Confocal scanning laser ophthalmoscopy

In conventional fundus photography, the entire fundus is flooded with a bright flash to visualize its composite structures. In confocal scanning laser ophthalmoscopy (cSLO), a small focused laser point is rapidly swept across the retina pixel by pixel in a raster pattern. Because imaging is confocal, interfering stray light from adjacent structures is minimized, thereby improving contrast. The use of several laser sources permits the imaging of the retina, RPE, and the optic nerve by different wavelengths according to their respective absorption, reflection, and excitation properties (Fig. 9.1). Confocal imaging also enables in-depth structural analysis of the retina and optic nerve, layer by layer, and three-dimensional digital reconstruction, e.g. in the Heidelberg Retina Tomogram (HRT, Heidelberg Engineering, Heidelberg, Germany). The latest SLOs offer not only facilities for digital fluorescein/indocyanine green angiography, but also autofluorescence, red-free and infrared imaging as well as high-resolution Fourier-domain optical coherence tomography (OCT) all in one machine (Spectralis, Heidelberg Engineering, Heidelberg, Germany).

Fig. 9.1 Light of different wavelengths penetrates and is reflected and absorbed differently by different retinal structures. This is why the same fundus, in this case of a patient with Stargardt’s disease, reveals different patterns and extent of involvement with conventional color photography (A) and confocal scanning laser autofluorescence (B), infrared (C), and red-free (D) imaging.

Spectralis, Heidelberg Engineering, Heidelberg, Germany.

Autofluorescence

Retinal autofluorescence² relies primarily on the content of fluorophores in the lipofuscin granules of RPE cells. Therefore, it serves as a non-invasive indicator of the health of the RPE and outer retina: increased autofluorescence indicates abnormal accumulation of lipofuscin in the post-mitotic RPE cell. It therefore serves as an indicator of RPE dysfunction and is seen in a large number of retinal disorders, for instance in Best’s and Stargardt’s disease. Loss of autofluorescence is an indicator of RPE atrophy.

Normally, the optic disc is not autofluorescent due to the absence of RPE cells in the optic disc area. However, focal hyper-autofluorescence is pathgnomonic for superficial optic disc drusen. Because the autofluorescence signal is two orders of magnitude smaller than the fluorescence in fluorescein angiography, autofluorescence scanning needs to be performed before fluorescein is administered for angiography.

Fluorescein and indocyanine green angiography

Digital SLO angiography provides much greater temporal resolution and detail than is possible with conventional serial photography.³ Unlike in adults, fluorescein (excitation maximum at 490 nm) and indocyanine green (excitation maximum at 805 nm) angiography is uncommonly used in children due to a combination of factors: rarer appropriate pathology and practical concerns of more difficult intravenous access (though oral administration is possible) and administration of intravenous drugs in a child in an eye unit. If angiography in a child is deemed necessary, it must only be carried out with all necessary equipment, drugs, and medical staff trained in pediatric resuscitation available.

Red-free and infrared imaging

Red-free imaging is particularly useful for highlighting vascular structures and nerve fiber layer defects in the inner retina. Red-free imaging is available with some scanning laser ophthalmoscopes (see Fig. 9.1) and, of course, by using the green filter on the slit-lamp or direct ophthalmoscope. Infrared imaging has been studied in Stargardt’s disease and may play a particular role in visualizing subretinal structures.^4,5

Wide-field imaging

The RetCam system (Clarity Medical, Pleasanton, California, US) provides wide-field imaging of up to 130° (Fig. 9.2). Because it can be used to visualize and document the posterior and much of the peripheral retina, it is common in screening for retinopathy of prematurity and the documentation of non-accidental injuries in babies. In addition to color images, it can also be used for fluorescein angiography. It requires eye contact.

Fig. 9.2 Wide-field RetCam imaging of a normal posterior fundus in a premature baby.

The Staurenghi 150° contact lens has been used in suitable older patients to obtain high-resolution wide-field cSLO autofluorescence, infrared, red-free, fluorescein, and indocyanine angiography imaging.

Ultra-wide-field confocal scanning laser

A further technological advance has been the development of ultra-wide-field confocal scanning laser imaging (Optos, Dunfermline, UK). Using an internal parabolic mirror, the scanner can capture up to 200° internal angle (Fig. 9.3

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