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SCANNING LASER OPHTHALMOSCOPYIN MACULAR DEGENERATION
MANISH NAGPAL, SHASHANK BANAIT
Retina Foundation, Near Shahibaug Underbridge, Shahibaug, Ahmedabad - 4, Gujarat.
Scanning laser ophthalmoscopy is a new diagnostic imaging technique in which the fundus is illuminated by a narrow laser beam scanned over the surface of the retina and a detector captures the reflected light. The image of the fundus is viewed in real time on the monitor which helps locate feeder vessels. The infrared wavelength can penetrate media opacities like cataract, corneal opacities and haemorrhage better. The images, taken using a digital camera are easier to store and retrieve.
HISTORY
The development of ophthalmic retinal photography has its origin in 1850, with the development of Ophthalmoscope by Von Helmholtz. The fundus photograph of optic nerve was obtained in 1885 by Jackson and Websterwith exposure time of 20 minutes. Although the picture quality was primitive, it was an important milestone in Ophthalmic imaging.[1] Thorner (1899) invented reflexes Ophthalmoscope which was later perfected by Gullstrand (1910) these have made it possible to have fundus images that we have today. Digital videoangiography was described by Guyer and Pulifitoin 1992 which was further modified by Yannuzi et al. Scanning laser ophthalmoscope was first introduced by Robert Webb and colleagues in 1979 in an article Flying spot TV Ophthalmoscope and was available commercially in 1980s.[2] The major advantages of scanning laser ophthalmoscope over the traditional cameras are:
1. Pupillary dilatation is not mandatory for scanning laser ophthalmoscopy as it can collect images even from a small aperature.
2. The patient can tolerate the procedure better because it does not involve bright flashes.
3. The iridescence from the retinal surface is less.
4. The resolution provided by SLO is better as compared to conventional cameras.
5. The infrared wavelengths of the SLO can penetrate media opacities as cataract, corneal opacities, haemorrhage and can thus provide useful information.
In addition the data storage which in the form of 35 mm slides or prints in the conventional imaging system, is not only difficult in terms of the space they occupy but also can degenerate over a period of time. It also makes it necessary for the patient to make an additional visit to the Ophthalmologist if treatment is required which further increases the problem of patients of ARMD considering their age, and other physical handicaps and socioeconomic considerations they have.
PRINCIPLES OF SCANNING LASER OPHTHALMOSCOPY
The fundus is illuminated with a narrow laser beam that is scanned over surface of retina and a detector captures the reflected light. A laser scans across the object area pixel by pixel. Pixelation is the term used to describe number of picture elements in an image. Typical image Pixelation numbers are 512 x 512 or 1024 x 1024 pixel per image. In SLO the laser beam is scanned horizontally and vertically and the timing is designated to synchronize with a wide range of computer and video equipment. The image of the fundus is viewed in real time on a monitor, typically with frame rate ranging 20-30 frames per second. Imaging with SLOs enables better depth perception partly due to larger stereopsis possible in these instruments.
