CPO CPO Ophthalmic Optics and Dispensing 2 β Questions and Answers
Question 1: A patient with anisometropia of 2.50D complains of vertical double vision when reading through their new bifocals. The prescription is OD: -1.00 SPH, OS: -3.50 SPH, with a reading depth of 10mm below the optical center. To correct the vertical imbalance, what is the appropriate slab-off prism to order?
- 2.50Ξ Base Up on the right lens (OD)
- 1.50Ξ Base Down on the left lens (OS)
- 2.50Ξ Base Up on the left lens (OS) (Correct answer)
- 1.50Ξ Base Up on the right lens (OD)
Correct answer: 2.50Ξ Base Up on the left lens (OS)
To calculate the required prism, first determine the vertical imbalance at the reading level using Prentice's Rule (Prism = Power x Decentration in cm). OD: |-1.00| x 1.0 cm = 1.0Ξ Base Down. OS: |-3.50| x 1.0 cm = 3.5Ξ Base Down. The total imbalance is 3.5 - 1.0 = 2.5Ξ. A conventional slab-off is ground with a Base Up prism on the most minus (or least plus) lens to counteract the Base Down effect. Therefore, 2.5Ξ Base Up prism is applied to the left lens (OS).
Question 2: A patient's prescription is +11.50 D sphere. The refraction was performed at a vertex distance of 10 mm, but the patient's chosen frame will sit at 15 mm from the cornea. What compensated power should be ordered to ensure the patient experiences the intended correction?
- +12.12 D
- +10.94 D (Correct answer)
- +11.50 D (no change needed)
- +11.00 D
Correct answer: +10.94 D
When a plus lens is moved farther away from the eye, its effective power increases. To compensate, the ordered power must be reduced. The formula for compensated power (Fc) is Fc = F / (1 + dF), where F is the original power and 'd' is the change in vertex distance in meters. Here, d = 0.005m (15mm - 10mm). Fc = 11.50 / (1 + 0.005 * 11.50) = 11.50 / 1.0575 β +10.87 D. The closest answer is +10.94 D. A common mistake is to increase the power, which would be done if the lens were moved closer.
Question 3: A patient presents with new progressive lenses and complains that they must tilt their head down excessively to see clearly in the distance. The prescription and lens verification are accurate. Which of the following is the MOST likely fitting error?
- The pantoscopic tilt is too great.
- The fitting height is too low.
- The pupillary distance (PD) is too narrow.
- The fitting height is too high. (Correct answer)
Correct answer: The fitting height is too high.
If the fitting height is set too high, the patient's pupils will be positioned in the upper part of the progressive corridor or near the near-vision zone when looking straight ahead. To access the distance portion of the lens, which is located at the top, they must unnaturally tilt their head down to look through the fitting cross area.
Question 4: Which of the following scenarios is the primary indication for prescribing iseikonic lenses?
- A patient with high astigmatism causing meridional distortion.
- A patient with significant anisometropia causing a perceived difference in image size between the two eyes. (Correct answer)
- A patient with presbyopia who cannot adapt to progressive addition lenses.
- A patient with vertical heterophoria requiring slab-off prism.
Correct answer: A patient with significant anisometropia causing a perceived difference in image size between the two eyes.
Iseikonic lenses are specifically designed to address aniseikonia, which is a condition where a patient perceives a difference in image size between their two eyes. This is most commonly caused by significant anisometropia (a large difference in prescription power). The lenses are manipulated by adjusting factors like base curve, thickness, and vertex distance to change magnification properties without altering the refractive power, thus equalizing the perceived image sizes.
Question 5: A patient wearing a new pair of high-wrap sunglasses with a prescription of -6.00 DS OU complains of peripheral distortion and a 'fishbowl' effect. The original refraction was performed in a standard phoropter. What position-of-wear parameter is most likely contributing to these complaints?
- Incorrect monocular pupillary distances
- Excessive retroscopic tilt
- Uncompensated wrap angle (Correct answer)
- Insufficient B measurement of the frame
Correct answer: Uncompensated wrap angle
High-wrap frames induce significant changes in how the patient views through the lens compared to the flat plane of a phoropter. The wrap angle introduces unwanted oblique astigmatism and prismatic effects, which can cause the sensation of peripheral distortion or a 'fishbowl' effect. Modern digital lenses can be compensated for the wrap angle, pantoscopic tilt, and vertex distance to provide clear vision. Failing to account for the wrap angle is a common cause of such complaints in high-wrap frames.
Question 6: A reverse slab-off is being ordered for a patient with anisometropia. On which lens is the base-down prism effect created?
- The lens with the most minus power in the vertical meridian.
- The lens with the highest cylinder power.
- The lens with the least minus or most plus power in the vertical meridian. (Correct answer)
- Either lens, as it is determined by the lab.
Correct answer: The lens with the least minus or most plus power in the vertical meridian.
A conventional slab-off creates a base-up prism effect on the most minus or least plus lens. A reverse slab-off achieves the same vertical correction by creating a base-down prism effect on the opposite lensβthe one with the least minus or most plus power in the vertical meridian. This is often done with molded lenses and can be a more cosmetically appealing option.
CPO CPO Ophthalmic Optics and Dispensing 2
A patient with anisometropia of 2.50D complains of vertical double vision when reading through their new bifocals.
The prescription is OD: -1.00 SPH, OS: -3.50 SPH, with a reading depth of 10mm below the optical center.
To correct the vertical imbalance, what is the appropriate slab-off prism to order?