Radial Optic Neurotomy for Central Retinal Vein Occlusion

In 1978, a neurologist proposed the theory of neurovascular compression at the optic nerve head in the pathogenesis of anterior ischemic optic neuropathy and suggested an external approach for relaxation of the optic nerve head for treatment of this condition. This approach was technically difficult and therefore abandoned very soon. In 2001, Opremcak1 reconsidered the same theory and suggested that the anatomy of the optic nerve head plays an important role in the pathogenesis of CRVO which may be also considered a “scleral outlet compartment syndrome“. In addition to containing the optic nerve, the scleral outlet is the space through which the central retinal artery (CRA) and central retinal vein (CRV) pass into and out of the eye (Fig. 1). Furthermore, it contains the lamina cribrosa and is encompassed by the scleral ring. As the optic nerve approaches the eye, it consists of myelinated nerve fibers, CRA, and CRV and has a diameter of 3 mm; however, the internal diameter of the optic disc and scleral outlet is 1.5 mm (Fig. 2). Figure 1 A schematic figure showing elements passing through the scleral outlet. Figure 2 Histological section demonstrating internal and external structures of the scleral outlet at the optic nerve head. The pathophysiology of certain conditions such as CRVO and nonarteritic anterior ischemic optic neuropathy is thought to be related to the confined space of the scleral outlet resulting in neurovascular compression of the CRA, CRV and optic nerve. Many factors such as changes in the scleral ring, increase in arterial diameter, and presence of anatomical anomalies as well as systemic factors may act in concert to decrease venous lumen, increase intravenous turbulence leading to endothelial damage and thrombus formation. Optic neurotomy has been the focus of attention by many vitreoretinal surgeons. Relieving the presumed "compartment syndrome" at the scleral outlet may improve venous outflow in an eye with CRVO thereby decreasing congestive macular edema and improving visual acuity. Since the first report of promising results of RON, its opponents have raised several concerns about this procedure.2–4 Herein, I provide evidence-based answers for all these concerns. What is the mechanism behind RON in the treatment of CRVO? This treatment is based on a pathophysiologic mechanism called the "scleral outlet compartment syndrome". In eyes with susceptible anatomy, the confined space of the optic nerve head might play a role in the pathogenesis of CRVO. This region is surrounded by the firm surrounding scleral ring. This unique tissue architecture at the scleral outlet results in an anatomical "bottle neck-like" configuration. Certain factors might increase pressure on elements within this ring and compress the lumen of the non-muscular vein, resulting in vein occlusion. Therefore, an incision on the margin of the disk could theoretically relieve this compression in eyes with CRVO, and improve blood flow and visual acuity.1 Is the scleral ring a relaxable tissue? In a histopathologic study on cadaver eyes, we demonstrated that optic neurotomy using a microvitreoretinal (MVR) blade seems effective for scleral outlet relaxation (Figures 3, ​,4). 4). This study also demonstrated that the excimer laser can make non-mechanical cuts with relative ease and reliability.5 Figure 3 The site of radial optic neurotomy in cadaver eyes with a mechanical cut using MVR blade (Fig. 3) and non-mechanical cut by an excimer laser (Fig. 4). Note that the incisions are well away from the central retinal artery and vein. Figure 4 The site of radial optic neurotomy in cadaver eyes with a mechanical cut using MVR blade (Fig. 3) and non-mechanical cut by an excimer laser (Fig. 4). Note that the incisions are well away from the central retinal artery and vein. Can an MVR blade perforate the sclera or lacerate vessels? The findings of the above-mentioned study revealed that no eye sustained macroscopic or microscopic scleral perforation and there was no injury to the central retinal vein or artery5 (Fig. 5). Figure 5 Histologic depth of optic neurotomy incision showing no scleral perforation Can RON damage nerve fibers leading to or exacerbating optic atrophy? Yes, it definitely can. However, this damage involves nasal nerve fibers which are not related to central vision. In other words, in this procedure macular fibers are saved at the expense of some nasal fibers (Fig. 6). Figure 6 Improvement of visual field (b) in comparison to the preoperative field (a) after radial optic neurotomy in a patient with central retinal vein occlusion. Note that isopter I4e is represented after radial optic neurotomy. Do all cases with CRVO demonstrate the same response? Depending on the underlying pathophysiologic mechanism, the response to RON varies among different patients. Some cases respond dramatically and some do not benefit at all. We had a patient who had received 8 intravitreal bevacizumab injections with no effect, but demonstrated improvement in vision from counting fingers to 20/200 only 10 days after RON (Figures 7, ​,88). Figure 7 Color fundus photograph (a) and optical coherence tomography (b) in a case of central retinal vein occlusion after 8 injections of intravitreal bevacizumab. Figure 8 Color fundus photograph (a) and optical coherence tomography (b) in the same patient 10 days after radial optic neurotomy demonstrating marked improvement of macular edema. Are there any studies comparing RON with other recently suggested interventions for CRVO? Yes. One study has compared the natural course of recent CRVO with four newly introduced treatment options including: (1) intravitreal triamcinolone acetonide, (2) RON, (3) combined RON and intraocular triamcinolone injection, and (4) combined internal limiting membrane peeling, RON and intraocular triamcinolone injection. This report demonstrated that among the four different approaches, RON combined with intraocular triamcinolone injection may lead to better visual outcomes.6 However, no study has compared RON with anti-VEGF agents yet. Does this technique entail complications? Like any other surgical procedure, this technique may entail adverse effects. In addition to complications inherent to vitrectomy in general, the surgeon may encounter vitreous hemorrhage, retinal detachment7 and peripapillary choroidal neovascularization8 following RON. The Pan-American Collaborative Retina Study Group has classified complications of RON for CRVO into three major categories: manageable, difficult to manage, and non-salvageable.7 Most complications encountered after RON are considered manageable. Which eyes with CRVO may benefit most from optic neurotomy? Eyes in which the underlying cause of CRVO is scleral outlet compartment syndrome with some viable tissue seem to benefit most from RON. Examination of the fellow eye oftendiscloses signs such as a small crowded disc with absent or small physiologic cup which are suggestive of a compartment syndrome. In contrast, visual prognosis for ischemic CRVO with severe initial visual loss is poor in general; loss of vision is the rule and a cause of great disability irrespective of treatment.9 In conclusion in light of exisiting knowledge and available evidence, RON can be considered beneficial for some patients with CRVO. Our experience demonstrated that outcomes of RON are superior to the natural history of the condition. We have provided before and after photographs, (Figures 9, ​,10)10) fluorescein angiograms and visual fields to provide evidence supporting the role of RON in CRVO. Complications do occur but the great majorities are manageable. From another perspective, surgery for CRVO is analogous to vascular surgery therefore any attempt to restore retinal circulation has considerable merit. We need to extrapolate from the experience of colleagues in the fields of cardiovascular, neurosurgical and vascular surgery and to reemphasize the goal of vascular surgery and the mission of the vascular surgeon: to restore blood flow and allow reperfusion of salvageable tissue. Figure 9 Color fundus photograph (a) and fluorescein angiogram (b) in a case of central retinal vein occlusion before radial optic neurotomy. Figure 10 Color fundus photograph (a) and fluorescein angiogram (b) in the same eye after radial optic neurotomy demonstrating restoration of laminar flow and a marked decrease in retinal edema and hemorrhages. Visual acuity improved from counting fingers to 20/200 .