How to assess central vision loss?

Assessment of Central Vision Loss

Central vision loss should be assessed using best-corrected visual acuity testing with ETDRS charts as the gold standard, combined with central visual field evaluation using either macular microperimetry for precise scotoma mapping or Amsler grid testing for screening purposes. 1

Visual Acuity Testing

Begin with distance visual acuity measurement using the Early Treatment Diabetic Retinopathy Study (ETDRS) chart, which is the gold standard for central vision assessment. 2, 3

• Document visual acuity with the patient's current correction at distance as the first priority to establish a critical baseline 2
• Record the power of any current correction being worn, as this information is crucial for accurate assessment 2
• For patients with acuity less than 20/100, extend the measurement range by using portable test charts at closer distances to avoid imprecise measurements 2
• Test both monocularly and binocularly, as binocular testing may reveal functional differences 2
• ETDRS charts provide significantly better accuracy than Snellen charts, particularly in patients with poor vision (<20/200), showing approximately 10 letters (0.2 logMAR) better detection 4

Important caveat: Snellen charts systematically underestimate visual function compared to ETDRS charts, with the greatest discrepancy occurring in patients with exudative AMD and those with vision worse than 20/200. 4

Central Visual Field Assessment

Gold Standard: Macular Microperimetry

Fundus-related macular microperimetry (microperimetry) is the most accurate method for detecting both central scotomas and fixation patterns in patients with central vision loss. 1

• Eye-tracking technology ensures stimuli are presented to the correct retinal position, overcoming the limitations of traditional automated perimetry in patients with eccentric fixation 1
• Microperimetry accurately determines both fixation location (foveal vs. eccentric) and fixation stability 1
• This technology prevents the common error where central scotomas appear paracentral due to eccentric fixation displacement 1
• Devices image the retina with either a camera or scanning laser ophthalmoscope while testing monocular central field 1

Alternative Screening Methods

The Amsler grid serves as a practical screening tool for central field defects, though it detects only about half of central scotomas due to perceptual completion. 1

• The Amsler grid test has 80% sensitivity and 100% specificity for detecting maculopathy 5
• Administer at 30 cm distance as a test of central field vision to detect AMD 1, 5
• The Berkeley Central Field Test (BCFT) offers comparable performance to Amsler grid with 71% sensitivity and 99% specificity 5

Automated Perimetry Considerations

When using automated static perimetry, employ 10-2 testing programs specifically designed to evaluate the central 10 degrees, as standard 24-2 or 30-2 programs sample fixation with only four test points. 1

• Traditional automated field tests have reduced accuracy in patients with unstable or nonfoveal fixation secondary to macular disease 1
• Scotomas may be mapped in the wrong location when patients have eccentric fixation 1
• Central 10-degree programs facilitate measurement of areas that encroach upon or involve fixation 1

Contrast Sensitivity Testing

Assess contrast sensitivity in addition to visual acuity, as many patients with central vision loss have disproportionate difficulty with low-contrast targets despite preserved high-contrast acuity. 1

• Use printed tests like Pelli-Robson (single spatial frequency) or VISTECH (range of spatial frequencies) 1
• Computer-based tests (e.g., SPARCS) allow adaptive testing with varied stimuli 1
• Patients with severe contrast sensitivity loss require devices supplying illumination or contrast enhancement 1
• Recognizing faces and distinguishing between similarly colored objects requires sensitivity to low-contrast targets 1

Fundoscopic Examination

Perform fundoscopy to visually inspect the interior of the eye, specifically examining the macula, retinal vasculature, and optic nerve. 1, 2

• Examine the fundus including vitreous, retina, vasculature, and optic nerve to identify vision-threatening conditions 2
• Check red reflex bilaterally, as any asymmetry in color, brightness, or size indicates serious pathology 2
• Dilated fundus examination has 79% sensitivity and 82% specificity for detecting eye disease 6

Pupillary Examination

Assess for relative afferent pupillary defect (RAPD), which indicates significant retinal or optic nerve dysfunction that may accompany central vision loss. 2

• Evaluate pupil size, shape, and reactivity to light in both eyes 2
• Slow or poorly reactive pupils suggest significant retinal or optic nerve pathology 2

Clinical Algorithm for Assessment

1. Measure best-corrected visual acuity using ETDRS charts at distance, testing monocularly and binocularly 2, 3
2. Perform central field testing with either microperimetry (preferred) or Amsler grid (screening) 1
3. Add contrast sensitivity testing to identify functional deficits not captured by acuity alone 1
4. Complete fundoscopic examination with dilated pupils to identify underlying pathology 2, 6
5. Document fixation pattern and presence of scotomas for rehabilitation planning 1

Critical pitfall: Do not rely solely on visual acuity measurements, as patients may maintain relatively good acuity despite significant central scotomas that severely impact function. 1 Conversely, poor acuity correlates with worse functional performance even among patients with vision 20/100 or worse. 7