Chapter 10  (Optics of Ametropia)

1. The following are true about myopia:

a. the second principal focus lies behind the retina

b. the presence of posterior staphyloma suggests axial myopia

c. axial myopia may be caused by the cornea having too strong 
    a refractive power

d. nucleosclerosis is a cause of index myopia

e. high myopia may be treated with clear lens extraction
 

Answera.F   b.T   c.F   d.T   e.T
In myopia, the second principal focus lies in front of the retina. 
Myopia may be classified into axial myopia (in which the eye
is abnormally long as in high myopia which can produce staphyloma)
or refractive or index myopia in which the refractive power of 
the eye is increased as in keratoconus and nucleosclerosis. 
 
 

2. The following are true about hypermetropia::

a. the second principal focus lies in front of the retina

b. accommodation is used to achieve normal vision

c. aphakia is a form of hypermetropia

d. patients require reading glasses earlier than the normal population

e. patients who has hypermetropic refraction following cataract surgery 
    will have problem for both near and distant reading
 

Answera.F   b.T   c.T   d.T   e.T
The second principal focus of hypermetropia lies behind the retina. 
Accommodation is used to correct the emmetropia to certain extent.
They require reading glasses earlier than the normal population. 
Aphakia is a form of hypermetropia. Following cataract surgery, 
the accommodative ability of the eye is lost. Therefore, if the patient 
is hypermetropic, the vision will be blurred for distant and near.
This is the reason why most patients are made slightly myopic 
so that they can achieve a reasonable vision without glasses for distance.
 
 

3. The following are true about hypermetropia:

a. manifest hypermetropia is the strongest plus lens which the patient 
    can accept for clear distant vision 

b. latent hypermetropia is the residual hypermetropia masked by ciliary 
    tone and involuntary accommodation

c. latent hypermetropia can be unmasked by cycloplegic refraction

d. falcultative hypermetropia refers to hypermetropia that can not be 
    overcome by accommodation

e. absolute hypermetropia cannot be overcome by accommodation
 

Answera.T   b.T   c.T   d.F   e.T 
Four terms are used to define hypermetropia:
Manifest hypermetropia refers tot he strongest plus/convex lens that the patient can accept for clear distant vision. 
Latent hypermetropia is the remaining hypermetropia masked by
the ciliary tone and involuntary accommodation. It can be unmasked by cycloplegic refraction. 
Facultative hypermetropia refers to hypermetropia that can be overcome by accommodation.
Absolute hypermetropia refers to hypermetropia in excess of the amplitude of accommodation..
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4. With regard to astigmatism:

a. regular astigmatism has the principal meridians at 90⁰ to each other

b. oblique astigmatism occurs when the principal meridians do not lie 
    at 90⁰ to each other

c. irregular astigmatism is seen in patients with keratoconus

d. astigmatic eye produces an image known as a Sturm's conoid

e. astigmatic image can not be fully corrected with a spherical lens
 

Answera.T   b.F   c.T   d.T  e.T
Regular astigmatism has the principal meridians at 90 degrees to each other. Oblique astigmatism also has its principal meridians at 90 degrees to each other but they do not lie near to 90 or 180 degrees. Irregular astigmatism occurs when the principal meridians are not at 90 degrees to each other. An image known as Sturm's conoid is produced by an astigmatic eye; such image can only be corrected with a spherocylindrical lens.
 
 

5. Regarding the images from by astigmatic eyes:

a. rays in all meridians are focused behind the eye in compound 
    hypermetropic astigmatism

b. ray from one meridian is focused on the retina while the other is focused 
    behind the retina in simple hypermetropic astigmatism 

c. rays in all meridians are focused in front of the eye in compound myopic 
    astigmatism 

d. ray from one meridian is focused on the retina while the other is focused in 
    front of the retina in simple myopic astigmatism 

e. ray from one meridian is focused in front of the retina and the other behind 
    the retina in mixed astigmatism
 

