**4.2 Epiretinal membrane**

Epiretinal membrane comprises a layer of cellular proliferation on the inner surface of the retina. Epiretinal membranes are frequently seen in premature neonates [17]. Lee et al. reported that epiretinal membranes were present in 32% of cases evaluated for ROP while. They were detected by OCT imaging while not seen on indirect ophthalmoscopy examination. In nearly a third of the patients the epiretinal membrane generation foveal deformation with loss of the fovea depression [29]. The association was made between epiretinal membrane and the development of vitreous bands suggesting a tractional pathogenesis of this finding in the affected infants [20, 30]. Nonetheless, the exact clinical significance of epiretinal membrane in premature infants with ROP remains unknown, and more studies are needed to evaluate its value.

#### **4.3 Immature retinal and choroidal morphology**

Overall inner retina layer thickness at the foveal center decreases and outer retinal thickness at foveal center increases over time in the preterm period, which is driven by centrifugal and centripetal displacement of inner and outer retinal cells, respectively.

#### **Figure 1.**

*Differences of foveal OCT B-scan in the healthy adult and developing retina. (A, C) Foveal OCT B-scan image from a 24-year-old adult born at term age. (B, D) Foveal OCT B-scan image from a 34-week postmenstrual age (PMA) infant (born at 25 weeks GA, birth weight 605 g). From top to bottom, the retinal layers are: Retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), external limiting membrane (ELM), myoid zone of photoreceptors (MZ), ellipsoid zone of photoreceptors (EZ), outer segments of photoreceptors (OS), interdigitation zone (IZ), and retinal pigment epithelium (RPE)/Bruch's complex. Inner retinal layers (IRL) are indicated by the blue vertical line. Outer retinal layers (ORL) are indicated by the orange vertical line. Note that the ELM, MZ, EZ, and IZ are not apparent in the immature developing retina (B, D). Adapted from [31] with permission.*

It has been reported that compared to term born infant, premature infant or ROP infant eyes usually has shallower foveal pit, retain inner retinal layers at foveal center, thinner outer retinal layers, and indistinctive external limiting membrane band and ellipsoid zone (**Figure 1**) [32–34].

Choroid is another component of the eye that could be impacted in premature or ROP infancy. It has been reported that several factors including gestational age, ROP status, pulmonary status and oxygen supplementation may affect choroidal thickness [8, 22, 35, 36]. However, the impact of changes in choroidal thickness on long-term visual outcomes is still under investigation.
