Exam 2 Flashcards
Which age group has the largest distribution of refractive errors?
Newborns
Change in refractive errors after birth
Rapid change toward emmetropia. The greater RE at birth, the faster the shift in the first 18 months
______% of newborns are myopes
25%
Change in refractive error during preschool age
Majority of emmetropization in the first 18-36 months
When does the incidence of myopia begin to increase?
School age ~8-10, with the greatest incidence around puberty
____% are myopic by age 14-15
40%
Myopia averages tend to stabilize around age _____
15-8
____% of school age children change less than 0.50 diopters?
75%
Mean RE at 6 years old
+0.75D
Mean RE at 12 years old
+0.25D
Adult onset myopia
more likely to be male, tens to remain in only low amounts
RE distribution in middle adulthood
Previously unRxed patients are coming in for exams! Guess what? They’re hyperopes! Mean RE is now +1.00
RE trends in late adulthood
Slight myopic shift in average RE, increased incidence in myopia again.
Those who started hyperopic drift in middle age may continue along that path
What is the most common anatomical feature that causes RE
Deviant axial length
When, and how quickly does the process of emmetropization take place?
Major changes in the first 18-36 months, continues more slowly through age 6-8
Axial length changes in age 0-5
Rate of increase is slower in myopic patients, faster in hyperopic patients
RE at age 13-14 can roughly be predicted based on RE when starting school. What will happen for
> +1.50 hyperopes?
+0.50 –> +1.25 hyperopes?
0 –> +0.49 hyperopes?
> +1.50 hyperopes: still hyperopic
+0.50 –> +1.25 hyperopes: close to emmetropic
0 –> +0.49 hyperopes: will be myopic (especially with ATR astigmatism)
Prevalence of myopia by age
birth?
age 6?
age 10?
age 20?
birth: 25%
age 6: 2% due to emmetropization
age 10: >10%
age 20: >40%
Incidence clusters of myopia
Congenital 8-9 12-13 19-24 55+
Predicting myopia using ratio of Axial Length / Corneal Radius (aka AL/CR)
At age 8…
AL/CR >3.0, most likely will become myopic
AL/CR
Rate of change of myopia
The younger it starts, the greater the rate of change
Elimination of hyperopia happens slowly, but once the individual passes across the threshold of emmetropia to myopia, the rate of change increases dramatically.
Girls start sooner, but also end progression sooner. Girls also have a higher rate of change
Final ammount of myopia
Later onset means slower progression into a lower final amount
Females have higher myopic mean than males
Late onset has a high chance of reversing
Females and myopia
Start and end progression sooner
Faster rate of progression
Higher incidence
Less likely to develop late-onset
Myopia and education
More highly educated people have more myopia. Surprise!
HS Grads: 25% Undergrads: 40% Grad Students: 55% Opt Students: 71% Military Academy Students: 50-60%
Environment and myopia
More time spent outdoors is associated with less myopia, regardless of activity
Rate of progression is slower in summer than winter, thought to be due to school
Rate of myopic change overall is about ____ diopters per year
0.4
Definition: manifest hyperopia
Amount of hyperopia revealed in a standard refraction
Definition: latent hyperopia
Additional hyperopia reveled by a cycloplegic refraction, which is normally masked by accommodation
Prevalence of hyperopia by age
Birth
5-6
75
Birth: 75% either hyperopic or emmetropic
5-6: 98% hyperopic or emmetropic
75: 50% are hyperopic (greatest prevalence)
RE Prognosis based on ammount of hyperopia at age 6
0.00-+0.50: most become myopes
+0.75-+1.25 lose some hyperopia
+1.50 to +2.00: will stay fairly constant
+2.50 to ?: will stay stable or increase slightly
Orientation of astigmatism by age
Birth to 4: ATR
4 to 40: WTR
40+: ATR becomes increasingly prevalent, but only overtakes WTR after age 70 or so
Most go toward WTR before age 4-6. Only ____% go toward ATR by age 6
8%
At age 80, ____% have ATR astigmatism
35%
Approximately ____ D toward ATR astigmatism after age 35-40
0.25 - 0.37 D
What part of the cornea does the keratometer take readings on?
Only the center!
What instrument gives us the most complete information on corneal shape?
