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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
*more than 2 principal meridian *no principal meridians *often acquired like trauma or disease
Irregular astigmatism
25
Irregular astigmatism causes
*distortion(s) of refractive surfaces *irregularities in refractive indices *disease/degeneration *scars *diabetes
26
refractive power of the vertical meridian is the greatest
with-the-rule
27
correcting of with-the-rule
minus cylinder axis 0-30deg, 150-180deg
28
refractive power of horizontal meridian is the greatest
against-the-rule
29
correcting of against-the-rule
minus cylinder axis 60-120 deg
30
two principal meridians lie somewhere between the axes defining either WTR or ATR astigmatism
Oblique astigmatism
31
correcting of oblique astigmatism
minus cylinder axis 31-59deg, 121-149deg
32
Classification of astigmatism by magnitude
*negligible *low *moderate *high
33
≤ 0.75D cyl
Negligible
34
1.00 - 1.50D cyl
Low
35
1.75 - 2.50D cyl
Moderate
36
against-the-rule to oblique/with-the-rule
Infants
37
>2.00D cyl
High
38
with-the-rule to against-the-rule/oblique
Adults
39
Astigmatism components
*total *corneal *lenticular/internal
40
Astigmatism compoents relationship
Total = corneal + lneticular
41
Javal's rule (1890)
Total astigmatism = corneal astigmatism - 0.50
42
measurement of astigmatism in front surface curvature
*keratometer *photokeratoscopy *computer-assisted corneal mapping system
43
measurement of astigmatism in back surface curvature
*Purkinje images
44
astigmatic refractive error that is present when a contact lens ids placed upon the cornea to correct the existing ametropia
Residual astigmatism
44
Physiological residual astigmatism
*Unneturalized corneal astigmatism *Posterior corneal astigmatism *Lenticular astigmatism *Tilted crystalline lens *Refractive index anomalies *Oblique aberrations *Misalignment of components
45
Residual astigmatism can create
tears or tear lens
45
interpret as , 'with spherical contact lens' unless stated otherwise
Residual astigmatism
46
47
Induces residual astigmatism
*Tilted and/or decentered CL *Toricity and/or bitoricity of CL *CL mislocation (rotation) *Warpage and/or flexture of CL
48
Residual astigmatism formula
Residual = Total - Corneal

CONTACT LENS 2 (2 decks)

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