FREE ELECTRONIC LIBRARY - Dissertations, online materials

Pages:   || 2 | 3 |

«Colenbrander - Basic Low Vision Exam THE BASIC LOW VISION EXAM August Colenbrander, MD, San Francisco This chapter is based (with some modifications) ...»

-- [ Page 1 ] --

Colenbrander - Basic Low Vision Exam


August Colenbrander, MD, San Francisco

This chapter is based (with some modifications) on a chapter that appeared in the (1994)

Vision Rehabilitation issue of the Ophthalmology Clinics of North America, 7-2: 151-162


Before we embark on any intervention we should know our starting points. This holds true for

Low Vision Rehabilitation as it holds for traditional medical interventions. When patients arrive for a Low Vision evaluation, they often ask why they should have another eye exam. "My doctor just dilated me and took lots of pictures". We explain that their regular ophthalmologist is concerned with the causes of their eye condition and with the treatment thereof, our service is aimed at dealing with the consequences of the eye condition. A dilated exam and dazzling indirect ophthalmoscopy are essential for determining proper treatment, but they prevent us from assessing the patient's undilated and undazzled reading performance.

Several aspects that represent subtle but important differences between a Low Vision evaluation and a traditional eye examination deserve mention.

In a traditional examination the patient is seated for the examiner's convenience and for easy access to the slitlamp, phoropter and ophthalmoscope. For the Low Vision evaluation the arrangement should be for the patient's viewing and reading convenience. An adjustable hospital bed table placed in front of the exam chair facilitates this. For many patients who have a need to search around their scotomata, an old fashioned trial frame with full diameter wire-rim lenses is more appropriate than a phoropter.

To indicate that macular function is below normal, vague statements such as 'count fingers' may be sufficient. To accurately predict the amount of magnification needed, more accurate measurements are necessary than are possible with a standard projector chart. Since most people do not spend their days in a semi-dark room looking at brightly projected images, visual acuity measurement on a printed chart in a lighted room is a more appropriate predictor of daily performance than is the use of a projector chart. We will discuss the details later in this paper.

In the prevalent model of medical and surgical treatment, the doctor has the active role and generally is the decision maker; the patient is asked for compliance and (informed) consent. In rehabilitation the roles are reversed: the doctor provides guidance and shows the patient options; the patient has the active role and must work at re-learning certain skills (whether that is to walk with crutches or to read with a magnifier) and must decide which of several options is most appropriate for a particular living situation. The doctor cannot select and 'prescribe' low vision aids in the way he/she prescribes glasses or medications; the doctor can only recommend solutions, the patient must choose which one to use. This is one of the reasons why the use of a loaner system is so successful, since it allows the patient to verify that what seemed to work best in the clinician's office is also the best solution at home, at work or at school. Failure to recognize this role reversal from the 'medical model' to the 'rehabilitation model', whether it is on the part of the doctor, the patient or both, may lead to failure of successful rehabilitation. Sometimes the role reversal is more readily accepted if the Low Vision adviser is an associate or consultant, rather than the primary ophthalmologist, since this avoids the need for repetitive role reversals between the medical and the rehabilitative model.

Colenbrander - Basic Low Vision Exam Yet it is important that the primary ophthalmologist presents the referral as a natural extension and completion of the medical and surgical treatment, rather than as an admission of failure.


Visual acuity notations are used so often that the underlying mathematical meaning is often forgotten. An eye is said to have a visual acuity of one half of normal if that eye can just recognize standard symbols that are twice as large or twice as close as those just recognized by a standard eye. If the symbols need to be ten times larger (or ten times closer), visual acuity is said to be one tenth of normal. Thus, visual acuity provides a direct estimate of the amount of magnification required to perform standard tasks. To determine that a macula is abnormal it does not make much difference whether the visual acuity is measured as 20/200 or 20/400; the latter measurement, however, indicates a need for twice as much magnification as the former.

The following pages will describe a simple, systematic approach for the accurate determination of visual acuity in low vision patients using a 1-meter test distance for letter chart acuity and a variable test distance for reading tests.


Distance visual acuity measurement is traditionally carried out at 6 meters (20 feet). For routine eye examinations this measurement is used to answer one of two questions. Is the visual system normal or abnormal, and can the refractive correction be improved? For both purposes the range of normal and near-normal acuity is most important. Our regular charts reflect this. In the 20/15 - 20/20 - 20/25 - 20/30 - 20/40 - 20/50 - 20/60 - 20/80 - 20/100 range three steps equal a change in the visual angle by factor of two. In the low vision range, accuracy on most charts drops significantly. For 20/100 - 20/200 - 20/400 each single step represents a factor of two. The common notations of 'Count Fingers' and 'Hand Movement' are even less accurate.

