Evaluating fontanels in the newborn skull
Palpating an infant’s anterior and posterior fontanels provides a window into what may be occurring in the newborn brain, but also examining skull shape and size will identify underlying problems that require further evaluation or intervention.
As innate as admiring a new baby and congratulating the parents when we enter the room, our tendency as pediatricians to palpate the anterior fontanel when we meet an infant, the “pediatrician’s handshake,” is universal. Why do we do this? Does it allow for a more intimate connection with our patient who cannot yet return a greeting? Does it reassure us to feel a subtle pulsation? Perhaps feeling its size helps convince us that the brain is growing well, or maybe we appreciate the fontanel because this unique window to the brain will not persist for long.
The information we gain from examining the fontanel is extensive. This article reviews the development of the fontanel, its clinical significance, the wide range of normal presentation, and discusses abnormalities of the fontanel and what this can teach us about our patients.
Fontanels in the newborn skull
The newborn calvaria is normally comprised of 7 bones: the paired frontal, temporal, and parietal bones, and the single occipital bone. As these bones grow radially from membranous ossification centers, sutures form at the junctions of the calvaria and fontanels form at the intersection of sutures.
Six fontanels are in the newborn skull, including the anterior and posterior fontanels and the paired mastoid and sphenoid fontanels (Figure 1). The triangular posterior fontanel is found at the junction of the sagittal and lambdoid sutures, and normally closes by 8 weeks.1 Although the fontanel does not truly close until the second decade of life, for purposes of this discussion it is closed when a fontanel is too small to be identified on physical exam. The diamond-shaped anterior fontanel forms at the junction of the coronal, metopic, and sagittal sutures. In addition, the mastoid fontanel (the asterion or star) forms at the posterior end of the parietomastoid suture, at the junction of the squamosal suture, mendosal suture, and extraoccipital synchondroses. The sphenoid fontanel (pterion) forms from the juncture of the sphenoparietal, sphenofrontal, and coronal sutures.
Fontanel and suture widths are determined by a balance between the growth of the calvaria and brain (Figure 2). The flexible sutures allow for molding during birth, as well as both prenatal and postnatal brain growth. The brain grows at its fastest rate during infancy and is about 66% of adult size by age 2 years.2 The cranial vault does not have intrinsic growth potential and will not expand without growth of the brain, as is seen in cases of primary microcephaly or poor growth related to perinatal injury (secondary microcephaly).3 Normally the processes of brain and calvarial growth are tightly linked and regulated, but there are conditions in which this balance is not maintained.
Although the majority of calvarial sutures remain patent into the second decade, the metopic suture normally fuses during infancy, usually within the first 3 to 9 months of life.4,5 Craniosynostosis results from premature closure of the sutures, a pathologic event that normally occurs in utero. Providers must recognize this condition during early infancy so that patients can be referred to a craniofacial center for timely management.6 Infants with craniosynostosis require cranial vault surgery to restore sufficient room for brain growth and prevent increased intracranial pressure (ICP) and developmental delay.