Astigmatism - Week 6

Astigmatism definition

anomaly of refraction where the image of a point object is not a single point, but rather two focal lines that are generally mutually perpindicular

are refractive surfaces (lens and cornea) spherical?

no

how is refractive surfaces described

orientation and power of the steepest and flattest curves

principal meridians

point of maximum and minimum curvuture

interval of sturm

dioptric distance between the line foci

circle of least confusion

lies midway between the focal lines

spherical lenses move _____ to the retina but they cannot correct _______

circle of least confusion

asitgmatism

what lenses are used for correction in an individual meridian

cylindrical

name the 3 positions of focal lines relative to the retina

1 simple

2 compound

3 mixed astigmatism

what is simple astigmatism

one foci is on the retina (plano power) and one is in front or behind the retina

what is compound astigmatism

both foci are in front or behind the retina

what is mixed astigmatism

one foci is in front of the retina and one is behind

how to know compound myopic astigmatism is present on prescription?

sign of the sphere is “minus” regardless which sign (+ or -) is used to represent the cylinder

how to tell compound hypermetropic astigmatism is present from prescription?

sign of the sphere is “plus” regardless o which sign is used to represent the cylinder

how to tell if mixed astigmatism is present by looking at prescription

sign of the sphere changes when transcription is transposed

simple astigmatism is present if?

if the sphere is plano orbecomes plano when the prescription is transposed

simple myopic astigmatism present when?

cylinder is minus

simple hypermetropic astigmatism present when?

cylinder is plus

with rule astigmatism

the steepest meridian is vertical (between 70-100 degrees)

against rule astigmatism

the steepest meridian is horizontal (between 160-20 degrees)

oblique astigmatism

the steepest meridian is neither vertical or horizontal (21-69 degrees or 111-159 degrees)

with rule astigmatism definition

where the highest refractive power is vertical

how is with rule astigmatism corrected

-ve cyl with horizontal

+ve cyl with vertical

what is ‘against the rule’ astigmatism

where the highest refractive power is horizontal

how is against the rule astigmatism corrected

• -ve cyl with vertical

• +ve cyl with horizontal axis

what is oblique astigmatism

the highest refractive power is neither vertical or horizontal, both meridians are oblique

how is oblique astigmatism corrected

• -ve cyl

• + cyl

  both in an oblique orientation

low magnitude

below 1.00DC

moderate magnitude

1.00-2.00DC

high magnitude

greater than 2.00DC

how is astigmatism categorised

regular or irregular

what is regular astigmatism

where the meridians of maximum and minimum power are mutually perpendicular and the refractive error is usually fully corrected with a sphero-cylindrical combination

what is irregular astigmatism

the meridians of maximum and minimum power are seperated by an amount other than 90degrees

why does irregular astigmatism occur

when there is a pathological defect of the eye, e.g corneal scarring

how else is irregular astigmatism termed

irregular refraction

symptoms of astigmatism

• blurred vision

  • asthenopia and frontal headache

signs of astigmatism

• patient may screw up eye