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CONTACT LENS 2 > ASTIGMATISM > Flashcards

ASTIGMATISM Flashcards

1
Q

defined as a refractive condition

A

Astigmatism

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2
Q

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

A

Astigmatism

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3
Q

astigmatism s caused by

A

Irregular shape of the cornea

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4
Q

internal cause of astigmatism are

A

aqueous humor, back index of the cornea, crystalline lens, vitreous humor

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5
Q

2 Meridians of the eye

A

Power meridian
Axis meridian

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6
Q

meridian of greatest optical power

A

Power meridian

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7
Q

meridian of least optical power

A

Axis meridian

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8
Q

2 Meridian descriptions

A

*Refractive power (Diopters)
* Radius of curvature (mm)

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9
Q

Astigmatism cuases

A

*Meridional differences
*Shape of posterior pole

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10
Q

astigmatism causes meridional differences in

A

*curvature and/or flattening rates of cornea and crystalline lens
*refractive index of optical components

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11
Q

Structural classification of astigmatism

A

*Corneal
*Lenticular
*Other internal astigmatism
*Total

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12
Q

cornea is seldom

A

spherical

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13
Q

it implies anterior

A

Corneal astigmatism

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14
Q

true corneal astigmatism must include

A

posterior

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15
Q

posterior neutralizes

A

10-14% of anterior astimgatism

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16
Q

corneal astigmatism is always in a

A

minus form

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17
Q

optical description

A

Corneal astigmatism

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18
Q

it is due to differences in
*meridional refractive powers
*refractive indices of lens layers
*surface flattening rates of lens layers
*symmetry of lens

A

Lenticular astigmatism

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18
Q

anatomical description

A

Corneal toricity

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19
Q

tilting and/or decentration of crystalline lens

A

Lenticular astigmatism

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20
Q

the posterior pole (retina) my be:

A

*toric in shape
*tilted
*decentered

21
Q

total astigmatism is the combination of

A

*corneal astigmatism
*lenticular astigmatism
*other internal astigmatism

22
Q

principal meridians that is 90deg apart

A

Regular astigmatism

23
Q

principal meridians that is not 90deg apart

A

Irregular astigmatism

24
Q

*more than 2 principal meridian
*no principal meridians
*often acquired like trauma or disease

A

Irregular astigmatism

25
Q

Irregular astigmatism causes

A

*distortion(s) of refractive surfaces
*irregularities in refractive indices
*disease/degeneration
*scars
*diabetes

26
Q

refractive power of the vertical meridian is the greatest

A

with-the-rule

27
Q

correcting of with-the-rule

A

minus cylinder axis 0-30deg, 150-180deg

28
Q

refractive power of horizontal meridian is the greatest

A

against-the-rule

29
Q

correcting of against-the-rule

A

minus cylinder axis 60-120 deg

30
Q

two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism

A

Oblique astigmatism

31
Q

correcting of oblique astigmatism

A

minus cylinder axis 31-59deg, 121-149deg

32
Q

Classification of astigmatism by magnitude

A

*negligible
*low
*moderate
*high

33
Q

≤ 0.75D cyl

A

Negligible

34
Q

1.00 - 1.50D cyl

A

Low

35
Q

1.75 - 2.50D cyl

A

Moderate

36
Q

against-the-rule to oblique/with-the-rule

A

Infants

37
Q

> 2.00D cyl

A

High

38
Q

with-the-rule to against-the-rule/oblique

A

Adults

39
Q

Astigmatism components

A

*total
*corneal
*lenticular/internal

40
Q

Astigmatism compoents relationship

A

Total = corneal + lneticular

41
Q

Javal’s rule (1890)

A

Total astigmatism = corneal astigmatism - 0.50

42
Q

measurement of astigmatism in front surface curvature

A

*keratometer
*photokeratoscopy
*computer-assisted corneal mapping system

43
Q

measurement of astigmatism in back surface curvature

A

*Purkinje images

44
Q

astigmatic refractive error that is present when a contact lens ids placed upon the cornea to correct the existing ametropia

A

Residual astigmatism

44
Q

Physiological residual astigmatism

A

*Unneturalized corneal astigmatism
*Posterior corneal astigmatism
*Lenticular astigmatism
*Tilted crystalline lens
*Refractive index anomalies
*Oblique aberrations
*Misalignment of components

45
Q

Residual astigmatism can create

A

tears or tear lens

45
Q

interpret as , ‘with spherical contact lens’ unless stated otherwise

A

Residual astigmatism

46
Q
A
47
Q

Induces residual astigmatism

A

*Tilted and/or decentered CL
*Toricity and/or bitoricity of CL
*CL mislocation (rotation)
*Warpage and/or flexture of CL

48
Q

Residual astigmatism formula

A

Residual = Total - Corneal

CONTACT LENS 2 (2 decks)

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