Chapter 3

Normal Swallowing and Development in the Term and Preterm Infant

JO PUNTIL-SHELTMAN

OBJECTIVES

1. Identify and understand core concepts of newborn development.

2. Identify the difference between a nonnutritive suck and a nutritive suck.

3. Identify and understand the benefits of breast milk.

4. Identify and understand the major swallowing milestones between birth and age 12 months.

It is important for the feeding specialist to understand the acquisition of normal swallowing in infancy. Term infants have the appropriate time in utero to achieve mature neurologic and physiologic development, whereas preterm infant development is shortened. One characteristic of term infants is a natural physiologic flexion; this flexion occurs whether the infant is in the prone or supine position. This position helps the infant remain neurologically organized and stable in awake states and throughout the feeding process. Preterm infants need to develop the skills necessary to maintain physiologic stability outside the uterus.

EMBRYOLOGY AND FETAL DEVELOPMENT

Knowledge of embryology and fetal development is vital in understanding the acquisition of swallowing. There are approximately 23 stages of development from prenatal to fetal development. Early embryonic development is described in stages because of the morphologic characteristics. The embryonic period terminates at the end of the eighth week, when the beginnings of all essential human structures are present. The fetal period extends from 9 weeks to birth and is characterized by growth and elaboration of structures. Fetuses are viable at 23 weeks after fertilization. The survival rate of fetuses younger than 23 weeks is poor. Gastrulation is the formative process by which the three germ layers and axial orientation are established in embryos. Gastrulation begins with the formation of the primitive streak. Each of the three germ layers (ectoderm, endoderm, and mesoderm) gives rise to specific tissues and organs.¹

The embryonic ectoderm gives rise to the epidermis, central and peripheral nervous systems, retina of the eye, and various other structures. The embryonic endoderm is the source of the epithelial linings of the respiratory and gastrointestinal tracts, including the glands opening into the gastrointestinal tract and glandular cells of associated organs such as the liver and pancreas. The embryonic mesoderm gives rise to the smooth muscular coatings, connective tissues, and vessels associated with tissues and organs. The mesoderm also forms most of the cardiovascular system and is the source of blood cells and bone marrow, the skeleton, striated muscles, and reproductive and excretory organs. This process occurs during the third week of gestation. By this time the central nervous system develops from the neural plate, which transforms into the neural tube.

Early in the fourth week after conception, the pharyngeal apparatus develops. At this point, the human embryo somewhat resembles the regions of the fish embryo at the comparable stage of development. This would explain the former use of the term branchial apparatus (from the Greek word brancha, meaning gill). Past literature references the term “branchial arch,” whereas current literature refers to this important area as the pharyngeal arch. The pharyngeal apparatus consists of pharyngeal arches, pouches, grooves, and membranes. All these embryonic structures contribute to the formation of the head and neck. Most congenital anomalies in these regions originate during transformation of the pharyngeal apparatus into its adult derivatives. By the end of the fourth week, four pairs of pharyngeal arches are visible externally. The first pair of pharyngeal arches plays a major role in facial development. More specifically, the face, neck, nasal cavities, mouth, larynx, and pharynx, along with muscular attachments of the head and neck, are all derived from the pharyngeal arches. These arches include the muscular component that differentiates into the muscles of the head and neck and a nerve that supplies the mucosa and muscles derived from the arch. In the fourth week the development of the forebrain begins, producing a prominent elevation of the head and a C-shaped curvature of the embryo. Upper and lower limb buds are recognizable by the end of the fourth week. The respiratory and gastrointestinal systems are developing at this same embryonic time. Both systems develop from the laryngotracheal groove in the primitive pharynx. The laryngotracheal groove gives rise to the larynx, trachea, bronchi, lungs, and esophagus.¹

The fourth to eighth weeks are crucial for neural and organ development. The 12 cranial nerves are formed during the fifth to sixth weeks of development. During these weeks, all major organs and systems of the body form three germ layers. The three germ layers differentiate into various tissues and organs so that by the end of the embryonic period, the beginnings of all the main organ systems have been established. The beginnings of the most essential external and internal structures are formed during the fourth to eighth weeks. This is the most critical period of development. Any disturbance during this period may cause major congenital anomalies of the embryo.

