The optic disc is the beginning of the optic nerve, is traversed by the retinal ganglion axon fibers in the nerve fiber layer and converges to the optic disc following the fine structure of the retina. All retinal ganglion axons pass through the optic disc. The optic disc consists of the front area of the screen (between the screen and the glass body), the screen area and the rear area of the screen. The blood circulation of the optic disc is mainly from choroid around the optic disk which supplied by the Haller-Zinn arterial ring and the posterior ciliary artery. Since the structure of the optic disc includes all retinal ganglion cell axons and retinal blood vessels, even relatively small optic disc damage can cause serious clinical consequences. Any case in the anatomical pathway from the retina to the brain would affects the ganglion axons, causing a characteristic visual field defect. Therefore, many researchers have paid great attention and enthusiasm to the study of the morphology of optic discs over the years.
Understanding the characteristics and developmental rules of normal optic disc is of great significance for the diagnosis and treatment of diseases such as optic nerve disease, retinopathy and glaucoma. The size and shape of the optic disc in normal healthy eyes may vary greatly in different regions, races, stages of development and age, the appearance may be extremely asymmetric. Therefore, before identifying the pathological changes of the optic disc, it is necessary to grasp the morphological characteristics and developmental rules of the optic disc of the normal eye, so as to avoid missed diagnosis and misdiagnosis. The morphological characteristics of adult optic discs are significantly different from those of infants and young children, the size of optic disc and optic nerve at birth is 75% of that of adults. Neonatal eye disease has special characteristics in anatomy, physiology, pathology, clinical manifestations, diagnosis and treatment methods, as well as other aspects [11–13].
Previous studies mostly focused on the characteristics of adult optic discs. Fundus data measured from normal adults are not suitable screening criteria for neonatal fundus screening. Therefore, to establish a visual disc parameter system for newborns is of necessity, which provides a reference and basis for the diagnosis of neonatal fundus diseases, especially for the diagnosis and treatment of optic nerve diseases and glaucoma.
Because of the lack of neonatal fundus examination equipment, previous study of neonatal fundus was to analyzed the optic disc shape of children over 10 years old, and to evaluate the relationship between optic disc parameter morphology and birth gestational age or birth weight in the past. Fledelius et al. [14] noted that children with birth weight less than 2000 g had significantly greater disc C/D than full-term infants, but there was no significant difference between children born less than 2000 g and children born less than 1500 g. Hellstrom et al. [15, 8] reported that children at the age under 7 years had an average premature birth of 27 weeks, the optic disc and disc area were significantly smaller compared with full-term birth, and there was no difference in the C/D area between the two groups. In a study of 12-year-old children, Samarawickrama et al. [16] indicated that infants with intrauterine growth retardation had significantly greater optic disc C/D ratios than the normal group. It is suggested that intrauterine growth retardation may be a risk factor of glaucoma. Other studies also reported a significant increase in optic disc C/D ratio in infants with ischemic brain injury [17, 18]. However, these studies analyzed the disc shape of premature or full-term children over 7 years of age and did not represent the optic disc characteristics of the neonatal period.
In our study, 60 preterm infants were at a chronological gestational age of 31.74 to 36.86 weeks, with an average gestational age of 33.57 ± 1.11 weeks, average corrected gestational age of 34.19 ± 0.70 weeks, and the average birth weight of 2110.5 ± 106.07 g. The full-term gestational age of 60 full-term newborns ranged from 37.14 weeks to 40.85 weeks. The average gestational age was 39.50 ± 1.52 weeks, the average corrected gestational age was 39.97 ± 1.52 weeks, and the average birth weight was 3646.68 ± 106.67 g. Neonatal fundus screening was performed within one week of birth. There were significant differences in gestational age, corrected gestational age, and birth weight between the preterm and term newborns. Disc parameters of premature infants showed that longitudinal diameter of the optic disc is 1.43 ± 0.00 mm, the transverse diameter of the optic disc is 1.11 ± 0.04 mm, the longitudinal diameter of the optic cup is 0.44 ± 0.01 mm, the transverse diameter of the optic cup is 0.40 ± 0.01 mm, the ratio is 0.31 ± 0.01, and the transverse cup-to-disk ratio is 0.36 ± 0.01. Full-term neonatal optic disc parameters showed that longitudinal diameter of the optic disc is 1.42 ± 0.11 mm, the transverse diameter of the optic disc is 1.13 ± 0.05 mm, the longitudinal diameter of the optic cup is 0.44 ± 0.03 mm, the transverse diameter of the optic cup is 0.40 ± 0.05 mm, longitudinal the cup-to-disk ratio is 0.31 ± 0.05, and the transverse cup-to-disk ratio is 0.35 ± 0.03.
