Tasmanian Eye Clinics G.P. Seminar

Tasmanian Eye Clinics G.P. Seminar

Session 2. Visual Acuity

VISUAL ACUITY

(20 mins)

VISUAL ACUITY

(20 mins)

RETINAL ARCHITECTURE

RETINA

• ROD AND CONES

BIPOLAR CELLS

GANGLION CELLS

NERVE FIBRE LAYER

•RETINAL FUNCTION

• Outer segments of Cones and Rods contain visual pigments,which absorb photons & generate an electrical current. _

Bipolar cells amacrine cells

Ganglion cells

Ganglion cell axone OPTIC NERVE

Central vs Peripheral Vision

Central vs Peripheral Vision

• Macula function.• Detailed with high

definition.• Reading and

colour vision.• Reliance on cones.• Parvocellular

pathway.

• Macula function.• Detailed with high

definition.• Reading and

colour vision.• Reliance on cones.• Parvocellular

pathway.

• Discerns movement and action.

• Relies on rods.• Dark adaptation

function.• Magnocellular

pathway

• Discerns movement and action.

• Relies on rods.• Dark adaptation

function.• Magnocellular

pathway

maculamacula

foveafovea

Recording Visual Acuity is not complex and

measures the most essential function of the eye.

Form acuity is only one way of assessing visual function.

Other functions are contrast sensitivity and dark adaptation.

Recording Visual Acuity is not complex and

measures the most essential function of the eye.

Form acuity is only one way of assessing visual function.

Other functions are contrast sensitivity and dark adaptation.

VISUAL ACUITYVISUAL ACUITY

• RECORD• RECORD• RECORD• You are not defendable if there is

no recorded acuity.

• RECORD• RECORD• RECORD• You are not defendable if there is

no recorded acuity.

RecordingRecording

• Record the result in the following manner.

• Remember no record = no defence

• Record the result in the following manner.

• Remember no record = no defence

Visual Acuity Visual Acuity

• Va’s Recorded at 6metres or 3metres.• The Numerator represents the distance used

and the denominator the line read.• Ie 6/6 represents reading the line at 6 metres

that should be read at 6 metres.• The Americans use 20/20 ( ie 6 metres ) as

the convention.• Young people can read better than

6/6 ,usually 6/5 or 6/4.• The chart used is actually based on a fallacy

but become imbedded in law

• Va’s Recorded at 6metres or 3metres.• The Numerator represents the distance used

and the denominator the line read.• Ie 6/6 represents reading the line at 6 metres

that should be read at 6 metres.• The Americans use 20/20 ( ie 6 metres ) as

the convention.• Young people can read better than

6/6 ,usually 6/5 or 6/4.• The chart used is actually based on a fallacy

but become imbedded in law

A fallacy but imbedded in LAWA fallcy but imbedded in LAW

Snellens E chart Landolts Rings

SHERIDAN GARDNER for Children,SHERIDAN GARDNER for Children,

“One thing like other thing”“One thing like other thing”

Recording Visual Acuity.Recording Visual Acuity.

• V.r (V.r (uncorrected) at distance.

• corrected ie with glasses)

• No glasses with Pin Hole.

• V.r. for near.

• V.r (V.r (uncorrected) at distance.

• corrected ie with glasses)

• No glasses with Pin Hole.

• V.r. for near.

• V.l.( uncorrected) at distance.

• V.l.(corrected ie with glasses).

• No glasses with Pin Hole.

• V.l. for near.

• V.l.( uncorrected) at distance.

• V.l.(corrected ie with glasses).

• No glasses with Pin Hole.

• V.l. for near.

PIN HOLE PIN HOLE

• A pinhole or stenopaeic device converts the eye into a pinhole camera and neutralises refractive error.

• A pinhole is used to create a shorthand refraction.

• If Visual Acuity is depressed it may be due to a simple refractive error .

• The pinhole test allows a non optical person to assess the visual potential of the eye.

• A pinhole or stenopaeic device converts the eye into a pinhole camera and neutralises refractive error.

• A pinhole is used to create a shorthand refraction.

• If Visual Acuity is depressed it may be due to a simple refractive error .

• The pinhole test allows a non optical person to assess the visual potential of the eye.

DEVICE PIN HOLE DEVICE PIN HOLE

Near VisionNear Vision

• Recorded in point print form.(remember the readers!)

• N8 is approximately the size of newsprint.

• N6 the phone book.• N10 most magazines.• Record with reading glasses if used.

• Recorded in point print form.(remember the readers!)

• N8 is approximately the size of newsprint.