For low vision, more accurate measurements are critical. The measurement range can be extended by moving the test charts normally used at 20 ft. (6 m) to a closer distance. Many low vision practitioners do this by moving from 20 ft. to 10 ft. (3 m), thus gaining a factor of two. We recommend bringing the chart to 1 meter, thus gaining a factor of six. We use a printed chart with external illumination, since a printed chart in a lighted room is a more appropriate predictor of everyday function, than is a projector chart in a dark or semi-dark room.

Using a 1-meter viewing distance also brings us in line with the metric system and provides for much easier calculations than does the use of feet and inches. Since we use the metric system to measure the viewing distance, we should also use the metric letter size unit. Snellen used the letter 'D' for this unit; Louise Sloan introduced the term 'M-unit' to prevent confusion with the symbol 'D' for diopters. Standard acuity (1.0, 20/20) is defined as the ability to just recognize a standard letter (1M-unit) at a standard distance (1 meter) or a letter with the same visual angle (x M-units at x meters).

Using a letter chart at one meter has significant advantages:

• It extends the measurable range down to 1/50 (20/1000) if a chart with 50M letters (20/200 at 5 m) is used and makes more letters per line available at each level. Table 1 demonstrates how a standard chart at 20 ft. covers mainly the normal and near-normal ranges; bringing the chart to 10 ft. includes the moderate low vision range (The single 'big E' at 20/200 is a memory test rather than a visual acuity test); bringing the chart to 1 meter extends accurate measurement to the ranges of severe and profound low vision. (see Table 1)

Colenbrander - Basic Low Vision Exam

• It is easy to maintain the exact testing distance by attaching a one-meter long string to the chart that can be extended to the patient's forehead.

• The chart is within arm's reach of the examiner, who does not have to get up to point to the chart.

• One meter is 1 diopter from optical infinity; thus, a refraction carried out at this distance simply needs 1 diopter subtracted for distance correction; conversely, a 1 diopter trial lens placed over the distance glasses will focus the patient for 1 meter testing. This can be done with a trial lens in a Halberg clip. A convenient alternative is to place a wide pair of +1D drugstore reading glasses over the patient's own correction.

• A simple metric Snellen fraction 1/... results from testing at one meter. The denominator of this fraction represents the theoretical number of diopters of add necessary for reading of 1M print. This is known as Kestenbaum's rule. Multiplying both numerator and denominator of the 1/... Snellen fraction by 20 will give the familiar 20/... notation.

–  –  –


• Do not push the patient too far for marginal performance, nor help by pointing to each individual letter. Determine the level at which patients can perform with reasonable comfort using their own fixation ability.

Colenbrander - Basic Low Vision Exam

• For occasional use and portability, a plastic chart with a 1-meter string attached to an occluder is useful. Ask the patient to hold the occluder to one eye and the chart will automatically be at 1 meter. For more frequent use it is helpful to place the chart on a movable easel. Again, use a 1-meter string so that the distance of the easel can be adjusted when the patient leans forward.

• In principle any letter chart can be used. However, charts with a consistent geometric progression of letter sizes and with letter size notation in M-units are preferred. Table 2 provides conversions if a chart without M-unit notation is used. A special chart with a 1 m cord and occluder attached and with an extra wide range of letter sizes from 50 M (20/1000 at 1m) to 1M (20/20 at 1m) is now commercially available [1].

–  –  –

Older charts and recent charts often carry metric letter size notations. When using a 20 ft. chart without such notations, the above conversions can be used for a chart calibrated for use at 20ft.

When in doubt, measure the height of the letters in mm. Remember that 1 M = 1.45 mm or 7 M = 1 cm. For a convenient clinical approximation 1 M = 1/16" may be used.

–  –  –

Jaeger numbers refer to the catalogue numbers in the print shop where Jaeger selected his reading tests in 1854. They have no mathematical meaning and are implemented inconsistently on different cards.

Point sizes are used in printing. Within one type style they are proportional to the letter size. However, actual letter sizes may vary for characters of different styles.

M-units are the units used in Snellen's visual acuity formula. They apply for distance charts and reading cards. Characters with the same M-unit designation have the same recognizability.

