The physiological process of human embryonic retinal vascularization consists of vasculogenesis and angiogenesis. Through vasculogenesis, blood vessels are generated by the endothelial cells' differentiation from their progenitors. This process takes place between the 12th and 21st week of gestational age. Angiogenesis is the process that leads to the formation of the superficial plexus. During angiogenesis, the retinal capillary plexus is generated from the optic nerve retinal vessels, branched and elongated towards the peripheral retina. These new vessels reach the nasal and temporal periphery at 32 and 36–40 weeks of gestational age, respectively (10).
ROP is a vasoproliferative disease caused by Vascular endothelial growth factor (VEGF) production due to retinal ischemia. IVB has been suggested as a treatment for ROP in the presence of aggressive posterior ROP, miotic pupil, or media opacity. Its advantages over laser therapy are availability, a simple technique of injection, and the preservation of the visual field. It has some disadvantages compared to laser therapy, including endophthalmitis, cataract formation, and a higher recurrence rate, especially after a more extended period (9). The major risk factors for ROP are prematurity (low gestational age and birth weight) and supplemental Oxygen therapy. More advanced neonatal care and specialized hospitals could reduce the ROP incidence rate, as small hospital studies compared with tertiary referral hospitals approve of it (11). When the gestational age is lower, the area of the ischemic retina is more extensive. As a result, the concentration of VEGF in the vitreous cavity would be higher, so the probability of type 1 ROP requiring treatment would be higher (12).
ROP recurrence is the redevelopment of plus disease, pathological new vessels, or elevated ridge following a complete regression of ROP following treatment (13). The recurrence rate in our study was 13.26%, which was relatively lower than in similar studies. A study comparing laser photocoagulation and intravitreal Ranibizumab injection showed an 18.3% recurrence rate in the Ranibizumab group. However, the mean interval between the injection and recurrence was similar (8.08 vs. 9.3 weeks) (14). On the other hand, another study showed a much lower recurrence rate (4.04%) after IVB injection in ROP cases. This lower recurrence rate might be related to the lower prevalence of Zone I ROP (21.6%) in the sample size (8).
In our study, low birth weight was a risk factor for ROP recurrence. However, lower gestational age was not statistically significant (the median gestational age was 26 weeks in the group with ROP recurrence compared to 29 weeks in those without recurrence). This statistically insignificant difference can be due to the study's low recurrence rate and small sample size. Most studies have confirmed that lower gestational age and weight are risk factors for ROP and its recurrence (7–9). However, logistic regression in a large study revealed that the Gestational age (GA) and Birth weight are not independent risk factors for the recurrence after Ranibizumab (14). The possible relationship between lower GA and Birth weight with ROP recurrence can be explained by more immature retinal vasculature, which leads to a larger area of ischemic retina, as mentioned above. Although GA was not a statistically significant risk factor for recurrence in our study, the lower postmenstrual age at IVB injection was associated with ROP recurrence. The Iwahashi et al showed a higher rate of recurrence in neonates receiving anti-VEGF threpy earlier than 35-week of postmenstrual age (15).
Supplemental oxygen therapy has been described as a risk factor for ROP development (2, 11, 16). In this study, we investigated the role of oxygen therapy after IVB injection. The number of children who received Oxygen after IVB injection and the duration of oxygen therapy were higher in the group with ROP recurrence. Similar to our study, the study by Ling et al. showed that oxygen therapy after IVB injection or intravitreal ranibizumab injection was associated with higher ROP relapse (9). It has been declared that O2 saturation fluctuations are more related to higher oxidative stress and ROP occurrence than steady prolonged hyperoxia (17). So, infants who need oxygen therapy even after IVB injection may have more unstable respiratory and metabolic conditions, leading to more oxidative stress, which induces ROP recurrence.
Besides, we have to take into account that these children are hospitalized due to respiratory problems or other systemic conditions that can worsen retinal hypoxia or ischemia. Premature infants are more prone to other exacerbating conditions, such as infections, and may have more severe forms of retinopathy due to the lower compliance of anti-oxidant metabolic pathways (8). With lower oxygen saturation, the rate of mortality increases; with higher oxygen saturation, the rate of ROP increases (18). So, it is hard to determine which oxygen saturation level suits these patients.
Similar to Ling et al. study (9), those infants who received IVB injection sooner after birth had a higher probability of ROP recurrence (the median of 30 days in those with ROP recurrence versus 50 days in those without relapse). This difference shows that the imbalance between angiogenic and anti-angiogenic agents occurs at a shorter period in these children, leading to the development of type 1 ROP. So, the probability of reaching this unbalanced state in the future after IVB injection is higher among these infants.
Recent studies hypothesized that lower Insulin-like growth factor-1 (IGF-1) levels at birth could be related to the ROP occurrence, and infants' nutrition could alter the IGF-1 levels (19). In addition, a systematic review demonstrated a protective role for human milk intake against the ROP and its severe forms (20). We aimed to find any possible association between the infants' diet type and ROP recurrence, which is not addressed well in the literature; however, we did not find any significant relationship.