clinical methods

Objective measures of refractive error

Keratometry, retinoscopy, autorefraction

get values without patient providing feedback

Subjective measures of refractive error

Manifest (subjective) refraction

requires subjective responses from patient

Why are both types of measurements of refractive error needed?

Working with children, patient preference, nonverbal patient, language barrier, can’t sit up in phoropter

refractive error

eye’s Rx

refractive correction

glasses Rx

emmetropia

no glasses needed

image point: at retina

far point: infinity

ametropia

myopia, hyperopia, astigmatism

factors that contribute to ametropia: power of optical system and axial length of eye

myopia

near-sighted, minus lenses needed

Image point: in front of the retina/ “in the eye”; Far point:

closer than infinity

hyperopia

far-sighted, plus lenses needed

Image point: behind the retina/ “behind the eye”; Far

point: “beyond” infinity

image point

– With an object at infinity, where light rays

converge in the back of the eye

– When light is coming IN to the system

power of optical system

cornea + lens

more plus → more powerful eye → light rays converge sooner

higher power cornea → myopia

power of optical system

cornea + lens

more plus → more powerful eye → light rays converge sooner

higher power cornea → myopia

axial length of eye

distance to retina (screen)

the shorter the distance from the front of the eye to the retina, the less distance rays have to converge and so they may not be fully converged at the retina

shorter eye = hyperopia. longer eye = myopia.

astigmatism

image in two different focus places

power of the eye is not perfectly spherical → two line images with a blur circle (circle of least confusion/CLC) somewhere between

two principle meridians give rise to two image lines (vertical meridian forms horizontal line images while horizontal meridian forms vertical line image)

simple myopic

one at retina one in front of retina

simple hyperopic

one at retina one behind

compound hyperopic

both behind retina

compound myopic

both in front of retina

mixed astigmatism

one in front and one behind

retinoscopy

goal: objectively neutralize patient’s refractive error

place far point of patient’s principle meridian coincident with doctor’s retinoscope, get to infinity by removing amount of lenses equal to working distance

mechanism: move streak across eye produces movement of retinal reflex, add lenses until no movement for neutralization (far point at retinoscope/working distance)

sleeve down for divergent beam

with-motion

far point of eye is behind viewing aperture (between aperture and infinity)

add + lenses

hyperopes, emmetropes, low myopes

against-motion

far point of the eye is between the eye and the viewing aperture

add - lenses

myopes (but not low myopes)

neutral-motion (neutrality)

retina and aperture of retinoscope in conjugate focus

retinoscopic reflex

affected by location of far point in relation to viewing aperture

far point closer to aperature → width, brightness, and speed of reflex increase

streak orientation and meridians of eye

orientation of retinoscope streak is 90 degrees away (perpendicular) from meridian being evaluated (neutralized)

astigmatic reflex

break: reflex misaligned with streak beam

skew: reflex movement misaligned with streak beam

brightness: reflex dimmer when misaligned with streak beam

width: reflex wider with less defined margins when misaligned with streak beam

working distance

distance from eye you hold retinoscope

50cm=2.00D, 67cm=1.50D

gross retinoscopy findings

when lenses findings for retinoscopy is when lenses are in place that allow “neutral” motion to be seen in all meridians (far point at plane of retinoscope)

net retinoscopy findings

Your working distance (50cm or 67cm) will determine the lens amount that will be removed to find the patient’s refractive correction (net findings) for retinoscopy

gross - working distance = NET

subtract 2.00D for 50cm WD

visual acuity

the ability of the visual system to resolve fine detail

spatial visual acuity

the finest spatial detail that can be detected, discriminated, or resolved

detection

Minimum visible acuity

Detection acuity = angular size of the smallest visible target

recognition/identification

minimum recognizable acuity

task: identify letters by resolving their details

types of acuity

distance visual acuity (DVA)

near visual acuity (NVA)

entering VA

VA presenting to you in clinic (can be with glasses, without)

first test performed at start of almost every eye exam

medical-legal issue

unaided/uncorrected VA

without correction/glasses/contacts

spectacle VA, cc

with correction

best corrected VA (BCVA)

VA after refraction (ideally)

OD

oculus dexter, right eye

OS

oculus sinister, left eye

OU

oculus uterque, both eyes

pinhole optics

reduces effective pupils size, increases depth of field/depth of focus, reduces refractive blur, reduces illuminance of retinal image (dimmer)

pinhole acuity

can help determine if reduction in VA due to uncorrected refractive error or non-refractive conditions (ocular disease, amblyopia)

PH 20/20

PHNI: pinhole no improvement

ambylopia

something preventing retina from getting clear image during development (congenital cataract, uncorrected high prescription, ptosis)

uses of visual acuity

• to measure visual function

• to prescribe glasses or contact lenses

• to track disease progression

• to document eligibility standards (insurance, licensing, benefits)

types of charts

Snellen, Bailey-Lovey/logMAR chart, Feinbloom

optotype

font

spacing

space between letters and lines

progression

size progression

Snellen chart

optotype: 5X5 grid, serifs

spacing: irregular

progression: irregular

Snellen notation

numerator = testing distance

denominator = letter size

letter size

distance at which letter height subtends 5 minutes of arc

1° = 60 minutes (‘)

ex. 20 foot letter size subtends 5’ at 20ft, 8.87mm height

test distance

US: in feet (20/20)

Europe: in meters (6/6)

ft to mm conversion

20ft = 6096 mm

1ft = 304.8mm

Bailey-Lovie/ETDRS/logMAR chart

optotype: sloan letters

spacing: between letter and between row spacings equal to lettering size

progression: logarithmic

each line has same number of test letters

minimum angle resolution (MAR)

