In this study, we described the presence at fundus examination of HSs, consisting of rounded, hyperpigmented areas of variable size and number with blurred margins, in 24.1% out of 249 patients diagnosed with NF1. Of note, HSs were not detectable within the control group.
Interestingly, no significant difference was observed between the group of patients with ocular HSs and the group of patients without HSs in terms of age, gender or severity grading of the disease.
Specifically, the absence of significant correlation between the severity of NF1 and the presence of HSs suggests that they are not a negative prognostic factor of the disease.
After comparative analysis, we noticed that pigmentary lesions corresponded in location, size and morphology to lesions previously described as NF1-related choroidal nodules.
Choroidal nodules are known as ovoid bodies consisting of proliferating Schwann cells arranged in concentric rings around an axon. Histologic studies showed hyperplastic Schwann cells, melanocytes, and ganglion cells in the ovoid bodies [19, 20]. These abnormalities appear as hypofluorescent patches in the early phase of indocyanine-green angiography and as bright patchy lesions under infrared monochromatic light; they are undetectable in conventional ophthalmoscopic examination or by means of autofluorescence (FAF) and fluorescein angiography (FA) [21, 22]. More recently, Kumar V. et al. identified hyper-flow areas on optical coherence tomography angiography (OCTA) of deep choroid corresponding to the bright patches of the choroidal nodules on NIR imaging .
Our results suggest that choroidal nodules, all visible on infrared light, may have different level of extension in the deeper choroid and different degree of pigmentation, reaching the level of visibility at fundus examination only in a minority of cases. In this respect, optical coherence tomography B-scan through the fovea showed choroidal nodules as having different levels of inward extension in the deep choroid. The most hyperreflective areas on infrared light matched well with the most pigmented areas on indirect ophthalmoscopy, UWF color and red laser images, and with the maximum extent from the outer to the inner choroid on cross-sectional SD-OCT scans. This explains the different frequency of presentation of choroidal nodules and HSs in our sample, showing percentages of 95.6 Vs 24.1, respectively. Furthermore, in this series, HSs were predominantly distributed to the posterior pole similar to the location of choroidal nodules, as the choroid within the main retinal vascular arcades is considerably rich in melanocytes and neural cells, from previous evidence . With reference to the pathogenesis of HSs, melanocytes embryologically recognize a dual origin: the neural ectoderm, more specifically, the outer layer of the optic cup, which generates the melanoblasts that migrate in the pigmented epithelium of retina, iris and ciliary body, and the neural crest that gives rise to the melanoblasts that populate the uveal portion of the eye composed of choroid, the stroma of iris and ciliary body, in addition to the melanocytes of skin and hair . The same origin of melanocytes, meningoblasts and Schwann cells from the neural crest is responsible for the occasional association between pigmented lesions of the uvea or skin, meningiomas and neurofibromas . However, patients with NF1 also develop tumors that are not derived from the neural crest. For example, the optic glioma and retinal hamartomas are of astrocytic and, therefore, of neuroectodermal origin. The café-au-lait spots of the skin are the result of an increased production of melanin in the basal and spinous layers of the epidermis. The hyperpigmentation is imputable to an hyperactivity of the melanosomes and / or to an increase in the number of melanocytes [26–28]. At ocular level we could hypothesize the same pathogenic mechanism, ascribing the origin of the HSs of the fundus to an increase in pigment production and/or an increase in the proportion of melanocytes in the choroidal nodules. The interaction between light and intraocular tissues plays an important role in the interpretation of optical methods for diagnosing ocular disease. The retina is nearly transparent, however, the pigments contained in the RPE and choroid interact strongly with the imaging lights over a range of wavelengths. The most representative pigments of RPE are lipofuscin and melanin. The choroid also contains melanin, with an embryonic origin and different optical and biochemical properties from its RPE counterpart. The proliferation of melanocytes in NF1 patients causes a patchy choroidal thickening, resulting in a strong absorption and a subsequent backscattering of near-infrared light through the high content of melanin [29–31]. In our series, the contribution of the choroid melanin is definitely more relevant since HSs are visible under indirect ophthalmoscopy exam, NIR-OCT and red laser images, but not under GAF. More in detail, the green laser light that is used in FAF is markedly absorbed by the pigment epithelium and therefore may only detect alterations if this cell layer is also affected. In this context, we combined NIR-OCT and UWF scans to achieve a complete instrumental investigation.
Previous articles described pigmentary lesions at fundus examination in NF1. However, they were confined to a report of two cases published in 1978, and a more recent single patient report, both describing retinal café au-lait macules [32, 33].
Specifically, Cotlier E. reported pigmentary changes at fundus examination, resembling cutaneous café au-lait spots, in association with retinal hamartomas . The lesions were described as lightly pigmented, sinuous, not elevated and not well defined, differing considerably from those observed in our group in terms of color, morphology, number and size. In addition, the concomitant presence of retinal hamartomas may be likely to indicate an exudative nature of such lesions. Then, mild choroidal hypofluorescence was described in the areas of café-au-lait pigmentation on fluorescein angiography, whereas choroidal nodules are known to be undetectable on FA. However, it is not possible to carry out adequate comparison with our findings given the absence of further instrumental exams and the scarce technical means available at that time. More recently, Venkatesh R. et al. reported retinal café-au-lait macules in a patient with NF1 . Specifically, the lesions appeared as pale, light-brown coloured and flat, hypo-autofluorescent on FAF, and hyperreflective on blue, green and infrared reflectance images. Thus, clinical and instrumental appearance of these abnormalities seems to deviate much from both the findings of Cotlier E. and the HSs identified in our series.
Intriguingly, our results showed a statistically significant association between the presence of HSs and neurofibromas (p = 0.047), that could possibly be related to the high content in Schwann cells and melanocytes and the same derivation from the neural crest [25, 34].
In addition, a statistically significant association was found between the presence of HSs and NF1-related retinal microvascular abnormalities (p = 0.017).
Previous studies investigated the relationship between choroidal nodules and overlying retinal microvascular changes in NF-1 [23, 35–37]. It was hypothesized that angiogenic factors secreted by the underlying choroidal nodules could have an effect on retinal vasculature . Other Authors speculated that functional disorders of vasomotor nerve cells, which originate in the embryonal neural crest, could have a role in the development of retinal microvascular alterations in NF1 patients . A recent study reported an abnormal retinal vessel along with thinning and low flow areas overlying the choroidal nodules at the level of choriocapillaris on OCTA in NF-1. Thus, the nature of retinal vascular changes in NF-1 and the association with the underlying nodules remain unclear. Based on previous evidence, we speculated that HSs, corresponding to hyperpigmented and inward extended choroidal nodules, could possibly have a role in the development of retinal microvascular abnormalities.
Therefore, we believe that investigation on angiographic retinal and choroidal features in relation to the presence of HSs is firmly recommended.
In our sample, the HSs showed a diagnostic sensitivity of 24.1%, with high diagnostic specificity and positive predictive value (100%). However, given the rarity of HSs, it would be advisable to implement the study sample to reach conclusions on diagnostic accuracy.