Colenbrander - Basic Low Vision Exam


When the purpose of measurements at different distances is to determine accommodative range and to prescribe for presbyopia, it is proper to distinguish between 'distance' and 'near' vision. For rehabilitation purposes the distinction between 'letter acuity' and 'reading acuity' is more relevant because reading difficulties constitute the most frequent complaint of low vision patients. Letter recognition requires only a very limited visual area; word recognition requires a larger area and fluent reading requires the availability of additional areas to the right of fixation to guide successive saccades. Letter acuity may be measured at far distances (4, 5, 6 m, 20 ft, most useful for patients with normal vision), at an intermediate distance (1 m, most useful for low vision) or at near distances (pocket screener, not recommended); reading acuity can also be measured at far distances (bill boards) or near (continuous text reading card).

Traditionally, reading vision is most often tested at a fixed distance, with some practitioners using 40 cm (16", 2.5D add), some 14" (35 cm, 2.87D add), some 3 D (33.3 cm, 13"). For low vision patients these reading distances may be too far away. An additional problem often is that the patient's current correction is not optimal for the standard test distance. Therefore, a method is needed that allows simple calculations for various short reading distances.

Such a method will be described.

As with distance acuity, two values are needed to calculate visual acuity: letter size and reading distance. Many practitioners are in the habit of recording just the letter size read (e.g.: 'J7') without specifying the reading distance. This is a bad habit in general, but for Low Vision work such notations are completely meaningless. The ability to read news print at 5" is very different from the ability to do so at 15" or 20".

Another problem is the use of 'Jaeger numbers' to describe letter size. Jaeger numbers refer to the catalogue numbers in the print shop where Jaeger selected his reading tests in 1854. They have no mathematical meaning. Worse, they are inconsistently used as one can easily determine by comparing charts from different manufacturers.

If both distance and letter size are recorded properly, the same visual acuity value should be calculated for all distances. The problem is that the most prevalent measures: inches and J numbers make such calculations impossible. The solution, again, is to adopt metric measurements. Recording the reading distance in cm and the letter size in M-units gives us the information we want and makes calculations possible. A remaining problem, however, is that the traditional notation of the Snellen fraction still leaves us with the need for division of two numbers and that the reading distance in meters becomes a fraction-within-a-fraction.

Since most people are better at mental multiplication than at mental division, could a multiplication be used instead? Indeed, this can be done. Since many optical formulas require the reciprocal of a distance, Monoyer (1872) suggested using the term 'diopter' to indicate the reciprocal of a distance in meters. We are thoroughly familiar with the use of diopters in connection with lenses (lens power expressed as the reciprocal of the focal distance), but tend to forget that diopters can be used in any formula where the reciprocal of a distance is needed.

Pages:   || 2 | 3 |

Similar works:

«Curriculum Vitae: Francesca C. Fortenbaugh January 2014 Francesca Cowden Fortenbaugh Boston Attention and Learning Lab V.A. Boston Healthcare System E-mail: francescafortenbaugh@gmail.com 150 S. Huntington Street, Room 12-A2 Jamaica Plain, MA 02130 EDUCATION 2012 Ph.D. University of California, Berkeley Department of Psychology; Cognition, Brain & Behavior Advisor: Lynn C. Robertson, PhD Dissertation: The Many Maps of Visual Space: Examining the Perception and Representation of Space in the...»

«Nutrition Guide Opportunity is missed by most people because it is dressed in overalls and looks like work. — Thomas Edison This is a no-nonsense, quick and dirty guide to optimal health. This guide includes all the steps you need to take. It has weekly shopping lists paired with meal plans and all the necessary recipes. Follow this guide and you will see and feel results! We also offer personal nutritional guidance. Please contact Rochelle@crossfitportland.com for details and to create a...»

«ITEM 20 GOVERNING BODY Date of meeting Agenda item number Tuesday 24 May 2016 20 Title of report Registers of Interest Paper Presented by: Jennifer Aldridge, Chief Nursing Officer Paper prepared by: Pam Bowling, Corporate Affairs/Governing Body Secretary  CCG strategic Develop and maintain an effective organisation  objective supported Commission high quality, safe and cost effective services which by this paper: (please reduce health inequalities and improve access to healthcare tick ...»

