Defects of the Eye

6 Key excerpts on "Defects of the Eye"

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  eBook - ePub

  Ophthalmic Disease Mechanisms And Drug Discovery

  • Kang Zhang(Author)
  • 2016(Publication Date)
  • WSPC

    (Publisher)

  Chapter 1:  Pathology and Mechanism of Eye Diseases

  Katherine J. Wert † , Heike Kroeger ‡ , Frances Wu ‡ ,Stephen H. Tsang § , and Jonathan H. Lin

  ‡ ,

  **

  † The Whitehead Institute for Biomedical Research,Cambridge, MA 02142

  ‡ Departments of Pathology and Ophthalmology,University of California San Diego, La Jolla, CA 92093

  § Departments of Ophthalmology, Pathology, and Cell Biology,Columbia University College of Physicians and Surgeons,New York, NY 10032

  ** VA San Diego Healthcare System, San Diego, CA 92161

  1.Introduction

  Retinal degenerative diseases are the leading cause of irreversible blindness in Western nations, and the loss of sight affects over 3.4 million people in the United States alone.1 These diseases have high genetic and allelic heterogeneity, and therefore our understanding of the pathophysiology of these conditions is limited. However, progress in molecular genetics has determined specific factors that play significant roles in the pathogenesis of retinal degeneration. This chapter is focused on understanding the pathology of the human eye, the molecular genetics identified in degenerative diseases of the eye, and the current known mechanisms for these diseases.

  2.The Eye

  The human eye is a complex organ that consists of many different cell types. The eye transmits information to be processed by the brain on the color, form, and light intensity of objects. The human eye can be divided into two main parts: the anterior and the posterior segments.2

  2.1.Anterior eye

  In the anterior portion, the sclera and cornea act as outer protective layers of the eye. Also anterior in the eye is the uveal tract, composed of the iris, ciliary body, and anterior choroid. The components of the uveal tract mainly function under the control of the autonomic nervous system. The uveal tract allows for the exchange of nutrition and gases into the posterior portion of the eye. The ciliary body and the iris are directly supplied by the uveal vessels, while these vessels indirectly support the sclera, lens, and outer retina via diffusible nutrients. The uveal tract contains numerous melanocytes that reduce the light reflected within the eye and absorb light transmitted through the sclera, in order to improve the retinal image.2

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  Modern Ophthalmic Optics

  • José Alonso, José A. Gómez-Pedrero, Juan A. Quiroga(Authors)
  • 2019(Publication Date)
  • Cambridge University Press

    (Publisher)

  5 The Lens-Eye System 5.1 Introduction The eyes are optical systems whose function is the formation of real images of the environ- ment that the brain can detect, process, and interpret as visual information. Like any optical system, natural or artificial, eyes cannot produce perfect images; they have aberrations, the most common of which are spherical defocus and astigmatism, both known as refractive errors. Defocus and astigmatism are also known as second-order aberrations. They are nothing but errors of curvature of the wavefront refracted through the eye, and we already know that curvature is determined by the quadratic – second order – terms of the polynomial expansion used to describe the wavefronts. As eyes are biological organs, the percentage of individuals presenting significant second-order aberrations is known as the prevalence of refractive errors. Although prevalence is a clinical term typically used to measure the extent a disease affects a population, refractive errors are not diseases. An eye affected by refractive error can be perfectly healthy; it simply has a mismatch between its power and its size. Refractive errors can be corrected by surgery, not free from risks, that changes the curvature of the cornea, henceforth changing the power of the eye. But still today a majority of refractive errors are compensated with lenses that, once annexed to the eye, create a new optical system, the lens-eye system, that should be free from second-order aberrations and should produce a sharp image on the retina. There are basically three types of these ophthalmic compensations: spectacle lenses, contact lenses, and intraocular lenses (IOL). Probably, the reader already has a general idea about the differences between the three types of lenses and may think that they mainly differ in their location with respect to the eye.

