Lec 2-Visual Acuity.pdf

Visual Acuity Dr. Aljoharah A. Al-kanaan – PhD Intended Learning Outcomes Explain the meaning of visual acuity. Identify the indications for measuring visual acuity. Explain different visual acuity tasks. Explain the principle of visual acuity charts. Explain the significance...

Visual Acuity Dr. Aljoharah A. Al-kanaan – PhD Intended Learning Outcomes Explain the meaning of visual acuity. Identify the indications for measuring visual acuity. Explain different visual acuity tasks. Explain the principle of visual acuity charts. Explain the significance of pinhole test. Explain the principle of pinhole test. Measure visual acuity. Record visual acuity professionally.  Definition Visual Acuity is the measure of the resolving power of the eye. OR is the measure of the capacity of the eye to see fine detail.  Definition Visual acuity is a measurement of central vision only. Definition  Factors affecting VA Optical Neural Integrity of the Integrity of the Integrity of the Integrity of the cornea lens retina brain  Clinical Use Routine examination (standard procedure following case history). Diagnostic tool. Decision making. Measures progression of disease. Evaluates treatment. Legal requirement.  Classification Weymouth Riggs Minimum distinguishable (or visible) Detection Minimum separable Discrimination Minimum cognizable (or legible) Recognition - Identification  Classification Minimum visible or Detection The ability to tell whether or not an object is present in an otherwise empty visual field. In other words, to see if something is being distinguished from the background. It reflects the absolute threshold of vision.  Classification Minimum visible or Detection Tests used to assess minimum visible acuity include: Dot visual acuity test Boek candy bead test Catford drum test  Classification Minimum visible or Detection Tests used to assess minimum visible acuity include: Dot visual acuity test Boek candy bead test Catford drum test  Classification Minimum Separable or Discrimination Is the ability to determine that a group of points or lines are separate and distinct. Example: grating acuity. When grating is used a minimum difference in the position of the line from 52-64 seconds of arc is required for detection.  Classification Minimum Separable or Discrimination Tests used to assess minimum separable acuity include: OKN test Preferential looking test Visually evoked response  Classification Minimum Separable or Discrimination Vernier Acuity: Ability to distinguish between two lines placed end-end with one of them laterally displaced. It is a special case of minimum separable acuity. Resolution here is reported to be 2-12 seconds of arc.  Classification Minimum legible (recognition & identification) The ability not only to discriminate the spatial characteristics of the test pattern but also to identify the pattern with which one had some experience. Clinical recognition acuity is tested by Snellen’s E-chart, Landolt C charts, and Snellen’s letter charts. Separated into 2 forms: Form sense and True minimum legible.  Classification Minimum legible (recognition & identification) Form sense: A simple form is used and some tasks such as orientation is used to determine visual acuity. True minimum legible: A complex pattern (number or letter [optotype]) is used as a test stimulus. Principle  Principle Notation Fraction Equivalent  Principle Notation Snellen acuity is based on the fact that two distant points can be visible as separate only when they subtend an angle of 1 minute of arc at the retina (minimum angle of resolution MAR). The test assumes 20ft (6m) is equivalent to optical infinity (no stimulus to accommodation).  Principle Notation The Snellen optotypes are designed to have an overall size equivalent to 5 times the details. In other words, an optotype subtends an angle of 5min of arc on the retina and the details subtends an angle of 1min of arc each.  Principle Fraction Snellen fraction = testing distance (ft or m)/distance at which the optotype subtends an angle of 5min of arc on the retina (ft or m). The “Snellen fraction” Test distance Distance at which the letter detail subtends 1 minute of arc e.g. If the patient can see the 30ft letter at a distance of 20ft then her/his visual acuity would be: 20 /30. e.g. If the patient can see the 30ft letter at a distance of 15ft then his/her visual acuity would be: 15/30 (equivalent to 20 /40).  Principle Equivalent The measured Snellen fraction can be used to predict what the patient visual acuity would be for any viewing distance and using any unit. e.g. The patient that can see the 30ft letter at a distance of 15ft would have a visual acuity of 15/30. (equivalent to 20 /40). What if the examiner prefers to use meters or decimals instead of feet as a unit in recording the acuity?  Principle Equivalent e.g. An acuity of 20/40 is equivalent to 0.5 decimal or 6/12 in meters. The Snellen fraction is the reciprocal of the MAR: the smallest target (measured in angular substance) that an individual is capable of resolving. A patient with a visual acuity 20/30 will have a MAR of 1.5.  Measurement of VA Distance VA Near VA  Distance VA Wall chart Printed on cardboard and mounted on a wall. Testing VA in literate adults. Well-suited for vision screenings and non- specialized doctors’ clinics.  Distance VA Wall chart Variations: Tumbling E chart, Landolt C chart, Bailey-Lovie chart.  Distance VA Hand-held VA cards The simplest form is the letter E in different sizes printed onto hand-held cards. Primarily for children in vision screenings. Has the advantage of free space testing.  Distance VA Chart projectors Projects test objects onto a screen. It comes with a system of lenses that allows compensation in target size to accommodate different testing distances. However, a visual acuity testing distance shorter than 20 feet can become a problem because of the increased stimulus of accommodation. Additionally, it artificially increases the visual acuity of myopic patients. To overcome these disadvantages, some systems were developed with mirrors to optically “fold” the testing distance in other words, create a viewing distance of 20 feet.  Pinhole An opaque disk with a single or multiple holes. Used following VA measurement when the patient does not reach 20/20. Used to differentiate a refractive error-induced visual loss from a pathological-induced visual loss.  Pinhole Principle Improves vision by blocking peripheral rays which will reduce the aberration and the size of the blur circle.  Pinhole Use pinhole VA improves VA deteriorates VA static Amblyopia Refractive error Ocular disease Ocular disease Intended Learning Outcomes Explain the meaning of visual acuity. Explain different visual acuity tasks. Identify the indications for measuring visual acuity. Explain the principle of visual acuity charts. Explain the significance of pinhole test. Explain the principle of pinhole test. Measure visual acuity. Record visual acuity professionally.