10. Ametropia Flashcards
in myopia the second principle focus is
in front of the retina
in hypermetropia, the second principle focus is
behind the retina
myopia can be
axial - abnormally long
refractive with refractive power increased
increased refractive power
Keratoconus – corneal power increased
Neuclosclerosis – lens power is increased
hypermetropia can be
o Axial hypermetropia: eye is short relative to its power
o Refractive hypermetropia: refractive power is inadequate
how can hypermetropia be overcome by some patient s
- Some patients can overcome it by using accommodation for distance
o Because accommodation reduces with age require reading glasses at a younger age
total hypermetropia
amoint of hypermetropia with ALL accommodation SUSPENDED with cycloplegic drugs –> cycloplegic refraction
after cycloplegic refraction what two types are hypermetropia are there
manifest and latent
manifest hypermetropia
STRONGEST convex lens correction accepted for clear vision
Facultative: amount overcome by accommodation
Absolute: amount which cannot be overcome by accommodation –> WEAKEST convex lens for clear vision
latent hypermetropia
remainder that is MASKED by ciliary tone and involuntary accommodation
Difference between non-cyclopelgic and cycloplegic refraction
Can account for several dioptres in children
breakdown of hypermetropia
astigmatism
- Refractive power of the eye varies in different meridians image is formed as a Sturm’s conoid
types of astigmatism
o Regular astigmatism – principal meridians are at 900 to each other
o Irregular astigmatism – principal meridians are NOT at 900 to each other and cannot be corrected with glasses
regular astigmatism type
o Against-the-rule astigmatism (0-300)
o Oblique astigmatism (30-600)
o With-the-rule astigmatism (60-900)
anisometropia
- = different refraction between the eyes
- Small degrees can be tolerated
anisometropia and hypermetropai
o Disparity of more than 1D will cause symptoms
o Rely on accommodation which is a binocular function and the eyes cannot accommodate to different degrees
anisometropia and myopia
o Can tolerate up to 2D of disparity from childhood
o Older patients who develop myopia due to cataract will not cope as well
tolerance of anisometropia
Hypermetropia: >1D spherical
Astigmatism: >1D
Myopia: >2D
i.e. myopes can tolerate more
pin hole
optimal size 1.2 mm
o VA reduced due to refractive error
pin-hole acuity will improve
o VA reduced due to other pathology
= no improvement
o VA reduced due to macular pathology
= pin-hole may be worse
Stenopaeic slit
- 1.2mm X 20mm = elongated pinhole
- Can be used to determine refraction and principal axis in astigmatism
optical correction of ametropia
- Far point of the eye must coincide with the focal point of the correcting lens which deviates parallel incident light so that is appears:
o Coming from the far point in myopia
o Converging towards the virtual fair point in hypermetropia
myopic correction
hypermetropic correction
- If a convex lens is moved forward (i.e. away from the eye)
image moved forward –> effective power increases
- If a concave lens is moved forward (i.e. away from the eye)
image moved forward effective power decreases
- Power of a correcting lens
o Fn = Fo / (1 – dFo)
o Fn = power of lens in new position
o Fo = power of lens in original position
o dFo = distance in metres
- Correcting lens of ametropia
changes the retinal image size
anterior focal lengths
- Emmetropic anterior focal length = 17.05mm
- Ammetropic focal length = 23.23mm
- Correction of aphakia (refractive hypermetropia) produces a RSM of
o 1.33 (magnification = 33% larger) when spectacles are used
o 1.10 (magnification = 10% larger) when a contact lens is used
o 1.00 (magnification = 0%) when an intra-ocular implant is used.
- Aniseikonia develops if one eye i
aphakic 1.33 magnification cannot be fused, overcome by using CLs or IOLs
SRK variebles
corneal refractive power (keratometer) and axial length (a-scan)
A-scan
- Used to measure the axial length – along the visual axis of the eye
- Axial length = straight line that passes through the centre of the pupil and the centre of the fovea
Peaks of A-scan
- 5 peaks: probe/cornea interface, anterior lens, posterior lens, retina and sclera
types of multifocal IOLS
o Concentric zones of graded power – near zone being the centre as the pupil constricts for near vision
o Concentric annular zones – graded near to distance
o Multiple ring steps – waves of light are differetact by the various zones of the lens