Recently, a new scanning laser ophthalmoscope has been introduced that has a pixelation 2000 x 2000 for a single frame and a field view oa120 degrees.[3] As against the digital fundus cameras, the SLO used for ICG angiography uses diode laser which is in the infra red portion (790). After intravenous injection of ICG the peak light absorption of ICG shifts from 780 nm to 805 nm, therefore fluorescence from the dye could be excited more efficiently than a xenon flash. The excitation by diode laser is so efficient that one diode laser may be enough to observe rapid entry of the dye throughout the choroid.[4] RPE and choroid absorb 59-75% of 500 nm light but only 21-38% of near infrared wavelength. Therefore, ICGA done on SLO in the infrared range allows visualisation of pathological conditions through overlying haemorrhage, serous fluid, lipid, pigments that block fluorescein otherwise Infrared light is reflected from the fundus to a greater extent than visible light permitting lower illumination power and, as it penetrates the retinal pigment epithelium, choroidal structures can be readily imaged. The conventional infrared illumination and detection systems are not well suited to ophthalmoscopy, this area is underdeveloped as a potential source of useful clinical data. Confocal, direct and indirect imaging modes have been used to image fundi of normal volunteers and patients with fundus disease. In comparison with conventional fundus photography confocal infrared SLO imaging improves visualisation of choroidal vasculature, retinal pigment epithelial abnormalities, laser photocoagulation scars, and optic disc pores in the lamina cribrosa. Direct infrared SLO imaging enables fundus visualisation through nuclear lens opacities. Furthermore, indirect mode imaging enhances significantly the appearance of macular drusen.[5]
SLO allows better discrimination against out of focus objects and provides higher contrast than other systems. ICG facilitates the study of the choroidal circulation. It better delineates the choroidal circulation than fluorescein, because the near-infrared light absorbed by ICG (795-810 nm) penetrates the RPE better than the shorter wavelength absorbed by fluorescein. Unlike fluorescein, ICG is strongly bound to plasma proteins, which prevents diffusion of the compound through the fenestrated choroidal capillaries and permits better delineation of choroidal details. ICG can facilitate visualization of choroidal vasculature and CNVM through haemorrhage. Hence, more chances of imaging CNVs or PED. Schieder and co investigators reported enhanced imaging of CNVs in a study of 80 patients using SLO with ICGA.[6]
The use of scanning laser ophthalmoscopy for ICG angiography further improves the technique because the confocal design eliminates scattered and reflected light, while the single-spot laser illumination improves contrast. Numerous investigators have described ICG angiography of CNVMs using scanning laser videoangiography. Scanning ophthalmoscopy also is an extremely versatile adjunct to ICG angiography. Results of ICGA carried out using SLO and by digital fundus camera were compared indicated that SLO assisted ICGA revealed well defined vessel structure in 28% as compared to 8% by digital fundus camera.[7] The newer scanning laser devices can image both retinal and subretinal features with remarkable contrast. As compared to traditional cameras, only a very small portion of retina is illuminated, hence less energy is required. Therefore all the light returning from the fundus has to originate from the point that has been illuminated by the scanning beam. Hence a very light sensitive photo multiplier or photodiode can be used to detect light. Also in some cases laser light can be steered around cata-racts and media opacities providing improved image fidelity in comparison to the conventional techniques, as there is less image distortion attributable to scattering of light. AUTOFLUORESCENCE
The intensity of autofluorescence in atrophic areas is typically decreased. Ageing changes of the retinal pigment epithelium (RPE) play a key role in the pathogenesis of the disease. In postmitotic RPE cells autofluorescent lipofuscin granules accumulate with age in the lysosomal compartment mainly as a byproduct of constant phagocytosis of membranous discs shed from distal photoreceptor outer segments. With the advent of confocal scanning laser ophthalmoscopy fundus autofluorescence and its topographic variation mainly mediated by RPE-lipofuscin accumulation can be visualized in vivo. Fundus autofluorescence with SLO imaging provides a reliable means to delineate areas of Geographical Atrophy. The automated image analysis allows more accurate detection and quantitation documentation of atrophic areas then the manually outlining. This method is useful in longitudinal natural history studies and for monitoring effects of future therapeutic interventions to slow down progression in AMD-patients with Geographical Atrophy.[8]
DRY ARMD
