Podcast
Questions and Answers
What difference in diopters does the patient's corneal astigmatism indicate?
What difference in diopters does the patient's corneal astigmatism indicate?
Which instrument is NOT mentioned for the assessment of corneal shape?
Which instrument is NOT mentioned for the assessment of corneal shape?
Which of the following factors does NOT affect the choice of contact lens diameter according to the visible iris diameter (HVID)?
Which of the following factors does NOT affect the choice of contact lens diameter according to the visible iris diameter (HVID)?
What is the expected effect of high lid tension on rigid gas permeable (RGP) lenses?
What is the expected effect of high lid tension on rigid gas permeable (RGP) lenses?
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When correcting the corneal astigmatism, which of the following will be assessed?
When correcting the corneal astigmatism, which of the following will be assessed?
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What ocular parameter is used for selecting the total diameter of soft contact lenses?
What ocular parameter is used for selecting the total diameter of soft contact lenses?
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Which parameter is primarily assessed to determine the fitting requirements for rigid gas permeable (RGP) lenses?
Which parameter is primarily assessed to determine the fitting requirements for rigid gas permeable (RGP) lenses?
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What is the normal photopic pupil diameter range?
What is the normal photopic pupil diameter range?
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Which measurement is essential for assessing the expected values of the cornea in healthy individuals?
Which measurement is essential for assessing the expected values of the cornea in healthy individuals?
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When the pupil diameter exceeds 6 mm, which contact lens type is typically prioritized?
When the pupil diameter exceeds 6 mm, which contact lens type is typically prioritized?
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What is the purpose of recording baseline measurements in optometry?
What is the purpose of recording baseline measurements in optometry?
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What influence does palpebral aperture size have on contact lens selection?
What influence does palpebral aperture size have on contact lens selection?
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How does the visible iris diameter assist in contact lens fitting?
How does the visible iris diameter assist in contact lens fitting?
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What is the primary purpose of keratometry in contact lens fitting?
What is the primary purpose of keratometry in contact lens fitting?
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In keratometry, what do the readings of the power meridian represent?
In keratometry, what do the readings of the power meridian represent?
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Which of the following tools can be utilized for keratometry?
Which of the following tools can be utilized for keratometry?
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What should be recorded when measuring the corneal curvature?
What should be recorded when measuring the corneal curvature?
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What is indicated by irregular and distorted mires during keratometry?
What is indicated by irregular and distorted mires during keratometry?
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What affects the image size of Y' in the keratometry process?
What affects the image size of Y' in the keratometry process?
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Which statement about the recording of keratometry results is true?
Which statement about the recording of keratometry results is true?
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What is the appropriate action during keratometry when mires are not clear?
What is the appropriate action during keratometry when mires are not clear?
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What is the normal blink rate in healthy individuals?
What is the normal blink rate in healthy individuals?
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What effect does too high lid tension have on contact lens wear?
What effect does too high lid tension have on contact lens wear?
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How does a low blink rate influence contact lenses?
How does a low blink rate influence contact lenses?
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Which of the following statements about lid angles is correct?
Which of the following statements about lid angles is correct?
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What is the recommended lens choice for individuals with reduced blink rates?
What is the recommended lens choice for individuals with reduced blink rates?
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Which factor does NOT affect the handling of contact lenses?
Which factor does NOT affect the handling of contact lenses?
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What is a likely consequence of a high blink rate when wearing contact lenses?
What is a likely consequence of a high blink rate when wearing contact lenses?
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What complication can arise from using a contact lens when having too low lid tension?
What complication can arise from using a contact lens when having too low lid tension?
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What are the two main components that contribute to total astigmatism?
What are the two main components that contribute to total astigmatism?
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How is the power meridian expressed in terms of optical power?
How is the power meridian expressed in terms of optical power?
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If the total astigmatism (TA) is less than internal astigmatism (IA), which type of contact lens would be indicated?
If the total astigmatism (TA) is less than internal astigmatism (IA), which type of contact lens would be indicated?
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What does a small radius indicate about the surface curvature of the cornea?
What does a small radius indicate about the surface curvature of the cornea?
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In the context of astigmatism, what does TA = IA + CA signify?
In the context of astigmatism, what does TA = IA + CA signify?
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What does a difference of 0.05mm in curvature correspond to in terms of dioptric power?
What does a difference of 0.05mm in curvature correspond to in terms of dioptric power?
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What is the primary effect of correcting corneal astigmatism in a patient who has it?
What is the primary effect of correcting corneal astigmatism in a patient who has it?
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When measuring corneal astigmatism, how is it calculated using keratometry readings?
When measuring corneal astigmatism, how is it calculated using keratometry readings?
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What does a difference in central corneal radii (∆r) indicate?
What does a difference in central corneal radii (∆r) indicate?
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Which measurement indicates a steep cornea?
Which measurement indicates a steep cornea?
