Excavation which is also described as cupping is the characteristic feature of the glaucomatous disc. Non-glaucomatous optic atrophies are flat (non-excavated). Histology of the end-stage glaucomatous disc reveals an empty crater devoid of all the structures once component of a normal disc.
I hypothesize that excavation of the glaucomatous disc is due to severance of the nerve fibers and the vasculature, not due to their atrophy. Notching at the poles in early glaucomatous disc is the initial localized excavation as a result of severance and depletion of the arcuate fibers which being fewer in number are depleted earlier. Thank you for feedback.
Dear Dr Kara, Can you please explain what do you mean by excavation in the glaucomatous disc? As I understand excavation is the creation of empty spaces after removal of the material it is composed of just as we produce excavation in the ground after removal of dirt. In glaucoma excavation and cupping are same thing which I fail to to understood. In cupping we assume that nerve fibers are being shrunk and atrophied but not their total disappearance. In the end-stage histology of the glaucomatous disc there is nothing left. I even don't understand what structure of the disc has really cupped!
When you say that excavation is due to biomechanics properties of sclera: then are you ignoring the fact that excavation is occurring in the disc which is lying in the scleral opening and not in the scleral itself. I fail to understand the role of sclera in excavation, probably you meant Lamina cribrosa. The message I am conveying is simple: the nerve fibers are being severed, not atrophied in glaucoma and severance is resulting in disappearance of the nerve fibers and thus resulting in excavation. If you can prove that severance of the nerve fibers is not taking place in glaucomatous disc I would abandon my hypothesis of severance of nerve fibers in glaucoma. Whether the nerve fibers are severed or atrophied in glaucoma is the real issue and would solve the mystery of glaucoma. Thank you for your interest in my hypothesis.
Dear Dr. Kara, In my humble opinion the mystery of glaucoma lies in three main things. 1. Histology of the end-stage glaucomatous disc ( ESGD). 2. Arrangement of the nerve fibers in the retina and disc 3. Glaucomatous visual field defects showing that peripheral vision fibers are always destroyed first and central fibers last until the end-stage in an orderly fashion and never haphazardly. Whatever theory we put forward it must answer the aforementioned three facts.
We have to critically re-evaluate the ESGD which is described as fully cupped disc in textbooks and elsewhere. Is ESGD really a 100 % cupped disc? If so, then what structure has really become cupped? Or is it a crater left over after the disappearance of all the structures of the disc including LC. Then we should move backward and investigate where did all the structures of the normal disc disappeared?
My hypothesis of sinking disc/severing of the nerve fibers attemps to answer the events taking place in glaucoma. ESGD may not be a cupped LC but a crater left over after the severance and disappearance of nerve fibers including vasculature and empty LC may be lying at the bottom of the crater. Analogy: a tree has been pulled out of ground, leaving behind a crater. The term excavation appears more appropriate to describe glaucomatous disc than the term cupping which is misleading and mistakenly given 160 years ago.
Why are the peripheral fibers being destroyed first? Since the peripheral fibers lie deeper and exit closer to the scleral edge therefore peripheral fibers would be the first to get stretched and broken due to sinking disc followed by next in line central nerve fibers. As the sinking of disc advances this process would continue until the central most fiber is moved to the scleral edge and severed and whole disc area becomes excavated or turned into a crater.
Why is the disc/LC sinking? I hypothesize it is due to atrophy of the border tissue of Elschnig (not LC itself) due to chronic ischemia resulting from high IOP in HTG whereas due to poor systemic circulatory problems in NTG. I don’t believe any pathology within the LC pores or beams can be so precise and perfect as to destroy always the peripheral fibers first and central at the end in an orderly fashion and never haphazardly. Glaucoma may not be an optic disc neuropathy but an optic disc axotomy.
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