Transcript Astigmatism
You must know!!! Write these on a sheet of paper to take notes on. • What causes cataracts & glaucoma. • Definition of – – – – – – Emmetropia, Myopia, Hypermetropia, Astigmatism, diplopia, Amblyopia • Equilibrium tests • Definition of Deafness • Hearing tests Problems with the Special Senses Clinical Note: Cataracts • Occurs as the lens fibers darken with age, fluid filled bubbles and clefts appear between the lens fibers. • When the lens loses its transparency • Can result from drug reactions, injuries, or radiation, viruses, and other causes. • Causes vision to appear milky or as if one were looking from behind a waterfall. • Senile cataracts: most common form; over time, the lens becomes yellowish and cloudy • Can be treated by replacing the natural lens with a plastic one. Clinical Note: Glaucoma • Group of eye diseases in which the optic nerve at the back of the eye is slowly destroyed, slowly destroying vision • Various causes: – Increased pressure inside the eye - a result of blockage of the circulation of aqueous, or its drainage. – Poor blood supply to the vital optic nerve fibers – A weakness in the structure of the nerve, and/or a problem in the health of the nerve fibers themselves. • Symptoms include illusory flashes of light, dimness of vision, narrowed visual field, and colored halos around artificial lights. • Vision cannot be restored Cataract (opaque lens) Other Conditions of the Eye • Hyperopia: far-sightedness • Myopia: near-sightedness Process of Vision • Convergence – Many rays focusing into one spot – Convex lens • Divergence – going from one point to many points – Concave lens • The cornea acts as a converging surface – convex lens • Helps to focus light on the back of your eye Normal Eye • Emmetropia is the optical term for normal vision. The light from an object is focused directly onto the retina forming a perfect image. • An emmetropic person can see distance and close objects clearly. Astigmatism • Astigmatism is the optical term for more than one point of focus. It occurs when the surface of the cornea or crystalline lens is not spherical. Light from an object may focus on the retina in one direction but not the other. An astigmatic eye has curves that are steeper in one direction than the other, like a rugby ball. A normal, spherical eye has curves that are the same in all directions, like a football. An astigmatic person will have distorted vision. Hypermetropia – Farsighted • The image on the right shows a hypermetropic eye. • Hypermetropia means long sight and is where the image is formed behind the retina. This could be because the eye is too short, or the cornea or crystalline lens does not refract the light enough. Myopia – Near Sighted • This image on the left shows a myopic eye. Myopia means short sight and is where light from an object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens refracts (bends) the light too steeply. A myopic person has clear vision when looking at objects close to them, and blurred vision when looking at objects in the distance. • Double vision: diplopia (what the patient experiences) – Eyes do not look at the same point in the visual field • Misalignment: strabismus (what is observed when shine a light: not reflected in the same place on both eyes) – can be a cause of diplopia – – – – Cross eyed Gaze & movements not conjugate (together) Medial or lateral, fixed or not Many causes • Weakness or paralysis of extrinsic muscle of eye – Surgical correction necessary • Oculomotor nerve problem, other problems • Lazy eye: amblyopia – Cover/uncover test at 5 yo – If don’t patch good eye by 6, brain ignores lazy eye and visual pathway degenerates: eye functionally blind NOTE: some neurological development and connections have a window of time - need stimuli to develop, or ability lost Links • Near & distant vision Problems with Hearing Equilibrium • Lesions of the inner ear, vestibular apparatus of the inner ear, the vestibulocochlear nerve, cerebellum, and other parts of the nervous system can affect a person’s ability to maintain balance or execute smooth movement. • Romberg test- put feet together and close your eyes. – If defective, you should lose your balance and fall to one side. Deafness • Any partial or complete loss of hearing. • Conductive deafness- inability to transfer vibrations through the auditory canal and middle ear ossicles to inner ear. • Sensorineural deafness (nerve)- results from defects in the cochlea, vestibulocochlear nerve, or CNS. – Most common cause is cochlear damage resulting from exposure to loud noise. Hearing tests • Rhine test- tuning fork against mastoid process, behind the ear. – When the patient no longer hears it, the examiner holds it next to the auditory meatus. – Normal hearing or partial nerve deafness hear the hum, because air conduction is more efficient than bone conduction. – With conductive deafness, the airborne sound disappears more quickly than the bone conducted sound. Hearing test • Audiometry – Patient listens to audiometer that plays pure tones of specific frequencies and volumes. – Shows degree of hearing loss. – Auditory brainstem responses can be used to test hearing in patients who cannot or will not report whether they hear a sound. • (comatose patients, infants, people feigning deafness) Cochlear Implants • Implants that lead to the cochlear nerve. • Detects sound and randomly stimulates cochlear nerve fibers. • Patients learn to associate the frequency responses they hear with the origin and relevance of the sound. • Aids in distinguishing the rhythm of speech, when words begin & end, car horns, & alarms.