Three mothers with 4 children exposed to vigabatrin in utero (but not breast fed) underwent perimetry and imaging of the retinal nerve fiber layer (RNFL) at the University Hospital of Wales and School of Optometry, Cardiff, UK. Two mothers showed vigabatrin-attributed visual loss and an abnormally attenuated RNFL. The third had an upper left quadrantanopia, consistent with previous temporal lobe surgery, and a normal RNFL. All four children, ages 6, 10, 15 and 18 years, had normal visual fields and RNFL thickness. Estimates of the in utero exposure to vigabatrin varied from 600 to 1410 mg/kg/day with a mean of 1100. Children exposed pre-natally may be spared the visual toxicity of vigabatrin. [1]

COMMENT. Vigabatrin-induced visual field loss manifests as a bilateral concentric constriction. It occurs in 30% of patients treated, and the incidence increases with duration and extent of exposure to the drug. Visual field perimetry examination is unsatisfactory under 9 years of age, but attenuation of the retinal nerve fiber layer thickness, estimated by optical coherence tomography, is a sensitive and specific test for vigabatrin toxicity.(Wild et al, 2006) Unlike the temporal quadrant atrophy seen in optic neuritis, vigabatrin toxicity is characterized by nasal quadrant constriction while the temporal quadrant is spared. Children exposed to vigabatrin by placental transfer only, in a dose 10 times that given to infants with infantile spasms, appear to be spared the visual field defect. Infants exposed after 6 months of age are 2.5 times more likely to show vigabatrin toxicity compared with those exposed before 6 months of age (Westall et al, 2007; cited by Lawthom et al).

Vigabatrin-induced visual field loss and age of exposure. Visual fields of 16 children treated with vigabatrin for infantile spasms were examined by Goldmann kinetic perimetry at age 6-12 years, in a study at Helsinki University Central Hospital, Finland [2]. Vigabatrin was started at a mean of 7.6 (range, 3.2-20.3) months. Mean duration of therapy was 21 months. Visual fields were normal in 15 children; a mild visual field loss occurred in one child who was treated with vigabatrin for 19 months. Children treated in infancy are less susceptible to vigabatrin-induced visual field loss than patients treated at a later age.