PC - Ametropia and Emmetropia - Week 1 Flashcards
What two structures do the majority of light bending within the eye?
Cornea and lens
Define emmetropia.
Lack of need for glasses/correction
Termed emmetrope.
Define accommodation.
Lens changing shape due to cilliary muscle contraction/relaxation to focus light
Contraction - Fat round lens
Relaxation - Thin curved lens
Describe what happens to the conjugate focus when looking at an object at a distance, when focus is shifted to a nearer object.
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.
What is the approximate power range needed to be considered an emmetrope?
-0.25 to +0.75
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?
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.
Are myopic eyes too powerful or too weak? What kind of correction is needed for myopes?
Myopic eyes are too powerful, and require diverging (concave) correction.
Refractive surgery reducing corneal power is a permanent solution.
What is the DS of myopic eyes?
≤ -0.5
Define optometric units of power.
Dioptre.
Define zero plano.
Zero power/dioptre
How does eye power relate to how far you can see? Describe the formula.
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.
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?
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.
Are hyperopic eyes too powerful or too weak? What kind of correction is needed for hyperopes?
Hyperopic eyes are too weak, and require converging (convex) correction.
Which of the two, myopia and hyperopia, is often asymptomatic? Why is this the case?
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.
What is the DS of hyperopic eyes?
≥ +0.75
What 3 secondary conditions can moderate to high degrees of hyperopia cause?
Binoccular disorders and with age
Risk of convergent strabismus and amblyopia developing in infancy
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?
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.
What secondary condition as a result of aging causes myopia in the geriatric population?
Cataracts.
Define astigmatism.
Eye is football shaped, resulting in two focal points.
Define the following astigmatism types in terms of focal point position: Mixed Simple myopic Compound myopic Simple hyperopic Compound hyperopic
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
Describe the following types of astigmatism:
Against the rule
With the rule
Oblique
Mention the axis angles and which lines are blurry.
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
Define OD and OS
OD - right eye
OS - left eye
Define DS and DC
DS - dioptre sphere
DC - dioptre cylinder
Define the script format for astigmatism.
±sphere power DS/ −cyl-power DC x axis (1 to 180)
Cylinder powers are negative in Australia
Describe the three steps to write a script.
- Pick the most positive sphere power
- Calculate the difference between the two spheres for cylinder power. This is made negative regardless of its value.
- Pick the axis of the most negative power as the cyl axis.
Define presbyopia, its symptoms (in emmetropes, hyperopes, and myopes), and onset.
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
What kind of optical correction is needed for presbyopia?
Positive/concave for near vision
Some may be bifocal/multifocal lens
Can pupillary distance vary in an invidual?
Yes, when focused at distance they are wider than they are focused at near objects.