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  Physics for O.N.C. Courses

  • R.A. Edwards(Author)
  • 2014(Publication Date)
  • Pergamon

    (Publisher)

  CHAPTER 22

  The Eye. Defects of Vision and Optical Instruments

  Publisher Summary

  This chapter focuses on various experimental results related to defects of vision and optical instruments. Image formation, in a good eye, is achieved and perfected by the action of the eye as a whole, namely, cornea, lens, aqueous, and vitreous humors. The angle subtended at the eye lens by the object is the same as that which is subtended by the image at the lens. If the eyeball is of such a length from lens to retina that parallel light from infinity is focused in front of the retina when the ciliary muscles are completely relaxed, short sight, or myopia results. Long sight or hypermetropia is a condition in which the effective power of the eye lens is too small in relation to the length so that when there is no accommodation, parallel light from infinity is focused behind the retina. Presbyopia is the loss of power of accommodation, which is normally associated with the advancing age of an individual. A photographic enlarger is a slightly less elaborate device to the projector. The photographic negative is used in place of a slide and the light-sensitive paper on which the print is to be made takes the place of the screen.

  22.1 Structure of the Eye

  The human eye (Fig. 22.1 ) contains a converging lens L of a gelatinous, transparent material which is not of uniform refractive index throughout and the surfaces of which have different curvatures. The power of the lens is controlled by the ciliary muscles M which act in order to increase the curvature of the lens surfaces. When these muscles are fully relaxed the eye is said to be unaccommodated and it is adjusted for viewing objects at a great distance. When the muscles are fully tensed the eye is fully accommodated for close vision. The point closest to the eye at which an object may be focused clearly when the eye is fully accommodated is called the near point. The distance from the eye of the near point varies considerably from one person to another but is often regarded as being at about 25 cm. The far point is at the greatest distance for which vision is clear when the ciliary muscles are completely relaxed. Ideally this point should be at infinity. The hard, opaque coating of the eyeball, called the sclerotic S , becomes transparent at the front of the eye in order that light may enter. This transparent “window” is called the cornea C . In fact most of the refraction of the light occurs at the cornea, the lens being employed largely for accommodation. Behind the cornea and in front of the lens is the aperture through which light is permitted to enter the lens. This is the pupil P , the size of which is adjusted by a diaphragm called the iris I , which is the familiar coloured part of the eye. The pupil has its largest diameter when illumination is poor, and vice versa. The space A between the lens and the cornea is filled with a salt solution called the aqueous humour . The front of the cornea is also moistened with salt solution and the action of blinking with the eyelids keeps the surface of the cornea clean. The inner wall of the sclerotic at the back of the eye forms the light-sensitive surface R on which the light entering the eye is focused to form images. This surface is called the retina and is covered with light-sensitive cells situated at the ends of nerve fibres which form a network over the surface of the retina and which all leave the the eyeball in a bundle known as the optic nerve O . This leads to the brain which interprets the image on the retina resulting in “sight” or vision. The point at which the optic nerve leaves the eyeball is insensitive to light and is called the blind spot B . In contrast to this is the fovea , or yellow spot Y , at which the retina is most sensitive. This point lies at the intersection of the principal axis of the lens and the retina. The space V between the retina and the lens is filled with a jelly-like substance called the vitreous humour

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  Principles of Ophthalmology

  • H. E. Hobbs(Author)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Details of the object focussed by the former lie in front of the retina and those focussed by the latter behind it. well pass unnoticed. Pathological changes in the eye are responsible for only a small proportion of refractive errors and systemic disease for fewer cases still. Nevertheless, the visual effects of errors which in health have remained latent are frequently made manifest through the onset of fatigue, debility or febrile illness. The symptomatology of refractive errors. Lack of a clear retinal image of the object viewed may result from any type of refractive error and provides its presenting symptom in a large number of cases. Blurred vision results; but this is described by the patient in many ways according to his command of language, to the circumstances in which he has become aware of it, its degree, etc. He may mistakenly refer to it as diplopia. If he has possessed the disability from his early years he may be quite unaware of it and examination of the visual acuity of school-children to detect this visual hindrance to learning is generally recognised as a necessity in civilised communities. A good deal may be learnt about the nature of the error from the history of an intelligent patient : the distance vision of myopes is indistinct but when objects The Optical Mechanism of the Eye 1 are brought to the anterior focus of the eye they may be seen clearly unless there is associated astigmatism. Hypermétropes, with or without astigmatism, see more clearly in the distance than close to and, if the error is small enough to be reduced appreciably by accommodation, they may be aware of the fact that greater clarity is to be achieved by this effort. Astigmatism is to be deduced less readily from the history unless the subject is an acute observer and happens to have been presented with an object with straight lines at right angles, coinciding with the meridians of his astigmatism.

