Low-vision assessment and magnification

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Visual impairment and functional assessment

WHO distance-impairment categories use presenting distance acuity in the better eye: mild impairment is worse than 6/12 to at least 6/18; moderate is worse than 6/18 to at least 6/60; severe is worse than 6/60 to at least 3/60; blindness is worse than 3/60. Near impairment is presenting near acuity worse than N6 or M0.8 at 40 cm. These population categories should not be conflated with every country's legal certification criteria.

Rehabilitation is based on the patient's goals, not solely a numerical category. Determine whether the priority is reading medication labels, sustained text, mobility, recognising faces or a work task. Evaluate refraction and treatable pathology, then assess near performance and an appropriate aid. Certification and rehabilitation can be relevant even when acuity alone understates field loss.

Source: WHO vision impairment fact sheet.

Assessment of Residual Visual Function

Standard clinical Snellen testing is inadequate in low vision due to unequal letter counts per line, variable letter difficulty, irregular progression ratios, and non-standardized letter crowding.

1. The LogMAR (ETDRS) Protocol

The Early Treatment Diabetic Retinopathy Study (ETDRS) LogMAR chart overcomes these limitations:

• Logarithmic Step Size: Line progression follows a constant geometric progression ratio of $0.1\text{ LogMAR}$ ($10^{0.1} \approx 1.2589\times$ change in letter height per line).
• Standardized Letter Allocation: Exactly 5 Sloan letters per line; each letter read correctly confers $0.02\text{ LogMAR}$ credit.
• Proportional Spacing: Inter-letter spacing equals the width of one letter on that line; inter-line spacing equals the height of the lower line.
• Testing Distance Flexibility: Standard testing is conducted at $4\text{ m}$; in low vision, testing moves to $2\text{ m}$ (add $+0.30\text{ LogMAR}$) or $1\text{ m}$ (add $+0.60\text{ LogMAR}$) to prevent ceiling/floor truncation.

2. Contrast Sensitivity: The Pelli-Robson Chart

High-contrast acuity measures only the high spatial frequency limit of vision. Patients with age-related macular degeneration or glaucoma often complain of severe visual disability despite near-normal high-contrast acuity ($6/12$).

• The Pelli-Robson chart presents triplets of Sloan letters of uniform large size ($4.9 \times 4.9\text{ cm}$, subtending $2.8^\circ$ at $1\text{ m}$, isolating low-to-intermediate spatial frequencies of $\approx 1\text{ cycle/degree}$).
• Contrast declines across triplets in steps of $0.15\text{ log units}$.
• Normal adult contrast sensitivity is $\ge 1.65 - 1.95\text{ log units}$. A score $\lt 1.50\text{ log units}$ correlates with reading fatigue, impaired facial recognition, and elevated fall risk.

3. Near Vision Acuity & M-Units

Near acuity can be recorded in M-units, noting distance and chart; Jaeger labels are poorly standardised:

• $1.0\text{M}$ Letter Height: Subtends $5\text{ minutes of arc}$ at a distance of $1\text{ meter}$ ($1.454\text{ mm}$ capital letter height), equivalent to standard $8\text{-point}$ newsprint.
• Snellen Near Fraction: Recorded as testing distance (meters) over M-size (e.g., $0.4 / 1.0\text{M} = 20/50$ Snellen equivalent at $40\text{ cm}$).

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Principles & Forms of Magnification

Magnification increases the angle subtended by the retinal image, stimulating healthy extra-foveal parafoveal photoreceptors.

The Four Fundamental Mechanisms

1. Relative Size Magnification ($M_S$): The physical size of the object is increased while the viewing distance remains constant: $M_S = \frac{\text{New Object Size}}{\text{Original Object Size}}$ Examples: Large-print books, $24\text{-point}$ display fonts, jumbo keyboards.
2. Relative Distance Magnification ($M_D$): The object is moved closer to the eye without altering its physical dimensions: $M_D = \frac{\text{Original Working Distance}}{\text{New Working Distance}}$ Example: Moving a reading card from $40\text{ cm}$ to $10\text{ cm}$ yields $M_D = 40 / 10 = 4\times$ magnification.
3. Angular Magnification ($M_A$): An afocal optical instrument (telescope) enlarges the visual angle subtended at the eye without changing physical distance or size: $M_A = \frac{\tan \theta'}{\tan \theta} = -\frac{F_{\text{eyepiece}}}{F_{\text{objective}}}$
4. Electronic (Electro-Optical) Magnification ($M_E$): Closed-circuit television (CCTV) systems and portable digital magnifiers combine relative size magnification and electronic sensor processing, providing $2\times$ to $60\times$ magnification with adjustable contrast polarity (e.g., white letters on black background to reduce intraocular glare).

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