Retinopathy of prematurity (ROP) is a retinal vascular disease that affects premature infants in whom the retinal vasculature is not fully developed at the time of birth. The hyperoxic environment after birth—inclusive of room air—compared with the relative hypoxic intra-uterine environment suppresses the growth of retinal vessels (phase 1). Subsequently, growth of the retina increases metabolic demand and, in the background of incomplete retinal vascularisation, results in retinal hypoxia and ROP development (phase 2).1 2 3 Retinopathy of prematurity is characterised by the presence of abnormal retinal fibrovascular proliferation. Severe disease will progress to total retinal detachment and blindness if there is no intervention during the critical treatment period. Such retinopathy is one of the leading causes of visual impairment in children.4 5 6

The risk factors for ROP include low gestational age (GA), low birth weight (BW), presence of co-morbidities (eg patent ductus arteriosus [PDA], necrotising enterocolitis (NEC), intraventricular haemorrhage [IVH]), and a high level of supplemental oxygen.7 8 9 Those born at extreme prematurity and with extremely low birth weight (ELBW) are at high risk of severe ROP development. The British Guidelines published by the Royal College of Paediatrics and Child Health in 2008 recommended screening for ROP in premature infants with GA of <32 weeks or BW of <1501 g.10 The American Guidelines published by the American Academy of Pediatrics in 2013 recommended screening for ROP if GA was ≤30 weeks or BW ≤1500 g. Selected infants with BW of 1500 to 2000 g or GA of >30 weeks with an unstable clinical course should also be screened if they were assessed by the attending neonatologist to be at high risk of ROP.11 Since neonatal intensive care units and standards of health care have improved significantly in the past decade, more extreme preterm infants are surviving and a higher risk of ROP development is to be expected.12 13

The revised International Classification of Retinopathy of Prematurity was published in 2005.14 In this revised version, ROP was classified into five stages depending on the severity of retinal fibrovascular proliferation and into three zones depending on the location of vascularisation.14 Plus disease is characterised by the presence of severe retinal venous dilatation and arteriolar tortuosity, iris vascular engorgement, poor pupillary dilatation, and vitreous haze.14 Presence of plus disease indicates high disease activity.14 Aggressive posterior ROP is an uncommon, severe form of ROP characterised by posterior vascularisation, prominent plus disease, and rapid progression.14 Timely treatment is required for severe ROP to prevent retinal detachment and vision loss. Current guidelines suggest treatment if the ROP is type 1 pre-threshold defined by the Early Treatment for Retinopathy of Prematurity (ETROP) study,15 that is: (i) zone I, any stage of ROP, with plus disease; (ii) zone I, stage 3 ROP, with or without plus disease; or (iii) zone II, stage 2 or 3 ROP, with plus disease.10 11

The traditional mainstay of treatment is laser photocoagulation to ablate all avascular areas and reduce the ischaemic stress to allow the retinal fibrovascular proliferation to regress.10 11 Intravitreal injection of anti–vascular endothelial growth factor (anti-VEGF) agent has recently been advocated if ROP is in zone I, stage 3 with plus disease or aggressive posterior ROP.16 Operation with vitrectomy or scleral buckle surgery is required if retinal detachment has occurred but the results are often unsatisfactory.17 18 Proper screening and timely treatment are thus important measures to prevent retinal detachment and vision loss.

The prevalence of ROP and severe ROP that requires treatment vary among different countries. The reported prevalence of ROP ranged from 12.6% to 44.5%13 19 20 21 22 23 and that of severe ROP requiring treatment ranged from 1.5% to 11.7% in other countries.13 19 20 21 22 23 In Hong Kong, studies that report the prevalence and severity of ROP are scarce.24 25 The aim of this study was to evaluate the ROP prevalence, screening, and treatment outcome in a tertiary hospital in Hong Kong.

This was a retrospective cross-sectional study with internal comparison in which the medical records of eligible subjects were reviewed. All premature infants who were born at Queen Mary Hospital and had ROP screening performed between 1 January 2013 and 31 December 2013 were included. In this hospital, all infants are screened if the British screening criteria are met—GA of <32 weeks or BW of <1501 g. Those who died before ROP screening could be performed were excluded from this study. This study was done in accordance with the principles outlined in the Declaration of Helsinki.

