This study reports QAF development during childhood in a large healthy Caucasian cohort. Our major findings comprise the early detection of AF at the posterior pole, an age-related increase in AF, an AF distribution at the posterior pole similar to that of adults, and correlation of AF and retinal thickness.
Earlier reports on clinical AF included QAF data from only a few healthy children.
7 Greenberg et al.
7 examined around 10 Caucasians, 18 Hispanics, and 8 African Americans in the age range of 5 to 20 years. Since gender might also impact QAF,
7 larger studies on QAF in healthy children are essential for a better understanding of AF development.
QAF, although still at an experimental stage, resolves the limitations of only qualitative evaluation of FAF images and enables quantitative measurements among participants and in follow-ups. Since its release in 2011,
6 QAF values have then been reported for normal aging
7 and in several retinal diseases.
12 However, previous studies reported only one QAF value for each individual eye (QAF8, mean of QAF values from eight segments at the parafoveal region), with a risk of missing information for pathologic lesions from areas outside the QAF8 ring. Also, there were no appropriate patterns for structural and other functional tests that fit the QAF8 ring pattern, which complicates adequate structural-functional testing.
13
Structure-function correlations require measurements at the exact same locations. In this context, we recently discussed the need for alternative QAF analysis patterns that enable a more detailed analysis and correlation with other imaging modalities or functional tests at distinct posterior pole regions
8 or pathologies.
12 Our functional (QAF)–structural (retinal thickness in SD-OCT) correlation revealed that in our young cohort, increasing QAF is related to a thickening of the RPE, which parallels findings from other studies.
13 In addition, in our study, diminished QAF intensities was found with thickening of the inner retina.
Our data on retinal thickness along the horizontal meridian are comparable to other studies in children with Caucasian ethnicity.
14,15 While the thickness of the
whole retina nasal and temporal to the fovea did not change with age, the thickness at the foveola changed significantly, as shown in both other OCT studies and histologic studies in children´s retina.
14–17 The increase in thickness at the fovea can be explained by the elongation of the photoreceptor´s inner and outer segments and an increasing packing density until early adulthood.
18,19 Thus, the results presented support the findings of recent studies that the development of the fovea is not completed at the age 5 years, as previously assumed, but continues.
19 The QAF intensities were low at the fovea since blue light–absorbing macular pigment has its peak at the fovea.
20,21 Furthermore, the characteristic distribution of intracellular RPE granules in relation to the fovea with high amounts of melanosomes and melanolipofuscin granules at the fovea nicely explains the low QAF signals, although previously examined in adults only.
22
In our study, outer retinal thickness (external limiting membrane to Bruch membrane distance) at the 11 points along the horizontal meridian did not change with age, which seems to contrast with findings from others, but might be related to methodologic differences.
13 Using the ETDRS grid, we also observed a thickening of the RPE/Bruch membrane complex, which could reflect the increasing accumulation of autofluorescent intracellular granules (lipofuscin, melanolipofuscin). At birth, RPE cells contain melanosomes only.
4 Histologic studies showed that during the next years of life, RPE cells accumulate autofluorescent material within melanolysosomes, while only few lipofuscin granules can be found within RPE cells of young children.
4,23 The small number of RPE lipofuscin granules nicely mirrors the low QAF signal in the youngest of our examined children. This phenomenon can be explained by the immature posterior pole at birth (RPE and photoreceptor system),
24 which then, with aging, increasing photoreceptor density, and visual cycle metabolism, leads to augmented lipofuscin and melanolipofuscin accumulation.
4,5 As the number of lipofuscin and melanolipofuscin granules increases, also QAF intensity increases. However, overall QAF intensity remains at low levels during the first two decades and does not increase until the end of the second/early third decade (
Fig. 2C), also previously shown in histology.
4 In addition, an interesting finding is that young participants show a narrow QAF variability that seems to broaden in adults and the elderly (
Fig. 2C).
The finding that thickening of the inner retina could lead to diminished QAF intensities reminds us that autofluorescence is not only related to the accumulation of autofluorescent granules within the RPE. Any changes in tissue transparency in the inner or outer retina (e.g., normal and pathologic tissue thickening or thinning,
25,26 accumulation of light-blocking pigments or material,
27,28 accumulation of hyperautofluorescent material,
11 and others) could lead to altered FAF, both increased and decreased, independent from the amount of the underlying autofluorescent RPE granules.
Standard retinae were generated for two age groups (5–10 and 11–18 years) and can be used for comparisons with retinal pathologies in age-matched cohorts,
29 modified as needed, and supplemented in future studies. The standard retinae reveal an increase of QAF intensities throughout the posterior pole. Noteworthy, in our cohort, the AF pattern at the posterior pole with the highest QAF signals at the temporal/temporal-superior region is identical to the pattern observed in adults.
6,8 As for adults, the reason for this increased AF signal remains speculative.
Gender-related differences in QAF with higher values in females were previously reported in a group with multiple ethnicities.
7 In our Caucasian children cohort, these gender differences were not or not yet observable. However, we were able to detect differences in retinal thickness, with boys having significantly thicker retinae compared to girls, partly in line with results from other studies in children.
14–16 In adults, these gender differences seem to be even more obvious for the whole retina.
30,31
Limitations of our study include the cross-sectional character and the examination of each participant at one time point only, which does not allow any statements on the development of QAF in an individual. Also, the changing of axial length in the growing eye in children could affect QAF measurements.
6 Future studies following up these children could address these issues. A relatively high number of children (24%) had to be excluded due to lack of cooperation and subsequent poor QAF image quality, an issue that might be related to the young age of our participants or our strict rules for grading image quality. However, even 5-years-olds have perfectly cooperated during image acquisition. Variability in our study cohort was comparable (around 10%) to the QAF measurements in healthy adults,
7,8 which also underlines that QAF can perfectly be used for fundus imaging in children. Although interesting, it is still unclear how QAF develops in the first years of life (i.e., from birth to age 5 years). Strengths of our study include the development and use of instruments for precise structure–function correlation at exactly the same locations, a pioneer in QAF interpretation.
In conclusion, we present QAF of the largest cohort of Caucasian children so far. QAF is measurable early in life and increases with aging. The QAF distribution across the posterior pole is similar to adults, with highest QAF intensities at the superior-temporal parafovea. While overall retinal thickness did not differ between the two age groups, a thickening of the RPE was observable, which might relate to the increasing accumulation of autofluorescent RPE granules. Future studies with the same participants can report QAF development of individual participants.