The possibility of studying in vivo lipofuscin accumulation in the RPE, either by using a spectrophotometer or a confocal scanning laser ophthalmoscope, represents a step forward in recording the fundus changes with age and disease. It is hoped that this may provide a greater understanding of the pathogenetic mechanisms of retinal disease. There is clear value in recording the distribution of autofluorescence. By this method, local changes of fundus autofluorescence over time, which may be decisive in the development of a number of retinal diseases such as age related macular degeneration, can be detected. Measurements of background fundus autofluorescence, if reliable and reproducible, might allow an early diagnosis in certain retinal disorders characterised by diffuse accumulation of lipofuscin in the RPE, such as Stargardt fundus flavimaculatus, when ophthalmoscopic features and functional loss are difficult to detect, and the recognition of carriers of the abnormal gene in retinal disease such as Best’s disease. Furthermore, the detection of high levels of background fundus autofluorescence may be helpful when the differential diagnosis between pattern dystrophies and age related macular degeneration has to be established. Finally, recording the levels of autofluorescence with age may be important in detecting those at risk of visual loss and distinguishing the different phenotypes of age related macular degeneration. The potential value of this technique as used currently can be tested by comparing the variation in measurements of fundus autofluorescence obtained by different observers and that which occurs with age and retinal disease. It is evident that the latter is far larger than the former indicating that such measurements are useful in the study of disease.[9]
Schneider described ICG angiography with simultaneous microperimetry using the SLO to facilitate precise point-to-point correlation between visual function and macular pathology. They detected a relative scotoma in 19/40 eyes and an absolute scotoma in 2140 eyes, noting that eyes with well-defined CNVM had significantly deeper scotomas than eyes with occult CNVM. They suggested that the depth of the scotoma might guide physicians in selecting appropriate treatment for the CNVM.[10]
The SLO examination with an argon laser and a large confocal aperture was useful for conducting kinetic examination of the vitreous opacity above the macula. With a diode laser and a ring aperture (dark-field mode), it was possible to examine the retina from the deeper retinal layer to the choroids. On the other hand, the SLO also allows us to conduct a functional examination of fixation. It was demonstrated that the referred retinal locus of fixation may change during the follow-up period in patients whose central fixation is impaired due to macular disease, and it was showed that the fixation behaviour was related to the visual acuity. Therefore, the SLO is an ideal instrument for determining the visual field and the visual acuity before and after treatment in patients with macular disease, because of its precise localization of the examination point by directly observing the fundus and by monitoring fixation behaviour. A new programme installed in the SLO allows completion of quantitative retinal sensitivity evaluation within 2 minutes, which is difficult to do using a conventional SLO programme. In addition, the extrafoveal visual acuity of normal subjects and patients with macular disease was studied using this new SLO programme. The iso-acuity lines could be illustrated by summarizing these results in normal subjects. The SLO acuity of the horizontal meridian is significantly better than that of the vertical meridian, and even in the nasal area adjacent to the optic disc, acuity of better than 0.1 could be achieved.[11]
WET ARMD
The uncontrolled growth of choroidal new vessels (CNV) is the major cause of permanent vision loss in adults; initial treatment is unsuccessful in a large proportion of patients, particularly those with age-related macular degeneration. There are several potential factors leading to the poor prognosis. Chiefly, it is difficult to detect the onset and to localize the components of the CNV. Treatable components remain controversial.
The use of scanning laser ophthalmoscopy for ICG angiography further improves the technique because the confocal design eliminates scattered and reflected light, while the single-spot laser illumination improves contrast. Numerous investigators have described ICG angiography of CNVMs using scanning laser videoangiography. Scanning ophthalmoscopy also is an extremely versatile adjunct to ICG angiography.
Current treatment standards are based upon a careful clinical examination including fundus biomicroscopy, plus evidence from a fluorescein angiographic (FA) study that there is indeed leakage from blood vessels. These techniques have severe problems from using bright, visible wavelength light in elderly patients. Increasingly used, indocyanine green angiography (ICGA), which uses near IR light, alleviates several of these problems. However, angiographic procedures are invasive and expensive in time and personnel and carry risk.
Recent clinico-pathological correlations indicate that CNV often is much larger than indicated by FA. The structure of the membrane is complex: it is not a well-localized vascular bed, but also has surrounding components such as fibrin and can invade outer retinal structures. Often CNV is associated with a pigment epithelial detachment (PED), which further obscures the view. Detection and localization of CNV remain clinical problems.[12]
Infrared imaging (IR imaging) with a Scanning Laser Ophthalmoscope, proves a comfortable, safe alternative: rapid noninvasive detection and localization CNV, which can complement angiographic interpretation or extend treatment possibilities when angiography fails.
