Optics of Ametropia

Question 1

What is ametropia

Answer 1

In contrast to emmetropia the ametropic eye fails to bring parallel light to a focus on the retina, i.e. the second principal focus of the eye does not fall on the retina

Question 2

What is myopia

Answer 2

the second principal focus lies in front of the retina.

Question 3

Causes of myopia

Answer 3

1) This may be because the eye is abnormally long. This is called axial myopia and includes high myopia in which there may be a posterior staphyloma.
2) Alternatively, the eye may be of normal length, but the dioptric power may be increased. This is called refractive or index myopia. Examples of this are keratoconus, where the corneal refractive power is increased, and nucleosclerosis, where the refractive power of the lens increases as the nucleus becomes more dense

Question 4

What is hypermetropia

Answer 4

the second principal focus lies behind the retina

Question 5

Causes of hypermetropia

Answer 5

1) If the eye is short relative to its focal power, then axial hypermetropia results. 2) Alternatively, if the refractive power of the eye is inadequate, then refractive hypermetropia results. Aphakia is an extreme example of refractive hypermetropia

Question 6

How do phakic patients overcome some or all of their hypermetropia

Answer 6

by using accommodation for distance vision. They then have to exercise extra accommodation for near vision.

Question 7

How to classify hypermetropia

Answer 7

manifest and latent hypermetropia

Question 8

What is manifest hypermetropia

Answer 8

the strongest convex lens correction accepted for clear distance vision

Question 9

What is latent hypermetropia

Answer 9

the remainder of the hypermetropia which is masked by ciliary tone and involuntary accommodation

Question 10

What is facultative hypermetropia

Answer 10

Hypermetropia which can be overcome by accommodation

Question 11

What is absolute hypermetropia

Answer 11

hypermetropia in excess of the amplitude of accommodation is called absolute

Question 12

What is regular astigmatism

Answer 12

If the principal meridians are at 90° to each other

Question 13

What is oblique astigmatism

Answer 13

If the principal meridians are at 90° to each other but do not lie at or near 90° and 180°

Question 14

What is irregular astigmatism

Answer 14

If the principal meridians are not at 90° to each other

Question 15

What is compound hypermetropic astigmatism

Answer 15

rays in all meridians come to a focus behind the retina

Question 16

What is simple hypermetropic astigmatism

Answer 16

rays in one meridian focus on the retina, the other focus lies behind the retina

Question 17

What is mixed astigmatism

Answer 17

one line focus lies in front of the retina, the other behind the retina

Question 18

What is simple myopic astigmatism

Answer 18

one line focus lies on the retina, the other focus lies in front of the retina

Question 19

What is compound myopic astigmatism

Answer 19

rays in all meridians come to a focus in front of the retina

Question 20

What is anisometropia

Answer 20

When the refraction of the two eyes is different.
Larger degrees are a significant cause of amblyopia

Question 21

How does the stenopaeic slit work

Answer 21

when the slit lies in one principal axis of the astigmatic eye, the second line focus is eliminated and the blur of Sturm’s conoid reduced thus allowing a clearer image to be formed

Question 22

What is the far point

Answer 22

is the position of an object such that its image falls on the retina of the relaxed eye, i.e. in the absence of accommodation

The distance of the far point from the principal plane of the eye is denoted by r, which according to sign convention carries a negative sign in front of the principal plane and a positive sign behind the principal plane

Question 23

What is the static refraction/ametropic error

Answer 23

The reciprocal of the far point distance r, in metres, is symbolised by R, expressed in dioptres

Question 24

What happens when the correcting lens is moved further from the eye in hypermetropia

Answer 24

the image is brought still further forward.
The effectivity of the lens is said to be increased. Therefore in this position a weaker convex lens throws the image onto the retina and corrects the hypermetropia

Question 25

What happens when the correcting lens is moved further from the eye in myopia

Answer 25

In the myopic eye the image falls in front of the retina. The purpose of the correcting concave lens is to take the image back on to the retina. When the correcting lens is moved further away from the eye, the image moves forward again. Thus the effectivity of the lens is said to be reduced. Therefore, in this position, a stronger concave lens is needed to throw the image on to the retina

Question 26

What is Back vertex distance

Answer 26

Distance between back of spectacle lens and anterior cornea

Question 27

When is back vertex distance important

Answer 27

for any lens of power greater than 5 dioptres

Question 28

Back vertex distance formula in contact lens prescriptions

Answer 28

F2=F1/(1-dF1) F2= CL power F1= Spectacle lens power d= BVD in cm

Question 29

What happens to the size of image in optical correction of ametropia

Answer 29

The optical correction of ametropia is associated with a change in the retinal image size

Question 30

What is the spectacle magnification

Answer 30

the ratio between the corrected and uncorrected image size corrected image size/uncorrected image size

Question 31

What is Relative Spectacle Magnification (RSM)

Answer 31

Clinically, it is more useful to compare the corrected ametropic image size with the emmetropic image size. Corrected ametropic image size/emmetropic image size

Question 32

What happens in axial myopia if the correcting lens is worn nearer to the eye than the anterior focal point

Answer 32

Image size is increased. The relative spectacle magnification is therefore greater than unity. Contact lenses in axial myopia thus have a magnifying effect

Question 33

What happens in refractive hypermetropia when the correcting lens is at the anterior focal point of the eye

Answer 33

The image size in refractive hypermetropia is increased, thus the relative spectacle magnification is greater than. unity. In refractive myopia the image size is diminished, and thus the relative spectacle magnification is less than unity

Question 34

What is the RSM encountered by aphakic spectacle correction

Answer 34

1.33- This means that the image produced in the corrected aphakic eye is one third larger than the image formed in an emmetropic eye

Question 35

What is anisekonia

Answer 35

Spectacle correction of a unilateral aphakic eye can achieve a clear retinal image, but with an RSM of 1.33 the image in the aphakic eye is one third larger than the image in the normal fellow eye. The patient is unable to fuse images of such unequal size (aniseikonia) and complains of seeing double.

Question 36

How to reduce anisekonia

Answer 36

The use of a contact lens or intra-ocular implant reduces the RSM to 1.1 or 1.0 respectively

Question 37

SRK Formula

Answer 37

P=A-B(AL) - C(K) P= A- (2.5 x AL) - (0.9 x K) P= IOL power in dioptres A= Constant which reflects the position of the particular model of IOL within the eye B= Multiplication constant for axial length AL= Axial length in mm C= Multiplication constant for average keratometry reading K= Average keratometry reading in dioptres Values for multiplication constants are B=2.5 C=0.9

Question 38

For eyes with Axial lengths <22mm or long>24.5mm which formula is used

Answer 38

SRK II

Question 39

What errors in biometry may indicate inaccuracy

Answer 39

1) AL different between 2 eyes (>0.5mm difference) in absence of clinical anisometropia 2) If predicted post operative refraction is not achieved in the first eye

Question 40

How much power does the lens contribute to the eye in situ vs in isolation

Answer 40

It will be recalled that the actual power of the crystalline lens in isolation is +19 D although it only contributes +15 D to the overall refractive power of the eye. Standard intraocular implant lenses are therefore usually of approximately +19 D power