10. Ametropia

Question 1

in myopia the second principle focus is

Answer 1

in front of the retina

Question 2

in hypermetropia, the second principle focus is

Answer 2

behind the retina

Question 3

myopia can be

Answer 3

axial - abnormally long
refractive with refractive power increased

Question 4

increased refractive power

Answer 4

 Keratoconus – corneal power increased
 Neuclosclerosis – lens power is increased

Question 5

hypermetropia can be

Answer 5

o Axial hypermetropia: eye is short relative to its power
o Refractive hypermetropia: refractive power is inadequate

Question 6

how can hypermetropia be overcome by some patient s

Answer 6

• Some patients can overcome it by using accommodation for distance
  o Because accommodation reduces with age  require reading glasses at a younger age

Question 7

total hypermetropia

Answer 7

amoint of hypermetropia with ALL accommodation SUSPENDED with cycloplegic drugs –> cycloplegic refraction

Question 8

after cycloplegic refraction what two types are hypermetropia are there

Answer 8

manifest and latent

Question 9

manifest hypermetropia

Answer 9

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

Question 10

latent hypermetropia

Answer 10

remainder that is MASKED by ciliary tone and involuntary accommodation
 Difference between non-cyclopelgic and cycloplegic refraction
 Can account for several dioptres in children

Question 11

breakdown of hypermetropia

Answer 11

Question 12

astigmatism

Answer 12

• Refractive power of the eye varies in different meridians  image is formed as a Sturm’s conoid

Question 13

types of astigmatism

Answer 13

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

Question 14

regular astigmatism type

Answer 14

o Against-the-rule astigmatism (0-300)
o Oblique astigmatism (30-600)
o With-the-rule astigmatism (60-900)

Question 15

anisometropia

Answer 15

• = different refraction between the eyes
• Small degrees can be tolerated

Question 16

anisometropia and hypermetropai

Answer 16

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

Question 17

anisometropia and myopia

Answer 17

o Can tolerate up to 2D of disparity from childhood
o Older patients who develop myopia due to cataract will not cope as well

Question 18

tolerance of anisometropia

Answer 18

Hypermetropia: >1D spherical
Astigmatism: >1D
Myopia: >2D
i.e. myopes can tolerate more

Question 19

pin hole

Answer 19

optimal size 1.2 mm

Question 20

o VA reduced due to refractive error

Answer 20

pin-hole acuity will improve

Question 21

o VA reduced due to other pathology

Answer 21

= no improvement

Question 22

o VA reduced due to macular pathology

Answer 22

= pin-hole may be worse

Question 23

Stenopaeic slit

Answer 23

• 1.2mm X 20mm = elongated pinhole
• Can be used to determine refraction and principal axis in astigmatism

Question 24

optical correction of ametropia

Answer 24

• 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

Question 25

Answer 25

myopic correction

Question 26

Answer 26

hypermetropic correction

Question 27

• If a convex lens is moved forward (i.e. away from the eye)

Answer 27

image moved forward –> effective power increases

Question 28

• If a concave lens is moved forward (i.e. away from the eye)

Answer 28

image moved forward  effective power decreases

Question 29

• Power of a correcting lens

Answer 29

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

Question 30

• Correcting lens of ametropia

Answer 30

 changes the retinal image size

Question 31

anterior focal lengths

Answer 31

• Emmetropic anterior focal length = 17.05mm
• Ammetropic focal length = 23.23mm

Question 32

• Correction of aphakia (refractive hypermetropia) produces a RSM of

Answer 32

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.

Question 33

• Aniseikonia develops if one eye i

Answer 33

aphakic  1.33 magnification cannot be fused, overcome by using CLs or IOLs

Question 34

SRK variebles

Answer 34

corneal refractive power (keratometer) and axial length (a-scan)

Question 35

A-scan

Answer 35

• 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

Question 36

Peaks of A-scan

Answer 36

• 5 peaks: probe/cornea interface, anterior lens, posterior lens, retina and sclera

Question 37

types of multifocal IOLS

Answer 37

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