Low Vision

When vision cannot be restored by glasses, contact lenses, medicine or surgery, it can still be used better. Low vision rehabilitation starts from what the patient wants to do, most often to read, and uses magnification, field expansion, light, contrast and training to make it possible. The optics are those of Part One, applied to eyes that can no longer resolve fine detail.

Low vision is not the same as legal blindness. In the United States, legal blindness is a best-corrected acuity of 20/200 or worse in the better eye, or a visual field no wider than 20° in its widest diameter. The World Health Organization grades visual impairment by presenting acuity in the better eye: moderate (worse than 6/18), severe (worse than 6/60) and blindness (worse than 3/60, or a field of 10° or less in radius around fixation). Rehabilitation helps long before either line is reached: the American Academy of Ophthalmology recommends that patients with acuity worse than 20/40, a scotoma, field loss or reduced contrast sensitivity be told about vision rehabilitation.

#Assessing low vision

• Distance acuity with a logMAR chart, brought as close as needed (a patient who reads the 6/60 line at 1 m has an acuity of 1/60). A precise figure matters: it predicts the magnification needed.
• Near acuity with continuous text in M units, recording both the smallest print read and the critical print size, below which reading slows.
• Contrast sensitivity (Chapter 11): low contrast sensitivity predicts difficulty with mobility and recognising faces better than acuity does, adds to acuity in predicting reading, and means more magnification and more light will be needed.
• Visual fields: a central scotoma, as in macular degeneration, forces the patient to use a preferred retinal locus outside it, with poorer acuity; a hemianopia or a constricted field makes reading and mobility difficult in other ways.
• Goals: reading mail and medicine labels, recognising faces, watching television, cooking, getting around, perhaps driving.

#Refraction in low vision

A careful refraction is the first low vision aid. It is done in a trial frame, so the patient can look around, with retinoscopy if the answers are unreliable; if the reflex is dim, the examiner moves closer (radical retinoscopy, at 20 cm or less) and allows for the larger working-distance lens. Small lens changes are invisible to an eye that sees only large letters, so the steps are scaled to the acuity: the just-noticeable difference in dioptres is about the Snellen denominator (in feet) divided by 100. At 20/200 it is 2 D, so lenses of ±1.00 D are shown, and a ±1.00 D cross cylinder replaces the usual ±0.25 D. A highly myopic patient may read well without glasses, holding the page at their far point: a natural high add.

#Four kinds of magnification

• Relative size: making the object bigger, as with large print. Doubling the print size doubles the retinal image.
• Relative distance: bringing the object closer. Moving from 40 cm to 10 cm magnifies 4 times, but the eye then needs 10 D of accommodation or a +10 D add to focus.
• Angular: an optical device enlarges the angle the object subtends, as magnifiers and telescopes do (Chapter 9).
• Electronic: a camera and screen enlarge the image, and can enhance its contrast and reverse its polarity.

Magnifications combined multiply: large print at twice the size, read from half the distance, gives 4 times. A magnification only means something against a reference distance. Relative to a page at 40 cm, a page at 5 cm (a +20 D lens) is magnified 8 times; by the magnifier convention, which compares with 25 cm, the same lens is labelled 5× (or 6×, by P/4 + 1, as some makers do). Comparing devices by their dioptres, or by the equivalent viewing distance (1/P), avoids the confusion.

#How much magnification?

Kestenbaum’s rule gives a starting point: the reading add, in dioptres, needed to read 1M print (newsprint) is the reciprocal of the best decimal distance acuity:

So 20/200 (6/60) suggests +10 D, with the page at 10 cm. That lets the patient read 1M print at threshold; reading fluently needs print two to three times larger than threshold (the acuity reserve, Chapter 11), so for sustained reading the add is often increased, for example doubled. Reduced contrast sensitivity also calls for more. Alternatively, start from the near acuity: a patient who just reads 4M print at 40 cm, and wants to read 1M print with a twofold reserve, needs 8 times the magnification, which brings the page to 5 cm, a +20 D add.

#Devices for near

• High-add spectacles (microscopic spectacles) leave both hands free and give the widest field, but the page must be held at the focal distance, very close for strong adds. Binocular reading needs base-in prism to help the eyes converge, about the add plus 2 prism dioptres in each eye (8 Δ for a +6 D add), and works up to about +10 to +12 D if the two eyes see similarly; stronger adds are used for one eye.
• Hand magnifiers magnify by about P/4 (Chapter 9). The field of view is largest with the eye close to the lens; a stronger lens is smaller and gives a smaller field. With the page at its focal point the light leaves parallel, so the patient looks through their distance correction. Viewed through a reading add instead, the magnifier and the add form a two-lens system of equivalent power $F_e=F_1+F_2-d{F}_1{F}_2$ (Chapter 5): held against the spectacles the two powers add, but held at its own focal length from them the combination is no stronger than the magnifier alone, and further away it is weaker still.
• Stand magnifiers rest on the page at a fixed distance, which helps patients with a tremor. The page usually lies inside the lens’s focal length, so the lens forms a virtual image at a finite distance, not at infinity, and the patient views it through their reading add or with accommodation.
• Electronic magnifiers (video magnifiers, tablets) give high magnification at a comfortable working distance, with adjustable contrast and reversed polarity (light text on dark), which many patients prefer. Text-to-speech and screen readers replace reading altogether.

