Communication Topic 5: Visual Defects

CommunicationTopic 5: Visual Defects Biology in Focus, HSC Course Glenda Childrawi, Margaret Robson and Stephanie Hollis

DOT Point(s) • distinguish between myopia and hyperopia and outline how technologies can be used to correct these conditions.

Introduction The ability of the eye to function effectively in refracting and transmitting light depends on the shape and size of the eyeball and the clarity of the structures within the eye. Much like the variations in other features of the body, the variations in the eye are often hereditary. www.eyedesignbook.com

Introduction If the cornea or lens is not the right shape, or the eyeball is too elongated or too round, this will affect the ability of the eye to refract light and refocus it accordingly on the retina. The resulting difficulties in seeing are called visual defects and include: • Myopia (short-sightedness) • Hyperopia (long-sightedness) • Astigmatism (This is common but not part of our course) These defects are not usually due to disease or illness but are simply a result of how the body grows.

Myopia • The correct medical term for short-sightedness is myopia. This is when a person can see near objects clearly, but distant objects appear blurred. psych.ucalgary.ca

Myopia • In an eye that is myopic, distant objects being viewed from a focused image that falls in front of the retina. When near objects are viewed, the image falls at the correct depth on the retina, enabling the person to see near objects clearly, hence the term ‘short-sightedness’.

Myopia Myopia has several causes: • The eyeball may be too elongated in shape • The refractive power of the cornea may be inadequate • The lens may not become flat enough when the ciliary muscles contract www.walgreens.com

Hyperopia • Hyperopia is the correct medical term for long-sightedness. This is when distant things are seen clearly but near objects cannot be seen in focus because the image falls behind the retina. www.clipart.dk.co.uk

Hyperopia Causes are: • An eyeball that is too short from front to back (the eyeball is too round) • A lens that is too flat and is unable to alter its shape adequately (perhaps due to a loss in elasticity as in old age) • The refractive power of the cornea is too great for the shape of the eye. visianinfo.com

Technologies to Correct Myopia and Hyperopia Visual defects such as myopia and hyperopia cause distinct disadvantages to people if not corrected. Myopic people would have difficulty seeing traffic signs, watching TV and playing sport. People who are long-sighted would have difficulty reading books, newspapers or a computer screen. www.experthow.com

Technologies to Correct Myopia and Hyperopia • To compensate for refractive errors in the eye, technologies offer people the option of using corrective lenses or more recently, the option of laser eye surgery. Technologies available to correct these defects are therefore of major advantage in society. www.visualphotos.com

Technologies to Correct Myopia and Hyperopia Spectacles The first spectacles were made in Italy in about 1280. Single magnifying glasses had been used prior to this. Spectacles (glasses) are frames that hold corrective lenses in front of the eye. www.goggleyes.com

Technologies to Correct Myopia and Hyperopia In the past, lenses were made of glass, but nowadays many of them are made of clear, hard plastic. This advance in technology has the advantage of the spectacles being lighter in weight and less easily broken. www.ellisandthompson.com

Technologies to Correct Myopia and Hyperopia Of further benefit is the fact that shattering of the spectacles no longer carries the risk of tiny splinters of glass entering the eye. Glass, however, does not scratch as easily as plastic and does not become hazy with time. hooptometrist.wordpress.com

Technologies to Correct Myopia and Hyperopia • Myopia can be corrected by wearing spectacles with concave lenses (thicker towards the outside and thinner towards the inside). These lenses bend the light rays outwards, causing them to diverge before they reach the eye. This extends the focal length of the light rays, allowing a focused image to fall on the retina instead of in front of it.

Technologies to Correct Myopia and Hyperopia • Hyperopia can be remedied by wearing spectacles with convex lenses (thinner towards the outside and thicker towards the centre). This lens will bend the incoming light rays inwards, causing them to begin converging before they reach the eye, shortening the focal length and allowing a focused image to fall on the retina rather than behind it.

