How is severe visual impairment diagnosed?

How to Diagnose Severe Visual Impairment (SVI)

Severe visual impairment is diagnosed by measuring best corrected visual acuity worse than 20/40 but better than 20/200 using standardized visual acuity testing, most commonly the Snellen visual acuity chart at 20 feet. 1

Diagnostic Criteria

The formal definition of SVI is best corrected vision worse than 20/40 (the cutoff for unrestricted driver's licenses in many states) but better than 20/200, which is the threshold for legal blindness. 1

• Visual acuity of 20/200 or worse, or visual field of 20 degrees or less, defines blindness rather than SVI 1
• The 20/40 cutoff is clinically significant as it represents the threshold below which functional impairments in activities of daily living, driving, and fall risk become evident 1

Diagnostic Testing Methods

Primary Diagnostic Tool

Standardized visual acuity testing is the standard diagnostic method for identifying SVI. 1

• The Snellen visual acuity test assesses the ability to recognize letters of different sizes arranged in rows from a prespecified distance of typically 20 feet 1
• Testing must be performed with best correction (optimal refractive correction in place) to distinguish true visual impairment from uncorrected refractive error 1
• In children, Teller Acuity Cards-II (TAC-II) demonstrate better repeatability than app-based alternatives for measuring grating acuity 2

Inadequate Screening Methods

Self-reported vision problems and screening questions are not accurate for diagnosing SVI and should not be relied upon. 1

• Compared to formal visual acuity testing or ophthalmologic examination, screening questions lack diagnostic accuracy 1
• Older adults may underreport visual impairment because symptoms progress slowly or due to comorbid cognitive impairment 1

Limitations of Primary Care Testing

Visual acuity screening tests alone cannot diagnose the underlying cause of visual impairment (such as AMD, cataracts, or refractive errors) and require referral to ophthalmology for definitive diagnosis. 1

• The Amsler grid shows poor accuracy for detecting visual conditions compared to ophthalmologic examination 1
• Pinhole testing, fundus examination, and clinical examination in primary care have limited evidence for diagnostic accuracy 1

Special Populations

Infants and Young Children

In infants below age 2 years with SVI, cerebral visual impairment (CVI) is the most common diagnosis in both preterm (39.1%) and term (11.4%) infants. 3

• Mean age at first eye examination for infants with SVI is 6.61 ± 5.25 months 3
• 30% of infants with SVI have multiple disabilities, rising to 94% in those with CVI 3
• Children with CVI and good visual acuity may have higher visual function deficits that remain undiagnosed without specific assessment 4

Children with Sensorineural Deafness

Children with severe and profound sensorineural deafness have a 34.9% incidence of visual impairment and require comprehensive ophthalmic assessment soon after diagnosis. 5

• Visual impairment criteria in this population include visual acuity <6/9 Snellen or equivalent and/or abnormal binocular vision 5
• 44% of visual impairment occurs in children without identifiable risk factors, necessitating universal screening 5

Clinical Pitfalls

• Do not rely on patient self-report alone, as many individuals are unaware of gradual visual decline 1
• Always test with best correction to avoid misclassifying uncorrected refractive error as SVI 1
• In children with good visual acuity but concerning history of brain damage, assess for higher visual function deficits that may indicate CVI 4
• Visual acuity testing does not assess other important visual functions including contrast sensitivity, visual field, color vision, or depth perception 1