Nursing: Visual Acuity

VISUAL ACUITY AIM: So demonstrate the visual acuity. Visual acuity is the ability to see the detailk and contours of objecte clearly. It is tested for both distant as well as never vision. This acuity of vision repere to the ability of the Teye to recognize two point sources of light or two parallel lines, as seperate rother one. It is expressed as minimum separable, ie, the minimum distance between the two pointe or liner when they can be recognized by two. If the distance is bes than this, they will be seen as a When porcieved as two they subtend the angle of I of arx at the nodal point. The nodal pois lies at about the middle of the lens and is the optical center of the eye, stay my passing through this point does not suffer refraction. This angle of I corresponds to a distance of 4.5 Mm between the retinal images of the two points Since the diamter of a poveal cone i about 1.5 Mm, one unstimulated cone seperates the two images some people can repetite the twn things even when the visual angle is only 25". FACTORS AFFECTING VISUAL ACUITY The visual acuity is a complex retinal and cortial mechanism that is appfected by the following factors: A. Sterulus factors. These include the following: 1. Allumination 2 redface. Bright illumination courer edge enhancement. i. of the between the bright field and a dark field is emphasized. ie the boundary 12. Size of the object and ite distance from the eye. 13. Wavelength of light. She chromatic abbercrations of the optical syeteres tond to reduce to VA får mixed light sources, while monochromatic light increases VA. The VA is maximum for white light or compared to other colours. 4. Brightres contrast. It depends so the luminescence of the dark and darker regions of the pattern 15. Sime of expositie od Light Hosh of concise duration may not be perceived. B. Optical Frors. Obriously, the he VA is reduced by any adr I of refraction because the images on the retina are blurred. In such cales, the VA will be raised by making the person look Horough a small apertiore, which increases the depth of focus and reduces the amount of burring caused by any error in focusing or by abberrations. TEST FOR DISTANT VISION The distant vision is clinically tested with Brellen's chart. It has a series of printer letters of varying sizes, black on a white background, arranged in eight liner. The top letter, the largest is visible to the naked eye at the distance of 60 meters, and the subsequent Letters at distarxes of 36, 24. 18. 12,9.6 and 5 metere respectively. She distances for each line are indicated above it. The letters are so designed that, from a specified distance, the letter as a whole subtende an angle of 5 of are at the Inadal point. Further, the breadth of each line in stroke, and the breadth of gops between two lines or two curves subtend an angle of one minute of an are at the nodal point. In the Landolt ring chart the gap in the ring is positioned at random in these 8 lines. She width of the gap subtends an angle of i'at the nodal point. In the E text chart the legs of the letters pointing in different directions. A person who cannot read, has to indicat the direction in which the legs of each letter are pointing.. TESTING THE DISTANT VISUAL ACUITY 1. The subject is reated at a distance of 6 meters from a well-lighted choort and is asted to read the lettere down the chart os far as she can read. Each eye is tested seperately. 2. The VA is recorded according to the formula VA = d/D whored is the distance at which the letters are read and D is the distance at which the letter should be rand by a person with normal vision. 3. Shure if only the top letter can be read, the VA ie 6/60. If the subject can read only the forst 4 lines, then the VA is 6/18, and es on, et normal pereen should be able to read at least the 7th line, ie, have a VA of 6/6. 14. If the VA is less than 6/60, the subject is moved towards the chart until she con rend the tap letter. The VA is 2/60 if she can read it at & meters. The VA lose than 1/60 in recorded or "counting fingers: TEST FOR NEAR VISION The subject is asked to read Inggoi's chart held at the ordinary reading distance of 15 linches. This chart is minde 2 up of reading material of various sizes with the 2 smallest size at bottom. The subject reade the material down a the chotet and VA is recorded or the smallest type that The eye accomodates for near vision by increasing the refractive power of the land, it the lens becomes more convex, mainly on the anterior surface. At reet, ie when the eye is looking at a distant object and the cliary musck is relaxed, the mallenble and elastic lens is hold under tension by its suspensery ligament and is pulled into a platened shape. In the State, parallel rays of light coming from the distant object are brought to focus on the retina and the object is seen clearly However, when the eye looks at a near object, the divergent rays are brought to a focus behind the retina so so that the image on the rot is blurred ine.. out of fexus. When the gaze is shifted to a distant object, the ciliary muede relaxes and the lens becomes less convex. RANGE OF ACCOMODATION far point. It is the forthest point from the eye at which an object is een clearly. Shis point is infinity in the normal eye. A distance of 6 meters from the eye is consideren as the practical for point because light rays from the distance are parallel. Near point. It is the nearest distance from the eye at which an object can be seen clearly. It is 9-10 cm at age 10 years. AMPLITUDE OF ACCOMODATION The difference in the refractive power of the lens in the two states of complex relaxation and maximal accomodation is called the amplitude of accomodation The total refractive power of whib the the eye 1. Of this 60 D. the cornea contributes 44D ie about 60 dipsters. Of refractive power of the ore is 16 D in a young person. During maximal accomodatie the refractive power of the lens can add another 14D to ite refractive power. Shue the amplitude of accomodation is 14 Diopters. COMMON ERRORS OF REFRACTION Myopia. In this condition, parallel rays of light coming from a distant object are brought to a focus in front of the retine, the rays then diverge and form a burred image on the retina. This distant objecte cannot be leon clearly. This Sorori corrected with concave lentes. Hypermetropia. This is the condition in which parallel rays of light from a distance dre focused behind the retina. Thus the near objecte as not seen clearly. The condition is covected with convex lenses. Presbyopia. The near point recedes throughout life, slowly at foret and then rapidly after age 40-45, being 9 cm at age 10 to about 80cm by age 65. This is due to hardering of the lens that resulte in loss of accomodation. Reading and close work graduat becomes difficult and the person holds the reading material farther and farther away from the eye. The condition is corrected wing convex lenses.