As described in the Age-Related Macular Degeneration section, subretinal choroidal neovascular membranes (SRN), when they grow under the center part of vision, can cause profound visual loss, not only in age-related macular degeneration but in the presumed ocular histoplasmosis syndrome, myopia, pseudoxanthoma elasticum, idiopathic membranes (no cause known), and many other disorders. These membranes are nothing more than scar tissue that is growing under the retina in an attempt to repair damage caused by one of these disorders. Subretinal membranes, being scar tissue, scar down both the photoreceptors above them and the RPE cells below them as they age and mature causing profound visual loss.
In the 1990's researchers began looking at ways to remove these membranes in an attempt to improve vision in patients. In the Macular Photocoagulation Studies (see AMD section) it was determined that laser treatment of membranes outside the center part of vision or just adjacent to the center part of vision could keep approximately 50% of the membranes from progressing to under the center part of vision. Unfortunately, 90-95% of the patients who present with a subretinal neovascular membrane already have the membrane under the center part of the vision and have vision loss when first seen by an eye specialist. Laser treatment of these lesions will limit the size of the blind spot that the membranes can cause, and may improve reading speed somewhat, but the average visual acuity in patients that have undergone laser photocoagulation is 20/250. Laser photocoagulation, therefore, was not of much benefit in actually improving vision in patients that developed these membranes directly under the center of vision.
As a result, surgical therapy is being investigated and is the subject of a large clinical trial in the United States. Subretinal surgery involves entering the eye and performing a pars plana vitrectomy. The vitreous jelly is removed so that manipulation can be done in the posterior portion of the eye under the retina. A small incision is made in the retina, called a retinotomy. Fluid may or may not be placed in the subretinal space and sometimes tissue plasminogen activator (t-PA), the same stuff used to break up clots in heart attack victims, is placed in the subretinal space to break up the fibrin portion of the scar tissue (SRN).
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SRN Removal Fluid in injected (if needed) through the retinotomy into the exudative RD (left). The SRN is dissected free from the surrounding structures (right). |
The membrane is peeled from surrounding structures in the subretinal space. Some membranes will grow under the RPE, and some membranes will grow on top of the RPE, although it is very difficult to determine which location the membrane is in preoperatively. If the membrane is under the RPE, then the RPE may be removed with the membrane, thus decreasing the chance for improvement in vision. If the membrane is over the RPE then it can generally be removed without as much damage to the surrounding structures. With the RPE intact and the retina reattached, vision can improve markedly.
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The SRN is grasped with very small subretinal forceps (left) and removed from the subretinal space. In some instances blood and fluid are "squeegeed" from the subretinal space with PFO (right). |
In our studies, patients with the ocular histoplasmosis syndrome and idiopathic neovascularization have done the best with this surgery, and some have improved to as good as 20/20 following removal of the membrane even when their preoperative vision was very poor. In our recent publication in the journal Ophthalmology (May 1999) we evaluated 64 patients undergoing this surgery for age-related macular degeneration. We found that 30% of patients will have a significant visual improvement following surgery (compared with laser photocoagulation where no patients had visual improvement, but only had stabilization of vision), 40% of our patients stabilized their visual acuity, and 30% had progressive decrease in vision despite surgery. Investigations into the transplantation of RPE to patients in which RPE is no longer functional, movement of the retina to an area with new RPE (macular translocation) are underway.
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SRN Removal SRN and blood removed from subretinal space as described above (left preop, right postop). Vision improved from 5/200 to 20/20 and has remained 20/20 for six years postop. |
Members of Retina and Vitreous of Texas have been performing this procedure since the earliest days and have designed many of the instruments used in this surgery. The lead author on the first paper on the surgical excision of these membranes in AMD was produced by a member of Retina and Vitreous of Texas. We feel that this procedure, combined with transplantation of RPE cells and/or the use of growth factors in the subretinal space may be the best hope for improvement in vision in patients with these disorders.
Disclaimer Stuff: The opinions expressed in this website are those of RVT. Diagnosis and therapy should be based on a thorough examination by and recommendations of a qualified eye provider.
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