In this report we showed that autofluorescence lifetimes originating from areas with remaining photoreceptor cell layers feature short decay times independent of the presence or absence of the underlying RPE. As such, additional information can be obtained on the state of photoreceptors in patients with CHM and may be helpful to further our understanding of the pathophysiology of this rare disease.
Although the underlying defect in CHM can be specifically attributed to a mutation within the gene that encodes
REP-1 with subsequent retinal degeneration, characteristically for CHM,
18 the exact pathomechanism of this hereditary disease has not yet been resolved. There is controversy about which layers are primarily affected by the disease and which undergo consecutive degeneration. Recently, wave front analysis using adaptive optics confirmed evidence from previous reports that the RPE is one primary site of degeneration in CHM.
18 In our current study using FLIO for measurement of autofluorescence lifetimes of the ocular fundus in CHM, we were able to identify areas with remaining photoreceptors in the absence of the RPE. A striking finding is that, in contrast to areas with photoreceptor atrophy which display very long fluorescence lifetimes, these areas feature short fluorescence lifetimes. Two findings associated with short fluorescence lifetimes merit further discussion. In the first instance, data from other studies
19 and from our previous studies
7 suggest that components of the visual cycle such as all-
trans retinal (atRAL) display very short lifetimes. Under conditions of visual cycle dysfunction, retinoid cycle by-products have been shown to accumulate in outer segments of photoreceptor cells and the RPE.
20,21 Therefore, short autofluorescence lifetimes found in areas of remaining photoreceptors may indicate that there is some remaining activity of the visual cycle and generation of visual cycle by-products. This hypothesis is further supported by findings in a mouse model of retinal detachment where autofluorescence measurements revealed hyperfluorescent spots within the area of detached retina which were spectroscopically similar to the bisretinoids that constitute RPE lipofuscin.
22 This explanation for the short autofluorescence lifetimes observed in areas of RPE atrophy, but remaining photoreceptors are speculative but consistent with previous hypothesis on the pathophysiology of CHM.
On the other hand, recent data suggest that distinct patterns of short fluorescence lifetimes within the fovea derive from macular pigment.
9 In patients with CHM, macular pigment present in the plexiform layers and the photoreceptor axon layers in areas with RPE atrophy may therefore lead to short fluorescence lifetimes, at least in the fovea (
Fig. 2). It was previously shown in a case series that macular pigment optical densities in patients with CHM do not differ from those in age-matched healthy controls.
23 Although we did not quantify this, this appears to be in keeping with our findings. However, in cases of advanced chorioretinal atrophy involving the center, such as seen in
Figure 4, the area with short lifetimes disappears. Thus, the distribution of very short fluorescence lifetimes within the macular center may be used as a follow-up parameter and may serve as an indicator of the integrity of the Henle fiber layer and the outer plexiform layer containing the macular pigment.
However, even in areas with intact RPE verified by autofluorescence imaging, fluorescence lifetimes are already prolonged compared to corresponding areas in age-matched healthy controls. Accumulation of lipofuscin derivatives, which have been shown to display long lifetimes (1262 ps),
12,24 could explain our findings. This supports the hypothesis that the RPE might be the primary site of degeneration.
1 Additionally, loss of photoreceptors and concomitant decrease of visual cycle by-products may also lead to longer fluorescence lifetimes in areas with intact RPE. The latter is supported by adaptive optic findings with cone density loss in such areas of patchy RPE damage.
18 The information about RPE function and loss of photoreceptors in remaining RPE islands will be useful to assess disease activity and serve as control for future therapies in the field of gene therapy.
1,4,25
The option to obtain information on individual lifetime components such as
T1 and
T2 and the corresponding amplitudes by 2D plotting of individual lifetime clusters allows visualization of the distinct aspects of disease progression over time and will be useful to monitor therapeutic approaches by quantifying changes in lifetime clusters over time as seen in
Figure 5. Furthermore, FLIO could provide information about the location where to deliver the subretinal vector deposits: on one hand to not harm the remaining RPE and on the other hand having a higher chance to integrate in areas with still existing photoreceptor cell structures.