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  The Science of Paediatrics: MRCPCH Mastercourse

  • Tom Lissauer, Will Carroll, Tom Lissauer, Will Carroll(Authors)
  • 2016(Publication Date)
  • Elsevier

    (Publisher)

  Chapter 30

  Ophthalmology

  Louise Allen

  Learning objectives

  By the end of this chapter the reader should:

  • Know and understand the anatomy and embryology of the eye

  • Understand how the structure of the eye relates to function

  • Understand the normal development of vision and the pathophysiology of visual impairment

  • Know the physiology of the eye and its movement

  • Know the genetic and environmental factors in the aetiology of eye disorders

  • Recognize congenital eye disease, enabling early prevention and treatment of blinding conditions

  • Understand the action of pharmaceuticals used in eye disease and know which systemic drugs can cause ocular toxicity

  • Know when an ophthalmic phenotype can help to make a systemic/genetic diagnosis

  • Know when a systemic disease puts a child at risk of ophthalmic disease

  Although ophthalmology may seem a fairly minor topic to the generalist, paediatric eye and visual disorders are common in both primary and secondary care settings. The eyes, their visual pathways and higher visual processing mature throughout early childhood. Good visual function depends on all these factors. These stages of normal visual development and the way we assess them are covered in Chapter 4 , Normal child development.

  Epidemiology of childhood visual impairment

  Visual impairment in childhood impacts all areas of a child's development and influences their future prospects. In the UK, nearly 4% of the childhood population are registered severely visually impaired or blind (compared to 12% in developing countries) and half these children will have additional motor, sensory, learning impairments or systemic disease. Our screening programmes and access to specialist paediatric ophthalmic care prevent many cases of severe visual impairment, and the majority (75%) of children registered blind in the UK have an unpreventable and untreatable cause. The registration process facilitates the educational and social support these children need. The level of visual impairment is based on corrected binocular visual acuity (using a Snellen chart) and visual fields. The definitions of sight impairment are shown in Box 30.1

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  Eyes and Education

  • Vernon H. Smith, F. E. James(Authors)
  • 2014(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  36 Eyes and Education by a moat of blindness, outside which is another circle of vision that extends to the periphery. This type of defect is often associated with night blindness and is caused by a rare progressive hereditary condition called Retinitis Pigmentosa. (ii) Common causes of visual field defects in schoolchildren. The causes of visual field defects are many and varied. We have mentioned Retinitis Pigmentosa already. This disease is often present in several members of one family, and unfortunately there may be very little that can be done to arrest its progress. It is unusual for it to cause severe incapacity before the age of ten or eleven, but it could be well advanced by the time the pupil is taking his 'O' or 'A' levels. The macular or central scotoma is unfortunately more common. Some types of infection, which may even be present at birth, affect the central part of the retina in one eye (if both eyes are affected it is extremely unlikely that the child will see well enough to attend a normal school) and sometimes a severe blow on the eye can cause damage that destroys central vision. Unhappily also not uncommon in children is the game of looking at the sun—sometimes through binoculars or a telescope. This cannot be con-demned too strongly, because the rays of the sun are concentrated on the macula and produce a solar burn which has a disastrous effect on central vision producing a severe central scotoma. Defects affecting the peripheral visual field are perhaps more common than scotomata. Sometimes they are due to congenital malformation of the eye. A child born with a Keyhole pupil—the so-called coloboma of the iris may have a similar defect in his retina, and as the deficiency in the iris is usually below, he will have a visual field defect in the upper part of his visual field. Children are also sometimes born with a defect in one half of their visual field.

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