DISCUSSION

    Previous attempts at external decompression of the orbital portion of the optic nerve by optic nerve sheath fenestration and sectioning of the posterior scleral ring have not been validated as effective treatments for CRVO

    Since the RON has been advocated as a possible treatment of CRVO, Hayreh and someone else contest that the lamina cribrosa is a nonelastic and compact structure of rigid collagen tissue, a single radial incision at one side may not be able to achieve a decompression of central retinal vein. Hayreh also argues that the site of occlusion in CRVO is posterior to the lamina cribrosa. Thus, a RON, if it even were to decompress the lamina cribrosa, will have no beneficial effect on decompressing the nerve where the site of the occlusion lies. Furthermore, he argues that with a CRVO, the vein is completely thrombosed and with a decompression procedure, the vein cannot be opened[4].

    From this view, we think about these doubts and our results seriously. Firstly, in the lamina cribrosa of non-pathological eyes the collagen fibrils are arranged circularly around the points of passage of axons and vessels in coarse bundles. There was a large amount of elastic fiber around the scleral ring [5]. During the first 2 weeks after surgery, the appearance of fusiform of the incision indicates RON may relieve a "compartment syndrome" occurring at the sclera outlet. Also, our results suggest that the incision of RON connects with subarachnoid spaces of intraocular optic nerve. Thus, because the pressure of subarachnoid spaces half of intraocular pressure[6] a RON, is indeed decreasing the pressure which inside the intraocular optic nerve(Figure 8,9).

    Secondly, after venous occlusion, the retina becomes hypoxic, which leads to functional and structural changes in the retina capillaries. These changes lead to information of collaterals and chorioretinal venous anastomosis. The collaterals and chorioretinal venous anastomosis does increase venous blood outflow and allows for improvement in resolution of the intraretinal hemorrhages and the improvement in visual acuity. If RON can decrease the pressure which inside the intraocular optic nerve, CRA and collateral may develop in the area of incision earlier in the postoperative period(Figure 10).

    At last, Green's study [7] confirms that thrombus in CRV is just posterior to lamina cribrosa or at the site of lamina cribrosa. But Hayreh and Beaumont [8,9] contest retrocribrosal site of occlusion, which has access to the pial plexus that can provide collateral channels for retinal venous drainage and most of CRVO are of this condition. We think, whether the site of occlusion in CRVO is posterior to the lamina cribrosa(Figure 11) or at the site of lamian cribrosa(Figure 12), CRA or collateral that draining blood to choroidal vein is a power channel because of the shortest distance. So it is not the position of thrombus that determines the effectiveness of RON.

    The exact mechanism of reduction of RON is not clear. In general, most doctors who undergo RON suggest that inducing new chorioretinal shunts that drain retinal circulation to the choroids might underlie the efficiency of RON. However, as Meyer has suggested [10]. the prospective, randomized clinical trial is needed to fully put to rest the debate over the efficacy of RON in the treatment of CRVO.

    RON can cut scleral ring sharply and don't damage the major vessel. The incision of RON connects with subarachnoid spaces of intraocular optic nerve and become broaden gradually at the site of scleral ring.

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