Angular size of the critical detail of threshold optotype (1/5th of letter height)

Reciprocal of the Snellen fraction

Expressed in minutes of arc

20/20: MAR=1.0, can see 1 minute of arc detail or 1/5 of letter height

LogMAR

Logarithm of the Minimum Angle of Resolution

take log of MAR

decimal notation

Snellen fraction as decimal

other visual acuity charts

Tumbling E, Landolt C, LEA symbols, HOTV

measuring VA

OD, OS, OU

start at line slightly higher than patient‘s threshold

can do single line/letter presentation

partial credit with + or -

read at least half of the letters on a line correctly for credit

human visual limits

diffraction, aberrations, photoreceptor size and spacing

other factors affecting visual acuity

illumination, contrast

count fingers (CF)

CF @ x feet

not standardized, letters are better

hand motion (HM)

ability to detect movement of a hand

light projection (LP with projection)

able to perceive presence of light + direction of light source

light perception (LP)

able to perceive presence of light, unable to perceive shape or direction

no light perception (NLP)

total absence of vision, unable to perceive presence or absence of light

qualitative notations

best to worst acuity: HM > LP with projection > LP > NLP

move chart closer first

visual impairment

legally blind: 20/200 best corrected

near VA

testing: full room illumination, near reading light, reading add in place for presbyopes

test distance: 40cm

charts: reduced Snellen, M notation, Jaeger

reduced Snellen

print size expressive in equivalent Snellen DVA ratios

must specify near viewing distance (usually 40cm)

recorded as RS 20/20 (@40cm)

difficult to compare acuities at different distances, should really be measured at 40cm

M notation

numerator = testing distance (m)

denominator = letter size (meter-letter)

ex. 0.40/0.4M

M units expressed in meters

distance VA = near VA if

• Patient has proper correction and focused

• Chart design is the same for dist and near

• Lighting level is the same

• Pupil not changing optics

How could you get VA worse than the real VA?

• dim room

• don’t know letters

• dry eye

• is the patient trying?

• is the patient malingering?

• Is the patient looking at the chart?

• Using glasses correctly?

• can’t cognitively recognize letters?

• can’t find chart?

• didn’t cover eye completely

• covering wrong eye

• squinting

• leaning forward

• pushing glasses closer/farther away

• patient looking with better eye

interpupillary distance (IPD) / pupillary distance (PD)

the distance between the centers of the pupils for a specific viewing distance

use: place the optical center of phoropter/ trial frame in frame in front of patient’s visual axis, place optical center of glasses in front of the patient’s visual axis

monocular PD

the distance from the center of the pupil to the middle of the bridge

used for PALs and high Rx

PD procedure

• full room illumination

• remove patient spectacles

• testing distance near working distance (40cm)

• maintain landmarks: temporal limbus OD, nasal limbus OS

• sight with left eye first then right eye

• position ruler zero mark at temporal limbus OD

• near PD: open OS, find location of nasal limbus

• dist PD: don’t move ruler, close OS and open OD for patient to look at, new location of nasal limbus

recording PD

dist PD/near PD in mm

ex. 65/63

near PD average ~3mm less

pupillometer

uses corneal reflex to determine PD, can but used for monocular PDs

real eye vs schematic eye

• Pupil access (eyelids in the way)

• Smaller pupil (also changes with age)

• Patient bored/not paying attention/falling asleep

• Accommodation (need to fog eye, need to control accommodation, leaving in WD)

• Irregular corneal reflex (“scissoring”)

• More realistic! Changes in brightness of reflex

clinical retinoscopy set-up

• adjust phoropter: enter distance IPD, position phoropter before patient, adjust level, vertex distance, pantoscopic tilt

• low room illumination

• fixation target: large (20/400) letter, use reverse contrast, cycle letter

• can add ret lenses in Marco

clinical retinoscopy patient education

“i will be using my light to determine an initial starting point for your prescription”

examiner positioning

evaluate patient’s right eye: position yourself so visual axis of your right eye is coincident with visual axis of patient’s right eye

evaluate patient’s left eye: position yourself so visual axis of your left eye is coincident with visual axis of patient’s left eye

don’t block patient’s view of target

lenses in phoropter

turning lens wheel down adds plus lenses, when you see with

turning lens wheel up adds minus lenses, when you see against

sphero-cylinder method

1. scan all meridians, use break and skew to determine principle meridians

2. neutralize most “with” meridians with sphere lenses

3. check that other meridian shows “against” before adding cylinder lenses

4. set the axis on dial parallel to streak that is scoping “against” meridian (applies power 90 degrees away at the meridian you are scoping)

5. add minus cyl lenses in quarter diopter steps until you see neutral (add until with reversal and go back down to neutral)

cylinder lens optics

power meridian

axis: other meridian is plano

controlling accommodation

real eyes can accommodate or increase plus power through focusing

• Pick a big target (20/400 letter)

• Remind the patient to look at the letter, NOT your light

• Instruct the patient to tell you if you ever block them (want to make sure they are looking far away)

• Ensure both eyes are open and the eye NOT being scoped views the target

• Lights are kept low to avoid cues to accommodation

• three eye technique: scope first eye, second eye, and first eye a second time

manual vs auto ret

manual phoropter displays gross findings

automated phoropter displays net findings (remove ret lenses to begin refraction)

gross to net

hyperopes (plus): less plus

myopes (minus): more minus

move in more minus/less plus direction