Answera.T   b.T   c.T   d.T   e.T
The definitions of the terms are correct.
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6. Anisometropia:

a. occurs when the two eyes have different refractive errors

b. of more than 1D in hypermetropic patients can usually be controlled 
    through accommodation of the more hypermetropic eye

c. is a common cause of amblyopia in patients with uncorrected low myopia

d. of recent onset may be caused by the development of posterior 
     subcapsular cataract

e. may result from unilateral central serous retinopathy
 

Answera.T   b.F   c.F   d.F   e.T
Anisometropia refers to different refractive errors between the two
eyes. Anisometropia of more than 1D in hypermetropic patients can
not be controlled with unilateral accommodation because it is a binocular process. Therefore the image in one eye is often blurred and is a cause of amblyopia. On the other hands, anisometropia in low myope does not usually cause amblyopia because the near vision is normal. Unilateral nucleosclerosis and not posterior subcapsular cataract can lead to index myopia. Central serious retinopathy can cause hypermetropic shift.
 

7. With regard to the pin-hole:

a. the vision through the pin-hole is usually worse in patient with macular 
    disease 

b. it may allow presbyopic patients to read comfortably without glasses 
    correction

c. failure of the vision to improve to 6/6 with pin-hole always indicates 
    the presence of macular diseases 

d. too small a pin-hole can affect vision through interference 

e. it can improve vision in patients with posterior subcapsular cataract.

 
Answera.T   b.T   c.F   d.T  e.F
Pin-hole is used to improve the vision of patients with ametropia in
the range of +4 to -4. High ametropia may not improve with pin-hole. Patients with macular disease, posterior subcapsular cataract or central corneal scar may have worse vision with pin-hole. If the hole on the pin-hole is too small, interference of light can affect the visual acuity.
 
 

8. The stenopaeic slit:

a. can be used to find the principal axes of astigmatism

b. is an elongated pinhole that reduces blur resulting from ametropia 
    in the meridian perpendicular to the slit

c. gives a clear vision when held at 90⁰ with a +2.00DS and at 180⁰
    with a -1.00DS, the cylindrical power required is -3.00DC at 90⁰

d. gives a clear vision when held at 90⁰ with a +2.00DS and at 180⁰
    with a -1.00DS, the lens required to correct the vision is 
    +2.00 /-3.00 X 180⁰

e. can be used to find the best position for optical iridectomy in a patient 
    with corneal scarring
 

Answera.T   b.T   c.T   d.F   e.T
Remember that the power of cylinder is 90 degrees from the axis. 
The cylinder power required in question b is either -3.00 at 90 degrees or +3.00 at 180 degrees. The lens required is either +2.00 / -3.00 X 90 or 
-1.00 / +3.00 X 90.
 
 

9. True statements about the correcting lens include:

a. when a correcting lens is moved forward in a hypermetrope the image
    is moved forward

b. when a correcting lens is moved forward in a myope the image is 
    moved backward 

c. the effectivity of the lens is increased in a myope if the correcting lens 
    is moved backward

d. the effectivity of the lens is increased in a hypermetrope if the correcting 
    lens is moved forward 

e. a hypermetrope with early presbyopia may be able to read clearly by 
    pushing his glasses closer to his eyes 
 

Answera.T   b.F   c.T   d.T   e.F
The image is moved forward when a correcting lens is moved forward irrespective if the lens is concave or convex. The effectivity of the lens
is increased in hypermetrope when the lens is moved forward (which is expected of a convex lens) but in the case of the myope, the effectivity of the lens is decreased if the lens is moved forward as the lens fails to achieve its expected function which is to move the image backward. A hypermetrope with early presbyopia may be able to read clearly by moving his glasses away from the eyes.
 
 

10. The following are true:

a. the back vertex distance is the distance between the back of a correcting 
    lens and the cornea

b. the back vertex distance is not required if the correcting lens is less than 
    5 dioptre power

c. the contact lens for a myope is usually stronger than the glasses 

d. the contact lens for a hypermetrope is usually stronger than the glasses 

e. contact lens magnifies the image in a patient with axial myopia 
 

Answera.T   b.T   c.F   d.T   e.T
The position of the correcting lens affect its effective power. The back 
vertex distance is important if the power of the correcting lens is more
than 5 dioptres. The contact lens for a myope is usually weaker than
the glasses but for a hypermetrope it is usually stronger than the glasses. Contact lens magnifies the image in a patient with axial myopia.
 
