Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Elkington Optics > Optics of Ametropia > Flashcards

Optics of Ametropia Flashcards

1
Q

What is ametropia

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is myopia

A

the second principal focus lies in front of the retina.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Causes of myopia

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is hypermetropia

A

the second principal focus lies behind the retina

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Causes of hypermetropia

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

How do phakic patients overcome some or all of their hypermetropia

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

How to classify hypermetropia

A

manifest and latent hypermetropia

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is manifest hypermetropia

A

the strongest convex lens correction accepted for clear distance vision

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is latent hypermetropia

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is facultative hypermetropia

A

Hypermetropia which can be overcome by accommodation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is absolute hypermetropia

A

hypermetropia in excess of the amplitude of accommodation is called absolute

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is regular astigmatism

A

If the principal meridians are at 90° to each other

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is oblique astigmatism

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What is irregular astigmatism

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is compound hypermetropic astigmatism

A

rays in all meridians come to a focus behind the retina

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is simple hypermetropic astigmatism

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is mixed astigmatism

A

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

18
Q

What is simple myopic astigmatism

A

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

19
Q

What is compound myopic astigmatism

A

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

20
Q

What is anisometropia

A

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

21
Q

How does the stenopaeic slit work

A

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

22
Q

What is the far point

A

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

23
Q

What is the static refraction/ametropic error

A

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

24
Q

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

A

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

25
Q

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

A

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

26
Q

What is Back vertex distance

A

Distance between back of spectacle lens and anterior cornea

27
Q

When is back vertex distance important

A

for any lens of power greater than 5 dioptres

28
Q

Back vertex distance formula in contact lens prescriptions

A

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

29
Q

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

A

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

30
Q

What is the spectacle magnification

A

the ratio between the corrected and uncorrected image size

corrected image size/uncorrected image size

31
Q

What is Relative Spectacle Magnification (RSM)

A

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

Corrected ametropic image size/emmetropic image size

32
Q

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

A

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

33
Q

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

A

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

34
Q

What is the RSM encountered by aphakic spectacle correction

A

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

35
Q

What is anisekonia

A

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.

36
Q

How to reduce anisekonia

A

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

37
Q

SRK Formula

A

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

38
Q

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

A

SRK II

39
Q

What errors in biometry may indicate inaccuracy

A

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

40
Q

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

A

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

Elkington Optics (14 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions