Teaching Case

Newborn female (36 weeks gestation) with a large midline skull bony defect just anterior to the posterior fontanelle.

A newborn female presented at 36 weeks gestation following caesarean section for fetal distress. A small head was noted on clinical examination (5^th centile), with a palpable bony defect anterior to the posterior fontanelle, situated at the vertex of the skull. This measured 2 cm in maximal dimension. The skull radiographs at the time depicted a single large lucency in the vertex region of the skull vault (Fig 1). This had non-sclerotic margins and was well circumscribed. Computed tomography (CT) of the skull with additional shaded surface display further delineated the bony defect, confirming its diameter at 2cm (Fig 2). The differential diagnosis for this lesion included paired parietal foramina and cranium bifidum occultum. Follow up clinical examination at one year demonstrated a marked decrease in the size of the lesion with no new associated findings.

Cranium bifidum occultum ("cleft skull") is rare clinical entity in which there is a delayed ossification of the posterior or anterior parietal bones resulting in a large, confluent, midline cranial vault defect with an intact scalp, pericranium and dura. The defect usually occurs in the median sagittal plane, either anteriorly or posteriorly. It is characterized by a bony defect which may be associated with a cephalocele or a meningoencephalocele There is a higher incidence of systemic anomalies in this cohort including ocular, skeletal, dermatological and cardiac manifestations (3). It has also been documented that the gestational history of these children has been complicated by medication intake and infection (3). If there is no herniation of cranial contents, as in our case, it is termed cranium bifidum occultum (1,2). We report a case of a newborn with cranium bifidum occultum and present the plain film and 3-dimensional surface rendered CT cranial imaging. Two separate processes are involved in the development of the skull vault, namely enchondral and intramembranous ossification. The former process involves formation of bone directly from vascularized mesenchyme; whereas the latter encompasses the differentiation of mesenchyme into hyaline cartilage which subsequently ossifies (frontal and parietal bones) (2). The aforementioned process is identical for both cranium bifidum occultum and paired parietal foramina (2). In both conditions there is deficient ossification around the parietal notch that is normally obliterated by the fifth month during fetal development (4). Cranium bifidum occultum often closes in mid-childhood to leave paired symmetrical openings in the frontal or parietal regions (2). A putative etiology includes interruption of the normal late embryo-fetal growth. An autosomal dominant mode of inheritance with variable phenotypic expression has been described (2). This entity is more common in Asian and Native American populations (2). The documentation of facial malformations is important as this entity may be associated with frontonasal dysplasia (FND), a rare malformation complex characterized by hypertelorism, a broad nasal root, median facial clefting and an absent nasal tip. It was first described in 1967 by DeMeyer, with various synonyms such as frontonasal syndrome and frontonasal dysostosis (5). There is a high incidence of intracranial abnormalities such as agenesis of the corpus callosum, hemimegalencephaly, microcephaly, pachygyria and polymicrogyria with a 50% occurrence of mental retardation (5). Ocular, auricular, limb and cardiac abnormalities also have been documented prenatally (5).