Refractive Status Flashcards
Emmetropia
Normal
Ametropia
Abnormal
Influenced by difference in
-location of optical element in regards to the retina
-refractive power of the optical element.
Mypoia
- Light focuses in front of the retina
- correction: concave minus lenses
Axial Myopia
Relatively longer axial length
Refractive Myopia
Excessive refractive power
Hyperopia
- Light focuses behind the retina
- Correction: Convex plus lens
- hereditary factors influence more than environmental factors
Axial Hyperopia
Relatively shorter axial length
Refractive Hyperopia
Insufficient refractive power
Accomodation
the ability to temporarily INCREASE the dioptric power of the crystalline lens
- under the age of 40(presbyopia)
- changes shape of the lens
Anisometropia
A difference in refractive status between the two eyes
Isoanisometropia
Both eyes have the sign, one is just more significant than the other
Antimetropia
The eyes are opposite in sign
Problems with anisometropia
Induced prism with glasses
- stimulus to accomodation
- image magnification
- too high, brain shuts down (can’t get the object to focus at the same time)
Regular and irregular astigmatism
regular- Difference is 90 degrees
irregular- Difference is not
Corneal and Lenticular astigmatism
corneal- the shape of the cornea causes difference
lenticular- the shape of the lens causes difference
With the rule and against the rule
with the rule- weaker power is in the horizontal
against the rule- weaker power is in the vertical
Astigmatism
Light focused differently in different meridians
-correction: lenses with different power in different meridians
Corneal Power
- focusing strength of the cornea
- normal is 43.25D
- reaches adult level early in life
- determined by curvature
- increased in myopes, but not proven
Crystalline lens power
- focusing strength of crystalline lens
- normal is 20D
- changes occur through life (decrease 6-8)(increase the rest of the time)
Anterior Chamber Depth
- Distance fro cornea to lens
- normal is 4mm
- increases until teenager (to go a long with decrease of crystalline lens power) then decreases (again, because of crystalline lens power)
- effect on refractive status and dependent on axial length
- if axial length stays the same and increase ACD, decrease refractive power
Axial Length
- length of entire eye
- normal is 24mm
- rapid increase in early life, slows and stabilizes by 15(your body pretty much stops growing)
- not the case with adult onset myopia - longer=myopia
- shorter=hyperopia.
Emmetropization
The different components work together to compensate and result in more emmetropes than we would expect
-active emmetropization effect is not perfect
Keratometry and Corneal Topography
Measure anterior corneal curvature via reflected light
- kerotometry measures curvature
- corneal topography uses colors. red=steep(anterior chamber depth)
Ultrasound and doppler interferometry
Measure thickness of cornea and length between ocular structures via sound and light rays respectively
- ultrasound-pulse to eye and echoes are generated at each change in media density, so time it takes for echoes to come back determines separation of each boundary.
- interferometry can predict before cat surgery on how to increase vision. (axial length)
Theories of myopic development
Biological-statistical theory-refractive error ranges from high myopia to high hyperopia so ametropia is just a biological variation
Use and Abuse theory- myopia onset due to abusing the eyes during strained near vision (more prevalent in people who do lots of close work/ reading.
Theory of emmetropization- coordinated growth of the optical elements. more emmetropes due to negative feedback, self-focusing control system.
What are the main components of refraction?
- corneal power
- crystalline lens power
- anterior chamber depth
- axial length
