Since the American Academy of Pediatrics advocated for "supine sleep" in the 1990s, the incidence of sudden infant death syndrome decreased significantly. However, the incidence of PHD increased significantly [5]. Since then, PHD has been widely studied. However, to the best of our knowledge, there are no detailed statistical and analytical reports on infant cranial types in China.
In our results, PHD incidence among 4456 term infants in Chongqing was 81.5% (Fig. 2) according to the international general diagnosis standards. Among them, the incidences of plagiocephaly (44.5%; Table 3) and brachycephaly (82.0%; Table 4) were the highest in the 2-3 months group, suggesting that the PHD incidence gradually increases up to 2-3 months after birth before gradually declining. This is because the infant’s head is not vertically stable for up to three months after birth, and caregivers usually place the infant in a supine position, where the occipital force is greater, leading to higher and lower incidences of brachycephaly and dolichocephaly, respectively (Table 5). In addition, at this stage, the infants’ ability to keep their head centered is poor. In cases where the head is inclined when supine, long term compression of the side of the skull results in plagiocephaly. At four months of age, their head control improves, the time spent outside the bed increases, uneven stress of the skull reduces, and, therefore, further aggravation of PHD decreases. Therefore, the first four months after birth is the key period for monitoring cranial shape, which should be measured monthly. Early detection of PHD and the corresponding correction are often effective in treating PHD.
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 (Fig. 3), which was consistent with the findings of Kluba et al. [15]. 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, such 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 owing to sleeping posture because infants preferentially turn their heads to the right side to be comfortable, thus, aggravating the deformity on the right side [16].
This 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 was 82.0%, which was much higher than that reported by Ballardini et al. [17]. However, the rate of plagiocephaly (44.5%) was 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 head is in line with the esthetic preferences of Chinese parents. According to traditional Chinese parenting habits, the infant is mainly placed in the supine position after birth and, therefore, their head shape is relatively flat. In contrast, CVA, used to diagnose oblique head deformity, reflects the difference in stress conditions on 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 observed 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’ with PHD in this region.
At present, CVA ≥0.3 cm, CI ≥82%, and CI ≤76% indicate plagiocephaly, brachycephaly, and dolichocephaly [1, 7, 8], respectively. However, none of these suggestions were based on the "norm" of comprehensive statistical analysis, and there are fewer studies in Asian regions. Therefore, according to our findings, we considered the percentiles P25, P10, and P3 as the cutoff values for PHD, medium PHD, 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). The diagnostic standards for brachycephaly and dolichocephaly are quite different from the international standards and are more suitable for the heads of Chinese infants and in line with Chinese parenting habits and esthetic preferences. It is noteworthy that deviation from CVA or CI values in infants aged 1-2 months is lesser than in those aged >2 months. Nonetheless, we include all infants aged up to six 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 standards, which did not distinguish the diagnostic criteria for different ages in months in detail. However, for the same reason, when a 0-1-month-old infant develops 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 measurements are needed and, therefore, the accuracy and convenience of measurement methods are important issues for clinical workers to consider. This study adopted the manual measurement method performed by 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. Thus, it is an effective method that is 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 when infants are crying or are uncooperative and special care should be provided.
The effectiveness of PHD correction is closely related to the growth rate of the skull [18]. The head grows rapidly before six months of age and skull hardness is low. The earlier PHD is detected, the better is the correction effect and the lower is the treatment cost. However, after six months, hardness of the skull increases, growth speed of the head circumference decreases, and the therapeutic effect decreases significantly [19, 20]. Therefore, early screening, diagnosis, and intervention should be performed. There are several limitations to our study. China is vast and has significant regional differences in environment and ethnicity; howere, our study only covers Chongqing area, it is unclear whether the acquisition of major motor milestones by infants of different ethnicities has a direct impact on the development of cranial type, still need further study. Another limitation is the long-term follow-up of cranial changes with the development, intervention methods, and their effects in term infants might be more informative.