All ROP screening was performed by two ophthalmologists who had experience in screening and treating ROP. All examinations were performed with binocular indirect ophthalmoscopy following pupil dilatation by topical mydriatic medication. The severity of ROP was graded according to the revised International Classification of Retinopathy of Prematurity.14 The screening protocol followed the British Guidelines published in 200810:

Data recorded included GA, BW, presence of co-morbidities, most severe ROP stage, any treatment given, and the treatment outcome. If the ROP stage was asymmetrical between the two eyes in an individual infant, the more severe ROP stage was measured.

Primary outcome measures included the prevalence of ROP of any stage and severe ROP that required treatment. Secondary outcome measures included association between risk factors of interest and risk of ROP development, and treatment outcome. The risk factors of interest studied included low GA, low BW, presence of respiratory distress syndrome (RDS), PDA, sepsis, NEC, IVH and the need for blood transfusion.

The Statistical Package for the Social Sciences (Windows version 23.0; SPSS Inc, Chicago [IL], US) was used to perform the statistical analysis. All continuous demographic data are expressed as mean ± standard deviation and categorical data are expressed as number (%). Further, GA and PMA are represented as number of weeks and the remaining days not completing a week written in superscript, eg 32 weeks and 5 days represented by 32⁺⁵ weeks. Chi squared test was used to evaluate the difference among subgroups for ROP development. Fisher’s exact test was used when the expected frequency of a cell in a table was <5. Risk factors that might predict ROP development were evaluated in univariate logistic regression analyses to calculate the odds ratio (OR) and 95% confidence interval. If there was more than one factor associated with a P value of <0.05 in univariate level, the risk factors would be entered into a multivariate logistic regression analysis with backward stepwise method. P<0.05 was considered to be statistically significant. All tests were two-sided.

A total of 92 infants met the British screening criteria during the study period of whom three died before ROP screening could be performed and were excluded from this study. Among the 89 infants screened, 52.8% were male. There were 49 (55.1%) singletons, 37 (41.6%) twins, and three (3.4%) triplets. The mean GA was 30⁺² weeks ± 16.5 days (range, 24⁺¹ weeks to 35⁺⁵ weeks; median, 30⁺⁴ weeks). The mean BW was 1285 g ± 328 g (range, 580 g to 2030 g; median, 1340 g). The distribution of infants in relation to GA and BW is shown in Table 1.

Of the 89 infants screened, 15 (16.9%) developed ROP at a mean time of 34⁺¹ weeks ± 13.0 days (range, 31⁺⁵ weeks to 38⁺⁴ weeks; median, 33⁺⁴ weeks), and three (3.4%) required treatment at a mean time of 40⁺² weeks ± 9.6 days (range, 39⁺² weeks to 41⁺⁶ weeks; median, 39⁺⁵ weeks). Nine (10.1%) infants developed stage 1 ROP, three (3.4%) developed stage 2 ROP, three (3.4%) developed stage 3 ROP, and none developed stage 4 or 5 ROP (Table 2).

Among the 15 infants who developed ROP, their mean GA was 27⁺¹ weeks ± 14.4 days (range, 24⁺¹ weeks to 30⁺² weeks; median, 27⁺⁵ weeks) and mean BW was 846 g ± 276 g (range, 580 g to 1530 g; median, 790 g).

In subgroup analysis, among the 17 ELBW infants of <1000 g, 12 (70.6%) developed ROP and three (17.6%) required treatment. Among the 12 extremely preterm infants with GA of <28 weeks, eight (66.7%) developed ROP and three (25.0%) required treatment (Table 2).

When the 2013 American screening criteria were applied retrospectively, 78 (87.6%) infants met the criteria. In the 11 (12.4%) infants whose GA and BW exceeded the 2013 American screening criteria, none of them developed any ROP.

In univariate logistic regression analysis, factors associated with risk of ROP development included low GA (OR=1.129 for each day decrease; P<0.001), low BW (OR=1.007 for each g decrease; P<0.001), RDS (OR=4.952; P=0.044), PDA (OR=12.904; P<0.001), sepsis (OR=4.787; P=0.013), and need for blood transfusion (OR=11.786; P<0.001) [Table 3]. In multivariate logistic regression analysis, factors associated with risk of ROP development included low BW (OR=1.006 for each g decrease; P<0.001) and PDA (OR=5.749; P=0.035) [Table 3].

Three infants developed severe ROP that required treatment (Table 4). Their mean GA was 25⁺¹ weeks ± 7.5 days (range, 24⁺¹ weeks to 26⁺² weeks; median, 25⁺¹ weeks) and mean BW was 708 g ± 79 g (range, 660 g to 800 g; median, 665 g). All received indirect diode laser photocoagulation treatment and all regressed after one laser treatment. No supplementary laser, intravitreal injection of anti-VEGF agent, or surgery was necessary.