«High Performance Oilless Commercial Refrigerant Recovery Unit User Manual 2079-1130 Rev. 6 – May 2010 Introduction Congratulations on your purchase of the STINGER high performance oilless recovery unit. Bacharach has worked hard to make the STINGER the highest performing, most portable, and easiest to use recovery unit on the market. We are committed to your complete satisfaction! CAUTION: These instructions are for personnel trained and experienced in the handling of refrigerants....»

«Parkinson’s Disease: Guide to Deep Brain Stimulation Therapy By: Michael S. Okun, MD Medical Director, National Parkinson Foundation Center of Excellence Co-Director, Center for Movement Disorders and Neurorestoration University of Florida, Gainesville Pamela R. Zeilman, MSN, ANP-BC Center for Movement Disorders and Neurorestoration University of Florida, Gainesville Your generosity makes this publication possible The National Parkinson Foundation is proud to provide these educational...»

«We are happy to consider requests for other languages or formats. Please contact 0131 536 5500 or email nhs.healthscotland-alternativeformats@nhs.net Acknowledgements This booklet was written by Kate Fearnley. Thanks are due to members of the Senior Citizens’ Forum (Edinburgh West) and the Pilton Elderly Project; Dr Neill Anderson; Professor Jon Evans; Maruska Greenwood; Pauline Halliday; Lindsay Kinnaird; Ruth Lang; Richard Leckerman; Neil McGrory; Angela McLeod; Lorna McTernan and Evelyn...»

«JPET Fast Forward. Published on March 24, 2015 as DOI: 10.1124/jpet.114.222414 This article has not been copyedited and formatted. The final version may differ from this version. JPET #222414 The Journal of Pharmacology and Experimental Therapeutics Donepezil reverses intermittent stress-induced generalized chronic pain syndrome in mice Takehiro Mukae, Hitoshi Uchida, Hiroshi Ueda Downloaded from jpet.aspetjournals.org at ASPET Journals on November 15, 2016 Department of Pharmacology and...»

«Learning to handle a myoelectric upper-limb prosthesis The development of an evidence-based guideline for training Hanneke Bouwsema The studies described in this thesis have been conducted at the Center for Human Movement Sciences, part of the University Medical Center Groningen, University of Groningen, the Netherlands. The research in this thesis was financially supported by Otto Bock GmbH Healthcare, Vienna, Austria. The printing of this thesis and the organization of a mini-symposium that...»

«ASX RELEASE 30 March 2012 LONRHO MINING LIMITED (ASX: LOM) QUARTERLY REPORT FOR PERIOD ENDED 29 FEBRUARY 2012 HIGHLIGHTS · Former Rio Tinto Diamonds MD, Gordon Gilchrist, appointed Chairman as part of Board revamp. · $12.7 million capital raising completed to fund new program to drill test 61 priority kimberlite targets within the Lulo Concession. · New $3.5 million Dense Media Separation diamond recovery plant and $1 million fleet of earthmoving equipment purchased for kimberlite...»

«Household Cookstoves, Environment, Health, and Climate Change: A New Look at an Old Problem May 2011 The Environment Department (Climate Change) The World Bank (insert Copyright page; credits for cover design, photo, and technical editing credits here) Contents Preface Acknowledgments Acronyms and Abbreviations Executive Summary Chapter 1. Introduction Chapter 2. What Is an Improved Cookstove? Review of Past Experience Terminology Clarification Chapter 3. Importance of Improving Household...»

«MEDICS ON THE MOVE: THE EFFECT AND EFFECTIVENESS OF ON-THEWORKFLOOR LANGUAGE TRAINING Kris van de Poel, Universiteit Antwerpen, Belgium Summary Medical specialists working abroad are immersed in their professional practice and do not have time for in-depth language study. They can remain difficult to understand, hard to communicate with, may appear indecisive, insensitive or even rude. They risk miscommunication, which has implications for delicate doctor-patient and doctor-staff relationships....»

«Diagnosing Pain Beyond Qì and Blood Stagnation A Look at Painful Obstruction (Bi Syndromes) Written The student clinic is a great place to get fresh perspectives as well as refine by: one’s thinking on almost any topic. Students never cease to amaze me with their beginner’s mind ingenious approaches to common problems. On Jason the other hand, I have noticed many common errors in thinking that happen Blalack across the board. Precisely because such ideas are so entrenched, they lend a good...»

<<  HOME   |    CONTACTS
2016 www.dissertation.xlibx.info - Dissertations, online materials

Materials of this site are available for review, all rights belong to their respective owners.
If you do not agree with the fact that your material is placed on this site, please, email us, we will within 1-2 business days delete him.