Technologies to Correct Myopia and Hyperopia Contact Lenses Advances in lens technology led to the first successful use of contact lenses in 1888, made by a Swiss Physician, Dr Eugen Frick. www.earlytech.com

Technologies to Correct Myopia and Hyperopia Contact Lenses are a type of technology that is based on similar principles to spectacles in terms of shape of lens and refraction of light. Although the basic structure of the lens used is convex or concave in shape the overall lens is shaped to fit the curvature of the eyeball. m.rocdocs.democratandchronicle.com

Technologies to Correct Myopia and Hyperopia The first contact lenses were very large, made of glass and brought the wearer extreme discomfort. Further advances in technology have resulted in three types of more comfortable and effective contact lenses: • Hard lenses • Soft lenses • Gas permeable lenses www.thegeminigeek.com

Technologies to Correct Myopia and Hyperopia Hard lenses are small, made of glass and cover only the central area of the cornea. Soft lenses are larger, more flexible lenses made of a type of plastic compound that contains 30-70% water. These lenses cover the entire cornea and are more comfortable to wear, but not quite as efficient at correcting vision as the small hard lenses. www.primaryeyecenter.com

Technologies to Correct Myopia and Hyperopia Gas permeable lenses are similar to hard lenses but are porous, allowing an exchange of gases between the outside surface of the eye and the outside air. Their main advantage is that they can be worn for longer periods of time. scienceline.org

Activity The advantages of wearing contact lenses compared with spectacles have long been debated. Nowadays, advantages of contact lenses are not only for the correction of visual defects. Discuss this as a class and take into account some of the factors listed here: • Contact lenses used for cosmetic purposes • Advantages when playing sport • Use of contact lenses in the entertainment industry • Doing away with prescription sunglasses • Contact lenses that block UV radiation www.solution-lens.com

Technologies to Correct Myopia and Hyperopia Refractive laser eye surgery A fairly new and current direction of research into correcting the visual defects of myopia, hyperopia and astigmatism is refractive laser eye surgery. • This involves the use of lasers to change the curvature of the cornea to alter its refractive power, compensating for any visual defects. www.thelasikpro.com

Technologies to Correct Myopia and Hyperopia There are currently two techniques being used. LASIK (‘laser in situ’) surgery involves cutting and uplifting a flap of the cornea, leaving it partly intact. This allows the surgeon to introduce the laser into the middle layers of the cornea beneath the lifted flap, sculpting and shaping the cornea before replacing the flap. lovethatred.wordpress.com

Technologies to Correct Myopia and Hyperopia PRK (‘photorefractive keratectomy’) surgery involves the removal of the spithelium (outer membrane) and the surface of the cornea. The laser is then used to shape the uppermost surface of the cornea. The epithelium takes two to three days to grow back over the cornea. eyesurgerychoices.com

Technologies to Correct Myopia and Hyperopia Current laser surgery is replacing the previous technique of radial keratectomy where fine surgical instruments, rather than lasers, were used to shave the corneal surface. Discuss the advantages and disadvantages of laser surgery compared with wearing lenses or spectacles. www.refractiveeyesurgery.org

Technologies to Correct Myopia and Hyperopia Astigmatism Astigmatism is a fairly common visual defect that results from an irregularity in the curvature of the cornea or lens, resulting in part of an image appearing indistinct. The lenses of spectacles to correct this are more complex and must be ground in a particular way to correct vision in the defective area only. capecoraleyecenter.com

Technologies to Correct Myopia and Hyperopia Presbyopia This conditions occurs as people grow older the lens of the eye loses some of its elasticity and becomes less supple, which hinders its ability to change shape. The older lens tends to remain in its elongated, distance-focusing shape, resulting in people from the age of 40 onwards having a tendency to long-sightedness. www.doctorsvisioncenter.com

Technologies to Correct Myopia and Hyperopia Cataracts A cataract is a visual defect caused by the clouding of the lens of the eye. Cataracts also interfere with the ability of the eye to transmit a clear signal, but they do so by obstructing the path of light through the eye. The development of cataracts is considered to be a disease of the eye and is usually associated with old age. optometrist.com.au

Homework -Students to complete DOT Point 3.9 Cataracts

Communication Topic 5: Visual Defects