Clinical: Subjective Refraction Flashcards
What type of lenses correct astigmatic refractive error?
Spherocylindrical lenses
Two methods for estimating refractive error:
-far point (punctum remotum)
-egger’s table
Estimating refractive error is estimating the ______________ refractive error
Spherical
Far point
Furthest point from the eye at which an object is accurately focused on the retinue when there is no accommodation
Far point for hyperopes…
Is virtual as it is located behind the retina
Each line on the snellen chart represents about ___D of amertropia
0.25D
If your VA is 20/25 on egger’s table
-0.25 myope or +2.25 hyperope
If your VA is 20/50 on egger’s table:
-1.00 myope or +1.50 hyperope
If your VA is 20/100 on egger’s table
-2.00 myope or +0.50 hyperope
Hyperopes can ___________ through their refractive error
Accommodate
How can we use egger’s table to estimate hyperopic refraction?
-place +2.50 lens in front of eye being tested
-check distance VA
-take myopic value from egger’s table subtracted from +2.50
Equation for spherical equivalent
= sphere + 1/2 cyl
Subjective refraction
Determines the combination of spherical and cylindrical lenses necessary to put the far point of the eye at optical infinity using the patients responses
Goal of subjective refraction
Provide the best possible acuity with the accommodation fully relaxed
Objective tests used as a starting point for subjective refraction:
-retinoscopy
-auto refraction
-lensometry
-keratometry
Typical subjective refraction order:
-initial MPMVA
-JCC axis check
-JCC power check
-final MPMVA
-binocular balance
Fogging
Blurring vision using plus lenses in order to reduce accommodation
What happens if the patient is accommodating during refraction
The prescription will be inaccurate
Purpose of maximum plus to maximum visual acuity (MPMVA)
To determine the maximum plus spherical power which provides the patient with their maximum visual acuity
MPMVA procedure
-occudle eye
-fog pt with +0.75 sphere, or until pts vision is blurred by 3 line of acuity
-begin reducing the fog -0.25D at a time
-ask pt to read lowest line possible with each click
-if not additional letters read they do NOT get to keep the power
The only time you add more minus to a prescription is when…
That minus prescription helps the pt see an additional line lower
Jackson cross cylinder is used to…
Refine the astigmatism axis and power in subjective refraction
JCC test:
-monocular
-determines cyl axis adn power
-search for uncorrected cylinder, “search for cyl”
What is a JCC?
A lens with equally powered plus and minus cylinders with axes 90 degrees apart
Small central knob on JCC
Used to change cyl power
Large outer knob on JCC
Used to change cylinder axis
If cyl numbers are RED you are in…
Minus cyl
Room illumination for subjective refraction
Normal room illumination
Target for JCC test
Isolated line of letters 3 lines larger than the best acuity after MPMVA
Goal of JCC axis check
We want the pt to see no difference in clarity of the letters on the chart between the two different positions or views of the JCC
When performing axis check using JCC we always chase the…
Red
If options 1 and 2 on the JCC axis check are equally clear, then..
We are at the correct cylinder axis
Rules for JCC: axis check:
-return to preferred option/position before moving on
-follow the red dot in the patients preferred lens option
-move in 15 degree steps until you obtain reversal, then move in 5 degree steps
Rules for JCC: axis check:
-return to preferred option/position before moving on
-follow the red dot in the patients preferred lens option
-move in 15 degree steps until you obtain reversal, then move in 5 degree steps
Patients with higher cyl power may be ______ sensitive to small changes in cyl axis
More sensitive
If patient says equal you…
Check 5 degrees to each side
If the patient give a reversal 5 degrees apart…
Pick the midpoint
If patient give more than one equal…
Find the ends of range of equality and pick the midpoint
JCC power check
Refines the cylinder power
Target for JCC power check
3 lines larger than best acuity after initial MPMVA
When you are checking the cyl power you need to…
Maintain spherical equivalent power during testing
For every ____ change in cyl power, _____ change must be made in sphere power
0.50D, 0.25D
Why do you need to change sphere when changing cyl?
To keep the image at proper distance from the pts retina
How does JCC work?
Refines cyl power by producing mixed astigmatism of equal amounts but amounts but opposite power in two principal meridians
When you’re at the correct cylinder power, the JCC produces…
Equal but opposite, mixed astigmatism at each position
JCC power mode procedure:
-JCC is flipped
-give same instructions to JCC axis
-if pt prefers red dots, they want more cyl (increase cyl)
-if pt prefers white dots, they want less cyl power(decrease cyl)
JCC power check rules:
-add/remove sphere while adjusting the cyl
-after reversal is obtained, move in 0.25 cyl power steps
-once patient says equal, check again w 0.25 less cyl, to see if pt will accept less
-if pt chooses white with less then 0.5 cyl power, check 90 degrees away
JCC power end point:
-equal response = endpoint
-verify the equal by checking with 0.25D less cyl
If multiple equal points are found in JCC power…
Leave pt at lowest cyl power that gives equal response
If immediate reversal is found 0.25D apart on JCC power…
Leave the patient at the lower cyl power
JCC power search
performed if there is no cyl in objective findings
How to perform JCC power search:
-add -0.25 cyl power and either:
-flip JCC at 090, 180, 045, and 135 degrees
-rotate the JCC through all 180 degrees
-if pt reports clearer vision at any of these axis, add another -0.25 of cyl power, then +0.25 sph to keep sph equivalent
Typical subjective refraction order:
- Objective measurement
- Initial MPMVA OD
- JCC axis and power OD
- Final MPMVA OD
- Initial MPMVA OS
- JCC axis and power OS
- Final MPMVA OS
- Binocular balance
