I have seen three patients on Vigabatrin (a GABA transaminase inhibitor)
with symptomatic visual loss. All three had constricted visual fields
with sparing to about 15-20 deg. and pale optic nerves but normal acuity
and colour vision. Similar cases have been reported in the literature.
The question is what is the mechanism of the visual loss? There is no
clinical syndrome of which I am aware that gives similar findings.
Generally the retina other than the optic nerve appears normal and ERG and
EOG are essentially normal. The pale optic nerves might argue in favour
of an optic neuropathy yet the normal acuity, colour vision, and pattern
of peripheral field depression encroaching upon central field would be
unusual. The field loss might argue in favour of retina yet the normal
retinal appearance would seem unusual.
The question then is- How would a GABA transaminase inhibitor account for
this picture?
P.S. This drug is widely used in Canada and Europe but has not been
approved in the U.S.