Chapter 10: Optics of Ametropia Flashcards
What is the definition of absolute hypermetropia?
the least amount of plus lenses needed for clear distance vision without cycloplegia
What is the definition of manifest hypermetropia?
the most plus that can be tolerated without blurring of vision and without cycloplegia
What is the definition of facultative hypermetropia?
the difference between the absolute and manifest hypermetropia
What is the defintion of latent hypermetropia?
the difference between the manifest hypermetropia and the hypermetropia measured with cycloplegia
Calculate the following for the following scenario:
A patient requires +3.00D to see in the distance clearly and can tolerate up to +4.50D without getting blurred. A cycloplegic refraction is +5.50D.
Absolute, manifest, faculatative + latent hyeprmetropia
- Absolute = +3.00D
- Manifest = +4.50D
- Facultative = +1.50D
- Latent = +1.00D
What is the name of the type of image formed in an astigmatic eye?
Sturm’s conoid
What is regular astigmatism?
when the principal meridians are at 90 degrees to each other and lie at or near 90 degrees and 180 degrees
What is oblique astigmatism?
the principal meridians are at 90 degrees to each toerh but do not lie at or near 90 and 180 degrees
What is irregular astigmatism?
the principal meridians are not at 90 degrees to each other
Which type of astigmatism cannot be corrected by spectacles?
irregular astigmatism
What is compound hypermetropic astigmatism / compound myopic astigmatism?
when rays in all meridians come to a focus behind the retina (hypermetropic) or in front of the retina (myopic)
What is simple hypermetropic astigmatism / simple myopic astigmatism?
rays in one meridian focus on the retina, the other focus lies behind (hypermetropic) or in front of (myopic) the retina
What is mixed astigmatism?
one line focus lies in front of the retina, the other behind the retina
What is anisometropia?
when the refraction of the two eyes is different
What degree of anisometropia between eyes in hypermetropic patients is large enough to cause amblyopia?
> 1D can cause amblyopia in the more hypermetropic eye
What degree of anisometropia between eyes in myopic patients is large enough to cause amblyopia?
>2 D or higher
Why are myopic patients less likely to develop amblyopia in anisometropia?
in hypermetropia, the individual eyes cannot accommodate by different amounts as accommodation is a binocular function, therefore the more hypermetropia eye remains out of focus
however, in myopia both eyes have clear near vision
What can the pinhole test tell you about reduced visual acuity?
if due to refractive error, the pin hold test will be much better than unaided acuity
if due to ocular pathology or neurology cause, pinhole acuity will show no improvement (in macular disease it may be even worse than unaided acuity)
What is the range of refractive errors beyond which the pinhole will not correct vision to 6/6?
+4 Dto -4D
What is a stenopaeic slit and what is it used for?
elongated pinhole, only allows light in the axis of the slit to enter the eye
can be used to determine refraction and principal axes in astigmatism
How is the stenopaeic slit used?
- Slit is first rotated to a position in which the clearest vision is obtained.
- Spherical lenses are added to give further improvement in acuity.
- The slit is then rotated through 90° and the spherical lens power adjusted to give best subjective acuity.
How do you calculate cylindrical correction from the stenopaeic slit?
it is the algebraic difference between the two spherical corrections used, and its axis is that of the original direction of the slit
What is the far point of an eye?
position of an object such that its image falls on the retina of the relaxed eye i.e. in absence of accommodation
What is important about the far point of the eye when prescribing a lens?
the far point of the eye must coincide with the focal point of the lens
the focal length, f of the correcting lens must be approx. equal to the distance, r of the far point from the principal plane
How can you calculate the power of a lens required using the distance of the far point from the principal plane?
F = 1/f = 1/r
F = power of lens in dioptres, f = focal length in metres, r = distance of far point from principal plane in metres
What is R?
static refraction, or ametropic eror
reciprocal of the far point distance, r, in metres
What happens when a) a convex lens and b) a concave lens is moved away from the eye?
image is moved foward in both cases
When a spectacle correcting lens is moved away from the eye in hypermetropia, how should the convex lens be changed to throw the image onto the retina?
weaker convex lens should be used
When a spectacle correcting lens is moved away from the eye in myopia, how should the concave lens be changed to throw the image onto the retina?
stronger concave lens is needed
What is the formula to calculate the new refractive power of a lens after moving the lens towards or away from the eye?
