7.1 astigmatism basic definitions

Question 1

what is the clinical definition for astigmatism ?

Answer 1

difference in power between two principal perpendicular meridians

Question 2

what is the optics definition for astigmatism ?

Answer 2

variation in focusing distance with meridian

Question 3

what is the cause of astigmatism ?

Answer 3

astigmatism is caused by toric surfaces

occurs when front surface of the eye ( cornea or lens ) has irregular curvature

Question 4

what are toric surfaces ?

Answer 4

have different curvatures along two perpendicular meridians this causes a difference in refraction and therefore in focusing which will change in meridian
-difference in curvatures that refracts light differently

Question 5

what are the types of toric surfaces ?

Answer 5

ring torus
barrel
capstan

Question 6

where can toric surfaces occur ?

Answer 6

toric surfaces can occur at different points in the eye
if it occurs at cornea then it will be corneal astigmatism
if it occurs at lens it will be lenticular astigmatism

Question 7

how are cyl lenses designed ?

Answer 7

. cylinder lenses are plano in meridian
. axis is along plano direction
. labelled power is effective along a direction perpendicular to the cylinder axis (the etched markings of the cyl)- so for example if Plano at vertical meridian the cyl will be in that direction

Question 8

explain axis notation in lenses ?

Answer 8

.axis marking show standard notation
. anti clock wise direction
. observer view
. dont use degree sign 
. recored 180 instead of 0

Question 9

how common is astigamatism ?

Answer 9

common - 63 % of young adults have 0.25DC or more

but only 18.9% have >1DC in young adults

Question 10

where is astigmatism most prevalent ?

Answer 10

prevalence in younger children

28.4% have >1DC between ages 5 and 17

Question 11

which ethnicity has most astigmatism ?

Answer 11

east asians and native Americans

Question 12

where is WTR most prevalent ?

Answer 12

in young children but shifts to ATR (against the rule) after age 40

Question 13

what is WTR ?

Answer 13

with the rule astigmatism

. steepest curvature in vertical meridian - vertical meridian is most myopic or least hypermetropic

Question 14

how to correct WTR ?

Answer 14

negative CYL in horizontal meridian with axis 180+/- 15

Question 15

what is ATR ?

Answer 15

against the rule astigmatism

. steepest curvature in horizontal meridian

Question 16

how to correct ATR ?

Answer 16

negative cyl in vertical meridian with axis 90 +/- 15

Question 17

what are the 5 classification of astigmatism ?

Answer 17

1. compound hypermetropic astigmatism ( CHA)
2. simple hypermetropic astigmatism ( SHA )
3. mixed astigmatism ( MA )
4. simple myopic astigmatism ( SMA )
5. compound myopic astigmatism ( CMA )

Question 18

what is Compound Myopic Astigmatism (CMA) ?

Answer 18

Both astigmatic focal lines lie behind the retina. (Both principal meridians are myopic)
. negative CYL and SPH to correct

Question 19

what is Simple myopic astigmatism (SMA) ?

Answer 19

The focus of the least powerful principal meridian lies on the retina with the other principal meridian focus behind the retina. (One meridian is emmetropic and the
other is myopic)
. negative CYL and no SPH to correct

Question 20

what is Mixed Astigmatism (MA) ?

Answer 20

The astigmatic focal line lie either side of the retina. (One meridian is myopic and the other is hypermetropic)

Question 21

what is Simple hypermetropic astigmatism (SHA) ?

Answer 21

The focus of the most powerful meridian lies on the retina with the other meridian focus behind the retina. (One meridian is emmetropic and the other is
hypermetropic)

Question 22

what is Compound Hypermetropic Astigmatism (CHA) ?

Answer 22

Both astigmatic focal line lie behind the retina. (Both

principal meridians are hypermetropic)