Optics and refractive errors Flashcards
how is axial length of eyeball measured
from corneal surface to RPE/Bruch membrane - most lengthening occurs in first 3-6 months of life
axial length of newborn eye
16mm
axial length of eye at 3 years
22.5mm
axial length of eye at 13-18 years (adult)
24mm
length of lens to retina in adults
17mm
low, moderate and high myopia is
low -6D
2 causes of myopia
axial (>24mm - most common)
index (high refractive power - often in keratoconus or nuclear sclerotic cataract)
3 potential methods in slowing myopic progression
atropine and pirenzepine drops
outdoor activity
bifocals and progressive lenses
when should contact lens wearers stop wearing them before LASIK/LASEK
soft - 14 days
rigid - 1 month
low, moderate and high hypermetropia
low +5D
2 causes of hypermetropia
small eyes (<24mm) low refractive power (aphakic patients i.e absence of lens, flat cornea)
5 conditions associated with hypermetropia
esotropia angle-closure glaucoma retinoschisis uveal effusion syndrome ( nanophthalmos) ambylopia in children
how does LASIK/LASEK differ in hypermetropia surgery
peripheral corneal tissue ablated = steeper central cornea
how to calculate power of lenses
power = 1/f
f = focal length in m
diurnal variation of corneal shape in normal eyes
flattest cornea in morning (changes in eyelid pressure and muscle tension)
2 types of astigmatism
regular - principle meridians (steepest and flattest) are 90 degrees from each other
irregular - principle meridians are not perpendicular to each other
3 further classifications of regular astigmatism
- with-the-rule astigmatism = vertical meridian (90 degrees) is steepest
- against-the-rule astigmatism = horizontal meridian (180) is steepest
- oblique astigmatism = when principle meridians are neither at 90 nor 180
how to correct with-the-rule astigmatism
plus cylinder lens between 60 and 120 degrees
how to correct against-the-rule astigmatism
plus cylinder lens between 150 and 30 degrees
how to correct oblique astigmatism
plus cylinder lens between 31 and 50 and 121-149
when does irregular astigmatism occur
conditions such as keratoconus or corneal ulcers
2 ways to manage astigmatism
soft toric lenses (combo of spherical and cylindrical lenses) RGP lenses (for irregular astigmatism)
difference between spherical and cylindrical lenses
spherical = same power in all meridians cylindrical = power in one meridian only (because focuses light onto a line rather than a point)
what is transposition of prescription lenses/glasses
converting a minus cylindrical lens to a plus cylindrical lens and vice versa (doesn’t change the optical properties - often used in toric lens prescriptions)
3 steps of transposition of lenses
1 = add cylinder and sphere power = new sphere power 2 = change sign of cylinder 3 = change axis by 90 degrees (if <90 then add 90, if >90 then subtract 90)
what is presbyopia
age-related loss of accommodative ability of eye
2 causes of presbyopia
increase in lens size and hardness
ciliary muscle dyfunction
accommodative power at age 8
14D - lost starting at age 40 then almost completely lost after age 60
what is the amplitude of accommodation
maximum increase in diopter power the eye can achieve through accommodation - to achieve comfortable vision, at least 1/3 of amplitude of accommodation should be kept in reserve
what is the near point of the eye
closest point where the image remains clear
2 broad classifications of squints
tropias
phorias
what is esotropia
eye is deviated nasally and moves temporally on cover testing to fixate
what is exotropia
eye is deviated temporally and moves nasally on cover testing to fixate
where should prisms be placed for esotropia/exotropia
should be placed for both eyes with power of prisms split evenly between the 2 eyes - base of prism should point away from deviation
what is hypertropia
eye deviated superiorly and moves inferiorly on cover testing to fixate
what is hypotropia
eye deviated inferiorly and moves superiorly on cover testing to fixate
where should prisms be placed for hyper/hypotropia
power split evenly between 2 eyes
base of prism away from deviation
what is a prism
transparent medium bound by 2 planes that are at an angle to each other - do not focus light but refract it towards the base
what is Snell’s law
when light moves from one transparent medium of higher density to another of lower density. the light refracts (applies to eyes)
