Physiological Optics 2 (KMK) Flashcards
Gullstrand’ s Exact Eye model
Cornea (front and back), humor, lens (core and outer layer) = 6 refractive surfaces
AL = 24mm
Reduced Eye Model
SSRI n=1.33, AL=22.22mm, total power = 60D
Far point
Farthest point of clear vision with no accommodation
Near point
Closest point of clear vision with maximum accommodation
Where is the far point for an emmetrope?
Optical infinity
Where is the far point for an uncorrected myope?
Located between the eye and infinity (somewhere in front of the retina), a real object
Night myopia
More myopic under low light levels, increased spherical aberration (large pupils) and light levels too low to fully relax accommodation when viewing distant objects
Myopia trends
-prevalence at birth 5-22%
-By age 1, few children are myopic
-age 6, myopia of at least -0.50D is only in 2% of kids
-myopia prevalence increases bw 6yo and 20yo, reaching a prevalence of about 20% at 20yo
-myopia prevalence for 20-40yo is about 30% and then declines slightly, with some chance of myopic shift from cataracts at older ages
Where is the far point for an uncorrected hyperope?
Located behind the retina and is considered virtual
Latent hyperopia
Young hyperopes will accommodate to neutralize their hyperopia, so shows lower amount in subjective vs objective refraction
Manifest hyperopia
The amount present in the subjective refraction
Absolute hyperopia
The amount of hyperopia that cannot be neutralized with pt’s accommodation
Facultative hyperopia
Amount of hyperopia that can be neutralized by accommodation
Hyperopia trends
-Prevalence of hyperopia is about 6% in ages 6-15yo
-hyperopic child at 5yo will likely be hyperopic still at 14yo if >+1.5D, or emmetropic at 14yo if bw +0.50 and 1.25, or myopic at age 14yo if <+0.50D
>45yo shows increase in hyperopia with age (for hyperopes and emmetrope)
Resolution acuity
Distinguishing a pattern from a uniforms patch of equal luminance (ex. Teller), typical cutoff 40-t-CPDG for young adult
Recognition acuity
Resolving high frequencies and being able to recognize what it is, ex. Snellen
Minimum Detectable acuity
Thinnest possible wire that is visible (thin wire against blue sky), lowest thresholds (about 1arcsec)
Hyperacuity
Directional relationships, tilt, parallel lines, low thresholds (about 3arcsec), result of higher cortical processing
MAR
Minimum angle of resolution, measured in arcmin (1/60 of a degree)
Snellen fraction to MAR
1/MAR = 20/x
Basically MAR is inverse of Snellen fraction
20/20 into MAR and logMAR
20/20 = 1MAR = 0 logMAR
How many arcmin does each letter of the Snellen chart subtend
5 arcmin when viewed at 20ft, each bar subtends 1 arcmin
Snellen to cpd
600/Snellen denom = cpd
Or
600/cpd = Snellen denom
Dynamic retinoscopy
Accommodation involved, ex. MEM, represents lag of accommodation
Nott’s Method
Example of dynamic ret, relies on moving the retinoscopy backwards instead of using lenses. The distance for neutrality can be used to find lag of accommodation (lag is further away ret, lead would be closer)
Mohindra ret
“Near retinoscopy” performed in dark, monocular (one eye patched), adjustment factor of 1.25D, not as effective as cyclo ret
JND
Approx equal to the denominator of the Snellen fraction divided by 100
If acuity is 20/150 what is the JND?
1.50D so make them choose between +0.75 and -0.75 lenses
0.50 JCC lens rx
+0.50 -1.00 x180
Astigmatic dial
Fog the eye to an acuity of approx 20/50, darkest and sharpest lines, add minus cyl until all equal, add minus sphere until BCVA. Rule of 30 is for the axis (line on clock x30 = axis)
Duochrome test / bichrome/red-green test
RAMGAP, red add minus, green add plus
(Green focuses first, and should be balanced in front of retina with red behind retina same amount)
Over minused will see green clearer (add plus)
When to use cycloplegic refraction
-child with convergent strab (must determine if accom is contributing)
-child with significant esophoria at near
-young adult complaining of HA with no uncorrected hyperopia (look for latent hyperopia)
Ocular accommodative demand
Amount of accommodation needed to see an object clearly
Ocular accommodation
Amount of accommodation that the eye actually provides (does not always match the demand)
Amplitude of accommodation (AoA)
Maximum amount of accommodation possible for a given eye (independent value, inherent ability)
Range of clear vision
Refers to all other points analogous to the near and far points but at varying degrees of accommodation
A person uses only about ___% of his/her accommodation ability comfortably.
50%
Accommodation and Age
Age 10 : 14.00D
Age 20 : 11.00D
Age 50 : <2.00D
Estimate amplitude formula
18.50D - 0.3 of age
Minimum amplitude formula
15 - 1/4 (age)
