Macular Function Tests

1. Amsler Grid

1. Pupils not dilated; performed before slit-lamp examination (to avoid photostress effect)
2. Appropriate reading correction worn; chart held ~33 cm in good illumination
3. One eye covered at a time
4. Patient fixates the central dot and reports any distortion, waviness, blurring, or missing areas
5. Patient outlines abnormal areas on the grid with a pencil

• Metamorphopsia (wavy/distorted lines) → macular disease (e.g., AMD, epiretinal membrane, central serous retinopathy)
• Scotoma (missing area) → macular hole, choroidal neovascularisation
• Patients at risk of macular neovascularisation (MNV) should use an Amsler grid at home for regular self-monitoring

Fig. Amsler grid chart — patient fixates the central dot (Kanski's Clinical Ophthalmology)

Fig. Amsler grid with red central dot (The Wills Eye Manual)

2. Photostress Test (Macular Dazzle Test)

• Maculopathy with equivocal ophthalmoscopic signs (e.g., mild cystoid macular oedema, central serous retinopathy)
• Differentiating macular disease from optic neuropathy as the cause of visual loss

1. Measure best-corrected distance visual acuity (BCVA) first
2. Patient fixates on the light of a pen torch or indirect ophthalmoscope held ~3 cm from the eye for ~10 seconds
3. Start timing immediately
4. Photostress Recovery Time (PSRT) = time taken to read any 3 letters on the pre-test acuity line
5. Repeat on the fellow (normal) eye for comparison

PSRT is prolonged in macular lesions but not in optic neuropathy — this is the key clinical distinguisher.

3. Light Brightness Comparison Test

• A bright light is shone into each eye alternately
• Patient rates the brightness in the affected eye relative to the normal eye (scale 1–5)
• In optic neuropathy, the light appears less bright in the affected eye
• Purely macular lesions cause minimal brightness asymmetry

4. Contrast Sensitivity Testing

• Assessed using the Pelli-Robson chart (rows of letters of decreasing contrast) or sinusoidal grating patterns
• More sensitive than standard Snellen acuity for detecting subtle macular or optic nerve dysfunction
• Reduced in macular disease, glaucoma, optic neuritis, and media opacity

5. Colour Vision Testing

• Ishihara plates: screen for red-green deficiency; mainly reflect macular function (as noted in Localization in Clinical Neurology)
• Farnsworth-Munsell 100 Hue and D-15: quantify the axis and severity of dyschromatopsia
• Acquired dyschromatopsia (especially red-green or blue-yellow) occurs in macular dystrophies, toxic maculopathy, and optic neuropathy
• Amsler Chart 3 (red squares) specifically targets colour scotomas in the macular region

6. Potential Acuity Meter (PAM) / Laser Interferometry

• Projects a Snellen chart or grating directly onto the retina through a clear portion of the lens
• Useful when cataract surgery is being considered in an eye with possible posterior segment disease
• Limitation: PAM and laser interferometry overestimate visual potential in macular holes, pigment epithelial detachments, and amblyopia
• Near vision testing may be the most accurate method of evaluating macular function when the cataract is not too dense

7. Multifocal Electroretinogram (mfERG)

• Records retinal responses simultaneously from multiple locations across the macular region while the patient fixates a central target
• Generates a topographic map of retinal sensitivity across the central ~20–25°
• Detects focal areas of macular dysfunction not visible clinically
• Used in monitoring drug toxicity (e.g., hydroxychloroquine), macular dystrophies, and unexplained central visual loss

Summary Table

Low Vision Aids in Ophthalmology

Definition & WHO Classification

Principles of Low Vision Aids

1. Magnification — enlarging the retinal image
2. Illumination enhancement — improving contrast and reducing glare
3. Field expansion — widening the usable visual field
4. Non-optical support — adaptive devices and environmental modification

Types of Magnification

Classification of Low Vision Aids

A. Optical LVAs

1. Spectacle-Mounted (Near) Magnifiers

• High-plus reading spectacles (e.g., +4D to +40D)
• Work on the principle that a higher plus lens brings the focal point closer, effectively magnifying the image
• Both hands are free — ideal for sustained reading tasks
• Must be held at the correct (short) focal distance
• High powers cause reduced field of view and require very close working distance
• Bifocal/prismatic half-eye spectacles: incorporate base-in prisms to compensate for convergence demand at very short reading distances (especially for binocular use)