Topography
What is an ophthalmometer?
And old-school, 2-position instrument that came before the keratometer
The keratometer measures the cornea as if it is a _____
convex mirror
What are the two variables that affect the keratometer reading?
1) distance between mires and cornea
2) curvature of the cornea
We want to get a precise reading of (2), so we keep (1) as constant as possible
A steep cornea makes the keratometer reflection _______
smaller
A flat cornea makes the keratometer reflection _____
larger
Explain the heleiscopic principle
If we split the reflection of the mires on the cornea, and line up the two circles side by side, the distance between the centers is the same distance as the full diameter of one circle. This allows us to measure the size of a mire reflection (and therefore the curvature of the cornea) without everything being too shaky to measure.
What is the application of Scheiner’s princile to keratometry?
If the distance between the keratometer and the cornea is not perfectly accurate, the mires will appear doubled
Keratometry power equation
P = (n1 - n)/r
n = index of air (1) n1 = index if eye (1.3375) r = radius of curvature in meters P = power in diopters
What is the likely cause of an incomplete mire ring in keratometry?
patient’s lid blocking part of the reflection; have them open wide!
What lens do you use to extend the keratometer range “upward”?
+1.25, used for especially steep corneas
What lens do you use to extend the keratometer range “downward”?
-1.00, used for especially flat corneas
A cornea that is steeper vertically is WTR or ATR?
WTR
When do you apply equivalent sphere to radial line testing?
For each -0.50D of cyl used BEYOND that used for the 20/40 blur. So if -0.50D of cyl was present during 20/40 blur, you don’t start adding +0.25 for equivalent sphere until the patient requests -1.00D of cyl.
Clockdial vs. Paraboline Radial Line Targets
Clockdial is cheap, easy to explain, and easy to use, but achieves less precise axis determination
Paraboline is more expensive, and more complicated to explain and use, but gives more precise results
Explain the procedure to determine cyl axis and power using the paraboline chart
1) 20/40 blur with no cyl in place!
2) Show a radial line chart. The patient will select the line that looks clearest. The paraboline chart is then aligned with this line. Axis has now been roughly determined.
2) The patient will select a parabola that looks clearer. Move the chart away from the clearer parabola. Repeat until both lines are equally clear. The axis has now been precisely determined. Set the axis in the phoropter to the solid line on the side of the parabola
3) Time to determine cyl power. The patient compares the sharpness of dotted line running between the parabolas and the dotted line running 90 degrees from the parabolas. The patient should respond that the one through the middle is darker initially. Add cyl power until they’re equal. Don’t forget to bracket!
What is the rule of 30?
For radial line test, multiply the lower number of the reporter clearer line by 30 to get the axis meridian of the cyl power needed.
Refining axis with clockdial chart
Patient has selected the clearest line. Now patient looks at lines next to selected clearest line. Is one clearer than the other? If so, move axis 10 degrees in that direction (remember to use rule of 30). If one of these two lines is just as clear as the initially selected one, place the axis directly between the two. You should now by within 5 degrees of your precise goal.
Determining cyl power with clockdial
Patient looks at clearest line and line 90 degrees away. Add cyl power until they are equally clear.
When doing cyl search for radial line testing, how do you know if the patient is rejecting the cyl you’ve given them
They report need for cyl 90 degrees away
If the red dots of a JCC lens are lined up with the axis in the phoropter, that means that…
the minus cyl of the JCC lens is being added to the minus cyl in the phoropter
When to consider binocular distance cyl testing?
When patient’s cyl is 2.50 or greater
How to do distance cyl testing binocularly
Vectographic charts and the Turville Infinity Balance allows testing without occlusion.
The plus fog technique (aka Humphriss technique) is to blur one eye to wash out the detail information from with, while still allows vergence to take place. Use somewhere between +0.75 to +1.50
Subjective cyl testing vs objective
Clockdial is less sensitive than JCC, but paraboline is probably just about as good
It’s a good idea to see if the patient actually wants the full cyl in their Rx. Just because it improves VAs doesn’t mean they’ll think it’s visually comfortable. How do you see what they want?
Trial lens flippers. Show them the full Rx vs a reduced cyl. Make sure you keep equivalent sphere the same!
