5.7 Disease Mapping

5.7Disease Mapping

Central serous chorioretinopathy produces predictable changes in FAF imaging in relation to the chronicity of disease. Soon after the separation of the retina, an increase in autofluorescence is observed. This is associated with an increase in thickness of the photoreceptor outer segments layer by the OCT [34]. Over time, increasing atrophy occurs and the autofluorescence signal decreases. The visual acuity in eyes with central serous chorioretinopathy was found to be correlated with the normalized level of autofluorescence from the foveal region [34]. Autofluorescence photography provides a simple method of mapping the areas of involvement in central serous chorioretinopathy (Fig. 5.5). Although serous detachment can be determined with OCT, a montage image of the fundus can be obtained in a matter of seconds using autofluorescence imaging.

Pseudoxanthoma elasticum (PXE) is caused by a mutation in the ABCC6 gene; more than 300 distinct loss-of-function mutations representative of over 1,000 mutant alleles in ABCC6 have been found [35]. Many of the missense mutations occur at locations in the protein involving domain–domain interactions in the ABCC6 transporter [36]. Even heterozygotes can show disease manifestations [37]. FAF abnormalities are very common in PXE. Eyes of patients with PXE can have angioid streaks, peau d’ orange, and drusen of the optic nerve, all of which have autofluorescence correlates. Eyes with PXE can show what has been termed a pattern dystrophy, this pigment patterning shows increased pigmentation and is associated with subretinal accumulations of the material [38]. Large areas of RPE atrophy can occur, even in areas not suspected to have RPE abnormalities, and were first

documented with autofluorescence photography [39]. Choroidal neovascularization is a common secondary consequence and is readily visible during FAF imaging. All of these changes are readily visible using autofluorescence imaging as a screening tool.

Fig. 5.5 Chronic central serous retinopathy as imaged by the modified fundus camera shows decreased FAF in the macula due to atrophy. Additional FAF abnormalities outside the central macula, including prominent descending tracts are visualized. Clinically, the patient has no signs of subretinal fluid

46 5  Autofluorescence Imaging

5.8  Functional Correlation

The relevance of alterations in FAF images can further be addressed by assessing corresponding retinal sensitivity.

5  Severe damage to the RPE such as atrophy, melanin pigment migration, or fibrosis leading to compromised photoreceptor function as confirmed by microperimetry is topographically confined to decreased autofluorescence [40, 41]. In patients with geographic atrophy secondary to AMD, it has been shown that – in addition to the absence of retinal sensitivity over atrophic areas – retinal function is relatively and significantly reduced over areas with increased FAF intensities when compared with areas with normal background signal [40]. Localized functional impairment over areas with increased FAF has also been recently confirmed in patients with early AMD. Using fine matrix mapping, it has been demonstrated that rod function is more severely affected than cone function over areas with increased FAF in AMD patients [41]. These studies are in accordance with the observation of increased accumulation of autofluorescent material at the level of the RPE prior to cell death. As the normal photoreceptor function is dependent on normal RPE function, in particular with regard to the constant phagocytosis of shed distal outer segment stacks for photoreceptor cell renewal, a negative feedback mechanism has been proposed, whereby cells with lipofuscin-loaded secondary lysosomes would phagocytize less-shed POS, subsequently leading to impaired retinal sensitivity. This would also be in line with the experimental data showing that compounds of lipofuscin such as A2-E possess toxic properties and may interfere with normal RPE cell function via various molecular mechanisms including impairment of lysosomal function [23].

In patients with retinitis pigmentosa and cone dystrophies, parafoveal rings of increased FAF have been identified in the absence of funduscopically visible correlates, which tend to shrink or enlarge with disease progression, respectively (Fig. 5.6) [42, 43 ]. Interestingly, functional testing using microperimetry and electrophysiology indicates that these rings demarcate areas of preserved photoreceptor function. In retinitis pigmentosa, a gradient loss of sensitivity is present outside the arc of the ring with increasing eccentricity.

5.9  Future Applications

for Therapeutic Interventions

In patients with advanced atrophic AMD, FAF imaging may also be helpful to develop and assess new emerging therapeutic strategies. Visual cycle modulators, which aim to

target the detrimental accumulation of toxic by-products of the visual cycle in the RPE, appear as promising pharmaceutical agents to slow down the progression of atrophy. Fenretinide (N-[4-hydroxyphenyl] retinamide), an oral compound, has been shown to lower the production of toxic fluorophores in the RPE in a dose-dependent manner in the albino ABCA4−/− mice [44]. This vitamin A derivate acts by competing with serum retinol for the binding sites of retinalbinding protein and promotes renal clearance of retinol. The bioavailability of retinol for the RPE and photoreceptors is consequently reduced and less toxic retinoid by-products such as A2-E may be generated. A Phase II randomized, double-masked, placebo-controlled multicenter study, which aims to include over 200 GA patients, has been already initiated in the USA in 2006 (Sirion Therapeutics, Inc.; Tampa, FL; http://www.siriontherapeutics.com). The therapeutic concept of Fenretinide is not only underscored by previous FAF findings. To reduce the observational period in a slowly progressive disease, to minimize the sample size, and to better demonstrate possible treatment effects, the patient recruitment in this study involves the identification of high-risk features in this first large interventional trial in patients with geographic atrophy secondary to AMD.

In retinal dystrophies, FAF imaging may be used to assess the potential functional preservation of the outer retina, and would therefore serve as an implication for future treatment. In patients with Leber congenital amaurosis having vision reduced to light perception and undetectable ERGs, normal or minimally decreased FAF intensities have been reported [45]. This suggests that the RPE/photoreceptor complex is, at least in part, functionally and anatomically intact and would indicate that the photoreceptor function may still be rescuable.

Because of the increased sensitivity and improved sig- nal-to-noise ratio, modern confocal scanning laser ophthalmoscopy allows – in addition to the well-known FAF with blue excitation light as described earlier – for the visualization of FAF phenomena by near-infrared light [8, 10]. Hereby, the so-called ICG-mode is used. Application in patients and case reports in animal models and donor eye suggest that near-infrared FAF detects fluorophores at the level of the RPE. Melanin has been postulated as the major candidate. However, no spectral analyses as for lipofuscin for blue light FAF have been reported.

The recent introduction of combined simultaneous spectral-domain optical coherence tomography (SD-OCT) with cSLO imaging in one instrument with real-time eye tracking that allows for the accurate orientation of OCT-scans and therefore for the 3D mapping of pathological changes at specific anatomic sites represents an important step forward to better investigate the origin of the FAF signal within the retina (Fig. 5.7) [46–50].