The neonatal optic disc parameters obtained in this study were compared with those of normal adults. Typical adult optic disc is elliptical with an average longitudinal diameter (1.9 mm) that is slightly larger than the average horizontal diameter (1.7 to 1.8 mm). In this study, we found that the visual disc longitudinal diameter/transverse diameter ratio was 1.29 in preterm infants and 1.27 in full-term infants, indicated the morphology of optic disc was vertical elliptical and the number of transverse elliptical optic disc was 0. This may indicate that the visual system in the neonatal period is at the beginning of development. The shape of the optic disc is mainly larger than the transverse diameter, and is mainly vertical and elliptical. The development of the eyeball and the development of the transverse diameter of the optic disc are accelerated with age, some may exceed the longitudinal diameter and develop into a transverse elliptical optic disc.
Current study of neonatal optic discs mainly focus on optic discs in preterm infants in different premature stages of preterm infant development, including premature infants with fixed eyeballs in formalin due to premature death [1, 19], birth babies [20, 21] and premature birth children [15, 18, 8, 22]. Different methods have also been introduced into analytical research. Binoculars and other methods [15, 17, 20, 21, 23] were used in some studies and the in vivo digital images used in this study all measured higher value than those measured by Rimmmer at autopsy. This may due to use of formalin fixation by Rimmmer and Liu, that may reduce the actual optic disc, or the amplification effect of the digital image in vivo.
In our study, we analyzed that birth weight and gestational age have no statistical significance on optic disc parameters at the gestational age of 31 to 40 weeks, which may indicate that the optic disc has developed to full-term birth levels around 31 weeks, which is related to JW Park [23]. In this study, the longitudinal cup-to-disk ratio and transverse cup-to-disk ratio of premature and full-term neonates were not statistically significant, with the values of 0.3 to 0.4.
In view of the fact that Hellstrom et al. pointed out that during childhood (at the age of 7 years) [14], the area of the edge of the optic disc was born 27 weeks before the child was significantly smaller than the full moon. This may due to premature infants, the degree of retinal maturity is different from normal newborns. In the last month of pregnancy, full-term infants are in a relatively stable environment in the mother's womb. However, premature babies are in different environments at the corresponding time, and may therefore change their physiology and metabolism. This change would lead to the optic nerve to increase to the maximum diameter at the age of 2 years, interfering with the natural apoptosis of the optic axon leading to excessive axonal apoptosis [24]. Therefore, the lower weight of premature babies, the lower the gestational age, the larger the cup and the smaller the area along the disc they will have after 2 years old.
In addition, our study observed that the scleral ring morphology of the optic disc has four forms, including acyclic, monocyclic, bicyclic, and irregular. Among the 60 eyes of premature infants, 8 were acyclic (10.0%), 22 were monocyclic (36.7%), 17 were bicyclic (28.3%), and 15 were irregular (25.0%). In the full-term neonates, the scleral ring morphology of the optic disc was acyclic (46.7%), monocyclic (18.3%), bicyclic (6.7%), and irregular (28.3%). The data showed that the ratio of the scleral ring monocyclic and bicyclic in the optic disc is significantly higher in the preterm infant group than in the full-term neonate group, while the acyclic ring ratio of the optic disc in the full-term neonatal group, significantly more than the proportion of premature infants. Among them, the double ring sign is considered to be a characteristic manifestation of optic nerve hypoplasia [25, 26]. Optic nerve hypoplasia is regarded as abnormal development of the optic nerve, characterized by a decrease in the number of optic nerve fibers in the optic nerve [27] and a small disc morphology [25], which may be only 1/3 − 1/2 of normal, and may be small or nearly normal. Comparing the proportion of the scleral ring bicyclic and monocyclic in the optic disc in premature and full-term neonates, we believe that the optic disc scleral ring bicyclic and monocyclic is a relatively early naive stage in the development of the optic disc during neonatal development.
Objects in our study ranged from premature to full-term neonates, from 31.74 weeks to 40.85 weeks of gestational age, and the proportion of double-rings decreased from 28.3–6.7% with increasing gestational age. This suggests that due to leaving the relatively stable uterus environment early in the final stages of pregnancy, oxygenation of the placenta changed to pulmonary oxygenation, fetal circulation of premature infants also changed. Oxygen saturation rises from mixed venous blood levels to arterial blood levels, while fetal lungs are still immature and do not perform the oxygen transport process well. These altered functions and metabolic requirements may have certain effects on the morphological development of the optic disc, slowing down the process of maturation of the scleral ring of the optic disc. Therefore, further follow-up studies are necessary for postnatal neonatal optic disc development.
In recent years, development of digital imaging technology, especially the neonatal digital wide-area fundus imaging system made it possible to conduct screening for neonatal eye diseases in ophthalmic hospitals [28, 29]. Neonatal digital wide-area fundus imaging systems overcome some of the shortcomings of traditional inspection equipment. Retcam takes only few minutes and could reduce the impact of crying on newborns, especially those with poor general conditions. Screening and recording of fundus diseases can be completed with a minimum of 5 photos for the entire examination (posterior pole and 4 quadrants). However,Retcam also has limitations: 1) The lens design is more suitable for smaller infants than infants with larger months. 2) Preparation for dilation before the examination of neonatal fundus diseases would cause the time consuming. 3) Children with poor fit need fundus examination under general anesthesia. The cumbersome procedure and invasive anesthesia examination limit the fundus examination of older children.