#Devices for distance

Telescopes magnify distant objects: for spotting a street sign or a bus number, watching a play, or seeing the board in class (Chapter 9). A Galilean telescope (plus objective, minus eyepiece) is short, light and gives an upright image, but its field narrows at higher magnification; a Keplerian telescope (two plus lenses) needs a prism to erect its image and is longer, but gives a wider field and better optics at higher powers. Every telescope narrows the field of view and exaggerates movement, which is why they are used for spotting rather than walking. Small telescopes mounted high in a spectacle lens (bioptics) are allowed for driving in most US states, usually with restrictions: daylight only, training, and minimum acuities through the carrier lens and through the telescope. Focused for distance, a telescope multiplies the accommodation a near object needs by roughly the square of its magnification: through a 3× telescope, something 1 m away needs about 8 D rather than 1 D. An implantable miniature telescope, placed in one eye in place of the crystalline lens, magnifies about 2 to 3 times for selected patients with end-stage macular degeneration, while the other eye keeps the peripheral field.

A telescope can be focused for near with a plus lens over its objective, a reading cap: the working distance is the focal length of the cap, and the magnification is that of the telescope multiplied by that of the cap (P/4). A 3× telescope with a +4 D cap works at 25 cm and magnifies 3 times; it gives a longer working distance than spectacles of the same magnification, at the cost of a small field.

#Field loss

A homonymous hemianopia removes half the field of each eye, and patients bump into things and miss objects on the blind side. Training to scan toward the blind side helps. In left-to-right scripts a right hemianopia slows reading more than a left one, because the words to come fall in the blind field. Peripheral prisms (Peli prisms) expand the field: strong Fresnel prism segments, of 40 to 57 Δ, are placed above and below the pupil on one spectacle lens, base toward the blind side. In peripheral vision they shift images from the blind field into the seeing field, by about 20 to 30°, warning the patient of obstacles while leaving central vision single.

A constricted field (advanced glaucoma, retinitis pigmentosa) can be widened with a reverse telescope, used backwards to minify: the field grows by the magnification, but acuity falls by the same factor, so it suits orientation rather than detail. A central scotoma is managed by eccentric viewing: training the patient to place what they want to see on the preferred retinal locus, with magnification to make up for its lower acuity. Prisms that shift images toward the preferred locus have little evidence of benefit.

#Beyond optics

• Light: a task lamp close to the page, directed from the side of the better eye and shielded from the eyes, is often the single most useful aid.
• Contrast: bold-lined paper, felt-tip pens, high-contrast markings on steps and dials, a reading slit (typoscope) that masks surrounding text and glare.
• Glare control: tinted lenses, side shields and hats for patients troubled by glare, as in cataract and retinitis pigmentosa.
• Other senses and services: audiobooks, screen readers, voice assistants, talking watches; orientation and mobility training with a long cane or guide dog; help at school or work; and screening for depression and falls.

#Recap

• US legal blindness: best-corrected 20/200 or worse in the better eye, or a field of 20° or less. Offer rehabilitation from 20/40, or any scotoma, field loss or contrast loss.
• Refract in a trial frame with steps of about the just-noticeable difference: Snellen denominator ÷ 100 (20/200: 2 D, shown as ±1.00 D).
• Magnification: relative size × relative distance × angular × electronic.
• Kestenbaum: add = 1 / decimal acuity for 1M print (20/200 → +10 D at 10 cm); double it for fluent reading.
• High-add spectacles: binocular up to about +10 to +12 D, with base-in prism of add + 2 Δ per eye.
• Magnifier $M=P/4$; field largest with the eye close. Stand magnifiers form a virtual image at a finite distance: use the reading add.
• Galilean: short, upright, narrow field. Keplerian: erecting prism, wider field. Reading cap: working distance = cap focal length. A telescope multiplies near accommodation by about M².
• Hemianopia: peripheral prisms of 40–57 Δ, base toward the blind side. Constricted fields: reverse telescope.
• Light and contrast matter as much as optics. Ask about Charles Bonnet hallucinations.

#Test yourself

#Further reading

• American Academy of Ophthalmology, Basic and Clinical Science Course, Section 3: Clinical Optics and Vision Rehabilitation, chapter on vision rehabilitation; and the Vision Rehabilitation Preferred Practice Pattern.
• R. L. Brilliant (ed.), Essentials of Low Vision Practice. Butterworth-Heinemann.
• E. Peli, Field expansion for homonymous hemianopia by optically induced peripheral exotropia. Optometry and Vision Science 77 (2000) 453–464.