Optometres & Keratometry Flashcards
(24 cards)
What is the main difference between objective and subjective methods for measuring refractive error?
Objective methods: Do not require patient input (e.g., keratometry, autorefractors).
Subjective methods: Require patient feedback (e.g., standard refraction tests).
primary purpose of keratometry in optometry?
To measure corneal curvature and estimate corneal astigmatism. It is also used to monitor contact lens fit and detect corneal surface distortions.
What is the central limitation of keratometry?
It only measures the central 2-4mm of the cornea and assumes uniformity across this area, which may not be accurate for the entire corneal surface.
key clinical uses of keratometry?
Diagnose and monitor corneal ectasia.
Calculate intraocular lens power.
Select contact lens base curves.
Detect and monitor corneal distortion, including gas permeable lens flexure.
Define an optometer in optometry.
optometer is an instrument used to measure refractive error objectively, with both manual and automated types.
What is the main principle behind a Badal optometer?
The Badal system uses a lens and target system to maintain a constant image size, ensuring a linear relationship between ametropia and target movement.
What is the main advantage of using the Badal optometer compared to simpler optometers?
It provides a more accurate, linear scale and constant target size, improving the assessment of refractive error.
: What is instrument myopia and why does it occur?
Instrument myopia is a temporary increase in near focusing power due to the proximity of the target in optometric instruments. It happens because the instrument’s target stimulates the eye to accommodate more than necessary for normal viewing.
How can instrument myopia be avoided or mitigated?
Techniques such as fogging the target or using open-field autorefractors can help reduce the accommodation response and correct for instrument myopia.
What is the principle behind Scheiner’s principle for optometers?
It states that the angular separation of retinal images depends on the ametropia. In a focused eye, two images overlap; in an unfocused eye, they separate, indicating the refractive error.
What is the primary clinical use of photorefraction?
Photorefraction is mainly used for screening refractive errors in infants and young children who cannot cooperate with traditional refraction methods.
how does photorefraction work??
A small camera and light source illuminate the patient’s face, and the retinal image is analysed to detect blur circles or ellipses. The size of these zones correlates with the degree of ametropia.
What is the advantage of using a phoropter in refraction testing?
Provides faster and more comfortable refraction.
Allows for large cross-cylinders to be used.
Ideal for patients with high refractive errors, strabismus, or low vision.
Offers flexibility for head positioning.
what are computerised phoropters and what are their advantages??
computerised phoropters are linked to autorefractors and focimeters, they provide more accurate and efficient refraction and the automatic alignment of cylinders makes the process quicker
How does a trial frame compare to a phoropter?
A trial frame is simpler, and it is especially useful in children or patients with specific needs (e.g., high refractive error, large strabismus). It may take longer but offers flexibility in lens choices.
difference between manual optometers and automated optometers?
Manual optometers require physical adjustment and patient feedback, while automated optometers (such as autorefractors) use sensors to measure refractive error objectively without patient involvement.
What is the principle of the Topcon RM-8900 autorefractor?
It uses the time difference for peak intensity and the Scheiner disc principle to measure the refractive error based on the separation of retinal images.
What does keratometry help differentiate between?
It helps differentiate between axial and refractive anisometropia.
How does a phoropter help in patients with hearing problems?
Phoropters are easier to use in patients with hearing impairments because they can quickly adjust to visual testing without relying heavily on verbal instructions.
advantage of using Risley prisms in a phoropter??
Risley prisms are used to measure heterophorias and fusional reserves which are imp for assessing binocular vision and eye coordination
phoropter???
a device used to test refractive error by using various lenses that can be rotated in front of the patient’s eyes to determine the correct prescription
what is the application of photorefraction in newborns??
photorefraction is used to screen newborns for significant refractive errors and conditions like amblyopia since they cannot communicate visual discomfort
In what cases is a manual optometer typically used?
A manual optometer is generally used when the patient is capable of providing feedback and when a more detailed, subjective measurement is necessary.
What is the main limitation of optometers?
Their non-linear scales and potential issues with target size variation, which can lead to inaccurate measurements due to accommodation.
