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I cannot find the word transvitreal in any dictionaries and only sources. Do you know what it means?

Context:
We investigated the feasibility of subretinal vector injection in patients with advanced retinal degeneration and found that we could achieve this outcome predictably and without immediate adverse events, using a transvitreal, transretinal approach after pars plana vitrectomy. A relatively degenerate vitreous facilitated detachment and removal of the posterior vitreous cortex, which otherwise might have resisted passage of the fine cannula.

Full context:
http://content.nejm.org/cgi/content/full/NEJMoa0802268
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All the references seem to spell it as you do. My guess is that it refers to any medical procedure which is performed by accessing the retina by going through the "vitreous humor" which fills the cavity behind the lens (of the eyeball.) Many references mentioned "fine needle biopsies," which would extract material through a hollow needle, to be examined in the lab, presumably material from the retina. There are references to the "scleral entrance site." The "sclera" is the outer covering of the eyeball.

"Trans" usually means "through" or "across." The "transvitreal route" is often mentioned, so that might mean through the lens, and through the vitreous humor to the retina.

You probably should check a medical dictionary.
Comments  
Thank you Avangi.

I've looked for it in whatever medical dictionaries that I could find, but none of them included the weird word.