Near Visual Acuity Measurement PDF

NEAR VISUAL ACUITY MEASUREMENT OBJECTIVES Measurement of near visual acuity. know the equipment and recording notations used. Clinical techniques for near VA. Overview of occupational NV standards.  WHEN ?? As part of routine eye examination Uncorrected NV Habitual correcte...

NEAR VISUAL ACUITY MEASUREMENT OBJECTIVES Measurement of near visual acuity. know the equipment and recording notations used. Clinical techniques for near VA. Overview of occupational NV standards.  WHEN ?? As part of routine eye examination Uncorrected NV Habitual corrected NV (i.e. current NV Rx) Best corrected NV Investigating Chief Complaint (i.e. reduced near vision) Occupational assessment (VDU, etc) Investigation of suspected pathology (e.g. third nerve palsy) ARE WE MEASURING OR TESTING NEAR VA ? Most NV charts test near vision adequacy (smallest letter size larger than acuity limit) Few actually give a measurement of near visual acuity threshold NEAR VA RECORDING SYSTEMS N point system Jaegar system Reduced Snellen M scale logMAR N POINT SYSTEM Based on printing industry font sizes Used in UK and Australia 1 point = 0.35 mm (1/72 inch) letter height Prefix ‘N’, e.g. N5 letter is 5 x 0.35 mm high = 1.75 mm Linear scale: N10 is twice the height of N5  Standard font is Times New Roman Letter height measured from extreme top to bottom of letter optometry Letter height JAEGAR SYSTEM 20 letter sizes: J1 (smallest) to J20 (largest) No standardization of letter size (large inter-manufacturer difference!) Not recommended for near VA assessment REDUCED SNELLEN SYSTEM Reduced size Snellen chart, designed for use at 40 cm. ‘6/6’ letters subtend 5 minutes of arc at 40 cm. Letter height = tan 0.083 x 0.4 m (5’ = 0.083º) Reduced 6/6 letter = 0.58 mm high M SCALE M units; commonly used in North America M units indicate distance at which a lower case letter ‘x’ subtends 5’ of arc (5 minutes of arc) 1.0M is equivalent to N8 and 6/15 reduced Snellen COMPARISON OF RECORDING SYSTEMS LOGMAR SYSTEM LogMAR (log Minimum Angle of Resolution) Rows of letters change in size in logarithmic steps To convert Snellen notation to MAR and logMAR: Invert Snellen fraction and calculate decimal, i.e. 6/5 to 5/6 to 0.83 = MAR (minutes of arc) Calculate log (base 10) of MAR: log 0.83 = -0.08 LOGMAR SYSTEM NV TEST CHARTS Faculty of Ophthalmologists NV chart Curpax Near Vision Test Type MNRead Acuity Chart Bailey-Lovie Word Reading Chart Maclure Reading Type for Children Jaeger Reading Test Type FACULTY OF OPHTHALMOLOGISTS APPROVED NV TEST CHART Non complete sentences. Not used frequently. Used for researches. CURPAX NEAR VISION TEST TYPE Based on Faculty of Ophthalmologists recommendations for NV tests (expanded) Font sizes N5 to N48 Meaningful sentences Also includes: industrial test (engineering drawing), music, phone directory sample, stock market report MNREAD ACUITY CHART Meaningful sentences M-scale, logMAR and reduced Snellen notation Testing distance 40 cm Acuity range: 1.0 (largest) to –0.5 logMAR BAILEY-LOVIE WORD READING CHART Words constructed from lower case characters. Not in meaning full sentences (prevents ‘guessing’ from context of sentence) N-point (N2 – N80) and M-scale notation logMAR scale correct at 25 cm Visual Acuity Rating (VAR) VISUAL ACUITY RATING (VAR) Bailey (1988) Scale 0 to 100 (0 = 6/600, 100 = 6/6) VAR = 100 – 50logMAR e.g. 6/9 = 0.18 (logMAR) VAR = 100 – 50 x 0.18 VAR = 91 MACLURE READING TYPE FOR CHILDREN Good NV test for younger children Non-serif text, i.e. M not M, and Times Roman font Seven reading grades to match text content to reading age N5 to N48 font size within each grade MACLURE TEST TYPE; READING GRADE VS AGE OF CHILD MACLURE TEST TYPE Extras: Selected (isolated) words section (N5 to N48) Reduced Snellen test types (6/60 to 6/6) for use at 35 cm Distance test type (Snellen) JAEGER READING TEST TYPE Booklet format Complete, meaningful sentences Font size J1 to J20 J1 to J3 very small, J3 to J6 small (!) Not very scientific. Variation in font size of up to a factor of 2! RECOMMENDED NV TEST PROCEDURE Test RE, LE and binocularly Use optimal distance vision correction (pre-presbyope) Use existing near vision Rx (presbyope) Px places NV chart at their normal reading distance (record this distance)  Instruct Px to read the smallest print/letters that they can Don’t routinely use additional lighting (makes task easier; may have less light at home/work) BUT, extra light can help Px with poor acuity NEAR CONTRAST SENSITIVITY Vistech VCTS 6000 (Vision Contrast Test System). Reduced size version of the VCTS 6500 distance CS system. 3 AFC. Sine wave gratings oriented upwards, or tilted left or right by 15º. VCTS 6000 Sine wave patches in 9 column by 5 row matrix. Contrast decreases in columns from left to right (33% to 0% in 0.2 log unit steps). Spatial frequency increases in rows from top to bottom (1.5, 3.0, 6.0, 12.0, 18.0 cpd). Patch subtends 1.4º at 46 cm. OCCUPATIONAL NV REQUIREMENTS VDU / DSE (Display Screen Equipment) users: N6 throughout the range 33 to 75 cm Notes: Employer has to pay for spectacles required specifically for VDU use (rare) VDU user entitled to eye examination, paid for by employer  Non-destructive testing: N4 or Jaeger 1 @ not less than 30 cm (with or without correction) London Underground: N6 with or without correction (no working distance stated) Train drivers: N8 JAR (JOINT AVIATION REQUIREMENTS) Class 1 (CPL) and Class 2 (PPL); near vision requirements: N5 at 30 – 50 cm (inclusive) N14 at 100 cm