Subjective refraction

$: Subjective refraction$
SUBJECTIVEREFRACTIONOPTOM FASLU MUHAMMED

REFRACTION PROCEDURE: DISTANCE

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1. Visual acuity (VA) with current Rx

VA versus spherical Rx table

• Spectacles• AR

• Retinoscope

• Unaided

2. Perform Pinhole testUnaided VA with existing Rx if aided VA is worse than 6/12


3. Determine Best Vision sphere

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• Letter Chart• Duochrome

4. Estimate Astigmatism• VA with BVS• Refractive Error Estimation table

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5. Refine Astigmatism (cylinder)• Axis location

• Astigmatic power- Fan & Block- Jackson X-cyl- Cyl trial lens Rotation &

Letter chart

6. Refine Sphere of Rx• +1.00 DS Fog to blur vision

Refraction Procedure: Distance


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7. Bi-Ocular balance• Simultaneous

• Alternating

- Prism Dissociation- Polaroid

8. Final Rx• Optio

ns

REFRACTION PROCEDURE: NEAR ADD

1. Establish Preferred Near Viewing Distance

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2. Determine Near Blur Point • Binocular• Monocular recheck

– near letter chart– measuring tape

• Binocular– measuring

tape

REFRACTION PROCEDURE: NEAR ADD

3. Calculate Near ADD required

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• Maintain half (50%) the Amplitude of Accommodation in reserve- nomogram, nomograph (in the workbook)

4. Refine ADD

• Range of clear vision- near letter chart- trial lens

•Visual acuity (VA) is a

measure of the patient’s

ability to resolve fine detail.

•There are three principal

measures of VA:

Unaided VA - vision.

Habitual VA, with the

patient’s own spectacles.

Optimal VA, with the

best refractive

correction, i.e. after

subjective refraction.

LOGMAR CHARTS•The design principles suggested by Bailey & Lovie

•More lines than a typical Snellen chart, particularly at poor VA levels

•Not truncated to 6/5 (20/15) or similar.

TYPES ADULTS

– Bailey–Lovie Charts– ETDRS charts

CHILDREN– The Glasgow Acuity Cards

Keeler crowded logMAR charts

PROCEDURE The luminance of the chart should be between 80

and 320cd/m2. Seat the patient comfortably with an unobstructed

view of the test chart. You should sit in front and to one side of the patient

in order to monitor facial expressions and reactions.

If you are going to measure both vision and habitual VA, measure vision first to avoid memorisation.

Measure the visual acuity of the ‘poorer’ eye first,

Ask the patient: ‘Please read the smallest line that you can see on the chart’ or similar

Do not permit the patient to screw their eyes up or look around the occluder or through their fingers

•They should be pushed to determine whether they can see any more.

•If they make four or more mistakes on a line of five

•Ask them to move closer if chart not seen

If the patient cannot see the letters even at the closest test distance

a) Hand Movements (HM) @ Y cm

b) Light Projection (Lproj.):50 cm

c) Light Perception (LP):

Repeat measurements for the other eye and binocularly.

Since each letter isworth an equal 0.02log units, you can most accurately determine visual acuityby accounting for everyletter correctly identified.

Typically, an equation isused to calculate VA:

logMAR VA = 1.1 - (0.02 x letters missed)

MOST COMMON ERRORS1. Allowing cautious patients to decide their acuity (i.e. not

pushing them to guess).2. Permitting the patient to screw their eyes up and improve their

VA. 3. Permitting the patient to look around the occluder or through

their fingers and view binocularly when measuring monocular VA.4. Taking distance VAs in a PAL or varifocal wearer when they are

not looking through the distance vision section of the lens. 5. Using an incorrect working distance.6. Not recording the result immediately and guessing the result at

the end of the examination.

VA MEASUREMENT USING SNELLEN CHART Procedure is similar to logMAR chart Vn is recorded as the smallest line in which

the majority of letters are seen,irrespective of subjective blurr.

Errors are recorded by appending a -1,-2, or -3 to snellen fraction

Eg:6/9(-2),6/9(-3)… If pt couldn’t see 6/60 letter @ 6m,but

could @ 2m,recorded as 2/60.

RECORDING The Snellen fraction is defined as:

Test Distance/Distance at which the letters subtend 5 min of arc.

