Embryological development

The rapid growth of the head begins during the fifth week of embryonic life, as the brain simultaneously undergoes a similar period of rapid growth. By the eighth week, the embryo is humanlike in form, but the head size is disproportionate to the body. During this early fetal development, the head is the fastest growing part of the body and constitutes 50% of the body length at 8 weeks’ gestation. The head growth then slows during the period from the ninth to the twelfth week in the developing fetus while spine growth accelerates. During the thirteenth week, ossification of the cranium begins in the skull, which is one of the primary ossification centers of the skeletal system; the other is located in the long bones. The hair patterns on the scalp also develop during the thirteenth week of fetal development, and the scalp hair present in the term infant is established by the twentieth week.¹ During the second and third trimesters of pregnancy, the head size becomes proportional to the body. The fetus continues to grow and gains 85% of its birth weight during the final period of growth.

Developmental variations

The normal growth of the skull depends on placental function, familial and hereditary factors, growth potential within the uterus, and optimum nutrition during pregnancy and early childhood. The contour of the cranium of the newborn reflects fetal positioning and the effects of the delivery presentation. The cranial bones are pliable and are loosely connected by the sutures and the fontanels, which allow the head to be molded during delivery.¹ Skull fractures are rare because the cranium is adaptable, but they can occur with vacuum-assisted vaginal births, forceps delivery, or a prolonged, difficult labor. The head is slightly flattened during the normal birth process then becomes ovoid by 1 to 2 days of age. If growth retardation does occur in the fetus as a result of either intrinsic or extrinsic factors, skull growth and brain development are impacted. Depending on the timing of the insult during fetal development, the infant can suffer long-term consequences of delayed growth and development.

Anatomy and physiology

The cranium or skull provides a protective housing for the brain and parts of the central nervous system. There are eight skull plates, or bones, in the cranium joined together by sutures (Figure 9-1). These skull plates are movable and separate at birth. The fontanels are the membranous spaces between the frontal and parietal bones and the parietal and occipital bones. The anterior fontanel lies along the coronal and frontal sutures (Figure 9-2). The posterior fontanel lies at the juncture of the sagittal and lambdoidal sutures. There are smaller fontanels located bilaterally in the lower skull. The sphenoid fontanel is located at the lower juncture of the frontal and parietal bones superior to the ear, and the mastoid fontanel is posterior to the ear at the juncture of the occipital and posterior parietal bones. The cranial sutures accommodate brain growth. The cranium is supported by the first cervical vertebra, the atlas. The atlas is a solid vertebra and rests on the second vertebra, the axis. These bones form the rotational bones of the skull. Ossification of the skull continues throughout infancy and childhood and into adulthood.

The facial bones are also pliable at birth, except for the maxilla and the mandible, which are then very small and underdeveloped. The facial skeleton consists of the larger bones of the frontal area, zygomatic processes, maxilla, and mandible (Figure 9-3). The two nasal plates and the lacrimal, ethmoid, and sphenoid bones comprise the smaller bones in the head. The maxillary and ethmoid sinuses are present at birth but are small, and the sphenoid and frontal sinuses develop during infancy and childhood.

The muscular structure of the head and neck is an intricate part of the underlying fascia of the cranium and neck structures. The connections between the muscular fascia and the facial orifices control facial expressions such as smiling, raising the eyebrows, and wrinkling the forehead. The superficial and deep muscles of the neck support the pivotal rotation of the head. The sternocleidomastoid or sternomastoid muscle is the largest muscle in the neck, running from the mastoid area at the base of the ear to the clavicle and sternum, and is primarily responsible for turning the head from side to side. It can be particularly susceptible to the effects of viral infections and gastrointestinal conditions in the pediatric patient. The trapezius muscle lies at the back of the neck and is triangular. The origin of the trapezius muscle is in the back at the twelfth thoracic vertebra extending to the lateral border of the clavicle and attaches at the posterior edge of the occipital bone. It supports the head movement from side to side and the shoulder movement.

The structures in the neck are protected by the deep vertebral muscles, which support the side movements of the head. The trachea is the cartilaginous tube that extends from the larynx to the bronchi in the upper chest beneath the sternum. In infants and children, it is more mobile and more deeply recessed in the vertebral muscles than in adults.

The thyroid gland lies in the anterior middle region of the neck just below the larynx surrounding the fifth or sixth tracheal ring. There are two lobes joined by an anterior isthmus connecting the lobes. The gland is composed of tiny follicles filled with thyroglobulin, which binds with iodine in thyroid synthesis. A dietary intake of 150 to 200 mcg of iodide is enough to produce sufficient quantities of thyroid hormone, and large reserves of iodide are concentrated in the thyroid gland. Thyroid hormone is necessary for normal growth and development in children and normal sexual maturation. The thyroid gland is extremely vascular and secretes thyroxine directly into the bloodstream, which promotes normal growth.

The four parathyroid glands are small endocrine glands generally located on the posterior side or dorsal surface of the thyroid gland. Like the thyroid gland, they do not contain ducts, and they secrete parathyroid hormone, which regulates calcium metabolism. Infants born with abnormal development of the parathyroid gland have an embryological defect known as DiGeorge syndrome caused by a chromosomal microdeletion (22q11). It is often associated with congenital heart defects, facial abnormalities, and other systemic defects.

The head and neck region are perfused by the carotid arteries. The external carotid supplies the head, face, and neck, and the internal carotid supplies the cranium. Blood from the cranium is drained through the subclavian and jugular veins. The thyroid and parathyroid are perfused by the superior and inferior thyroid arteries.

Physiological variations

Rapid growth in the infant brings rapid changes in the head and neck. At birth, the anterior fontanel ranges in size from an average of 2 cm to 4 to 5 cm in the term infant. The anterior fontanel may be small at birth, about the size of a fingertip, because the skull is compressed during vaginal delivery, and then the fontanel enlarges in the neonatal period. The posterior fontanel may or may not be palpable at birth. It enlarges by a range of 0.5 to 1 cm and closes by 6 weeks to 2 months of age. Suture lines can be overlapping or protuberant after birth and often are palpable until 6 months of age from molding at birth. Slow progression of closure of the anterior fontanel occurs during the first year of life, and closure in some infants may begin by 9 months of age with only a fingertip concavity present at the crown of the scalp. However, closure of the anterior fontanel normally occurs by 18 months of age. Head growth in the first year is determined not by the size of the fontanel, but by the normal increase of the occipital-frontal circumference (OFC) of the head. Microcephaly, a small head for gestational age, is considered 1 to 2 standard deviations (SD) below the norm for age and size, and macrocephaly, a large head for gestational age, is 1 to 2 SD above the norm for age and size.

Cephalhematoma is a soft, fluctuating effusion of blood trapped beneath the pericranium caused by rupture of the blood vessels. It does not cross the sagittal suture at the crown. It is usually unilateral and occurs over the parietal area. The bleeding into the periosteum of the cranium may occur slowly and therefore may not be apparent until the infant is 24 to 48 hours old. A cephalhematoma may be associated with hyperbilirubinemia. Resolution of a cephalhematoma may be slow and it may persist until 6 weeks to 2 months of age. See Table 9-1 for developmental variations in the head and neck.

Family, cultural, racial, and ethnic considerations

African-American infants have a slightly larger anterior fontanel than do Caucasian infants secondary to familial and genetic factors. Native American infants have an additional horizontal suture line over the occipital bone.

System-specific history

Obtaining a thorough history of growth and development of the head and neck area, including any injury or insult to the skull during the growth process, is an important part of accurate diagnosis. Refer to the Information Gathering table for pertinent questions to ask about the growth and development of the head and neck.