Certified Ophthalmic Assistant Practice Exam

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In keratometry, if the power of the vertical meridian is greater than the power of the horizontal meridian, what type of astigmatism does the patient have?

With the rule astigmatism
Against the rule astigmatism

The correct answer is: With the rule astigmatism

In keratometry, a patient's astigmatism is classified based on the difference between the power of the vertical and horizontal meridians. When the power of the vertical meridian is greater than the power of the horizontal meridian, the patient has "with the rule astigmatism." This means that the cornea is steeper in the vertical meridian and flatter in the horizontal meridian. This is the most common type of astigmatism, accounting for up to 80% of all cases. It is also important to note that the power difference between the meridians must be significant enough to classify as "with the rule astigmatism." If the power difference is minimal, it may not be clinically significant. The other option, "against the rule astigmatism," refers to cases where the power of the horizontal meridian is greater than the vertical meridian. This type of astigmatism is less common, accounting for about 10-20% of cases. It is important to accurately identify and classify astigmatism in order to determine the best course of treatment for the patient.