F2 = F1 / 1-dF1
F2 is the new dioptric power, F1 is the original dioptric power, d is the distance moved
d is positive if the lens is moved towards the eye and negative if moved away from the eye
For which prescriptions is it important to give the back vertex distance (distance from back of corrective lens to the eye surface)?
all prescriptions over 5 dioptres
How do you clinically calculate the change in retinal image size from spectacle magnification?
Relative spectacle magnification (RSM) = corrected ametropic image size/ emmetropic image size
What is the relative spectacle magnification in axial ametropia if the lens is at the anterior focal point?
image is same size - no magnification
What is the relative spectacle magnification in axial myopia if the lens is nearer to the eye than the nterior focal point?
magnifying effect - image size is greater
What happens in refractive ametropia when the correcting lens is at the anterior focal point?
image size differs from emmetropic image even when correcting lens is at this point (unlike axial)
What happens to image size in corrected refractive myopia?
image size is diminished
What happens to image size in corrected refractive hypermetropia?
image size is increased
In refractive ametropia what happens to the image size if the correcting lens is nearer the eye than the anterior focal point?
the image size approaches emmetropic image size
What is the RSM in spectacle corrected aphakia when lenses are at the anterior focal point (23.2mm in front of principal plane)?
magnification of 1.36 (is form of refractive hypermetropia)
What is the RSM in the aphakic eye when a contact lens is used?
1.1
How far are spectacles usually worn in relation to the cornea and what RSM does this produce in the aphakic eye?
12-15mm in front of the cornea
RSM of 1.33
Why is the image magnification (1.33) in 1 corrected aphakic eye an issue?
causes pt to misjudge distances - objects appear closer to the eye due to increased visual angle subtended at the eye
How does image magnification manifest in visual acuity tests e.g. Snellen?
enhanced performance (e.g. 6/9 for aphakic spectacle wearer is equivalent to 6/12 for emmetropia)
What is the benefit of a contact lens or IOL in aphakia correction vs glasses?
reduce RSM to 1.1 or 1.0 respectively which overcomes misjudging distance issues with aniseikonia
Why do spectacles in aphakia cause misjudging of distance?
because a standard object subtends a larger visual angle the closer it is to the eye; an artificially magnified object is therefore assumed to be closer to the eye than it really is
In addition to misjudge distances what are 4 other problems caused by aphakic spectacles?
- image distortion - pin cushion effect
- pristmatic effect - ring scotoma + jack in the box phenomenon
- high power spectacles are heavy and slip
- seeing double due to aniseikonia
How may patients overcome the pin cushion / image distortion from aphakic spectacles?
learn to restrict their gaze to the axial zone of their lenses and moving head rather than eye to look around
How can the ring scotoma result in the jack in the box phenomenon?
the direction of the ring scotoma changes as the patient moves eyes, objects may appear out of the scotoma or disappear into it
What are 2 ways of reducing heavy aphakic spectacles and what is an issue associated with each?
- plastic lenses: tendency to scratch
- plenticular lens (only central portion worked to prescription): reduced field of vision
Why does double vision occur in unilateral aphakic spectacles?
eyes see images of 2 different sizes (1.33 magnification in aphakic eye) and is unable to fuse the 2 images - aniseikonia
What is 1 way to overcome aniseikonia from unilateral aphakic spectacles?
iseikonic lens - spectacle lens with no focusing power but alters retinal image size by increasing visual angle subtended
What does magnification of iseikonic lenses rely on?
curvature of the front surface of the lens and on its thickness; back curvature adjusted to render the lens afocal
What is the maximum magnification that can be achieved by an iseikonic lens and what is the implication?
5% - insufficient to be of practical benefit in correction of unilateral aphakia
What is the equation in words that takes into account the 2 key measures required to calculate required IOL power?
required IOL power = required vergence in plane of IOL - effective power of cornea in plane of IOL
What is the SRK formula to calculate IOL power, P, in dioptres?
