Low Vision and Optical Aids Flashcards

A patient with a dense central scotoma secondary to Stargardt disease undergoes scanning laser ophthalmoscopy (SLO) for eccentric viewing training. The preferred retinal locus (PRL) is confirmed at the inferior retina, 5° below the fovea. In which direction should the patient be instructed to shift their gaze to optimally place a target of regard on the PRL?

Answer: Superiorly (upward)

The retinal image is inverted relative to the visual field. The inferior retina receives light from the superior visual field. Therefore, to project a target onto the inferior retina (where the PRL resides), the patient must direct their gaze upward (superiorly). This counterintuitive relationship — inferior PRL → look up — is a common source of error in eccentric viewing training.

A 58-year-old low vision patient requires a bioptic telescope for driving rehabilitation. The prescribing clinician is choosing between a Galilean and a Keplerian design at 6× magnification. Which statement BEST justifies the preferred design choice for this application?

Answer: Keplerian telescopes are preferred because they provide superior image resolution and a wider field of view at high magnifications

Keplerian (astronomical) telescopes are the design of choice for bioptic telescopes at magnifications above approximately 4×. They use a positive objective and positive ocular lens, producing an inverted intermediate image that is corrected by internal prisms (e.g., Porro or Schmidt). This yields superior image brightness, resolution, and field of view compared to Galilean designs. Galilean telescopes, while lighter and producing erect images without prisms, suffer from severely degraded field of view and image quality at magnifications above 3–4×, making them impractical for high-power bioptic use.

A 67-year-old patient with right homonymous hemianopia following a left parieto-occipital CVA is fitted with Peli sector prisms for visual field expansion. Where are these prisms positioned on the spectacle lens, and what is their primary optical mechanism?

Answer: In the superior and inferior peripheral zones of the right lens, deflecting peripheral images into the intact left visual field

Peli prisms (sector prisms) are placed in the upper and lower peripheral portions of the lens on the side of the hemianopia — in this case, the right lens. They are deliberately kept out of the central visual axis to avoid diplopia or image fusion conflicts. When the patient's gaze naturally moves toward the blind side, the sector prisms shift peripheral images optically into the intact visual field, expanding awareness of the hemianopic side without disrupting central binocular vision.

A low vision patient has best-corrected visual acuity of 20/400 and requires sufficient magnification to read standard newspaper print (approximately equivalent to 1M print at 40 cm, or ~20/80 acuity demand). Using Kestenbaum's rule to determine the starting reading add, which of the following calculations is correct, and what does this power correspond to in terms of working distance?

Answer: Add = 20D; working distance = 5 cm

Kestenbaum's rule states that the required add in diopters equals the reciprocal of the patient's distance visual acuity expressed as a decimal. For 20/400, the decimal acuity = 20/400 = 0.05. Therefore, starting add = 1/0.05 = 20D. A 20D lens has a focal length of 1/20 m = 5 cm, so the working distance is 5 cm. This represents the threshold power; in practice, a reserve factor (typically 2×) is applied, and the patient is positioned at or near the focal point for maximum benefit. This is distinct from simply calculating magnification for newsprint — Kestenbaum gives the add needed to bring the patient's resolution to 1M print at threshold.

A 78-year-old patient with end-stage glaucoma retains 20/40 central acuity but only 8° of remaining visual field. A CCTV (closed-circuit television) magnification system is being considered for near reading tasks. Which of the following is the MOST clinically significant contraindication to relying primarily on CCTV for this patient?

Answer: The large display area of a CCTV requires extensive scanning, which is highly inefficient with severely constricted tubular fields

In patients with severely constricted visual fields (tubular vision), a CCTV's large monitor is paradoxically difficult to use effectively. The patient can only perceive a small window of the screen at any moment and must perform extensive, laborious scanning across the display to track text lines — an exhausting task with an 8° field. Optical aids with shorter working distances (e.g., a high-add spectacle or stand magnifier) may allow more efficient use of the remaining field by reducing the physical extent of the reading area. The patient's 20/40 acuity also means that minimal magnification is needed, further reducing the benefit of a CCTV.

During a low vision evaluation, a patient demonstrates Pelli-Robson contrast sensitivity of 0.75 log units (severely impaired) with Snellen acuity of 20/60. The patient reports difficulty reading even with a +8D add at adequate distance. Which intervention addresses the PRIMARY limiting factor most directly?

Answer: Positioning a high-intensity directional lamp (e.g., halogen or warm LED) at 45° from the shoulder, aimed at the reading material

When contrast sensitivity is the primary limiting factor — not acuity — doubling magnification does not solve the problem; the letters get bigger but remain low-contrast. The highest-yield first-line intervention is optimizing illumination: a high-intensity directional lamp positioned at 45° from behind the patient's shoulder maximizes the luminance and contrast of the reading material. Increased illumination directly improves contrast sensitivity function in many patients with media opacities or retinal conditions. While reverse-polarity CCTV is beneficial, it is not universally optimal and represents a secondary tool; directional illumination is universally applicable and often produces immediate, dramatic improvement in functional reading performance.