Since the American Academy of Pediatrics advocated "supine sleep" in the 1990s, the incidence of sudden infant death syndrome has decreased significantly, but the incidence of PHD has increased significantly (5). Since then, PHD has been widely studied. In 2001, Boere-Boonekamp et al. (15) reported a PHD detection rate of 8.2% in 7609 infants younger than 6 months of age in the Netherlands, while in 2013, Mawji et al. (16) found that PHD detection rate in 7-12 week-old healthy term infants in Canada was 46.6%. However, to the best of our knowledge, there are no detailed statistical and analytical reports on infant cranial type in China.
In our results, PHD incidence among 4456 term infants in Chongqing is 81.5% (Figure 1) when using the international general diagnosis standard. Among them, incidence of plagiocephaly (44.5%; Table 3) and brachycephaly (82.0%; Table 4) was the highest in the 2-3 month group is the highest, suggesting that the PHD incidence gradually increases up to 2-3 months after birth, before gradually declining. This is because the baby’s head is not vertically stable up to 3 months after birth, and caregivers usually place the baby in a supine position, where the occipital force is greater, leading to a higher incidence of brachycephaly, while the incidence of dolichocephaly is lower (Table 5). In addition, at this stage, the baby's ability to keep their head centered is poor. If the head is inclined when supine, long term compression of the side of the skull results in plagiocephaly. At 4 months of age, head control improves, time spent outside the bed increases and uneven stress of the skull reduces, so further aggravation of PHD decreases. Therefore, the first 4 months after birth is the key period for monitoring cranial shape, which should be measured monthly. Early detection of PHD and corresponding correction are often effective.
In addition, this study found that the detection rate of right plagiocephaly in term infants of each age group was significantly higher than that of left (Figure 2), which was consistent with the findings of Kluba et al. (17). This may be because the apex of most fetal heads in the womb are located in the birth canal, with the left occipital side in front, so that the right occipital bone is pressed on the woman’s pelvis and the left forehead is in contact with the lumbosacral vertebrae. This is likely to continue after childbirth due to sleeping posture, because babies preferentially turn their heads to the right side to be comfortable, thus aggravating the deformity on the right side (16).
The present study also found that according to international general diagnostic standards, brachycephaly was frequent among the studied term infants. The rate of brachycephaly at the age of 2-3 months is 82.0%, which is much higher than that reported by Ballardini et al. (18), while the rate of plagiocephaly (44.5%) is similar to that reported by Mawji et al. (46.6%) (5), suggesting that the heads of infants in Chongqing are relatively flat. This relates to differences in parenting culture, customs, and esthetic preferences in different regions and nationalities. The flat baby head is in line with the esthetic views of Chinese parents. In traditional Chinese parenting habits, the baby is mainly placed in the supine position after birth, so their head shape is relatively flat. In contrast, CVA, used to diagnose oblique head deformity, reflects the different stress conditions of the left and right sides of the head; as the ideal of bilateral skull symmetry is shared by Chinese and international parents, little difference was seen regarding this aspect. There are obvious differences between the basic data of cranial types of infants in this region and internationally. Therefore, it is inappropriate to apply the commonly used international standards to diagnose infants’ PHD in this region.
Regarding PHD diagnostic criteria, Hutchison et al. (13) set the brachycephaly threshold at CI ≥93% and plagiocephaly at CV ≥0.3 cm, while Loveday et al. (19) suggested that normal CI was 75%-85%, but none of these suggestions were based on the "norm" of comprehensive statistical analysis. At present, CVA ≥0.3 cm indicates plagiocephaly, CI ≥82% indicates brachycephaly, and CI ≤76% indicates dolichocephaly (1,7,8). In 2012, Wilbrand et al. (11) defined the graduation standard based on the measurement data of the European infant's skull type from the past 20 years, thus ignoring the regional differences (20). Relevant research in Asian regions is rare. Therefore, according to the present study, we consider the percentile P25 as the cutoff value for PHD, and P10 and P3 as the cutoff value for medium and severe PHD, respectively, and put forward preliminary reference values for PHD diagnosis in infants younger than 6 months of age in Chongqing (Table 6). CVA ≥0.4 cm indicates plagiocephaly (mild: 0.4 cm≤ CVA <0.7 cm [P75-P90], medium: 0.7 cm≤ CVA <1.0 cm [P90-P97], severe: CVA ≥1.0 cm [≥P97]). CI ≥91% indicates brachycephaly (mild: 91%≤ CI <95% [P75-P90], medium: 95%≤ CI <99% [P90-P97], severe: CI ≥99% [≥P97]). CI ≤82% indicates dolichocephaly (mild: 79%< CI ≤82%, medium: 76%< CI ≤79%, severe: CI ≤76%). The diagnostic standards for brachycephaly and dolichocephaly are quite different from the international standards, and are more suitable for the heads of Chinese babies and in line with Chinese parenting habits and esthetics. It is noteworthy that deviation from CVA or CI values in infants aged 1-2 months old is lesser than that in infants older than 2 months of age. Nonetheless, we include all infants aged up to 6 months when we recommend the diagnostic standard, mainly because lower complexity makes it more convenient for primary health care institutions to diagnose children. In addition, we referred to the current international diagnostic standard, which did not distinguish the diagnostic criteria of different months in detail. However, for the same reason, if a 0-1-month-old infant has developed medium or severe PHD, it suggests that the infant’s head deformity may be more serious and the risk higher; hence, full attention should be paid to correcting it in time.
In the assessment, diagnosis, and treatment of infant cranial measurement and PHD, repeated measurement is needed, so the accuracy and convenience of measurement methods are important issues for clinical workers to consider. Carson et al. (21) state that visual assessment is the best diagnostic test tool, but visual assessment obviously relies on the personal experience and subjective judgment of the clinicians, which is not accurate enough either to quantify the degree of deformity or to make an objective assessment of the improvement after correction. The results of 3D laser and computed tomography can accurately and objectively determine the cranial type and correction effect, but are often time-consuming and labor-intensive; further, the radiation dose and cost must be considered (22). These are more suitable for children who need accurate head form assessment, such as children adjusting their helmet before or during helmet PHD correction treatment, or who need surgical treatment for craniosynostosis. This study adopts the manual measurement method based on Wilbrand et al. (11), which requires simple equipment, little time or effort, and can be used repeatedly. After training, the measurement values of different research centers can reach consistency, so it is an effective method suitable for use in primary health care institutions. However, in the process of using the bending foot gauge, there is a certain potential safety hazard if infants are crying or uncooperative, and special care should be taken.
The effectiveness of PHD correction is closely related to the growth rate of the skull (23). This grows rapidly before 6 months of age, and skull hardness is low. The earlier PHD is detected, the better the correction effect and the lower the treatment cost. However, after 6 months, hardness of the skull increases, growth speed of the head circumference decreases, and the therapeutic effect decreases significantly (24,25). Therefore, early screening, diagnosis, and intervention are important. This study is the first to analyze large measurement data samples of the cranial patterns of term infants in mainland China and propose preliminary local diagnostic reference standards according to the characteristics of the cranial patterns of Chinese infants. This will help the prevention and treatment of PHD in infants from this region and from China. However, China is vast and has significant regional differences in environment as well as ethnicity; it is therefore unclear whether the acquisition of major motor milestones by infants of different ethnicities has a direct impact on the development of cranial type (26). Future research should collect the cranio-type data of infants from different regions and races in China based on an in-depth understanding of PHD influencing factors and the growth and development process of skulls, to establish a unified Chinese standard.