How do you know it's chiasmatic?  Bilateral central scotomas with some preferential temporal loss in ethambutol toxicity has an explanation other than a chiasmatic location.  And check how pure the respect for the vertical really is.  

Ethambutol toxicity affects the smallest PMB fibers.  Those that course nasally to the disc are smaller than those that arc around the fovea.  If the former are more adversely involved, you will get more temporal and less nasal field loss.  

Alfredo

Yes, ethambutol is a chelator that produces a mitochondrial optic neuropathy by consuming the iron, zinc and copper that the mitochondrial ribosomes use.  It's usually the zinc and copper that's hardest to supplement which is why Emily Chu included them on the AREDS formula.  We proved the rescue in a rat RGC model of the disease.  

Best,

Alfredo  

Good question.  For all mitochondrial optic neuropathies (MON -- LHON, DOA, copper deficiency, ethambutol, etc.) the issue comes down to surface area to volume ratios.  The surface area of a fiber represents the workload as the membrane needs constant ATP to fuel the sodium/potassium pumps that restore the membrane potential.  But the volume represents the capacity to contain mitochondria.  If a fiber has twice the diameter, it has 4 times the surface area but 8 times the volume and hence a more favorable ratio.  All our histopathology shows a steady march in MON from smallest fibers to larger ones.  --Alfredo   Eg.