Low vision: visual impairment that cannot be corrected with standard spectacles, contact lenses, medical treatment or surgery, and that limits daily activities
ICD-11 categories by presenting acuity in the better eye (see Global Epidemiology of Blindness)

Mild: worse than 6/12. Moderate: worse than 6/18. Severe: worse than 6/60. Blindness: worse than 3/60, or a central field of less than 10° radius

Low vision rehabilitation aims to make the best use of remaining vision: optical aids, electronic aids, lighting and contrast, training, and support

## Assessment

The patient's goals: reading, faces, television, mobility, work
A careful refraction first: high adds and corrected astigmatism often help more than expected
Distance acuity (logMAR), near acuity in N-point or M units at a measured distance, contrast sensitivity, visual fields, glare sensitivity and the preferred retinal locus in central loss

## Magnification needed

Required magnification = acuity achieved ÷ acuity needed, at the same distance
Worked example: a patient reads N20 at 25 cm and wants to read newspaper print (N5). Magnification = 20 ÷ 5 = ×4. A magnifier giving ×4 has an equivalent power of 4 × 4 = +16D (M = F ÷ 4), with a working distance of 1 ÷ 16 = 6.25 cm
Kestenbaum's rule: the reading add needed to read 1M (about N8) print is the reciprocal of the distance Snellen fraction

Worked example: distance acuity 6/60 gives an add of 60 ÷ 6 = +10D. The working distance is 10 cm
A starting point only; increase it for smaller print or poor contrast

## Near magnifiers

Aid	Advantages	Disadvantages
High-add spectacles (microscopes)	Hands free, widest field for their power, cosmetically acceptable	Very short working distance (1 ÷ add). Binocular use needs base-in prisms to help convergence, up to about +10 to +12D
Hand magnifiers	Portable, cheap, flexible working distance, can be illuminated	Need a steady hand. The field of view shrinks as the eye moves away from the lens
Stand magnifiers	Fixed distance, good for tremor, arthritis and children, often illuminated	The page sits inside the focal length, so light leaving is divergent: the patient needs a reading add or accommodation. Awkward posture
Electronic (video magnifiers, tablets, phones)	High and adjustable magnification, contrast reversal (white on black), better posture, text-to-speech	Cost, power and training needs

## Telescopes

Telescopes magnify distant objects: television, signs, faces, the board in class (see Telescopes and Loupes)

Galilean: plus objective and minus eyepiece. Erect image, short and light, up to about ×4, but a small field
Keplerian: two plus lenses with erecting prisms. Higher magnification, a wider field and a brighter image, but longer and heavier

Forms: hand-held monocular (for spotting), spectacle mounted full-field, and bioptic (a small telescope mounted high in the carrier lens, used by glancing up)
Problems: a narrow field, a short depth of field, and magnified movement, so telescopes are used when still, not for walking
Telemicroscope (near telescope): a telescope with a reading cap. It gives a longer working distance than a magnifier of the same total magnification

Total magnification = telescope magnification × reading cap power ÷ 4
Worked example: a ×2 telescope with a +4D cap gives ×2 at a working distance of 25 cm. A simple +8D magnifier gives the same ×2 at only 12.5 cm

## Field expanders

Hemianopia: prisms with the base towards the blind field move images from the blind side into the seeing field

Peli prisms: high-power (about 40Δ) Fresnel segments above and below the pupil, so they work in primary gaze without causing central diplopia

Constricted fields (e.g. advanced glaucoma, retinitis pigmentosa): a reverse Galilean telescope (looking through the objective) minifies the scene, fitting more of it into the remaining field at the cost of acuity
Mirrors on spectacles, and scanning training, also help

## Non-optical aids and support

Lighting: task lighting directed onto the page; illuminance rises with the inverse square law as the lamp is brought closer (see Light and Physical Optics)
Contrast: bold-lined paper, felt pens, typoscopes (a black card slot that isolates a line of text), contrasting edges on steps
Glare: tinted filters (often yellow or amber), visors, side shields
Eccentric viewing training for central scotomas, using a preferred retinal locus
Large print, audio books, screen readers, mobility training and long canes, guide dogs
Emotional support, and registration for services and benefits where available

Charles Bonnet syndrome: vivid, formed visual hallucinations in people with visual loss and preserved insight. Reassurance that it is not a sign of mental illness is the main treatment
Low vision aids are most successful in motivated patients with specific, realistic goals. Age-related macular degeneration is the commonest indication in high-income countries; uncorrected refractive error and cataract are the commonest causes of impairment worldwide, and they need refraction and surgery, not low vision aids