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Macular Function Tests
Source: Kanski's Clinical Ophthalmology - A Systematic Approach, 10th Edition
The macula is responsible for central, high-resolution, and colour vision. Tests of macular function assess visual acuity, fixation, contrast sensitivity, colour vision, and photoreceptor integrity.
1. Visual Acuity (VA)
- Distance VA using Snellen or LogMAR (ETDRS) charts is the most fundamental macular function test, as the fovea drives fine-detail resolution.
- Near VA is a sensitive indicator for macular disease. Charts are held at a comfortable reading distance with appropriate corrections. The smallest legible type is recorded per eye.
- LogMAR charts are preferred in clinical trials due to greater accuracy.
2. Amsler Grid
The most practical bedside test of central macular function.
Purpose: Evaluates the central 20° of visual field centred on fixation. Detects metamorphopsia (distortion) and scotomas. Used for screening and home monitoring of macular disease (especially macular neovascularization).
Charts (7 total):
| Chart | Description | Use |
|---|---|---|
| 1 | White grid on black background; 400 squares, each 1° | Standard |
| 2 | Similar + diagonal lines | Aids fixation in central scotoma |
| 3 | Red squares on black | Detects subtle colour scotomas; toxic maculopathy, optic neuropathy |
| 4 | Random dots only | Distinguishes scotomas from metamorphopsia |
| 5 | Horizontal lines | Detects metamorphopsia along specific meridians; reading difficulty |
| 6 | White background, closer central lines | More detailed meridional evaluation |
| 7 | Fine central grid (0.5° squares) | More sensitive central testing |
Technique:
- Pupils should NOT be dilated; avoid slit-lamp examination beforehand (to prevent photostress effect)
- Chart held at ~33 cm, well illuminated
- One eye covered; patient fixes on central dot
- Asks about: distortion/waviness of lines, blank spots, ability to see all four corners and sides
- Macular disease - wavy/distorted lines (metamorphopsia)
- Optic neuropathy - missing or faint lines, no distortion
- Missing corner/border - raises possibility of glaucoma or retinitis pigmentosa
3. Photostress Test
A bedside test that differentiates macular disease from optic nerve disease.
Principle: Bleaches visual pigment with bright light, creating a temporary scotoma. Recovery time depends on photoreceptor ability to re-synthesize visual pigment. Prolonged in macular disease because the RPE (retinal pigment epithelium) is involved in pigment regeneration.
Technique:
- Record best-corrected distance VA
- Patient fixates on a pen torch or indirect ophthalmoscope light held ~3 cm away for ~10 seconds
- Photostress Recovery Time (PSRT) = time taken to read any 3 letters of the pre-test VA line
- Repeat on the fellow (normal) eye and compare
Interpretation:
- Normal PSRT: 15-30 seconds
- Prolonged PSRT (sometimes >50 seconds) = macular disease (e.g., mild cystoid macular oedema, central serous retinopathy)
- Normal PSRT = optic neuropathy (the RPE is intact)
Clinical use: Useful when ophthalmoscopy is equivocal; differentiates macular from optic nerve visual loss.
4. Contrast Sensitivity
Principle: Measures the ability to distinguish an object from its background - different from high-contrast VA. Can be reduced while standard VA is preserved (e.g., amblyopia, optic neuropathy, cataracts, higher-order aberrations).
Tests:
- Pelli-Robson Chart - viewed at 1 metre; rows of equal-sized letters with decreasing contrast (0.15 log units per group of 3 letters). Patient reads until lowest-contrast triplet is reached.
- Sinusoidal gratings - patient views increasingly lower-contrast gratings.
- SPARCS (Spaeth-Richman) - computer-based; ~5-10 min per eye; measures both central and peripheral contrast sensitivity; suitable for illiterate patients.
Use: Useful for objectively demonstrating functional deficit in patients with good VA but subjective visual symptoms (especially in low illumination).
5. Colour Vision Testing
Relevance to macular function: Acquired macular disease tends to produce blue-yellow (tritan) defects; optic nerve lesions produce red-green defects.
Tests:
| Test | Description | Detects |
|---|---|---|
| Ishihara | 16 plates + 1 test plate; dot matrix patterns concealing numbers | Congenital protan and deuteran (red-green) defects; also assesses optic nerve function |
| City University | 10 plates; central colour + 4 peripheral colours; patient picks closest match | All types of acquired and congenital defects |
| Hardy-Rand-Rittler (HRR) | Similar to Ishihara | All 3 congenital colour defects (protan, deutan, tritan) |
| Farnsworth-Munsell 100-Hue | 85 colour caps arranged in sequence | Sensitive; congenital + acquired defects; identifies axis of colour deficiency |
| Farnsworth D-15 | Abbreviated version | Screening; identifies type of defect |
6. Microperimetry
Purpose: Measures retinal sensitivity and fixation stability in macular disease. Allows exact correlation between fundus pathology and functional abnormality. More sensitive than standard automated perimetry (SAP).
Instrument: MAIA perimeter - table-top, uses a line-scanning laser ophthalmoscope (SLO) with 850 nm illumination.
Technique:
- 20 minutes mesopic dark adaptation
- Fundus tracking via SLO
- Goldmann size 3 stimuli projected onto central 9° of fundus
- 4-2 staircase thresholding; 200 ms stimulus duration
- Normal retinal sensitivity: 18 dB
- Results: colour-coded sensitivity maps + fixation loss indices
Uses:
- Assessing therapeutic interventions on the macula
- Early glaucoma with subtle change near fixation
- Monitoring retinal dystrophies
7. Electrophysiological Tests
Electroretinogram (ERG)
- Records the electrical response of the retina to light stimulation
- Full-field ERG - global retinal function
- Pattern ERG - predominantly macular/ganglion cell function
- Multifocal ERG (mfERG) - simultaneously records responses from a distribution of retinal areas across the macular region while patient fixates; used to evaluate macular function topographically
Electro-oculogram (EOG)
- Measures the standing potential of the eye (RPE function)
- Arden ratio (light peak:dark trough) - normally >1.85
- Abnormal in Best vitelliform macular dystrophy (even in carriers with no fundus changes)
Visual Evoked Potential (VEP)
- Records cortical responses to visual stimuli
- Used to monitor macular pathway function and investigate non-organic visual loss
- Flash VEP or pattern VEP (checkerboard reversal)
Summary Table: Macular vs Optic Nerve Disease
| Test | Macular Disease | Optic Nerve Disease |
|---|---|---|
| Amsler grid | Wavy/distorted lines (metamorphopsia) | Missing/faint lines (no distortion) |
| Photostress test | Prolonged PSRT (>50 sec) | Normal PSRT |
| Colour vision | Blue-yellow defect | Red-green defect |
| Light brightness test | Usually normal (early/moderate disease) | Reduced brightness (afferent defect) |
| EOG | Abnormal in RPE disease (Best disease) | Normal |
Kanski's Clinical Ophthalmology - A Systematic Approach, 10th Edition, Chapter 1 (Ophthalmic Assessment)
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