Vitreous buffering capacity as a mechanism for silicone oil optic neuropathy

Dear editors

I read with interest the article concerning silicone oil optic neuropathy by Knecht et al in October issue of BJO. In that article they suggest an active transport of silicone oil into the optic nerve as a mechanism for optic neuropathy. [1] I would like to suggest a new mechanism based on buffering capacity of normal vitreous as a cause of silicone oil optic neuropathy. Conway et al demonstrated buffering capacity for bovine vitreous, when it was incubated with 5%CO₂ -95% air mixture. [2]

Buffering capacity of vitreous is largely dependent on bicarbonate- CO₂ system. This system is the most important buffer in the extracellular fluid. [3] If we consider vitreous as like as the plasma regarding its CO₂ content, we will need 5%CO₂ over the vitreous to demonstrate vitreous buffering effect. (5%*760mm Hg=38mm Hg so closed to normal 40mmHg P CO₂ of plasma) Water in vitreous cavity may have buffering capacity comparable to vitreous itself. Remember that 99% of normal vitreous is water.In the presence of carbonic anhydrase as the catalyst and 40 mmHg of P CO₂ in nearby blood and interstitial tissue, which is in dynamic equilibrium with this water, this buffering capacity is expected either for normal vitreous or balanced salt solution. Buffering capacity is also dependent on volume of buffer, so with replacement of water content of vitreous with silicone oil the buffering capacity might be decreased significantly or even lost. Retinal nerve fiber in contact with a fluid which doesn't support PH homeostasis will be at risk for more degeneration, and silicone oil-associated optic nerve degeneration could occur. [4]

Mehdizadeh M MD
Shiraz University of Medical Sciences
Shiraz, IRAN

References

1. Knecht P, Groscurth P, Ziegler U,et al. Is silicone oil optic neuropathy caused by high intraocular pressure alone? A semi-biological model.Br J Ophthalmol. 2007;91:1293-5
2. Conway M D,Jermak CM,Peyman GA et al.Buffering capacity of bovine vitreous. Retina 2008;28: 150-153
3. Berne R M Levy M N :Principles of physiology.St Louis C.V.Mosby 1990:470
4. Budde M, Cursiefen C, Holbach LM, et al. Silicone oil-associated optic nerve degeneration. Am J Ophthalmol 2001; 131: 392-4