L12 Flashcards
(19 cards)
What is the principle behind the fan and block subjective refraction technique?
It allows correction of astigmatism by determining the principal meridians and powers using subjective feedback, without the need for retinoscopy.
What is spherical ametropia?
A refractive error where the eye has equal curvature in all meridians, resulting in a single focal point that may not fall on the retina (e.g., myopia or hyperopia).
What is astigmatic ametropia?
A refractive error where the eye has different curvatures in different meridians, leading to two principal meridians and two focal lines at different distances, causing a distorted image.
What is the astigmatic pencil?
The refracted pencil of light in an astigmatic eye, made up of two focal lines and the circle of least confusion.
What does the distance between the two focal lines indicate?
It indicates the magnitude of astigmatism.
What are the five main classifications of astigmatism?
Compound hyperopic, simple hyperopic, mixed, simple myopic, compound myopic.
What is the purpose of the fan and block method?
To determine the principal meridians and principal powers for correcting astigmatism, especially useful after a difficult retinoscopy or in patients with large astigmatism.
Why is it important to control accommodation during subjective refraction?
To ensure accurate measurement of refractive error and avoid over- or under-correction.
What is the Maddox V used for in the fan and block technique?
To locate the principal meridians by rotating it towards the clearest and darkest lines on the fan chart.
How is the axis refined using the Maddox V?
By rotating the V towards the blurry side until both sides are equally clear or equally blurred, indicating the principal meridian.
How are the blocks used to refine cylinder power in the fan and block method?
Minus cylinder is added at the axis found; as the power is refined, the blocks perpendicular to the axis become clearer until all blocks appear equally clear.
What does it mean if, after adding minus cylinder, the blocks perpendicular to the arrow direction are darker and clearer?
The cylinder power needs to be reduced.
What is the endpoint of the cylinder phase in the fan and block method?
When the blocks appear equally clear, indicating that astigmatism has been corrected.
What is the purpose of the -0.25DC check test in the fan and block routine?
To confirm that both focal lines were in front of the retina; if blocks reverse in clarity, the interval of Sturm has not collapsed and the cylinder power needs rechecking.
What is the purpose of the +0.50DS check test in the fan and block routine?
To ensure that the posterior focal line is just in front of the retina; both blocks should blur equally.
What is the purpose of the +1.00DS check test in the fan and block routine?
To ensure accommodation is relaxed; vision should blur to approximately 6/18.
How is the fan and block routine modified after retinoscopy?
Start with the retinoscopy result in the trial frame (after removing working distance), refine sphere and axis as usual, use the fan chart and Maddox V for axis determination, and use blocks for cylinder power refinement.
What is the role of subjective feedback in the fan and block method?
The patient’s perception of clarity or blur in the fan and block charts guides the refinement of axis and cylinder power.
What is the endpoint of the fan and block subjective refraction?
A point focus just in front of the retina, confirmed by check tests and subjective clarity of all blocks.
