Introduction

Before generating a treatment plan, the etiology of the asymmetry should be determined.

Asymmetries can be congenital or acquired. The congenital asymmetries will be either deformation or malformation occurring prenatally, some of which may be part of various syndromes. Some of the etiologies related to congenital asymmetries have been reviewed in the past (Bishara et al. 1994; Cohen 1995a, 1995b, 1995c), but almost 30 years later a lot of their aspects remain unclear (Medina‐Rivera 2016).

Congenital

The deformation generated prenatally will be dependent on the space available and, therefore, more frequent in the case of twins or triplets or after a hard delivery. Mild plagiocephaly is routinely diagnosed at birth as it may be the result of a restrictive environment (Flannery et al. 2012; Looman and Flannery 2012).

The congenital deformation will have strong tendency to self‐correct postnatal and this is underlined when advising the importance of the sleeping posture. Among the congenital deformations that led to an asymmetry of the craniofacial skeleton, in the side of the skull, the sleeping posture is considered important. A mild and widespread form is characterized by a flat spot on the back or on one side of the head caused by remaining in a supine position for prolonged periods (Laughlin et al. 2011). Plagiocephaly is a diagonal asymmetry across the head shape. Often it is a flattening of one side at the back of the head that will lead to some facial asymmetry. Depending on whether a synostosis is involved, plagiocephaly can be divided into two groups: If there is premature union of skull bones, this is more properly called craniosynostosis (malformation) or nonsynostotic (deformational) (Kadom and Sze 2010). Surgical treatment of these groups includes the deference method; however, the treatment of deformational plagiocephaly is controversial.

The incidence of deformational plagiocephaly has increased dramatically since the advent of recommendations for parents to keep their babies sleeping on their backs. Data also suggest that the rates of plagiocephaly are higher for twins and multiple births, premature babies, babies who were positioned in the breech position or back‐to‐back, as well as for babies born after a prolonged labor (Ditthakasem and Kolar 2017).

The most frequently seen asymmetry visible at birth is cleft palate followed by some kind of plagiocephaly or hemifacial microsomia. Hemifacial microsomia is the asymmetry the cause of which is mostly unknown. Chen et al. (2018) suggested different etiologies for a disruption which occur during the first weeks of gestation. One would be external factors as various types of medication, or maternal intrinsic factors as maternal diabetes or genetic factors. In addition, three other causes have been proposed for hemifacial microsomia including a physical damage to the Meckel’s cartilage, an abnormal development of the cranial neural crest cells, and a vascular abnormality and hemorrhage model. However, none of these proposed etiological factors can account for the asymmetry and the related deformation. The impact of the vascularization is, however, stressed also when analyzing the effect of maternal factors either genetic or related to disease as diabetes or medication. Contributing to some of the congenital asymmetries may be expression of genetically determined malformations that attack only tissues on one side. This abnormality may be of all tissues, cleft palate and hemifacial microsomia being the most prevalent. The abnormal growth may be of all parts of the craniofacial skeleton. It may be the size of all the tissues or only the skin. However, according to Tingaud‐Sequeira et al. (2022) none of these etiologies account for the abnormal development of the first and second branchial arches described by Kjær (2017).

Postnatally

Thumb Sucking

The etiology of asymmetry developed postnatally will, if not related to a congenital disease, be the result of lifestyle or trauma to hard or soft tissues. The most frequent lifestyle cause of asymmetries is the nonnutritional sucking either by pacifier or thumb sucking. During the nonnutritional sucking, the mandible is kept back and the baby does not have to move the mandible forward, a movement as is normally done when sucking and swallowing take place simultaneously. The nonnutritional sucking has been found to be related to open bite and lateral crossbite. The latter may lead to asymmetry and crowding (Dimberg et al. 2010). Apart from the narrow upper arch, an asymmetrical arch form can also be the result of a prolonged thumb sucking (Figure 2.1).