L4 Flashcards
(40 cards)
What is astigmatic ametropia?
A refractive error where the eye has different curvatures in different meridians, causing light to focus at different points and resulting in a distorted image.
How is the curvature of a rugby ball used as an analogy for astigmatism?
A rugby ball has a steepest and a flattest curvature at 90° to each other, like an astigmatic eye, resulting in different refraction in different meridians.
How does a spherical ball differ from a rugby ball in terms of curvature?
A spherical ball has equal curvatures in all meridians, so light is refracted equally in all directions.
What is the magnitude of astigmatism?
It is defined as the difference between the maximum and minimum meridional powers of the eye.
What are the principal meridians in astigmatism?
The two meridians of greatest and least refractive power, always perpendicular to each other.
How does an astigmatic eye focus a point object?
It forms two focal lines at different distances, not a single point.
What does the term ‘astigmatism’ mean?
From Greek: ‘A’ = not, ‘Stigma’ = point; refers to formation of line or elliptical images rather than point images.
What is the circle of least confusion (COLC)?
The blur circle formed midway between the two focal lines in an astigmatic eye.
What is the typical pattern of astigmatism with age?
Astigmatism tends to be ‘with the rule’ (vertical meridian steeper) in youth and ‘against the rule’ (horizontal meridian steeper) in older age.
What are the two main anatomical sources of ocular astigmatism?
Corneal astigmatism (measured by keratometer) and lenticular astigmatism (from the lens surfaces).
How can lenticular astigmatism be estimated?
By subtracting the corneal astigmatism (keratometer reading) from the total astigmatism found during refraction.
What is the astigmatic pencil?
The refracted pencil of light in an astigmatic eye, consisting of focal lines and the circle of least confusion.
How are the focal lines oriented relative to the power meridian?
The orientation of the focal lines is always at right angles to the power meridian.
What does the distance between the two focal lines indicate?
It indicates the size (magnitude) of the astigmatism.
What are the main classifications of astigmatism?
Compound hyperopic, simple hyperopic, mixed, simple myopic, compound myopic.
What is compound hyperopic astigmatism?
Both focal lines are behind the retina; both correcting lens powers are positive.
What is simple hyperopic astigmatism?
One focal line is on the retina, the other is behind; one correcting lens power is plano, the other positive.
What is mixed astigmatism?
The retina lies between the two focal lines; one correcting lens power is positive, the other negative.
What is simple myopic astigmatism?
One focal line is on the retina, the other is in front; one correcting lens power is plano, the other negative.
What is compound myopic astigmatism?
Both focal lines are in front of the retina; both correcting lens powers are negative.
How does astigmatism affect vision?
Vision is blurred for distance and near if one or both focal lines are in front of the retina; the type and axis of astigmatism also affect vision quality.
How does accommodation affect astigmatism?
Accommodation moves all features of the refracted pencil by the same amount, but the distance between the focal lines remains unchanged.
What factors affect unaided vision in astigmatism?
Amount of astigmatism, type of astigmatism, and axis direction (oblique astigmatism is worse).
When does optimum vision occur in astigmatism?
When the circle of least confusion is placed on the retina.