This retrospective study identified the prevalence of ROP and severe ROP requiring treatment among premature infants in a tertiary hospital in Hong Kong. We observed a prevalence of ROP of 16.9% and that of severe ROP requiring treatment was 3.4%. Our prevalence was comparable to or less than that reported in most other countries. The reported prevalences of ROP were 29.2% in Singapore,19 37.8% in Southern Taiwan,20 21.6% in Southern India,21 12.6% in England,13 21.9% in Netherlands,22 and 44.5% in Brazil.23 The reported prevalences of severe ROP requiring treatment were 4.8% to 5.0% in Singapore,19 11.7% in Southern Taiwan,20 6.7% in Southern India,21 1.5% in England,13 and 1.8% in Brazil.23 In mainland China, the ROP screening included bigger infants with BW of up to 2000 g and GA of up to 34 weeks, and the reported prevalences of ROP and severe ROP requiring treatment were 17.8% and 6.8%, respectively.26 Since the risk of ROP among large infants is known to be small, the prevalence of ROP in mainland China could not be directly compared with our study.

Severe ROP is more prevalent in ELBW infants. Our study showed the prevalence of ROP was 70.6% and that of severe ROP requiring treatment was 17.6% in ELBW infants of <1000 g. This was comparable to another local study in Hong Kong in which 53.4% of ELBW infants developed ROP and 14.5% developed severe ROP requiring treatment.25 Our results are also comparable to those of other countries, where the prevalences of ROP and severe ROP requiring treatment in ELBW infants were 55.4% and 13.7% respectively in Singapore,19 70.7% and 29.3% respectively in Southern Taiwan,20 61.3% and 28.4% respectively in Northern Taiwan,27 and 55.9% and 19.4% respectively in Turkey.28 In this study, multivariate logistic regression analysis showed the risk of ROP development was significantly associated with low BW and presence of PDA. The association between PDA and risk of ROP development has been shown in previous studies.8 9 It has been postulated that persistent left-to-right shunt results in low systemic blood flow and retinal ischaemia, and thus is associated with higher risk of ROP development.8 29 In addition, use of indomethacin to close PDA might reduce retinal blood flow and contribute to ROP development.8 30

Dilated fundal examination in ROP screening is a stressful event for premature infants. It is important to screen only those who are at risk to avoid unnecessary examination and stress. In this study, 11 infants would not have been screened if the 2013 American screening criteria were used and none of them developed any ROP. This may suggest that the 2013 American Guidelines are more appropriate than the 2008 British Guidelines in reducing unnecessary examination.

In our study, all severe ROP (100%) regressed after one laser treatment without the need for repeat treatment. No infants developed stage 4 or above ROP. This reflects a good standard of neonatal care in Hong Kong. In Southern Taiwan, 16.9% progressed to stage 4 or 5 ROP requiring further intervention after initial treatment.20 In mainland China, 4.2% of stage 3 ROP and 28.6% of aggressive posterior ROP progressed to retinal detachment after initial treatment.26

Our study highlights the importance of proper screening and timely treatment to prevent retinal detachment and severe vision loss due to ROP. We recommend strict adherence to international screening guidelines, and all ROP screening should be performed by ophthalmologists with dilated fundal examination.10 Since ELBW infants have a high risk of ROP and need for treatment, early parent education and good communication with anticipation for treatment will be helpful. For premature infants transferred from other hospitals for non-ophthalmological conditions, attention should be paid to the ROP screening record and examinations should be performed if the screening criteria are met. Good communication between hospital units is crucial to ensure continuous care and that these infants do not miss ROP screening and thus the window of opportunity for treatment.

There were several limitations in this study. First, the sample size was small. Second, due to the retrospective design, this study was not able to evaluate other potential risk factors that might increase the risk of ROP development. The level of oxygen therapy was not evaluated because it could not be assessed accurately in view of frequent changing of arterial oxygen saturation level and percentage of oxygen administered. Last, this study reviewed only those who had received ROP screening or treatment, therefore we were not able to evaluate those who died before being screened.

Retinopathy of prematurity is an important health problem among premature infants in Hong Kong, especially those with ELBW. The results of our study suggest that the current screening service and treatment outcome are satisfactory. Strict adherence to international screening guidelines and vigilance in infants at risk are key to successful ROP management.

All authors have disclosed no conflicts of interest.

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