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BASIC SCIENCE Paediatric anatomy first, being concave anteriorly. The cervical develops at around 3 months when the child is able to support the weight of its head and the lumbar when learning to walk at about 1 year. Anthony Lander Jeremy Newman Upper limbs These are well developed and long compared to the neonatal lower limbs. The elbow is unable to fully extend at birth by about 15 . The neonate has a relatively strong grasp reflex and is able to support its own weight within the first days of life. Abstract Technological and clinical advances in neonatal medicine have enabled the successful management of the preterm infant born at as early as 24 weeks’ gestation. An understanding of the anatomical differences between adults, infants and neonates is essential for the clinician managing newborns. This article illustrates the clinically important variations in anatomy, focussing mainly on the normal neonatal anatomy. Lower limbs These are under-developed and remain in a flexed and abducted position in the neonate. They appear bowed due to the relative immaturity of the medial head of gastrocnemius compared with the lateral. The general musculature for walking is also not well developed at this stage, giving a rather flat appearance to the buttocks. Keywords Anatomy; development; infant; neonate Respiratory system Musculoskeletal system Airway The tongue is relatively large and the nares small, in comparison with an adult. The larynx is anterior and cephalad (C3-4 vs C6) and the trachea and neck are short. Due to these differences, children up to the age of 5 years may be obligate nasal breathers. The trachea is short and the cricoid cartilage is the narrowest point of the airway in children under 5 years of age. Uncuffed endotracheal tubes can be used to intubate children under the age of 12 years, forming a good seal at the cricoid ring. The thyroid cartilage is shorter and broader in the child and lies nearer the hyoid and its superior notch and laryngeal prominence are less marked. The sexual differences in the larynx are evident by 3 years of age. The trachea is relatively soft in the first year of life and is easily compressed. Before birth the fetus is weightless within the amniotic sac and can assume any position and still develop normally. In premature babies, malleable developing bones, especially the skull, may become distorted due to gravity, pressure from mattresses or medical equipment. The normal skull is approximately circular in cross-section, whereas many premature skulls develop elliptical cross-sections. This can mean that measurements of head circumference, used as a surrogate for brain volume, may overestimate volume. Head circumference is serially measured, as an abnormally enlarging head could indicate the development of hydrocephalous after a bleed for example. Poor head growth may indicate poor nutrition or neurological impairment. The bones of all children have an important haematopoietic function, which in adults is limited to the red marrow of the ribs, sternum, vertebrae and proximal ends of the humerus and femur. Respiratory system Alveoli continue to increase in number and size until around 8 years of age. Growth beyond this is seen in both the airways and the alveoli. At term airway patency is maintained by surface active proteins, which are deficient in premature neonates, leading to a higher rate of respiratory failure. This may be treated with the administration of surfactants. Fontanelles Ossification has not reached the suture lines of the skull at birth and the junctions between the calvarial bones are known as fontanelles. Of the six normally present, the anterior is the largest and transmits the pulsation of the sagittal dural venous sinus, which it overlies. The paired sphenoid and posterior are closed by 6 months and the paired mastoid and anterior by 2 years of age. During parturition the calvarial bones of the neurocranium are displaced and may even overlap at the suture lines to allow passage of the head through the birth canal. Thorax (Figure 1) The neonatal thorax has a rounder circumference when compared to the adult more flattened appearance. It is very compliant and susceptible to collapse during negative intrathoracic pressure. The work of breathing is thus much greater in the child. The type 1 muscle fibres, which are fatigue resistant, seen in adult intercostals and diaphragms, are not prominent until about 2 years of age. The thymus is a large structure in the first year of life and easily causes confusion on chest X-rays. Vertebral column At birth the spinal column is very flexible and lacks the fixed curvatures present in adulthood. The thoracic curvature develops Gastrointestinal system (Figure 1) Anthony Lander FRCS(Paed) is a Consultant Paediatric Surgeon at the Birmingham Children’s Hospital, Birmingham, UK. Conflicts of interest: none declared. Oral cavity The large tongue is short and broad, lying entirely within the oral cavity. It begins to descend into the neck during the first year of life, the posterior third forming part of the anterior wall of the pharynx by age 4 years. During suckling the high position of Jeremy Newman FRCS is a Vascular Surgery Registrar at the Worcester Royal Infirmary, Worcester, UK. Conflicts of interest: none declared. SURGERY 31:3 101 Ó 2013 Elsevier Ltd. All rights reserved. BASIC SCIENCE Abdomen and chest Large thymus Small stomach under left lobe