From the ninth to twelfth weeks the head constitutes half the crown-heel length of the fetus. Growth in body length accelerates rapidly. At nine weeks the liver is the major site for the formation of red blood cells, whereas urine formation begins between the ninth and twelfth weeks and is discharged into the amniotic fluid. Nonnutritive sucking occurs between the eleventh and twelfth weeks. Between the twelfth and thirteenth weeks the fetus is capable of swallowing amniotic fluid. The fetus grows rapidly from the thirteenth to sixteenth weeks. Limb movements become coordinated by week 14. The sex of the external genitalia can be recognized at 12 to 14 weeks, and at 17 to 20 weeks the mother first detects movement of the fetus. By week 18 the gag reflex is present. The skin is covered with a cheeselike material, and the body of a 20-week fetus is completely covered with fine hair. Rapid growth occurs between weeks 20 and 25. This period gives rise to rapid eye movements, the lungs are beginning to secrete surfactant, and fingernails are present. Between 25 and 29 weeks the fetus can survive outside the womb with medical support. The central nervous system has matured to direct rhythmic breathing movements and attempts to control body temperature. Eyes are opened at this time, and hair is noted on the head. From 30 to 34 weeks the fetus continues to grow and can survive if born prematurely, likely with less medical intervention. The rooting reflex is noted by 32 weeks’ gestation, although the coordination of the suck-swallow-breathe bursts is usually more developed by 34 weeks.

NORMAL DEVELOPMENTAL MILESTONES

It is important that the feeding specialist develop an appreciation for normal developmental milestones in systems other than swallowing. Poor development, or lack of development, often may be seen in multiple systems and provides clues as to the child’s level of functioning. The normal development of sensory systems such as hearing, vision, taste, and smell may be crucial in understanding the child’s disordered feeding ability.

Auditory Development

The critical period for auditory development begins around 24 weeks’ gestation and continues until 3 to 4 years of age. The fetus is protected from high-frequency sounds by tissue absorption. The fetus hears low-frequency sounds through fluid and bone conduction. The fetus is now aware of the mother’s voice. The auditory system of the fetus is vulnerable to injury by exposure to intense low-frequency noise. After the birthing process, sound exposure shifts from fluid- and tissue-conducted sound to air-conducted, which adds high-frequency sounds. It is vital to protect the auditory system of a preterm infant, especially from 24 to 32 weeks’ gestational age. The infant should be protected from any noise above 45 dB.² Hearing in preterm infants from 32 to 34 weeks’ gestational age demonstrates rapid maturation of the cochlea and auditory nerve. Hearing in the infant older than 34 weeks shows an increase in speed of conduction and an increasing ability to localize and discriminate sound. It is recommended that the combination of continuous background sound and transient sound in any isolette not exceed an hourly average of 55 dB. Transient sounds produced by voices or equipment should not exceed 70 dB. Typically sounds from a ventilator or from bubbling tubing noise range from 60 to 80 dB. The American Academy of Pediatrics recommends that noise in the neonatal intensive care unit (NICU) be maintained at levels below 60 dB to prevent cochlear damage.

Visual Development

High noise levels, bright lights, sleep deprivation, and long-term sedation can affect the processes of early visual development. From conception to 20 weeks, the brain’s structural development and genetic mapping for vision occur. Gross structures are in place by 23 to 24 weeks. A critical period is from 20 weeks’ gestation to 3 years of age, when neuronal and cell realignment is being laid. At 24 to 28 weeks the fetus’s eyelids unfuse, the lens is cloudy, and the cornea is hazy until 27 weeks. At 25 weeks, vascularization begins and the retina demonstrates rod differentiation. During this same time the cortex is undergoing rapid dendritic growth. The infant demonstrates no pupillary response and is very myopic. From 30 weeks’ gestation to approximately 3 to 4 months the infant develops ocular dominance columns. At 31 to 34 weeks’ gestation these ocular dominance column formations include the maturity of the retina, the lateral geniculate nucleus, cortical radiations, and waves of endogenous retinal stimulation (during rapid eye movement sleep). The infant needs rapid eye movement sleep to build ocular columns. Visual column development occurs at approximately 40 weeks’ gestation (term) to 7 months. These include ocular dominance, lines and patterns, movement, depth, color, visual mapping, and complex visual processing.³ It is imperative that the infant’s visual system is protected, especially from 23 to 32 weeks of age. At 34 weeks to term the infant’s eyes begin to track, show visual preferences, and spontaneously orient toward soft light. By 40 weeks the eyes attend to form, objects, and faces. Infants begin to visually track and can see objects at 2 feet but attend best at 8 to 12 inches.

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