Generally, ICG angiography can show CNVM as localized hot spots or as diffuse hyperfluorescent plaques. Guyer and others reported the largest series of digital ICG videoangiography of occult choroidal neovascularization as determined by fluorescein angiography (FA). In 1000 consecutive cases, ICG could better reveal the occult CNVM with ICG angiography. Guyer’s[13] study produced the following results: 1) 283 cases (29%) of focal hyperfluorescent spots; 2) 597 cases (61%) of hyperfluorescent plaques that consisted of 265 cases (17%) of well-defined plaques and 332 cases (34%) of poorly defined plaques; and 3) 84 cases (8%) of combination lesions that consisted of 35 cases (3%) of marginal spots, 37 cases (4%) of overlying spots, and 12 cases (1%) of remote spots.
FEEDER VESSEL IDENTIFICATION AND THERAPY
It has been believed that feeder vessels could be rarely and exceptionally seen and generally are observed solely as extending from a laser scar to recurrent CNV along the perimeter of laser scar until the establishment of ICGA as a useful method in the diagnosis of CNV. The use of SLO makes the images of early transit phase (from filling of choroidal arterioles to that of neovascular nets) more distinct to infrared camera. In SLO 20 degrees images have double as high a resolution as 40 degrees field images. Nearly 22% of new CNVs secondary to AMD choroidal vessels were noted to be connected with neovascuar nets in an early transit phase with a 20 degree field.[14] Furthermore it has been shown that feeder vessels can be detected in 20 degrees field images with a scanning laser Ophthalmoscope.[15] Which were treated using laser photocoagulation.[16,19] In a study 227 simultaneous FA and ICGA were carried out using a confocal scanning laser ophthalmoscope. It provided high contrast images during all phases of angiography which allowed for accurate correlation of fluorescein and indocyanine green angiographies as it provided quasisimultaneous frames.[17] This procedure was safe and did not have more side effects as compared to individual procedures.
To diagnose the feeder vessel it is extremely important to detect it within the first 5 seconds of the post injection phase. A simultaneous injection of fluorescein mixed with ICG is given and a real time movie like images of the early phase is used to locate feeder vessels. Once this crucial phase has passed it becomes very difficult to isolate the presence of feeder vessel because of the enhanced fluorescence coming from the surrounding choroidal and retinal vessels which camouflage it. Feeder vessels have been classified into two distinct types.
1. Umbrella type wherein there is a central hyperfluorescence dot which branches radially into a full fledged net. Most of these vessels arise from the centre of the fovea and hence have only an academic identification value. One cannot do direct laser to these lesions since the fovea itself would get damaged.
2. Racquet type wherein there is a extrafoveal start of hyperfluorescence which eventually branches into a racquet type distribution forming a net of the membrane. These leaks can be treated with direct laser therapy since the original site is extrafoveal.
OUR EXPERIENCE
We have been able to locate feeder vessels in about 18 cases so far. Of these 11 have been the umbrella type feeder vessel and 7 were of the racquet type. Of the 7 racquet type of vessels which underwent direct laser treatment 5 resulted in closure of the vessel resulting in clearance of the exudation and surrounding haemorrhage over a period of 15 days. Two still persisted and we did transpupillary thermotherapy as an adjunct to the feeder vessel treatment.
Failure of this therapy also depends on the fact that it could be a wrong identification of feeder vessel or there are more than one feeder vessel in that membrane and so the leakage persists and of course recurrence is always a possibility.
We believe that this technique requires a lot of skill and has a learning curve especially related to identification of feeder vessels but definitely holds promise in certain cases which have a racquet type of feeder vessel.
Role of Scanning Laser Ophthalmoscope in Macular Hole Surgeries
The macular holes are seen as areas of central bright round disc outlined by a thin dark edge surrounded by a dark ring and a less dark area with ill defined limits. The SLO examination allows accurate assessment of the anatomic and functional results of macular hole surgery.[18]
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