of liver Relatively flat horizontal diaphragm Large liver Umbilical vein and falciform ligament Urachus Medial umbilical fold (umbilical artery) Lateral umbilical fold (inferior epigastrics) Figure 1 Small and large intestine The small intestine has fewer and less marked circular folds than are seen in adults. The mesentery contains very little fat and is much easier to manage when resecting intestine than in adults. The small intestine is between 300 and 350 cm long in a term baby. This is a measurement with the bowel under gentle tension and the mesentery removed. At a laparotomy with normal smooth muscle tone and a normal mesentery the small intestine appears closer to 120 cm than 300. The small intestine lies in a more transverse orientation than in the adult due to the abdominal bladder. The large intestine is approximately 60 cm long and has a very poorly developed muscularis. The ascending and descending colon are relatively short and the transverse colon relatively longer. The normal haustra and appendices epiploicae are not present, giving it a very smooth outline. The haustra appear over the first 6 months. the larynx is elevated further so that the fluid passes directly into the pharynx. This enables the infant to feed and breathe at the same time. Oesophagus At birth the oesophagus is approximately 8e10 cm long and extends from the cricoid cartilage to the gastric cardia (C4 to T9) and possesses the same constrictions as that of the adult. The adult oesophagus starts and finishes two vertebral bodies lower (C6 to T11). Abdomen In the adult the abdomen is generally rectangular with the long axis vertical and the most common open surgical approaches are made through vertical incisions. In babies the abdomen is broader than it is long and open procedures are generally made through transverse supraumbilical incisions. Liver The liver is relatively large in the neonate being 4% of body weight compared to the adult where it constitutes only 2.5e3%. The right lobe extends below the costal margin anteriorly and lies close to the iliac crest posteriorly. The left lobe can extend to the lateral wall of the abdomen, overlying the stomach and the spleen. Stomach The stomach is very small at birth and lies under the liver. If a gastrostomy is needed its placement may not be easy in the first few days of life. This is particularly so if there is no antenatal swallowing in the case of an isolated oesophageal atresia, when the stomach may be less than 5 ml in volume. The stomach distends fivefold in the first few days once swallowing and feeds commence. Acid secretion begins during the first day of life. The stomach’s anterior surface is nearly entirely covered by the left lobe of the liver, only a small portion of the greater curvature being visible below. Its size increases rapidly from 30 ml in a term baby to 100 ml by the fourth week. An adult’s stomach has a capacity of approximately 1 litre. SURGERY 31:3 Gallbladder The gallbladder does not extend to the edge of the liver and has a small peritoneal surface. The majority are embedded within the liver. After the second year of life it has proportionately similar characteristics to an adult. It is easy to miss the gallbladder at 102 Ó 2013 Elsevier Ltd. All rights reserved. BASIC SCIENCE a neonatal laparotomy, but its presence should be documented since an absent gallbladder is associated with some rare anomalies. million or so remaining at birth, only about 400 will actually ovulate. Pancreas The pancreas has a relatively large head and its body points upwards and to the left towards the tail. Uterus The uterus is influenced by the maternal hormones during fetal development and so usually decreases by about a third in size after birth until puberty is reached. At birth it is approximately 2.5e5 cm long and 2 cm wide, the uterine cervix accounting for two-thirds of this. Occasionally the early response to the withdrawal of maternal hormones is accompanied by a small uterine bleed. Peritoneal cavity The anterior abdominal wall bulges forwards in the neonate to accommodate the bladder, uterus and ovaries, which are pelvic in the adult. This is accentuated by the flattened diaphragm pushing down on the supracolic compartment. Testes The testes are situated at the deep ring by the sixth month of gestation and 98% in term babies and 80% in preterm babies will have descended into the scrotum by birth. The processus vaginalis is collapsed at birth, but not necessarily obliterated. Eighty percent are obliterated 10e20 days after birth. Undescended testes are a common surgical problem and if a testis has not descended to the scrotum by 3 months of age surgical referral is essential and an orchidopexy is typically performed around 1 year of age. Genitourinary system (Figures 2 and 3) The kidneys The kidneys are lobulated at birth, have wide-calibre ureters and lie under relatively large adrenal glands. Bladder The apex of the unfilled bladder lies midway between the pubis and the umbilicus and, when filled, may reach the umbilicus. Only the posterior surface is covered with peritoneum and, although considered intra-abdominal, about half lies within the pelvic cavity. It does not truly become pelvic until about the sixth year of life. The ureters correspondingly do not have a pelvic component until that time also. The top of the bladder is continuous with the urachal remnant (median umbilical ligament and the overlying median umbilical fold) reaching the