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When evaluating astigmatism, if the principal meridians are perpendicular, what type of astigmatism is present?
When evaluating astigmatism, if the principal meridians are perpendicular, what type of astigmatism is present?
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What should be administered before measuring in cases of unstable mire images?
What should be administered before measuring in cases of unstable mire images?
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What is indicated if ∆r is less than 0.1 mm?
What is indicated if ∆r is less than 0.1 mm?
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In which age range does astigmatism generally tend to be with-the-rule?
In which age range does astigmatism generally tend to be with-the-rule?
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What does a reading of r < 7.0 mm suggest?
What does a reading of r < 7.0 mm suggest?
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Which characteristic describes oblique astigmatism?
Which characteristic describes oblique astigmatism?
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What is indicated if the difference in corneal radii (∆r) exceeds 0.3 mm?
What is indicated if the difference in corneal radii (∆r) exceeds 0.3 mm?
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What does a measurement area on the cornea typically encompass?
What does a measurement area on the cornea typically encompass?
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Study Notes
Optometry Skills - Baseline Measurements & Keratometry
- Baseline measurements are crucial for contact lens fitting, encompassing corneal diameter (HVID), vertical palpebral aperture (VPA), and pupil size.
- Keratometry assesses the curvature of the cornea, providing essential data for lens selection.
- Ocular parameters like pupil diameter, visible iris diameter, and palpebral aperture are critical for contact lens fitting.
- Pupil diameter: normal values range from 3-4mm (photopic) to 4-5mm (mesopic); pupils larger than 6mm often necessitate SCLs as a first choice.
- Palpebral aperture (PA): small PA (<9mm) necessitates smaller contact lens diameters, while large PA (>11mm) requires larger diameters.
- SCL TD = HVID + 2mm; RGP TD = HVID - 2mm.
- Corneoscleral profile (CSP) describes the topographical shape between the cornea and sclera.
- The key to successful contact lens fitting is a thorough understanding of ocular parameters. Central corneal radii are measured on an area of approximately 3mm on the cornea; these radii are orthogonal—one flat and one steep—to each other.
- Measurement errors in keratometry can arise from distorted mires, unstable mire coincidence, incomplete mires, or very small radii, often attributable to corneal scars, dry eyes, unstable tear film, patient positioning difficulties, and lack of full eye opening.
- To ensure accurate measurements, re-center mires and adjust focus during procedures.
- Interpreting corneal radii involves determining if the cornea is flat (r > 8.2mm), normal (7.4 < r < 8.2 mm), steep (r < 7.4 mm), or indicative of keratoconus (r < 7.0 mm). A difference between the flat and steep meridians (Δr = rsteep - rflat) signifies corneal astigmatism.
- A spherical cornea has a difference of 0.0mm between the radii; small to moderate astigmatism is indicated by Δr values of 0.1 to 0.3 mm, while higher astigmatism typically presents Δr values greater than 0.3mm.
- To determine the specific astigmatism, the axis and power of the meridian with the "least" optical power are needed.
- Corneal astigmatism (CA) is obtained by evaluating keratometry findings combined with refractive measurements.
- Total astigmatism (TA) represents the sum of corneal and internal astigmatism. Internal astigmatism (IA) is influenced by the posterior corneal surface, the shape of the crystalline lens, and variations in refractive indices within the optical components.
- Astigmatism tendencies often change during development, with young individuals frequently displaying with-the-rule astigmatism.
- When using keratometers, ensure accurate readings by focusing and re-centering mires and recording conditions of the mires (regular or irregular). Also consider using the provided scales within workbooks, and the rule of thumb, that every 0.05 mm difference in curvature = 0.25 diopters
- Different types of keratometers, such as Zeiss-Bombe, Zeiss CL-150, Rodenstock C-Mess, Topcon OM-1, and Topcon OM-4, are employed to accurately measure corneal curvature.
Ocular Parameters and Contact Lens Fitting
- Blink rate, lid tension, and lid angles significantly influence contact lens fitting.
- High blink rates are often indicative of dry eye, while low blink rates suggest stable tear film.
- Lid tension, classified as high, normal, or low, affects lens positioning and handling.
- Lid angles influence the choice, handling, and positioning of contact lens types.
Additional Information
- Corneal topography employs corneal topographers (e.g., Orbscan and Pentacam) to assess corneal shape.
- Colour-coded maps generated by corneal topographers provide detailed information on corneal radius, power, and elevation.
- Key measurements include visible iris diameter, pupil diameter and palpebral aperture for contact lens diameter specification.
- Identifying problems during keratometry, and understanding the causes associated.
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Description
Test your knowledge on the essential baseline measurements and keratometry relevant to contact lens fitting. This quiz covers critical ocular parameters, including pupil size and palpebral aperture, and their implications for lens selection. Dive into the details that are vital for successful fitting and understanding of the corneoscleral profile.
