Low Vision and Optical Aids Flashcards

A patient with advanced macular degeneration has a best-corrected visual acuity of 20/400 and requires a stand magnifier for near tasks. The stand magnifier is rated at +20D. If the patient holds the reading material at the focal length of the lens, what is the effective magnification relative to the standard 40 cm reference distance?

Answer: 5×

Magnification of a simple lens relative to the standard 40 cm (0.4 m) reference distance is calculated as M = D/4, where D is the dioptric power. For a +20D lens: M = 20/4 = 5×. The 20× figure is the dioptric power, not the magnification.

A low vision patient using a spectacle-mounted telescope for distance tasks reports significant reduction in field of view. The technician notes the telescope is a 4× Galilean type. Which optical modification would MOST effectively improve the field of view while maintaining near-equivalent magnification?

Answer: Switch to a 4× Keplerian (astronomical) telescope

Keplerian (astronomical) telescopes have an erecting prism system and a positive eyepiece, which provides a wider field of view compared to Galilean telescopes of the same magnification. The Galilean design has an inherently narrow field at higher powers. A lower-power Galilean with a reading cap changes the use case rather than improving the distance field. Widening the objective alone does not sufficiently address field of view limitations in Galilean designs.

During eccentric viewing training for a patient with a central scotoma, the technician must determine the patient's preferred retinal locus (PRL). Which test is MOST appropriate for precisely mapping the scotoma boundaries and identifying the PRL location?

Answer: Scanning laser ophthalmoscopy (SLO) microperimetry

Microperimetry performed with scanning laser ophthalmoscopy (SLO) allows simultaneous fundus imaging and precise threshold perimetry, enabling exact mapping of scotoma boundaries and real-time identification of the preferred retinal locus relative to fundus landmarks. Standard Amsler grids and tangent screens lack the resolution and fundus-correlation capability needed for accurate PRL localization. Confrontation testing is far too gross for this purpose.

A patient with retinitis pigmentosa has a 10-degree remaining central field and 20/60 central acuity. They report difficulty with mobility in dim lighting. Which combination of low vision interventions is MOST appropriate for this patient's profile?

Answer: Field expansion reverse telescope plus NoIR ultraviolet-blocking amber filter

Retinitis pigmentosa causes constricted peripheral fields and poor dark adaptation. A reverse telescope (minification) expands the functional visual field, while NoIR amber filters block UV and short-wavelength light that exacerbates photophobia and glare sensitivity in rod-cone dystrophies. Prismatic relocation addresses only reading posture. Yellow absorptive lenses are less effective than amber for the scotopic sensitivity loss pattern in RP. Night-vision devices are rarely prescribed as standard ophthalmic low vision aids.

A COT is performing a low vision evaluation and measures the patient's near point acuity as 1.0 M print at 4 cm. The patient's goal is to read standard newsprint (0.4 M equivalent). Using Kestenbaum's rule as the STARTING point for a near add prescription, what is the calculated add, and what adjustment should be made for the reading distance goal?

Answer: Add of +8D; adjust to +20D to achieve comfortable 5 cm working distance for newsprint size

Kestenbaum's rule estimates the starting add as the reciprocal of the distance visual acuity in Snellen feet (or 1/acuity in meters). Here, near acuity is 1.0 M at 4 cm, and the goal is 0.4 M print — requiring a magnification factor of 1.0/0.4 = 2.5×. The initial Kestenbaum add based on distance acuity would need upward adjustment. To read 0.4 M print at a practical working distance (~5 cm), the required add is approximately +20D (working distance = 100/20 = 5 cm). The +8D figure is a common Kestenbaum starting estimate for moderate impairment, adjusted upward given the goal acuity level.

A low vision patient with hemianopic visual field loss following a right occipital stroke is being fitted with a sector prism (Fresnel press-on prism). For a right homonymous hemianopia, where should the prism be positioned on the spectacle lens, and in which direction should the base be oriented?

Answer: Base-right prism on the right half of both lenses, apex pointing toward the blind field

For right homonymous hemianopia, the blind field is to the right. Sector prisms are placed on the side of the blind field (right half of each lens). The base is oriented toward the right (the blind side), which displaces the image of objects in the right blind field toward the intact left visual field, alerting the patient to stimuli in the hemianopic region. Base-in on the left half would not address the right field loss. Full-lens coverage creates disabling diplopia.