umbilicus. This may rarely be patent and leak urine. Inguinal canals The inguinal canal is similar to the adult and it is rarely true that the internal and external rings overlap e this is a common misconception. Even in small premature neonates with large inguinal hernias the canal has some length and a repair through a small opening in the front of the canal is possible. The canal is short, but so are the arms and legs! Ovaries The ovaries are much larger than the testes at birth and weigh approximately 0.3 g. They lie in the iliac fossae at birth and descend into their pelvic position in early childhood. All the primary oocytes are present after the first trimester. Of the 1 Cardiovascular system Heart At birth the right ventricle has been working against systemic pressure and the muscular bulk is therefore only 25% smaller Lobulated newborn kidneys and large adrenals Inferior vena cava Large adrenals Right gonadal vessels Left gonadal vessels Figure 2 SURGERY 31:3 103 Ó 2013 Elsevier Ltd. All rights reserved. BASIC SCIENCE Pelvic anatomy Apex of bladder lies high Ovary in the iliac fossa Prominent uterus at birth 96% of foreskins adherent to glans at birth Up to 2% of testes may be undescended at birth, 0.8% by 1 year Straighter anorectal angle than in an adult Figure 3 than the left. However after birth, when the fetal circulation changes and pulmonary circulation is established, the left ventricle rapidly grows and its muscular bulk becomes about twice of the right at 2 years of age. This difference continues into adulthood. The ventricular volumes in a heart with normal connections are of course very similar. drops after birth. This causes constriction of the ductus arteriosus and the umbilical vein and artery. Occasionally the duct remains patent and problematic. If closure does not follow drugs such as indomethacin or a surgical ligation may be needed. Umbilical arteries These are a direct continuation of the internal iliac arteries. At birth the smooth muscle in the wall constricts and the arteries are obliterated. The remnants of these arteries become the medial umbilical ligaments seen on the undersurface of the anterior abdominal wall covered by the medial umbilical folds. For completeness remember that more lateral, still, are the lateral umbilical folds overlying the inferior epigastric vessels (see Figure 1). Foramen ovale This lies at the level of the third intercostal space between the right atrium and left atrium. It is approximately 5 mm vertically by 4 mm wide in size and allows blood to bypass the pulmonary circulation in the fetus. Once respiration starts and the pulmonary circulation is established it functionally closes. It is obliterated in 3% of infants by 2 weeks and 90% by 16 weeks. Ductus arteriosus The ductus arteriosus, roughly 8e12 mm long, bypasses the pulmonary trunk to the arch of the aorta in the fetus. It arises as a direct continuation of the pulmonary trunk at the point it divides into left and right pulmonary arteries. Its diameter is approximately the same size as the ascending aorta (5 mm) and joins the descending aorta just below the left subclavian artery. Like the umbilical artery and vein which also occlude after birth, the wall of the ductus arteriosus is populated by smooth muscle, connective tissue and elastic fibres which proliferate close to birth. Bradykinin is released by the lungs on adequate exposure to oxygen and from the umbilical cord when the temperature SURGERY 31:3 Umbilical vein This passes from the umbilicus, within the falciform ligament, superiorly and to the right for 2e3 cm to the porta hepatis. It gives off several branches to the liver before joining the portal vein. It also contracts after birth and its remnant is the ligamentum teres. Ductus venosus Before birth the ductus venosus shunts most of the umbilical venous blood into the inferior vena cava allowing oxygenated blood to bypass the liver. The ductus venosus closes during the first week of life in term neonates but may take longer to close in pre-term babies. The remnant of the ductus is the 104 Ó 2013 Elsevier Ltd. All rights reserved. BASIC SCIENCE Summary ligamentum venosum. The ductus can be used for venous access in the newborn. The anatomical and mechanical differences that distinguish babies and children from adults have implications for the management of the airway and surgical approaches to the abdomen. Some of the differences that persist into infancy and early childhood also affect the response to trauma and have implications for trauma management. A Lymphatic system Lymphoid tissue is in abundance in the neonate and continues to increase throughout childhood. Thymus This weighs approximately 10 g at birth and continues to increase in size until puberty, when it weighs about 30 g. It decreases in adulthood and weighs about 12 g in old age. It lies in the anterior mediastinum overlying the great vessels of the superior mediastinum and may reach up into the cervical region as far as the thyroid gland. FURTHER READING Advanced trauma life support for doctors, 8th edn. American College of Surgeons, 2009. Advanced paediatric life support, 4th edn. American Academy of Pediatrics, 2009. Gray’s anatomy: the anatomical basis of medicine and surgery, 38th edn (British Edition). Edinburgh: Churchill Livingstone, 2009. Spleen Accessory spleens are very common in the neonate and usually found in the greater omentum. SURGERY 31:3 105 Ó 2013 Elsevier Ltd. All rights reserved.