7.1 - Introduction to Astigmatism Flashcards
What is astigmatism
Difference in power between two principal (perpendicular) meridians
What happens in astigmatism
- Variation in focussing distance with meridian - each meridian varies = not same for both meridians like it is in spherical ametropia.
- Theres a difference in focussing because the geometry of surfaces will lead to light being refracted in different ways - changes with meridian
What is a toric surface
Surfaces which have different curvatures along two perpendicular meridians
What are classifications of toric surfaces
Ring torus
Barrel
Capstan
What happens in the different classifications of toric surfaces
Curvatures along different meridians changes
What happens in a capstan toric surface
Curvature along vertical section is shallow compared with curvature along horizontal along equator = this difference in curvature refracts the light differently
What is special about toric surfaces that give rise to astigmatism
They occur at different points in the eye
What are 2 examples of astigmatism in the eye
Corneal astigmatism - toric shape of cornea giving rise to it
Lenticular astigmatism - due to shape of lens
How many meridians do cyl lens have and what is the power of one of them
2 meridians
Plano in one meridian
Where is the cyl axis marking along in a cyl lens
Along the Plano direction - no cyl power here
Along the meridian with no cyl power - Plano
Where is the cyl power found in a cyl lens
cyl power is along a direction perpendicular to the cylinder axis
What is the standard axis notation
Going anticlockwise from 0 to 180 starting at horizontal
What is axis marking for observers view
0 on right
180 on left
What % of young adults have 0.25 power cyl or more
63%
What % of young adults have >1 power cyl or more
18.9%
What is prevalence of astigmatism higher in for age
Children
28.4% have >1DC between ages 5 and 17
Prevalence might increase in older adults
What is prevalence of astigmatism higher in for ethnicity
Higher prevalence in East Asians and Native Americans
Which has higher prevalence - with the rule or against the rule
WTR has a higher prevalence in the young but shifts to ATR after age 40
What is with the rule astigmatism
- Steepest curvature in vertical meridian
- Vertical meridian = least hypermetropic or most myopic
- Negative cyl axis 180 ± 15
What is against the rule astigmatism
Steepest curvature in horizontal meridian
negative cyl axis 90 ± 15
What is compound hypermetropic astigmatism
Both meridians hypermetropic as behind retina
What is simple hypermetropic astigmatism
One meridian on retina and other behind it
What is mixed astigmatism
Meridians either side of retina - one myopic and one hypermetropic
What is simple myopic astigmatism
Focus on retina and other infront = negative cyl and no sphere
What is compound myopic astigmatism
Negative sphere and negative cyl = both in front of retina = both meridians myopic
What is astigmatism corrected by
Toric lenses
