WEEK 1 CO2

What is the C:D ratio of this?

0.2

What is the C:D ratio of this?

0.4

What is the C:D ratio of this?

0.6

What is the C:D ratio of this?

0.8

What are the 4 bad signs when looking at the optic disc?

• eccentric cup (not placed centrally)

• Notch in rim

• Asymmetry between the 2 eyes

• Pale colour

What is this and should we be worried?

Choroidal crescent, it’s normal

• choroidal pigment visible because retinal tissue does not abut the optic nerve

How does a normal AV crossing look like?

Can you explain an AV crossing?

The artery overlaps on the vein

Can you describe an abnormal AV crossing and how it might occour?

AV nicking - The artery presses in the vein at the crossing causing constriction

Hypertension

What scenario would you use retinoscopy?

• new px

• Children

• Poor subjective responses

• Non-English speaking

• Learning difficulties

• Young hyperope (accommodate a lot)

During retinoscopy, which meridian do you neutralise first and why?

• positive/with meridian

• The other meridian will show an against movement, so we end up using minus cyl

How do you check your final rx is accurate with ret?

• move forward - with movement

• Moving back - against movement

When can the scissors reflex be seen?

• spherical aberration

• Distorted cornea

• Tilted crystalline lens

• Large pupil

• Astigmatism

• Kerataconus

What would you do if you saw the scissors reflex?

• increase room lighting to reduce pupil size to cut out peripheral aberrations

• Use bigger steps than 0.25D

What causes a dim reflex during ret?

• high rx (espc in young patients)

• Cataracts

• Small pupils

What would you do if you saw a dim reflex?

• Move closer to increase brightness of reflex, adjust WD accordingly

• Start with higher powered lens (+5/-5 and check what direction)

How does an amblyopic optometrist perform ret if they can’t use one eye?

• use good eye for both px eyes

• Px fixates on ret light

• Repeat with distance target using good eye on equivalent px’s eye

• Apply difference in sphere found at distance to other eye’s ret result

What are common errors of ret?

• wrong WD

• Performing off-axis

• Smudged lenses

• Not focusing on centre of pupils

• Blocking patients view, stimulating accommodation

What is an auto-refractor?

• measures refractive state of the eye

• No px or examiner judgement required

What are the advantages of auto refractors?

• quick and convenient

• Usually accurate and repeatable

• Good for astigmatism

• Useful for children, nonverbals

• Provides 2nd objective measure

• Reliable, valid

• faster than ret

• Can be delegated

• Accommodation reduced with IR light

• Accurate with cycloplegia (paralysis of eye muscle)

What are auto aspects of auto refractors?

• auto centration

• Auto firing

• Auto fogging

What are the main principles of an autorefractor?

• fundus is illuminated

• Light is scattered and reflected

• Reflected light is analysed

• Scheiner principle

What principle do most auto refractors use?

Scheiner principle - two small bundles of light are imaged at the pupil, separation depends on refractive error

What image will an emmetrope produce as a result of the Scheiner principle?

Single image

What image will an myope produce as a result of the Scheiner principle?

Double crossed

What image will an hyperope produce as a result of the Scheiner principle?

Double uncrossed

How does the Scheiner principle work for astigmatism?

• 2 IR LED’s imaged at pupil, separation depends on refractive error

• For astigmatism, 4 LED’s used, 1 for each meridian

• System moves back and forth until the diplopic images merge to give the refractive power for each meridian

What are some other autorefractor principles?

• retinoscopic - observes direction and motion of light reflex

• Basal optometry - uses a condensing lens and a moving target to form a sharp image on retina

What are the common characteristics of an autorefractor?

• Near IR radiation

• Pupil size dependant

• Vertex distance must be accounted for

• Accommodation needs to be controlled

• Fixation target

What are the advantages of IR radiation?

• Higher light yield

• Effectively reflected from fundus

• Invisible

• Doesn’t stimulate accommodation or pupil reflex

What are the disadvantages of near IR radiation?

• fundus reflects IR more diffusely than visible light

• More light used

• Unsure of surface of reflection

• -0.50/-0.75 must be added to compensate for refraction with visible light

What are the two types of end point auto refractors?

• nulling principle - instrument changes the optical system until refractive error is neutralised

• Non-nulling - measurement is made by analysing the characteristics of the radiation exiting the eye

For autorefractor to work, what size should they be bigger than?

More than 2.5-3.00 mm

Why is vertex distance important in autorefractors?

Autorefractors measure refraction at corneal plane but converted to spectacle plane at a choice of distances

Why are fixation targets important in autorefractors?

• helps control gaze and accommodation

• Px must fixate properly = accurate = reliable

How do autorefractors control accommodation?

• distant fixation targets (optically distance, subjectively near)

• Auto-fogging - blurring image to relax accommodation

• Near IR is used to prevent accommodation being triggered

What are uses of autorefractors?

• preliminary refraction

• Screening of children

• Confirmation of difficult/unusual refractions

• Refraction of non-verbal px

• Research

What are the main sources of error in autorefractors?

• changes in accommodation, pupil diameter and fixation

• Diurnal variation

• Media opacities

• Keratoconus

• Refractive surgery

• Unstable tear film

• Ptosis/long lashes

• Intrinsic inaccuracy of instrument

What are the disadvantages of autorefractors?

• expensive

• May be unreliable with patients w: Aphakia, cataracts, IOL implants, poor fixation, small pupils, media opacities

• Accommodation (proximal)

• range limitation (+15 to +23D, -12 to -20D)

• No qualitative info eg keratoconus, cataract

What is photorefraction?

• uses camera and infrared LED’s place eccentrically to visual axis

• Eye is focused = uniform brightness at pupil

• Defocused = light forms a gradient which is used to calculate rx

What does a myope look like during photorefraction?

Ilumiance at top, black at bottom

What does a hypermyope look like during photorefraction?

Illuminance at bottom, black on top

What are advantages of photorefraction?

• binocular - test both eyes simultaneously

• Good with young children and uncooperative px

• Screening tool

• iPhone attachment

What are disadvantages of photorefraction?

• calibration must be accurate

• Depend on ethnicity

• Calibration error increases with refractive error