astigmatism

Question 1

define astigmatism (3)

identify 2 types of astig

Answer 1

• irregular shape causing different powers at different principal meridians
• creating elongated image of a dot point source -> ie. parallel pencil of ray not coming to a single point focus on the retina
• amplitude of astigmatism is equal to difference of refracting power of the two principal meridians

1. corneal astigmatism (most common)
2. lenticular astigmatism

Question 2

outline alpha and beta principal meridians (2,2)

Answer 2

alpha:

• principal meridian of maximum power
• most myopic or least hyperopic

beta:

• principal meridian of minimum power
• least myopic or most hyperopic

Question 3

outline general rule of astigmatism (1+3)

Answer 3

• generally the STEEPER meridian alpha is VERTICAL
• with-the-rule astig: if alpha is vertical
• against-the-rule astig: if alpha is horizontal
• oblique astig: if alpha between 120°-150° and 30°-60°

Question 4

classify different types of astigmatism (1a-e + 3)

Answer 4

1. regular astigmatism
   - principal meridians are perpendicular

(a) compound myopic (both)
(b) compound hyperopic (both)
(c) simple myopic (one)
(d) simple hyperopic (one)
(e) mixed astigmatism (m+h)

2. irregular astigmatism
   - principal meridians are not perpendicular
   - curvature of any one meridian is not uniform
   - often associated with trauma, disease, surgical complication

Question 5

how to transpose +cyl to -cyl (3)

Answer 5

1. add sph and cyl for new sph
2. change sign of cyl
3. change axis by 90°

Question 6

outline sturm’s conoid (2)

Answer 6

• optical condition in which refractive power of cornea and lens in not the same in all meridians
• instead of 1 focal point -> there are 2 focal lines

Question 7

outline circle of least confusion (3)

where is it located (1+1)

• linear distance
• dioptres

Answer 7

• location of best visual acuity (want to place on retina)
• smallest circle -> therefore is the spherical equivalent meridian
• associated with depth of focus

linear distance:
- located between l’ₐ and l’₆ (circular cross-section)
dioptres:
- midway between l’ₐ and l’₆

Question 8

outline interval of sturm (1)

Answer 8

• distance between the two focal points

ie. from the horizontal line and vertical line

Question 9

describe the conoid of strum resulting from a refracting surface with 60D along 90 meridian and 10D along 180 meridian

Answer 9

90 meridian is alpha -> vertical meridian forms horizontal focal line -> horizontal focal line focuses closest to refracting surface -> followed by CoC -> followed by vertical focal line

Question 10

how are horizontal and vertical focal lines formed (1+1)

Answer 10

• horizontal meridians always forms a vertical focal line

- vertical meridians always forms a horizontal focal line

Question 11

outline causes of sturm’s conoid (3+2)

Answer 11

corneal causes:

• occurs due to abnormalities of curvature of cornea
• most common cause of astigmatism
• eg: pterygium, corneal degen

lenticular causes:

• comparatively rare
• curvatural, positional or index

Question 12

outline 1 clincal procedure used to determine patient’s astigmatism

Answer 12

• Jackson cross cylinder (JCC)
• most frequently used subjective test
• power in one of the principal meridians of a crossed cylinder is equal and opposite to the power in the other meridian

Question 13

how to calculate spherical equivalency

Answer 13

spherical equivalent power = sph + 1/2 cyl

Question 14

describe the effect of accommodation on astigmatism for:

• myope
• hyperope

(1+1)

Answer 14

hyperope:
- accommodation is likely to change change the postiion of the focal lines and CoC in younger people

myope:
- accommodation can’t help -> defocuses more

Question 15

when is vision of uncorrected astigmatism poorest

Answer 15

when occular astigmatism is at oblique axis