What causes plagiocephaly?
For nearly three decades, back sleeping has been recommended as the safest position for infants. One of the adverse side effects, however, is head flattening. Conservative estimates are that 25-40% of infants who sleep on their back will develop some degree of asymmetrical (plagiocephaly) or symmetrical (brachycephaly) head flattening. These changes occur when the infant’s rapidly growing head tries to expand against a flat mattress, car seat, or other resting surface. Contrary to popular belief, this process has nothing to do with a "soft" head or that the bones are malleable. It is exclusively a result of growth displacement around a point of contact. Newton's Law (equal and opposite forces) predicts that the force (weight X gravity) applied by the infant's head to the bed is met with an equal upward force from the bed to the head. This pressure restricts growth in the area of contact and growth is displaced to any area where there is no contact. As the growing head grows increases in weight, the flattening accelerates, which is why the observed flattening becomes very obvious between 6-8 weeks in prone kids. This process is analogous to how a pumpkin flattens when it grows in a field. Flattening can progress until the infant acquires independent head mobility, typically around age 4-5 months. Once the flattening stops progressing, subsequent head growth can make the shape look better, but residual flattening and facial asymmetry can persist well into adulthood. The best course is to avoid flattening before it becomes an issue.
As long as the head continues to contact the bed in the same spot, the head deformation and flattening will continue to progress. It is no surprise that conditions that limit independent head mobility in infants in the first few months of life make them prone to develop head flattening. The most significant is congenital muscular torticollis (CMT). CMT is a tightness of the neck muscles that arises in utero and forces affected infants to position their head to one side. Other causes include prematurity and developmental delay, although these are much less common. The faster an infant gains independent head mobility, the less severe the flattening. For example, in an infant with very mild torticollis and normal motor development, the process of head flattening may stop quickly as the child begins to move the head more freely and redistribute the point of contact to other areas of the posterior cranium. In fact, redistributing the area of contact between the cranium and the resting surface is the basis for preventative measures such as manual head reposition efforts, physical therapy (to try to accelerate improvement of the torticollis), and tummy time (enhance motor development).
Unfortunately, kids with more severe torticollis, prematurity, or developmental delay may not respond as readily to traditional early prevention and treatment strategies that work for some families (e.g., repositioning and tummy time). In fact, most parents of an infant with a head flattening have tirelessly tried all of these modalities only to find that their child's head continued to worsen in spite of their best efforts. It is not that they did it wrong or did not follow instructions. Instead, they are fighting a very different set of circumstances than parents of infants with less severe or no preconditions. These high-risk infants should be easy to identify and manage, but many do not receive any intervention until the head flattening has become quite significant. For example, the most common risk factor, torticollis, often presents only as a persistent head rotation or position preference in early infancy. Many pediatricians and some physical therapists do not consider a diagnosis of torticollis unless there is a characteristic head tilt and chin rotation with a restriction of head rotation with manual (passive) motion. This definition is far too restrictive and misses many infants with more subtle forms of torticollis, who may have asymmetry or limitations of active head rotation and no obvious head tilt (often shows in milder forms once the infant is supporting the weight of the head at 3 months). In my opinion, any child who is severely premature, has clear reason for developmental delay, or has a strong, persistent head position/rotation preference (torticollis) is at high risk for developing head flattening and should start a preventative strategy including physical therapy and sleep surface modification.