Lecture 5- Astigmatism Flashcards
what is the definition of astigmatism
a condition of refraction in which rays emanating from a single luminous point are not focused at a single point by an optical system, but instead are focused as two line images at different distances from the system…generally at right angles to each other
Astigmatism is a ____ anomaly due to unequal refraction of the incident light by the dioptric system in different meridians.
refractive
what is regular astigmatism
meridians of max and min power are sep by an angle of 90 degrees; primary meridians are perpendicular
what is irregular astigmatism
meridians of max and min power are sep by an angle other than 90 degrees. primary meridians are not perpendicular; usually secondary to injury or corneal disease
what are causes of irregular astigmatism
- ocular trauma
- surgery
- contact lens overwear
- keratoconus
- corneal disease
- corneal dystrophies
- dislocated lens
How do you classify astigmatism
- Classification with respect to the contributing ocular component
- classification by orientation
- classification with respect to the refractive error
how do you classify astigmatism with respect to the contributing ocular component
- anterior cornea
- posterior cornea
- crystalline lens
____ cornea is most frequently produced by the toricity of the anterior corneal surface. Air/tear film interface causes the greatest change in diopteric effect if the meridians do not have the same radii of curvature.
Anterior
____ the rule astigmatism deals with corneal meridian that has the least refractive power is horizontal (+/- 30 degrees)
With
____ the rule astigmatism deals with corneal meridian that has the least refractive power is vertical (+/-30) degrees
against
What are the classifications with respect to the refractive error
- simple astigmatism
- compound astigmatism
- mixed astigmatism
In regards to simple astigmatism, if myopic; one principal meridian is ____ and the other principal meridian is _____; if hyeropic; one principal meridian is ____ and the other principal meridian is ____
myopic; emmetropic; hyperopic; emmetropic
If compound astigmatism and myopic: two principal meridians of an eye are both ____. If hyperopic with compound astigmatism, two principal meridians of an eye are both _____
myopic; hyperopic
in mixed astigmatism; one principal meridian is ____ and the other principal meridian is _____
hyperopic; myopic
what is the prevalence of astigmatism
- native american indians have a lot of with the rule astigmatism.
- marfans syndrome (lens tilted)
- albinism
majority of eyes reach their adult level of astigmatism by age ___ to ___ years old and remains relatively stable until about age ____; after this age, some ind’s show a gradual shift toward against the rule astigmatism due to lenticular changes
3; 5; 60
what are symptoms of uncorrected astigmatism
- blur at distance and near
- “ghost” images next to to the primary image
- bright lights are more annoying due to increased glare inside the eye.
- asthenopia and tearing if the patient’s occupation recquires detailed visual work.
which type of astigmatism would show the greatest affect on VA? what about the least affect?
most affect: oblique
least: WTR
the level of uncorrected VA may have more to do with the location of the ___ ____ for the two meridians and how close they are to the retina
focal points
How can you determine if they have diplopia or uncorrected astigmatism when a pt reports they see double
have the patient cover one eye; does the diplopia resolve?
what do patients with uncorrected astigmatism complain about
photophobia and glare
unaided VA; what would you expect for uncorrected astigmatism?
blur at both distance and near
what is normal diff in meridian K’s for an average patient
K steep: 44
K flat: 43.50
Pts with ___ have slightly more trouble adapting to changes in the cylinder orientation/amount. Changes in this can cause changes in the percieved distance, size, shape, and orientation of objects.
cylinder
