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PHOTOCOAGULATION IN THE MANAGEMENTOF CHOROIDAL NEOVASCULAR MEMBRANE
BIJAYANANDA PATNAIK
Retina Associates Eye Institute, New Delhi.
Laser photocoagulation in the management of choroidal neovascular membrane has been found to be a rewarding treatment in suitable cases. This review article explains the principles, mechanism and technique of treatment and summarizes the results of the macular photocoagulation study (MPS) trial which evaluate the utility of photocoagulation in the management of CNVM in ARMD.
The commonest cause of choroidal neovascularisation (CNV) is age related macular degeneration (AMD). It is the major component of the exudative variety of AMD. Though exudative AMD is seen in only 10% of all cases of AMD, these account for nearly 90% of legally blind due to this cause. Colloid bodies and pigment epithelial detachments may precede the growth of CNV. Histopathologically there are two type of CNV, according to their location. In type-I CNV the new vessels initially grow between the inner and outer layers of the Bruch’s membrane, before spreading under the sub-pigment epithelial space. Often this NV breaks through the PE to appear under the retina. At any stage these could bleed. The type-II CNV on the other hand grows under the sensory retina. Seen in younger individuals, with relatively healthy RPE and has better prognosis. Choroidal new vessels are directly associated with such clinical findings as: RPE detachment, RPE tears, sub-retinal haemorrhages, sub-pigment epithelial haemorrhages, break-out vitreous haemorrhage on disciform macular scarring.
There are other less common causes of CNV. In fact, any pathology that damages the RPE and Bruch’s membrane can lead to choroidal neovascularisation. These include: pathologic myopia, angioid streaks, post traumatic choroidal tears, choroiditis, serpigenous choroidopathy and ocular histoplasmosis syndrome, etc.
While the CNV are the most sight threatening form of AMD and they are the only variety that is amenable to treatment, at least some of them. The most rewarding treatment in suitable cases has been by laser photocoagulation. In advanced cases, sub-retinal surgery has been used with some success. Lately subfoveal and ill defined (occult) CNV lesions are being treated by photodynamic therapy (PDT) and transpupillary thermal (TTT) with encouraging initial results. However, with the enthusiasm on PDT, partly novelty and partly driven by industry initiatives, the role of photocoagulation lasers in the management of CNV is being under stated. This needs correction, specially when the cost of PDT is beyond the reach of most patients in this country and best results are not impressive enough. PHOTOCOAGULATION IN CNV
It is desirable to understand the principles and mechanism by which photocoagulation destroys the CNV. The photocoagulation energy is absorbed by the pigments of RPE and the choroid under the CNV and is converted in to heat. The heat thus produced, coagulates the new blood vessels of the CNV and obliterate it. The ablasion of the CNV often is incomplete. The new vessels get enmeshed in layers upon layers of fibrous tissue originating from the growth of the RPE, induced by the photocoagulation.
Photocoagulation of the CNV is effective only when the outline of the CNV is clearly defined on a fluorescein angiogram. When the entire neovascular lesion is subretinal, the outline can be expected to be well defined. The fluorescence outline of even the subretinal new vessels would be obscured by (a) haemorrhage so often originating from the very same CNVs, (b) hyperplastic RPE or fibrous tissue there from or (c) serous detachment of the RPE, which would become fluorescent pretty fast, blurring the contrast. The CNV under the detached RPE can not be defined by fluorescein angiography. Some CNV are partly subpigment epithelial and part subretinal. Clearly defined CNV are popularly known as Classic and those which are ill defined or obscured are described as occult. Some occult CNV can be studied by Indocyanine Green Angiography. Because of lack of definition, over 50% of subjects considered for Macular Photocoagulation Study did not qualify.
Photocoagulation of the CNV is to be undertaken only after obtaining a good quality fluorescence photograph. Since it is estimated that the CNV can grow at an average of 10 to 18 microns a day, the fluorescein angiogram to be used as a reference must not be older than 72 to 96 hours. Ideally it should be less than 24 hours old. Since digital videoangiographic images are inferior in contrast and clarity to fluorescein angiograms on 35 mm celluloid films or their prints, the later is preferred as a reference for laser photocoagulation. The big problem in locating and outlining the CNV accurately arises out of a fundamental reality. The precise outline of a CNV is seen only on a fluorescein angiogram. But the laser photocoagulation is to be done on the fundus view, where the CNV is not visible with any degree of clarity. Therefore the entire procedure is to be completed on a guess work. The presumed outline of the CNV have to be determined by extrapolating the outline of the CNV in relation to vascular landmarks on the fluorescein angiogram to the same landmarks (vascular) on the fundus view. This process understandably be only approximate. To ensure that no part of the CNV remains untreated, usually the photocoagulation is carried out over the presumed CNV, plus 100 micron beyond. Therefore, the scar is at least 200 microns larger than the CNV.
