previously. Presbyopia is corrected by means of convex lenses added to the distance correction. This is known as “near add”. The power of the lens required to achieve clear near vision varies with an individual’s habits, age, occupation, and accustomed distance of doing near work. Generally the weakest possible “near add” is prescribed, and increases as the patient grows older; the maximum is usually +3.0 or +3.25 diopters needed to achieve clear vision at 1/3 of a meter. Individuals who do not require distance correction only require lenses that correct the presbyopia. For individuals who require distance correction, bifocal lenses may be the solution, or trifocal and multifocal if they require distance, intermediate distance and near correction. Astigmatism is an optical condition in which the refracting power of a lens or a cornea is not the same in all meridians, which produces an image with multiple focal points or lines (see Fig. 1-6). Astigmatism can occur by itself or in conjunction with myopia or hyperopia. Fig. 1-6. Astigmatism prod uces an image with multiple focal points or lines on the retina (Courtesy of Laser Sight Centres). 8 The anterior corneal surface in an emmetropic person is spherical, but it is flatter peripherally giving the cornea a hydrobolic shape.