PC - Ametropia and Emmetropia - Week 1

Question 1

What two structures do the majority of light bending within the eye?

Answer 1

Cornea and lens

Question 2

Define emmetropia.

Answer 2

Lack of need for glasses/correction

Termed emmetrope.

Question 3

Define accommodation.

Answer 3

Lens changing shape due to cilliary muscle contraction/relaxation to focus light
Contraction - Fat round lens
Relaxation - Thin curved lens

Question 4

Describe what happens to the conjugate focus when looking at an object at a distance, when focus is shifted to a nearer object.

Answer 4

At rest on a distant object, the conjugate focus on the retina is at infinity. When focus shifts to a nearer object, the focus is behind the retina, and accommodation must occur to bring it back to the retina.

Question 5

What is the approximate power range needed to be considered an emmetrope?

Answer 5

-0.25 to +0.75

Question 6

Define myopia, and where the conjugate focus falls. Describe what causes this condition, and how distant/near objects are brought into focus (or not). Is correction often required?

Answer 6

Myopia is shortsightedness, where the eyeball is too long. Focus of distant objects are therefore in front of the retina, and accommodation cannot rectify it.
Near objects focus behind the retina, but accommodation can rectify this onto the retina.
Correction typically required.

Question 7

Are myopic eyes too powerful or too weak? What kind of correction is needed for myopes?

Answer 7

Myopic eyes are too powerful, and require diverging (concave) correction.
Refractive surgery reducing corneal power is a permanent solution.

Question 8

What is the DS of myopic eyes?

Answer 8

≤ -0.5

Question 9

Define optometric units of power.

Answer 9

Dioptre.

Question 10

Define zero plano.

Answer 10

Zero power/dioptre

Question 11

How does eye power relate to how far you can see? Describe the formula.

Answer 11

Consider the power of the eye - x, and take its inverse - 1/x.
This is the distance in metres where accommodation can occur adequately without refractive aid.

Question 12

Define hyperopia, and where the conjugate focus falls. Describe what causes this condition, and how distant/near objects are brought into focus (or not). Is correction often required?

Answer 12

Hyperopia is longsightedness, where the eyeball is too short. Focus of close objects are therefore in behind of the retina, though accommodation can often rectify it.
Distant objects are focused adequately on the retina.
Correction not always required.

Question 13

Are hyperopic eyes too powerful or too weak? What kind of correction is needed for hyperopes?

Answer 13

Hyperopic eyes are too weak, and require converging (convex) correction.

Question 14

Which of the two, myopia and hyperopia, is often asymptomatic? Why is this the case?

Answer 14

Hyperopia can be asymptomatic as it affects close vision, and hyperopic eyes can often accommodate adequately. This is true in younger populations, however the aging process tends to weaken accommodation in the geriatric population.

Question 15

What is the DS of hyperopic eyes?

Answer 15

≥ +0.75

Question 16

What 3 secondary conditions can moderate to high degrees of hyperopia cause?

Answer 16

Binoccular disorders and with age

Risk of convergent strabismus and amblyopia developing in infancy

Question 17

Are individuals typically born hyperopic or myopic? How does this resolve over time, and at which particular time periods after-birth? What can affect this process?

Answer 17

Individuals are born hyperopic, this resolves with age as the eye grows in a controlled manner to achieve emmetropia.
85% of controlled growth occurs within the first 3 years.
Genetics plays a factor, however environmental factors also have a strong influence.

Question 18

What secondary condition as a result of aging causes myopia in the geriatric population?

Answer 18

Cataracts.

Question 19

Define astigmatism.

Answer 19

Eye is football shaped, resulting in two focal points.

Question 20

Define the following astigmatism types in terms of focal point position:
Mixed
Simple myopic
Compound myopic
Simple hyperopic
Compound hyperopic

Answer 20

Mixed - one focal point behind, and another in front of the retina
Simple myopic - one focal point on the retina, the other in front
Compound myopic - both focal points in front of the retina
Simple hyperopic - one focal point on the retina, the other behind
Compound hyperopic - both focal points behind the retina

Question 21

Describe the following types of astigmatism:
Against the rule
With the rule
Oblique
Mention the axis angles and which lines are blurry.

Answer 21

Against the rule
~90 degree axis, vertical lines are blurry
With the rule
~180 degree axis, horizontal lines are blurry
Oblique
~45 or ~135 degree axis, oblique lines are blurry

Question 22

Define OD and OS

Answer 22

OD - right eye

OS - left eye

Question 23

Define DS and DC

Answer 23

DS - dioptre sphere

DC - dioptre cylinder

Question 24

Define the script format for astigmatism.

Answer 24

±sphere power DS/ −cyl-power DC x axis (1 to 180)

Cylinder powers are negative in Australia

Question 25

Describe the three steps to write a script.

Answer 25

1. Pick the most positive sphere power
2. Calculate the difference between the two spheres for cylinder power. This is made negative regardless of its value.
3. Pick the axis of the most negative power as the cyl axis.

Question 26

Define presbyopia, its symptoms (in emmetropes, hyperopes, and myopes), and onset.

Answer 26

Difficulty with near vision due to lens inflexibility with age, resulting in decreased ability to accommodate.
Symptoms
-Eye strain and tiredness
Emmetropes/hyperopes
-Increased working distance
Myopes
-Removal of SVD

Question 27

What kind of optical correction is needed for presbyopia?

Answer 27

Positive/concave for near vision

Some may be bifocal/multifocal lens

Question 28

Can pupillary distance vary in an invidual?

Answer 28

Yes, when focused at distance they are wider than they are focused at near objects.