• N6 the phone book.• N10 most magazines.• Record with reading glasses if used.

Clues from the Visual Acuity

Clues from the Visual Acuity

• Reading one side of chart may indicate a hemianopia.

• “Searching” movements of the head when trying to read the letters may suggest macula changes.

• A poorer reading acuity compared to the distance acuity suggests macula dysfunction.

• Reading one side of chart may indicate a hemianopia.

• “Searching” movements of the head when trying to read the letters may suggest macula changes.

• A poorer reading acuity compared to the distance acuity suggests macula dysfunction.

Technique PointsTechnique Points

• Cover each eye for the patient carefully to avoid “peeping”.

• Record the best line the patient can see even if there are mistakes and record as eg 6/6-2

• Single letter vision charts are easier to read than multiple letters but mask “ crowding”.

• Beware the illiterate and the memorised chart.

• Cover each eye for the patient carefully to avoid “peeping”.

• Record the best line the patient can see even if there are mistakes and record as eg 6/6-2

• Single letter vision charts are easier to read than multiple letters but mask “ crowding”.

• Beware the illiterate and the memorised chart.

Standard OccluderStandard Occluder

ACUITY STANDARDSACUITY STANDARDS

• 6/12 (corrected or uncorrected ) in at least one eye is the driving standard for an ordinary licence. A visual field of 120 degrees (binocular ) is also required.

• 6/9 for public vehicle licences.• <6/60 for the blind pension or

<5degrees of field in the better eye.

• 6/12 (corrected or uncorrected ) in at least one eye is the driving standard for an ordinary licence. A visual field of 120 degrees (binocular ) is also required.

• 6/9 for public vehicle licences.• <6/60 for the blind pension or

<5degrees of field in the better eye.

•A little about optics and lenses

•A little about optics and lenses

OpticsOptics

• The power of an optical surface is at the interface between a less dense and more dense medium .

• This creates REFRACTION.• All optical devices depend on

refraction.• The following illustrate the main

types of lenses.

• The power of an optical surface is at the interface between a less dense and more dense medium .

• This creates REFRACTION.• All optical devices depend on

refraction.• The following illustrate the main

types of lenses.

OPTICAL POWER occurs at an optical surface ie between 2 materials of

different refractive index

Power = N2-N1/r

alpha

beta

Parallel rays of light

f

CONVEX LENS

CONCAVE LENS

CYLINDRICAL LENS

No power in this plane

Lenses.Lenses.

• Convex.• Image moves in

opposite direction.• Magnify.• Focal plane

concept• Identify.

• Convex.• Image moves in

opposite direction.• Magnify.• Focal plane

concept• Identify.

• - lenses.• Concave.• Image moves in

same direction.• Make object

smaller.• Focal plane

concept.• Identify.

• - lenses.• Concave.• Image moves in

same direction.• Make object

smaller.• Focal plane

concept.• Identify.

+ lenses+ lenses

Spectacles.Spectacles.

• Single vision.• Bifocals etc.• Multifocals.• High + and – and the problems.• Identify main types.

• Single vision.• Bifocals etc.• Multifocals.• High + and – and the problems.• Identify main types.

Single VisionSingle Vision

D Segment BifocalD Segment Bifocal

Executive BifocalExecutive Bifocal

Round Segment BifocalRound Segment Bifocal

MultifocalMultifocal

TrifocalTrifocal

Increasing power of lensIncreasing power of lens

No demarcation No demarcation

Spectacle MaterialsSpectacle Materials

• CR39 1.49

• High Index Plastics(1.67)

• CR39 1.49

• High Index Plastics(1.67)

Reducing Edge thicknessReducing Edge thickness

& weight& weight

Practical Training (20 mins)

• Visual Acuity Drills.• Pin Hole use.• Methods of recording.• Identification of basic optical lenses.• Identification of basic glasses types.• Chart types & advice re use.

Visual Acuity Multiple Choices  1. Visual Acuity is the only method of assessing visual function. T. F. 2.            Pinholes correct ametropia only in young subjects. T. F.3.            Patients with macula dysfunction perform better on distance acuity measurement than near acuity. T. F.4.            “Crowding” occurs in partially corrected amblyopia. T. F.5.            Visual acuity is classically recorded on the page as if the examiner was the patient. T. F.6.            Snellens Acuity charts accurately reflect the resolving power of the eye. T. F.7.            Recording the Acuity in the affected eye only is “good enough” to save consultation time. T. F.8.            Sheridan Gardiner charts are used to assess Acuity in pre verbal children only. T. F.