2. Hand Magnifiers

• Convex lenses mounted in a frame, held by the patient
• Portable, flexible, easy to use — best for spot tasks (price tags, menus)
• Working distance more flexible than spectacle magnifiers
• Require steady hand; can cause fatigue
• Available in powers from ~+4D to +60D
• Illuminated versions available

3. Stand Magnifiers

• Lens mounted on a fixed stand resting on the reading material
• Object-to-lens distance fixed (within the focal length)
• Produce a virtual, magnified, erect image at a finite distance — used with slight plus reading addition
• Hands-free; ideal for tremor, arthritis
• Available in illuminated versions
• Working distance is fixed, limiting use for some tasks

4. Telescopes (Distance LVAs)

• Based on Galilean (concave eyepiece + convex objective) or Keplerian (two convex lenses + erecting prism) designs
• Galilean telescopes: compact, lightweight, erect image without prism; lower magnification (up to ~4×)
• Keplerian telescopes: higher magnification, wider field, but longer and heavier
• Used for distance tasks: watching TV, reading boards, recognising faces
• Bioptic telescopes: mounted in the superior portion of spectacle lenses; patient views through the carrier lens most of the time, tilts head to use the telescope for specific distance tasks
• Reduced field of view; not suitable for mobility

5. Loupe

• Small, single-lens or compound magnifier used close to the eye (like a jeweller's eyeglass)
• High magnification with relatively large field
• Monocular; used for detailed near work

6. Absorptive/Tinted Lenses (Optical Filters)

• Reduce glare and enhance contrast
• Amber/yellow filters improve contrast in low-light conditions
• Useful in conditions like albinism, aniridia, rod dystrophies, and macular degeneration
• NoIR filters, Corning CPF filters: block short-wavelength light; reduce photophobia and improve contrast

B. Electronic / Electro-Optical LVAs

1. Closed-Circuit Television (CCTV) / Video Magnifiers

• Camera projects magnified image on a monitor
• Magnification up to 60× or more
• Adjustable contrast (positive/negative), brightness, and colour
• Large field of view; no need to hold anything steady
• Bulky; not portable (desktop units); expensive
• Portable/handheld video magnifiers now available (e.g., tablet-sized or pocket-sized)
• Ideal for sustained reading tasks

2. Screen Readers / Computer-Based Systems

• Text-to-speech software (e.g., JAWS, NVDA)
• Screen magnification software (e.g., ZoomText)
• Useful for those who need to use computers for work

3. Head-Mounted Displays (HMDs)

• Camera captures scene; displays magnified image on screens within the headset
• Wearable; allows both near and distance magnification
• Examples: eSight, OrCam, NuEyes
• Increasingly sophisticated with AI-based text reading

C. Non-Optical LVAs

D. Lighting and Environmental Modifications

• Adequate, directed illumination is critical — many patients benefit from a flexible-arm reading lamp positioned over the shoulder
• Avoid glare from windows; use anti-glare screens
• Contrast enhancement in home environment (e.g., stair edges, light switches)

Selection of Appropriate LVA

Eccentric Viewing Training

Low Vision Assessment Process

1. History — Tasks the patient needs/wants to perform; motivation
2. Visual acuity at near and distance (logMAR charts preferred)
3. Contrast sensitivity assessment
4. Visual field testing (confrontation or Humphrey)
5. Refraction — ensure optimal refractive correction first
6. Trial of aids — patient tries various aids in clinic
7. Prescription and training — patient trained in use at home/work
8. Referral — to social services, RNIB (or equivalent), vocational rehabilitation, orientation & mobility training

Common Conditions Requiring LVAs

• Age-related macular degeneration (AMD) — most common in elderly
• Diabetic maculopathy
• Glaucoma (advanced)
• Stargardt disease / other macular dystrophies
• Optic atrophy
• Albinism / aniridia
• Retinitis pigmentosa (field expansion aids particularly useful)
• Amblyopia (in children)

Summary