Keratometry Flashcards
What is the purpose of keratometry?
Measure the power and astigmatism of the anterior corneal surface, the most refractive element of the eye
How much power does the cornea have?
Between 40 and 45D
60 to 75% of the eye’s total refractive power
Why is keratometry important?
Determine site of ocular astigmatism (corneal or non corneal)
When fitting CL
When ocular refraction is difficult
Determine is ametropia is axial or refractive
Examine corneal health
How do we correct astigmatism using keratometry recordings?
Ex: 42.00 M 45 / 43.75 M 135
Correct with -1.75 x 45
-1.75 x 45 AM 42.00
How can we tell if a patient;s astigmatism is not corneal?
Keratometry reading shows spherical reading but refraction shows astigmatism
Describe with-the-rule astigmatism.
The vertical meridian (+/- 20 degrees) has the highest dioptric power
This can be corrected by a minus cyl axis 180 (+/- 20 degrees)
Describe against-the-rule astigmatism.
The horizontal meridian (+/- 20 degrees) has the highest dioptric power
This can be corrected by a minus cyl axis 90 (+/- 20 degrees)
At what ages are WTR and ATR astigmatisms more common?
Below 45 WTR is more common
Above 45 ATR is more common
What is an oblique astigmatism?
Astigmatism that does not meet the definition of with or against the rule
Ex: maximum power along 35 degrees or corrected by a minus cyl axis 120 degrees is termed oblique astigmatism
Define irregular astigmatism? What are they caused by? How can they corrected?
Astigmatism where the principal meridia are not 90 degrees apart
Can occur due to corneal irregularities associated with refractive surgery
Cannot be corrected with spectacle, may be helped with rigid CL to provide smooth anterior surface
How can we calculate the ocular (total) astigmatism?
Ocular (total) astigmatism (refraction)= corneal astigmatism (keratometry) + non-corneal astigmatism
Remember to consider wtr and atr when adding
Describe Javal’s rule.
OA= 1.25CA + 0.50atr
For when we can’t perform a refraction
How can we estimate if a refractive error is axial vs refractive using keratometry?
Very high or low corneal power for refractive
Normal cornea power for axial
What is the quality of reflected mires for a normal healthy cornea?
Clear and regular
Describe the appearance of the reflected mires for astigmatism and for CL overwear and dry eye.
Oval rather then round
Blurred and irregular (corneal warping)
Appear clear immediately after patient blinks but go out of focus until the patient reblinks
