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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
