What tests are used to monitor retinal defects caused by Vigabatrin (Sabril) in pediatric patients?

Monitoring Retinal Defects from Vigabatrin in Children

Children with cognitive age >9 years should undergo visual field examination with Goldmann perimeter or Humphrey field analyzer before starting vigabatrin and every 6 months thereafter, while children <9 years should have electroretinography (ERG) performed according to international standards when visual field testing is not feasible. 1

Age-Stratified Testing Approach

Children with Cognitive Age >9 Years

Visual field testing is the primary monitoring modality for older children who can cooperate with testing. 1, 2

• Goldmann perimetry using I1e or I2e isopter and I4e or V4e isopter 1
• Humphrey field analyzer with age-related, three-zone suprathreshold strategy and 135-point field 1
• Testing schedule: baseline (before starting vigabatrin or within 4 weeks) and every 6 months during treatment 1, 2, 3
• Additional testing 3-6 months after discontinuation is recommended 2, 3

Important caveat: Confrontation testing does not reliably identify vigabatrin-associated visual field defects and should not be used for screening. 1

Children with Cognitive Age <9 Years

Electroretinography (ERG) is the recommended alternative when visual field testing is not feasible due to cooperation limitations. 1, 2

• ERG should be performed according to international standards 1
• ERG can detect abnormalities of cone function that occur with vigabatrin toxicity 1
• Testing schedule mirrors that for older children: baseline and every 3 months during treatment 2, 3

Critical limitation: Field-specific visual evoked potential techniques to assess peripheral vision are being evaluated but are not yet routinely available for children <9 years. 1, 2

Specific Pattern of Vigabatrin-Associated Defect

The characteristic defect helps distinguish vigabatrin toxicity from other causes:

• Bilateral concentric constriction within 30° radius from fixation 1, 2
• Nasal loss extending in an annulus over the horizontal midline 1, 2
• Relative sparing of the temporal field 1, 2
• The pathogenesis involves cone and amacrine cell dysfunction in the retina 1

Additional Monitoring Considerations

Baseline Documentation

• Document any pre-existing visual abnormalities before starting vigabatrin 2, 3
• Children with existing visual defects or at risk for visual defects from other causes (e.g., previous surgery, retinal disease) should receive vigabatrin with extreme caution or not at all 1, 2

Emerging Evidence on ERG Parameters

Research suggests specific ERG parameters may be useful:

• Post-receptor sensitivity (log σ) derived from scotopic b-wave stimulus-response function is frequently abnormal 4
• Implicit time of the OFF response (d-wave) to long flash in photopic conditions shows abnormalities 4
• 30-Hz flicker response amplitude has been proposed as a predictor but occurs infrequently in pediatric patients 4, 5

Important caveat: Recent research cautions against over-reliance on ERG alone, as ERG parameters do not consistently correlate with visual field defects, and abnormalities may not be solely attributable to vigabatrin. 4, 6

Long-Term Monitoring

• Visual field defects are irreversible—neither recovery nor progression has been confirmed after drug withdrawal 1, 2
• The defect is unlikely to develop if perimetry remains normal after >3 years of treatment 1
• Vision loss may worsen even after stopping vigabatrin 3
• Prospective data demonstrate that ERG-detected retinal toxicity in infancy correlates with visual field loss and retinal nerve fiber layer attenuation in adolescence 7

Clinical Pitfalls to Avoid

• Do not rely on symptoms alone: Approximately 30-40% of adults develop visual field defects, and most children are asymptomatic until defects are severe 1, 2, 8
• Do not use confrontation testing: It is inadequate for detecting the characteristic peripheral constriction 1
• Do not automatically discontinue vigabatrin in well-controlled patients: The risk-benefit calculation must consider seizure control, particularly since vigabatrin remains the drug of choice for infantile spasms 1, 2
• Do not assume testing will prevent damage: Vision testing allows detection but cannot prevent the irreversible damage 2, 3

Real-World Feasibility

Recent real-world data show that formal visual field testing is completed in only 2.1% of pediatric patients, ERG in 13%, and OCT in 6.7%, with the incidence of definite vigabatrin-related toxicity at 0.7% in a large cohort. 6 This highlights the practical challenges of monitoring but does not negate guideline recommendations for attempting regular surveillance.