1 and 2 - accommodation basics

what is accommodation?

the eye's ability to bring objects located at different distances into focus by changing the shape of the lens

What does spectacle accommodation ignore?

differences in AD between correcting at the spec plane vs CL plane → simplification of determining AD

what is emmetropia?

parallel light entering eye focuses on retina

what is accommodation for emmetropes?

0 for objects near and at infinity → always clear

no extra calculations necessary

what does accommodation do?

adds more plus power to eye → focuses nearer objects

what do myopes have too much of?

plus power (built in add)

what happens when myopes are uncorrected?

nearer objects clear with no accommodation (depending on rx)

have a built in add → ↓ AD at near compared to emmetropia

what kind of lenses do myopes need to see clearly in the distance?

minus lenses → diverges light to retina

what do hyperopes not have enough of?

plus power (light focused behind the eye)

what happens when hyperopes are uncorrected?

focuses beyond infinity with no accommodation

↑ AD at near (and all distances) compared to emmetropia

what do hyperopes need to see clearly in the distance?

extra plus power → accommodation or lenses

How does accommodative demand change when fully correcting an ametrope (either myopes or hyperopes)?

their AD becomes the same as an emmetrope

what is the equation for standard accommodative demand (AD)?

1/distance

what is the standard accommodative demand for a far away target?

0

What is the true accommodative demand formula?

TAD = AD + RE - lens

what is the accommodative demand when fully corrected?

standard AD=1/distance

ex: @ 40 cm → AD=1/0.4m=+2.50D

what is the accommodative demand when over-plused/under-minused?

↓ AD

what is the accommodative demand when under-plused/over-minused?

↑ AD

what is our goal when doing a refraction?

least minus power for maximum VA

what does rx the least minus power for maximum VA do?

determines lens power that brings distance target into focus when all accommodation is relaxed (accommodating 0 D)

why won't the spectacle Rx given to pt not match the refraction?

1. sometimes due to aniseikonia (differences images between the eyes)

2. usually due to accommodation

What is the expected VAs for a myope?

decrease one line for every -0.25D of myopia

What is the expected VAs for a <+1.75 hyperope?

20/20

what is the expected VAs for a +1.75-(+2.25) hyperope?

20/25

what is the expected VAs for a +2.50, +2.75 hyperope?

20/30

what is the expected VAs for a +3.00 hyperope?

20/40

what is the expected VAs for a +3.25 and +3.50 hyperope?

20/50

What is the expected VAs for a low astigmat (≤-0.75)?

Use SE to estimate

What is the expected VA for a high astigmatism (>-0.75)?

VAs will be worse than SE estimate

How would you calculate the accommodative demand of someone with a sphero-cylinder prescription?

patients will accommodate to bring the circle of least confusion (COLC) near the retina

think of this as the average between the two meridians

What are the 4 components of ocular accommodation?

1. blur (defocus) accommodation

2. convergence accommodation

3. proximal accommodation

4. tonic accommodation

What is hyperopic defocus? What lenses induce hyperopic defocus?

image is behind the retina - created by minus lenses

image - blue lines

What is myopic defocus? What lenses induce myopic defocus?

image is in front of the retina - created by positive lenses

image - purple lines

What is the stimulus for blur (defocus) accommodation?

retinal image defocus

What is the only component of ocular accommodation that is driven by a negative feedback loop?

blur (defocus) accommodation

i.e. accommodation regulates itself, by either upping the accommodation if there's hyperopic defocus or reducing accommodation if there's myopic defocus

Symptoms in accommodative dysfunction are due to what?

strain on blur (defocus) accommodation

What is convergence accommodation?

accommodation that is driven by convergence

What is average CA/C (convergence accommodation driven by converging)?

0.1-0.4 D/pd

not commonly measured clinically

What is proximal accommodation?

accommodation that is driven by perceived nearness of the target

What do you need to know about proximal accommodation?

it can affect clinical testing (mirror rooms)

What is tonic accommodation?

rest accommodation, dark focus, empty space myopia, dark myopia

base-line value in absence of any stimulus to accommodation