My personal technique : To improve accuracy and precision of photocoagulation I have been using my original "precision photocoagulation" technique. While visualizing the fundus through a suitable contact less on the Laser slit lamp, cobalt blue filter is inserted. Then intravenous sodium fluorescein is injected. One can visualize the dye flow on the fundus. Argon blue-green aiming beam (which has been placed close to the approximate site of the CNV) is used to illuminate, locate and define the neovascular membrane. While keeping the intense fluorescence of the CNV in view, the beam size is reduced to effective 150-200 microns on the fundus and photocoagulation is done to mark the outline of the neovascular membrane. While marking spots are being placed using blue-green laser, the CNV closest to the fovea is avoided at this stage. Then under white light of the slit lamp these points are connected and the area so defined is filled up by intense laser burns. At this stage, we use an Argon green beam specially while working on the macula close to the fovea. In classical CNV we achieve a more precise and effective photocoagulation than the methods described in the literature. Unlike the MPS protocol we do not feel the necessity to cover 100 micron beyond the assumed margin of the CNV. Similarly we cover only around 50 microns in to any blocked fluorescence due to blood. Often the blood disperses in a couple of weeks, letting us identify any blocked fluorescence due to blood. Often the blood disperses in a couple of weeks, letting us identify any part of CNV not treated in the first instance and coagulate this. A series of 59 CNV lesions in 53 eyes of 50 patients, treated with this technique is under publication. Retreatment of the original CNV lesion was needed within 3 months of the first treatment on total of 10 occasions. In the majority (7) of these, the perfusing CNV was entirely inside the border of the treated area. Only in two the CNV was seen extending beyond the treated area, indicating failure to photocoagulate the entire CNV.[1]
We have been treating all classical or near classical extrafoveal CNVs, juxtafoveal or subfoveal CNV of size of 1 disc area or less by photocoagulation, since early 1970s. Only 3 CNV lesions were larger than 1 disc area, but less than 2 disc area. Thirty one cases of CNV were Age Related of the rest 22 young cases: myopics-10, angioid streaks-2, traumatic choroidal rupture-2, colloid bodies-1 and idiopathic-7. There could be little controversy about the fact that photocoagulation is the treatment of choice for extrafoveal CNV. The later two varieties were treated under the principle: treatment would be better than no treatment in the long run, even though there could be an immediate drop in the vision. The benefit has been in the way of preventing gross visual loss and improving contrast sensitivity and improved reading abilities. Smaller the lesion the better is the long term visual prognosis, specially in the subfoveal lesions.
Macular photocoagulation study (MPS), multicentric, randomized, prospective clinical trials have addressed this issue of utility of the photocoagulation in the management of CNV in Age Related Macular Degeneration has validated most of our treatment philosophies. It has been seen that treatment is better than no treatment in all three categories of Macular CNV, namely: Extrafoveal, Juxtafoveal and Subfoveal CNVs.
Extrafoveal CNV (Posterior border of the CNV is more than 200 microns from the foveal centre or FAZ)
The laser treatment was beneficial in reducing the risk of severe visual loss (6 lines or more) as compared to no treatment. The risk of severe visual loss was 24% in treated as against 41% in untreated at one year, 45% against 63% at 3 years and 46% against 64% at 5 years. However, the risk of recurrence was 54% at the end of 5 years. Eighty per cent of these eyes with recurrence had severe visual loss.
Juxtafoveal CNV (Posterior between 1 to 199 micron from the foveal centre or FAZ)
Again the treatment was better than no treatment. Only 25% of the laser treated eyes had vision of 20/400 or worse, as against 40% of the untreated eyes. The risk of severe visual loss was 31% in treated as against 40% in untreated at one year, 44% against 63% at 3 years and 49% against 58% at 5 years. The risk of recurrence was 45%.
Subfoveal CNV (Lies under the centre of the fovea or FAZ)
Treatment was better than no treatment with respect to visual acuity, contrast sensitivity and reading speed. This trend was maintained at least for 4 years. The risk of severe visual loss was 18% in treated as against 23% in untreated at one year, 20% against 37% at 2 years and 22% against 47% at 4 years.
The treatment benefit diminished with increasing size of the CNV. The treatment of lesions larger than 2 disc areas may not be beneficial. On the other hand, smaller or medium sized lesions (= or < 2 disc areas) with vision of 6/60 or worse do better. Small lesions with good vision (6/12 or better) would do much better with treatment than no treatment in the long run. However, the patients should be warned about the immediate fall in the visual acuity on laser treatment. CONCLUSION
Considering in necessity of repeated treatment (average of 5.6) with prohibitingly expensive photodynamic (PDT) therapy, not quite established usefulness of the trans pupillary thermal therapy (TTT) and high risk of available surgical approaches, laser photocoagulation remains a definitive and effective means of treating classical or near classical CNVs of moderate to small size. The occult lesions are difficult to treat with any degree of success using any of the modalities available today.
R EFERENCES
1.Macular Photocoagulation Study Group. Age related macular degeneration. Am J Ophthalmol 1984; 98 : 376.
2.Macular Photocoagulation Study Group. Recurrent choroidal neovascularisation after argon laser photocoagulation for neovascular lesions of neovascular retinopathy. Results of a randomized clinical trail. Arch Ophthalmol 1986; 104 : 503.
3.Macular Photocoagulation Study Group. Laser photocoagulation of subfoveal recurrent neovascular lesions in age related macular degeneration. Results of a randomized clinical trial. Arch Ophthalmol 1991; 109 : 1220.
4.Macular Photocoagulation Study Group. Laser photocoagulation of subfoveal recurrent neovascular lesions in age related macular degeneration. Results of a randomized clinical trial. Arch Ophthalmol 1991; 109 : 1232.
5.Macular Photocoagulation Study Group. Subfoveal neovascular lesions in age related macular degeneration: guidelines for evaluation and treatment in the MPS. Arch Ophthalmol 1991; 109 : 1242.
6.Photodynamic therapy of subfoveal choroidal neovascularisation in age related macular degeration with verteporfin: One year results of 2 randomized clinical trails-TAP report. Arch Ophthalmol 1999; 117 : 1329.
7.Photodynamic therapy of subfoveal choroidal neovascularisation in age related macular degeneration with verteporfin: two year results of 2 randomized clinical trails- TAP report-2. Arch Ophthalmol 2001; 119 : 198.
8. Photodynamic therapy of subfoveal choroidal neovascularisation in age related macular degeneration with verteporfin: two year results of 2 randomized clinical trials-TAP report 2. Arch Ophthalmol 2001; 119 : 198.
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