Fig. 1.1
Neonatal skull . Edge of the frontal bank of left coronal suture is marked with arrows. (1) Frontal bone, (2) parietal bone, (3) anterior fontanel, (4) metopic suture, (5) squamous temporal bone
In the newborn, the membranous bones of the vault are separated by the intervening sutures. At the points of intersection, sutures widen and assume the shape of fontanels. The larger anterior fontanel is at the intersection of the sagittal, coronal, and metopic sutures, and the posterior fontanel is at the intersection of the sagittal and lambdoid sutures (Fig. 1.2). The most significant growth of the skull occurs along the sagittal and coronal sutures [6]. Sutures facilitate deformation of the head during delivery and allow uniform expansion of the calvarium during brain growth by its fibrous connective tissue content [7, 8]. Sutures regulate the balance between proliferation and differentiation of the osteogenic precursors through multiple molecular pathways and are the principal growth centers in the skull for the first few years of life [5, 9]. As a rule, the growth of the skull is perpendicular to the open suture lines and parallel to a fused suture (Virchow’s law) (Fig. 1.3) [7]. Closure of the various fontanels and sutures takes place at specific age ranges, but it is rarely any cause for concern, in isolation, in a developmentally normal child with an otherwise normal cranial morphology when a fontanel closes earlier or later than expected (Table 1.1).
Fig. 1.2
Illustration of the vault of the skull in an infant viewed from above. AF anterior fontanel, C coronal suture, L lambdoid suture, M metopic suture, PF posterior fontanel, S sagittal suture
Fig. 1.3
Virchow’s law
Table 1.1
Normal age ranges of closure for fontanels and cranial sutures
Fontanel or suture | Age of closure |
|---|---|
• Anterior fontanel | 12–18 months |
• Posterior fontanel | 3–6 months |
• Posterolateral fontanel | 24 months |
• Anterolateral fontanel | 3 months |
• Metopic suture | 3–8 months |
• Coronal sutures | ~35 years |
• Lambdoid sutures | ~35 years |
• Sagittal suture | ~35 years |
• Squamosal sutures | ~35 years |
The base of the infant skull , on the other hand, contains multiple cartilaginous joints or synchondroses located between the sphenoid and ethmoid bones anteriorly and between the sphenoid and occipital bones posteriorly. Growth of the skull base and consequent cranial lengthening is largely independent of cerebral growth and takes place mostly at the synchondroses between the sphenoid and occipital bones. In the sphenoid region, three prominent synchondroses can be identified and are named the fronto-sphenoid, the inter-sphenoid, and the spheno-occipital. The first two usually close by age 2, but the spheno-occipital synchondrosis may be visible on lateral radiographs of the skull base until the age of 18 years [2, 10].
The single most important stimulus for head growth during infancy and childhood is brain growth [11]. Throughout the period of rapid development of the brain, pressure is exerted on the inner table of the skull, which accommodates to the increasing size of the brain. Such an adaptation is facilitated by the membranous fontanels, which remain open until maximal brain growth has been attained. Accurate assessment of head growth is therefore one of the most important aspects of the neurologic examination of infants and young children [11].
Normal Development of the Brain
The development of the nervous system occurs through the interaction of several synchronized processes, some of which are complete before birth, while others continue into adulthood [12]. The central nervous system begins to develop in the human fetus 2–3 weeks after fertilization of the oocyte. From 4 to 12 weeks of gestation, ectodermal tissues of the neural tube begin to differentiate into the precursors of the various structures of the nervous system. The forebrain and facial structures develop at one end and the spinal cord at the other. The hollow center of the tube in the region of the future brain eventually develops into the ventricles. Regions called proliferative or ventricular zones form in the vicinity of the ventricles and differentiate into the site for the division and origin of cortical and subcortical neurons [12, 13]. Between weeks 12 and 20 of gestation, neurons migrate from the ventricular and adjacent subventricular zones along a scaffolding of glial cells toward their determined final destinations in the cortex [12, 14].
Subsequently, a period of rapid programmed cell death occurs, reducing neuronal populations by half between 24 weeks of gestation and 4 weeks after birth. The cell bodies of the neurons are primarily found in the gray matter of the brain. Their myelinated axons form white matter. Myelination begins at the brain stem by 29 weeks and generally proceeds from inferior to superior and posterior to anterior. Proximal pathways tend to myelinate before distal, sensory before motor, and projection before association. Although most major tracts are significantly myelinated by early childhood, some axons continue to myelinate into the second and third decades of life [12].
Another major developmental process is the proliferation and organization of synapses , which begins around the 20th week of gestation [12]. The rate of synapse formation peaks after the 34th week of gestation reaching about 40,000 new synapses per second [13]. A rapid increase in synaptic density occurs after birth with a number estimate at the age of 2 years that is around 50 % greater than the typical number in adults [15]. Due to increasing cell and synaptic density, beginning at approximately 15 weeks of gestation, the surface of the growing brain begins to fold into sulci and gyri. The major sulci, except for the occipital lobe, are in place by 28 weeks of gestation, after which secondary and tertiary sulci are elaborated, with nearly all gyri present by birth. The sulcal and gyral patterns continue to increase in complexity after birth [12]. Rapid brain growth takes place in the first 2 years of life reaching 80–95 % of its adult size [16].
Abnormal Head Size
Evaluation of the Head Circumference
In 1968, Nellhaus compiled graphs of head circumference in children of both sexes from birth to 18 years of age [17]. Detailed tables and percentile charts based on measurements taken from a very large number of subjects have become available through the WHO Multicentre Growth Reference Study Group which established the WHO Child Growth Standards for head circumference for age. We have included these charts for easy accessibility inside the cover of this book. Detailed normative data and percentile charts for boys and girls, specific charts that account for prematurity, and specific chart for children with certain specific etiologies such as Down’s syndrome can be accessed online at the WHO website (http://www.who.int/childgrowth/standards/hc_for_age/en/) [18]. A head circumference that is two standard deviations above or below the mean for age requires investigation and explanation [11].
Macrocephaly
Macrocephaly is defined as a head circumference more than two standard deviations for age and sex above the mean. It can be caused by various benign or pathological conditions (Table 1.2). Common causes of macrocephaly include familial megalencephaly (larger-than-normal brain mass), benign extracerebral collections of infancy (BECC), and hydrocephalus [21], the workup and treatment of which will be discussed in more detail in Chaps. 13 and 14.
• Familial megalencephaly (larger-than-normal brain mass) |
• Benign extracerebral collections of infancy (BECC) |
• Hydrocephalus |
• Hydranencephaly |
• Brain tumors |
• Intracranial cysts |
• Pseudotumor cerebri |
• Subdural hematomas or hygroma |
• Rebound or “catch-up” brain growth (after prematurity or serious illness) |
• Genetic, metabolic, and dysplastic syndromes (e.g., neurofibromatosis, Soto syndrome, mucopolysaccharidoses, hemimegalencephaly, achondroplasia) |
• Lipid storage disease, leukodystrophies, cranial dysplasias, and marrow hyperplasia (chronic hemolytic anemias) |
Detailed history, neurological examination (especially for signs of high intracranial pressure), evaluation of developmental milestones, and assessment of head growth rate through serial head circumference measurements are important for differential diagnosis, urgency of imaging, and further management [19]. Macrocephaly with normal growth rate and normal neurological examination is reassuring and is characteristic of benign megalencephaly which is usually familial [21]. Rapid head growth rate with loss of developmental milestones or other neurologic findings, on the contrary, suggests an increased intracranial pressure, often caused by hydrocephalus or neoplasms [11].
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