what is the angle of incidence
angle the light travels as it hits the boundary of another medium
what is the critical angle
when the angle of refraction is equal to 90 degrees (happens when the angle of incidence increases)
when does total internal reflection occur
when the angle of incidence > critical angle = light doesn’t pass through medium and is completely reflected
what does one prism diopter (PD) produce
a deviation of a light ray of 1cm measured at 1m from the prism
2 equations to calculate PD
P = Fd - P = PD, F = lens power (D) and d = distance (cm) of pupil from optical centre)
P = 2 x angle of deviation (measured in degrees)
how to interpret Snellen chart readings
6/6 - first 6 = distance chart is away from patient
second 6 = distance at which an average person can read
what is a duochrome test
test using chromatic aberrations of the eye to refine the best vision sphere following optical correction
black letters on red and green background
how does duochrome test work
red = longer wavelength than green therefore focuses behind retina
green = shorter wavelength therefore focuses before retina
seeing no differene = focusing on retina and perfect sphere correction
clearer red = hypermetropia, clearer green = myopia
what is ishihara chart used for
to screen for red-green colour blindness
what is logmar chart used for
for research purposes instead of Snellen chart
what is 0 and 1 LogMAR equivalent to
0 = 6/6 Snellen 1 = 6/60 Snellen
how does a logMAR chart work
- patient placed 4m away
- 5 letters on each row with equal spacing
- letter of spacing = one letter width
- row spacing = height of a letter from row below
- each correct letter is worth 0.02 log units - one correct line = 0.1 log units
- scoring is from 1 to 0, so that each letter read correctly will result in subtraction of 0.02 from 1
how does retinoscopy work
light from retinoscope shone into retina at certain distance - observe patient’s red reflex while adding plus/minus lenses until a complete red reflex is observed
myopic = direction of reflex against direction of light
hypermetropic = direction of reflex with direction of light
how to achieve the perfect vision correction value from retinoscopy
once perfect reflex is observed, examiner should subtract the dioptric equivalent of their working distance from the correcting lens
e.g. if working distance is 67cm (2/3m), examiner should subtract 1.5D (power = 1/f = 1/0.67) from lens power
age at which we develop full colour vision
5 months
VA of newborns, 3 mo, 6 mo, 9 mo, 1-2 years
newborn = 6/200-6/60 3 mo = 6/90-6/60 6 mo = 6/30 9 mo = 6/24 1-2 years = 6/18-6/6
how to test VA in children <1 year
keeler or cardiff charts - forced preferential looking charts
how to test VA in children 6mo-2 years
Cardiff cards
how to test VA in children 2-3 years
Kay pictures
how to test VA in children 3-5 years
Sheridan-Gardiner
when to use Keeler LogMAR or illiterate E test
for preschool children and illiterate adults
5 types of illumination techniques using slit lamps
- direct (focal) illumination = most common
- diffuse illumination
- retroillumination
- specular reflection
- sclerotic scatter
when is diffuse illumination used
general examination of external eye strutures - where illumination light is out of focus
when is retroillumination used
uses light reflected from iris to look for corneal opaci
of from fundus to examine red reflex, patency of iridotomies and lens opacities
when is iris transillumination (type of retroillumination) used
uses light reflected from retina in an undilated pupil to view iris abnormalities e.g. pigment dispersion or pseudoexfoliation
when is specular reflection used
to view corneal endothelium e.g. Fuch’s corneal dystrophy
hwen is sclerotic scatter used
to evaluate general corneal opacities - light directed at limbus which is scattered through cornea
magnification of image produced by direct ophthalmoscopy
x15 (area of about 2 disc diameters)
virtual and erect
magnification of image produced by indirect ophthalmoscopy
x2-5 - as power of the biconcave aspheric lens (15-40D) increases the magnification decreases
20D lens = x3 magnification
30D lens = x2 magnification
(area of about 8 disc diameters)
inverted vertically and horizontally
when is the field of illumination in indirect ophthalmoscopy largest vs smallest
largest = high myopia
smallest = hypermetropia