P = A - 2.5(axial length in mm) - 0.9(average keratometry in diotpres)
What does A represent in the SRK formula for calculating IOL power?
constant which reflects the position of the particular model of IOL within the eye
How is the SRK formula modified if a refractive condition (R) other than emmetropia is desired?
P = A - B(AL) - C(K) - D(R)
D is the multiplcation constant for the desired refraction. D is 1.25 if IOL power for emmetropia is >14D, 1.0 if less than or equal to 14D
What calculation must be done if IOL power has been calculated for a posterior chamber lens and at surgery an anterior chamber lens has to be used?
required power must be reduced by the difference int he A constants of the IOLs (because anterior chamber lens is further forward in the eye and therefore has a lower A constant)
What is usually the difference between anterior chamber and posterior chamber IOLs for a patient?
usually anterior chamber IOL is 2 dioptres weaker than the posterior chamber lens
Which patients was the original SRK formula inaccurate for?
eyes of short (<22mm) or long (24.5mm or above) axial length
How can different axial lengths be adjusted for in the SRK II formula for IOL power?
A constant is adjusted (termed A1) over the range of axial lengths
What is the refinement of the SRK III formula for very high or very low IOL powers?
SRK-T formula
How is the axial length of an eye measured for the SRK formula?
A-scan ultrasonography
How does A-scan ultrasonography work?
sound and US energy transmtited by vibration of adjacent particles, travel different speeds depending on medium density. pulse echo times recorded; speeds of US in various ocular media are known so axial length may be calculated
When must a correction be applied after calculating axial length from A scan ultrasonography?
if vitreous has been replaced by silicone oil
What can A scan US also measure in addition to axial length?
corneal thickness
Along which axis must the axial length of the eye be measured?
visual axis of the eye
What can cause an erroneous reading of axial length from A scan ultrasound?
posterior staphyloma - outpouching of wall of globe
What should be done to overcome the issues posed by staphyloma in measuring axial length of globe?
B scan - 2 dimensional US scan
What are 2 important things to take into consideration for choice of post-operative refraction after cataract surgery?
- state of fellow eye - avoid anisometropia -→ aniseikonia + disruption of binocular vision
- previous refractive experience - lifelong myopes don’t like using convex reading glasses, prefer concave distance correction
How do multifocal IOLs work?
form multiple images over the whole lens aperture
How do bifocal IOLs work?
two images of an object formed; one falls on the retina when the object is at infinity, the other falls on the retina when the object is at the patient’s reading distance
What are 2 disadvantages of bifocal IOLs?
- neither of the 2 images formed is as clear and bright as monofocal
- second image forms a blur circle on the retina, may degrade clarify and cause glare
What are 3 options for ways to achieve a multifocal effect for an IOL?
- concentric zones of graded power - near zone at centre
- concentric annular zones - graded, near and distance over whole lens aperture
- multiple ring steps on posterior surface of distance power - diffraction
How do multiple ring steps on the posterior surface of the IOL produce multifocal vision by diffraction?
diffraction of light provides second image for near by constructive interference between waves of light diffracted by the various zones of the lens
At what degree of axial myopia does removal of the crystalling lens render the eye emmetropic?
-18 to -20 D
Why is the eye emmetropic after lens removal in axial myopia -18- -20D if the lens is +15D?
because of the change in effectivity of lenses, in order to achieve a correction of -15D in the plane of the crystalline lens, a spectacle correction of -15 to -20 D is required. Because degree of myopia is referred to by the strength of spectacle correction, myope of -18 to -20 becomes emmetropic
the actual power of the crystalline lens in isolation is +19D but contributes +15D to refractive power of eye
How do you calculate lens power needed for a patient to read newspaper print with 6/60 snellen acuity?
Kestenbaum’s rule: 6/60 → 60/6 = + 10D
In patient >40 years, little to no accommodate power (presbyopic) so no correction
If young, have 7-8D accommodative power, could use 50% comfortable (4D) so minus this 10-4 = +6 D for reading newspaper print
