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Contact Lens (MIDTERM | 2ND SEM) > Astigmatism > Flashcards

Astigmatism Flashcards

(45 cards)

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1
Q

it is the difference in refractive power between the
two principal meridians of the eye

A

astigmatism

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2
Q

The greater the curvature the:

A

greater the power

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3
Q

two meridians of the eye

A

power meridian
axis meridian

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4
Q

meridian of greatest optical power

A

power meridian

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5
Q

meridian of least optical power

A

axis meridian

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6
Q

Meridian Descriptions

A

Refractive Power (Dioptres)
Radius of Curvature (mm)

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7
Q

Causes of astigmatism

A

Meridional differences in curvature and/or flattening rates of cornea, crystalline lens, and refractive index of optical components

Shape of posterior pole

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8
Q

Astigmatism Classification

A

● Simple hyperopic astigmatism
● Compound hyperopic astigmatism
● Simple myopic astigmatism
● Compound myopic astigmatism
● Mixed astigmatism

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9
Q

Structural classification of astigmatism

A

Corneal
Lenticular
Other internal astigmatism
Total

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10
Q

most common classification of astigmatism

A

corneal astigmatism

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11
Q

Classification of astigmatism where the crystalline lens has a problem

A

lenticular astigmatism

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12
Q

Total Astigmism =

A

Internal + External

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13
Q

In corneal astigmatism cornea is seldom ________

A

spherical

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14
Q

True corneal astigmatism must include ___________
this neutralizes 10-14% of anterior astigmatism

A

posterior

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15
Q

optical description

A

corneal astigmaism

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16
Q

anatomical description

A

corneal toricity

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17
Q

Lenticular astigmatism is due to differences in

A
  • meridional refractive powers
  • refractive indices of lens layers
  • surface flattening rates of lens layers
  • symmetry of lens
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18
Q

Lenticular astigmatism is caused by tilting and/or decentration of __________________

A

crystalline lens

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19
Q

Internal astigmatism

The retina may be:

Study These Flashcards
A
  • toric in shape
  • tilted
  • decentred
20
Q

Total Astigmatism is the combination of

Study These Flashcards
A

● Corneal astigmatism
● Lenticular astigmatism
● Other internal astigmatism

21
Q

Astigmatism where the principal meridian is 90deg apart

22
Q

Astigmatism that is often acquired or decondary to trauma or disease

regular or irregular

23
Q

CAUSES IRREGULAR ASTIGMATISM

Study These Flashcards
A
  • Disease/Degeneration
  • Diabetes
  • Distortion of refractive surface
  • Irregularities of refractive indices
  • Scar
24
Q

Types of regular astigmatism

Study These Flashcards
A

WTR
ATR
Oblique

25
Ocular astigmatism in which the refractive power of the vertical (or near vertical) meridian is the greatest
WTR
26
Correcting minus cylinder axis 0-30°, 150-180°
WTR
27
Ocular astigmatism in which the refractive power of the horizontal (or near horizontal) meridian is the greatest
ATR
28
Correcting minus cylinder axis 60-120°
ATR
29
Astigmatism in which the two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism
Oblique
30
Correcting minus cylinder axis 31-59°, 121-149°
oblique
31
ASTIGMATISM REFRACTIVE CYLINDER Classification by magnitude ≤ 0.75 D Cyl
Negligible
32
ASTIGMATISM REFRACTIVE CYLINDER Classification by magnitude 1.00 to 1.50 D Cyl
Low
33
ASTIGMATISM REFRACTIVE CYLINDER Classification by magnitude 1.75 to 2.50 D Cyl
Moderate
34
ASTIGMATISM REFRACTIVE CYLINDER Classification by magnitude >2.50 D Cyl
High
35
ASTIGMATISM INCIDENCE SHIFT WITH AGE ATR -> OBLIQUE/WTR
INFANTS
36
ASTIGMATISM INCIDENCE SHIFT WITH AGE ADULTS
WTR -> ATR/OBLIQUE
37
ASTIGMATISM COMPONENTS
TOTAL CORNEAL LENTICULAR/INTERNAL
38
JAVAL’S RULE
If ATR Total = 1.25 (Ac) - 0.50 If WTR Total = 1.25 (Ac) + 0.50
39
Grosvenor et al rule for WTR
T = Ac + 0.50
40
Grosvenor et al rule for ATR
T = AC - 0.50
41
Measurement of astigmatism front surface curvature
keratometer photokeratoscopy computer-assisted corneal mapping system
42
Measurement of astigmatism back surface curvature
purkinje images
43
\ defined as the astigmatic refractive error that is present when a contact lens is placed upon the cornea to correct the existing ametropia.
residual astigmatism
44
PHYSIOLOGICAL RESIDUAL ASTIGMATISM
● Unneutralized corneal astigmatism ● Posterior corneal astigmatism ● Lenticular astigmatism ● Tilted crystalline lens ● Refractive index anomalies ● Oblique aberrations ● Misalignment of component
45
INDUCED RESIDUAL ASTIGMATISM
● Tilted and/or decentred CL ● Toricity and/or bitoricity of CL ● CL mislocation (rotation) ● Warpage and/or flexure of CL

Contact Lens (MIDTERM | 2ND SEM) (6 decks)

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