1. Test distance can be provided in metres (metric) or feet (imperial).

2. Snellen VA can be labelled in either decimal or

conventional Snellen notation

Snellen Fraction = Test DistanceDistance at which entire letter subtends 5’

The acuity test distance should be long enough to not stimulate the accommodative system. By convention, the standard test distance in the U.S. has been 20 feet. Everywhere else it is 6 meters:

20 is a Snellen fraction in feet60

6 is the same Snellen fraction 18 in metric units

Measuring visual acuity: The Snellen Fraction

Measuring Distance Visual Acuity

Testing Sequence

By convention you always test in the following order

1st OD ocular dexter Right eye (cover left eye)2nd OS ocular sinister Left eye (cover right eye)3rd OU ocular utrique Both eyes

Measuring Distance Visual Acuity: Example 2

A P E O T F 20/25 E V O C T Z 20/20 O H P N T C 20/15

What is the visual acuity of this patient?5/6 letters on the 20/25 line and only 2 letters on the 20/20 lineNo reason to even test the 20/15 line

The visual acuity can be expressed as 20/25

However, accounting for the missed and identified letters...

The VA can be more descriptively expressed as 20/25-1/+2

MONOCULAR SUBJECTIVE RX

Letter ChartDuochrome test

Combination22

Spherical Correction

= Best vision sphere (BVS)

Obtain spherical correction giving best VA


Starting point for BVSSphere component of Spec RxSphere component of Auto-RxUnaided VA Estimation Method

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BVS = (Sphere Rx) + (Astigmatic Rx ÷ 2)

Best Vision Sphere (BVS)

VISUAL ACUITYSnellen 6/VA Decimal MAR (Minutes of Arc) logMAR

3 2.00 0.50 -0.30

4 1.50 0.67 -0.18

5 1.20 0.83 -0.08

6 1.00 1.00 0.00

7.5 0.80 1.25 0.10

9 0.67 1.50 0.18

9.5 0.63 1.58 0.20

12 0.50 2.00 0.30

15 0.40 2.50 0.40

18 0.33 3.00 0.48

19 0.32 3.17 0.50

24 0.25 4.00 0.60

30 0.20 5.00 0.70

36 0.17 6.00 0.78

38 0.16 6.33 0.80

48 0.13 8.00 0.90

60 0.10 10.00 1.00

120 0.05 20.00 1.30

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RELATIONSHIP BETWEEN REFRACTIVE ERROR AND VA

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Vision6/6 (20/20)6/9 (20/30)6/12 (20/40)6/18 (20/60)6/24 (20/80)

6/36 (20/120)6/60 (20/200)

Spherical*small0.500.751.001.502.00

2.00 to 3.00

Astigmaticsmall1.001.502.003.004.00high

Refractive Error (D)Bennett and Rabbetts, 1984

* Myopia or absolute hypermetropia

RELATIONSHIP BETWEEN REFRACTIVE ERROR AND VA

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Vision6/6 (1.00)6/9 (0.67)6/12 (0.50)6/18 (0.33)6/24 (0.25)6/36 (0.13)6/60 (0.10)

Spherical *< 0.500.500.751.001.502.00

2.00 to 3.00

Astigmatic< 0.751.001.502.003.004.00> 4.00

Refractive Error (D)

Bennett and Rabbetts, 1984 * Myopia or absolute hypermetropia

ESTIMATE RESIDUAL SPHERICAL ERROR: DISTANCE RX

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• Note the Best Snellen VA obtained• Estimate the Amount of Spherical

Error

Procedure

Residual Spherical Error = Snellen (metric) VA Denominator ÷16

ESTIMATE RESIDUAL SPHERICAL ERROR: DISTANCE RX

Unaided Visual Acuity

RE LE

Patient ASpherical Rx

Snellen 6/12 Snellen 6/9

Patient BSpherical Rx

Snellen 20/30 Snellen 20/60

Patient CSpherical Rx

Decimal VA 0.1 Decimal VA 0.17

Patient DSpherical Rx

MAR 4 logMAR 0.7 28

0.75D 0.56D

0.56D 1.12D

2.25D3.75D

1.50D 1.75D


• Use the ‘highest plus’ sphere as a starting point

• Add plus lenses in 0.50 steps until VA begins to decrease

• Then add negative lenses in 0.25D steps until there is no further VA improvement:

“Does this lens make the letters clearer or just smaller and darker?”

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Refinement: Sphere (BVS)Letter Chart

SPHERICAL CORRECTION

Points to note: Accommodation

if the letters get darker and smaller only, then too much minus has been prescribed

When adding plus lenses, assure the patient you know it is getting worse

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Letter Chart

SPHERICAL CORRECTION

Duochrome(starting point)

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Combination TestsLetter chart(refinement)

Duochrome(end point)

Letter chart(BVS)

OR

Subjective refraction

Healthcare

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