**5.2 Inherited retinal dystrophies**

Inherited retinal dystrophies are a heterogenous group of diseases that result from mutations in various genes with consequent changes in retinal metabolism that result in photoreceptor loss. The most common retinal dystrophies include vitelliform dystrophy (**Figure 10**), Stargardt's disease (**Figure 11**), and retinitis pigmentosa (**Figure 12**). In Stargardt disease, mutations and subsequent decrease in ABCA4 gene activity results in excessive lipofuscin deposition in RPE. The characteristic dark choroid appearance in FFA of Stargardt results from absorption of the blue excitation light by the lipofuscin pigment in RPE. B-scan OCT in Stargardt disease demonstrates the outer retinal changes of RPE irregularities and shaggy photoreceptor layer with outer retinal loss in the advanced stages. Recently, OCTA showed severe choriocapillaris atrophy coinciding with the areas of RPE and photoreceptor loss that were previously undetectable by FFA due to RPE lipofuscin deposits. Vitelliform dystrophy is due to a mutation in the BEST gene and hence the other name, that is Best disease. In the early stages of the disease, there is progressive deposition of subretinal vitelliform, egg-yolk-colored material, that is vitelliform stage, which then starts to resolve leaving behind variable degrees of photoreceptor loss and may become complicated by CNV. Therefore, OCTA can be very useful to detect the CNV complication of vitelliform dystrophy which is mostly undetectable by either FFA or B-scan OCT due to masking and projection artifacts respectively. Retinitis pigmentosa is an umbrella term for a group of hereditary retinal diseases that share some common clinical and pathological features. Retinitis pigmentosa is historically characterized by the clinical triad of retinal arteriolar attenuation, waxy optic disc pallor, and RPE proliferation and intraretinal migration, which manifest as bone spicule retinal pigmentations. While the main cause of visual impairment in retinitis pigmentosa is photoreceptor loss, other complications may concur and contribute to visual morbidity, for example

#### **Figure 10.**

*Adult Vitelliform dystrophy, SD-OCT showed a hyper-reflective material lies between the RPE/Bruch's membrane complex and the photoreceptors layer (ellipsoid zone). In OCTA, there is displacement of blood vessels at the level of the superficial and deep capillary plexus caused by the accumulation of the subretinal material with rarefaction of the choriocapillaris layer.*

#### **Figure 11.**

*Stargardt's disease, OCTA showed rarefied superficial and deep capillary plexus with decreased vessel density, decreased flow index, and more exposure of the choroidal blood vessels.*

#### **Figure 12.**

*Retinitis pigmentosa case showed an irregular widened FAZ at the level of the superficial and deep plexus with areas void of flow mainly in the superficial plexus. Diffuse loss at the level of the RPE and the choriocapillaris with exposure of the large choroidal blood vessels findings confirmed by the B-scan showing cystoid oedema and loss of the inner/outer segment as well as the affection of the RPE/choriocapillaris complex.*

macular oedema. Consequently, OCTA can prove useful in detecting macular oedema as well as retinal vascular plexus changes in retinitis pigmentosa [17, 18].

The B-scan showed diffuse foveal atrophy with hyperreflective external limiting membrane, loss of the ellipsoid zone, and invariance of retinal tissue through Bruch's membrane.