 

11. Relative spectacle magnification:

a. is defined as the ratio of corrected image size to uncorrected image size

b. is 1.0 in axial ametropia if the correcting lens is placed at the anterior 
    focal point

c. is 1.0 in index or refractive ametropia if the correcting lens is placed at
    the anterior focal point

d. is about 1.33 in aphakia corrected with spectacle

e. is 1.0 in aphakia corrected with contact lens 
 

Answera.F   b.T   c.F   d.T   e.F
Relative spectacle magnification (RSM) is defined as the ratio of corrected ametropic image size to emmetropic image size. The definition in a is spectacle magnification. In axial ametropia if the correcting lens is placed at the anterior focal point, the RSM is 1.0. In index or refractive ametropia if the correcting lens is placed at the anterior focal point,
the RSM is >1.0 for hypermetropia and <1.0 for myopia. Aphakia is an index/refractive ametropia and the RSM is about 1.33. With contact lens, the RSM approaches unity but still measures about 1.1. In order to achieve RSM = 1, a secondary lens implant is the treatment of choice. 
 

12. Regarding the correction of aphakia with glasses:

a. there is an increasing image magnification as the correcting glasses 
    is moved forward from the position of the natural crystalline lens.

b. reduction in visual field is a common problem

c.  “pincushion” distortion of the visual field

d. ring scotoma of aphakia is created by the prismatic effect of the lens

e.  using head turning rather than eye movement to change gaze direction 
     can prevent 'Jack-in-the-box' phenomenon
 

Answera.T   b.T   c.T   d.T   e.T
There is an increasing image magnification as the correcting glasses 
is moved forward from the position of the natural crystalline lens. Therefore, aphakia is best corrected with either contact lens or secondary lens implant. 
The problem with aphakic glasses include:
a. large image magnification 
b.“pin-cushion ” distortion of the visual field
c. a ring scotoma, which has the “jack-in-the-box” effect of objects suddenly appearing from the edge of the scotoma.
d. reduction of visual field
.
 

13. Intraocular lens calculation:

a. is more affected by an error in axial length calculation than keratometric 
    measurement

b. most commonly used in the preoperative clinic is derived from theoretical 
    formulas 

c. uses a lower A constant for anterior chamber than posterior chamber lens

d. is affected by the velocity of the ultrasound through the cataract 

e. is affected by the absence of crystalline lens
 

Answera.T   b.F   c.T   d.T   e.T
In the SRK formula, the axial length is multiplied by 2.5 whereas the keratometric result by 0.9. Therefore, a difference in the axial length 
has a greater effect on the IOL calculation than the keratometry. 
The most commonly used formula is an empirical formulas using regression analysis. A lower A constant is used for the anterior chamber than one in the posterior chamber. The velocity of ultrasound through the lens can affect the IOL calculation and this velocity of ultrasound varies according to the density and type of cataract. It is important to set the ultrasound to phakic, pseudophakic or aphakic during biometry as the ultrasound travels at different speed through these variables.
a
a

14. The SRK formula for IOL calculation

a. the SRK formula is not accurate for eye shorter 22mm

b. the SRK formula is not accurate for eye longer than 24.5 mm 

c. the SRKII is a more accurate formula than SRK

d. the SRK II uses a higher A constant than SRK 

e. the SRK-T is a more accurate formula than SRK 
 

Answera.T   b.T   c.T   d.F   e.T
SRK formula is used for IOL calculation. However, it is not accurate for eye less than 22mm or longer than 24.5mm. SRK II and SRK-T 
are more accurate than SRK. The SRK II uses adjusted A constant depending on the length of the eye.
 

15. The crystalline lens:

a. has an in-situ effective power of +15.00D

b. contributes more power than the cornea towards the refraction of the eye 

c. if extracted without implant can correct myopia who needs spectacle 
    correction of -15.00D

d. has a longer radius of curvature anteriorly than posteriorly

e. has an uniform refractive index
 

Answera.T   b.F   c.F   d.T   e.F
The crystalline lens has a power of +19.00D but within the eye it contributes only +15.00D. If extracted without implant it can be used to correct myopia who needs spectacle correction of between -18.00 and -20.00DS. The anterior lens surface is less curved than the posterior lens surface. The lens has different refractive indexi within its substance; the nucleus tends to have higher refractive index than the cortex.