Optics of Ametropia Flashcards
What is ametropia
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
What is myopia
the second principal focus lies in front of the retina.
Causes of myopia
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
What is hypermetropia
the second principal focus lies behind the retina
Causes of hypermetropia
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 do phakic patients overcome some or all of their hypermetropia
by using accommodation for distance vision. They then have to exercise extra accommodation for near vision.
How to classify hypermetropia
manifest and latent hypermetropia
What is manifest hypermetropia
the strongest convex lens correction accepted for clear distance vision
What is latent hypermetropia
the remainder of the hypermetropia which is masked by ciliary tone and involuntary accommodation
What is facultative hypermetropia
Hypermetropia which can be overcome by accommodation
What is absolute hypermetropia
hypermetropia in excess of the amplitude of accommodation is called absolute
What is regular astigmatism
If the principal meridians are at 90° to each other
What is oblique astigmatism
If the principal meridians are at 90° to each other but do not lie at or near 90° and 180°
What is irregular astigmatism
If the principal meridians are not at 90° to each other
What is compound hypermetropic astigmatism
rays in all meridians come to a focus behind the retina
What is simple hypermetropic astigmatism
rays in one meridian focus on the retina, the other focus lies behind the retina
What is mixed astigmatism
one line focus lies in front of the retina, the other behind the retina
What is simple myopic astigmatism
one line focus lies on the retina, the other focus lies in front of the retina
What is compound myopic astigmatism
rays in all meridians come to a focus in front of the retina
What is anisometropia
When the refraction of the two eyes is different.
Larger degrees are a significant cause of amblyopia
How does the stenopaeic slit work
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
What is the far point
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
What is the static refraction/ametropic error
The reciprocal of the far point distance r, in metres, is symbolised by R, expressed in dioptres
What happens when the correcting lens is moved further from the eye in hypermetropia
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
What happens when the correcting lens is moved further from the eye in myopia
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
What is Back vertex distance
Distance between back of spectacle lens and anterior cornea
When is back vertex distance important
for any lens of power greater than 5 dioptres
Back vertex distance formula in contact lens prescriptions
F2=F1/(1-dF1)
F2= CL power
F1= Spectacle lens power
d= BVD in cm
What happens to the size of image in optical correction of ametropia
The optical correction of ametropia is associated with a change in the retinal image size
What is the spectacle magnification
the ratio between the corrected and uncorrected image size
corrected image size/uncorrected image size
What is Relative Spectacle Magnification (RSM)
Clinically, it is more useful to compare the corrected ametropic image size with the emmetropic image size.
Corrected ametropic image size/emmetropic image size
What happens in axial myopia if the correcting lens is worn nearer to the eye than the anterior focal point
Image size is increased. The relative spectacle magnification is therefore greater than unity. Contact lenses in axial myopia thus have a magnifying effect
What happens in refractive hypermetropia when the correcting lens is at the anterior focal point of the eye
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
What is the RSM encountered by aphakic spectacle correction
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
What is anisekonia
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.
How to reduce anisekonia
The use of a contact lens or intra-ocular implant reduces the RSM to 1.1 or 1.0 respectively
SRK Formula
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
For eyes with Axial lengths <22mm or long>24.5mm which formula is used
SRK II
What errors in biometry may indicate inaccuracy
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
How much power does the lens contribute to the eye in situ vs in isolation
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
