Sensory development is complex, with both morphologic and neural components. Development of the senses begins in early fetal life, initially with structures and then in-utero stimulation initiates perception. After birth, environmental stimulants accelerate each sensory organ to nearly complete maturity several months after birth. Vision and hearing are the best studied senses and the most crucial for learning. This article focuses on the cranial senses of vision, hearing, smell, and taste. Sensory function, embryogenesis, external and genetic effects, and common malformations that may affect development are discussed, and the corresponding sensory organs are examined and evaluated.
Key points
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Sensory development begins in early fetal life responding to in utero stimulation.
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Sound transmission from the mothers speech, heartbeat, and external noise stimulates fetal hearing development prior to birth.
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Color vision is absent in babies less than 34 weeks gestation and the first color perceived by newborns is red.
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Taste and smell in the newborn correlates with maternal dietary components in amniotic fluid.
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Primary care providers are poised to detect anatomic and sensory abnormalities and coordinate early intervention.
Introduction
Sensory development is complex, with both morphologic and neural components. The senses begin to develop well before birth based on in-utero stimuli. They all mature rapidly in the first year of life. This article focuses on the cranial senses of vision, hearing, smell, and taste. Tactile development and pain perception are not addressed. Sensory function, embryogenesis, external and genetic effects, and common malformations that may affect development are discussed, along with the corresponding sensory organ examination and evaluation.
Vision
Eye Development
The eye is derived from an outgrowth of neuroectoderm of the forebrain. By the 32nd embryonic day, a distinct optic cup with a ventral groove is detectable. The optic cup further invaginates to form the globe with anterior and posterior chambers. Surface ectoderm is pulled in to form the lens, iris, and other associated structures to separate the 2 chambers. The cornea is formed from surface ectoderm and a fine layer of mesoderm between the neuroectoderm and surface ectoderm. The eyelids and lacrimal glands are formed from surface ectoderm. The retina forms from the internal walls of the optic cup. A thick neuroepithelium differentiates into rods and cones. Myelination is incomplete before birth at term but, after light exposure for approximately 10 weeks, myelination is complete. This process is markedly delayed in babies born prematurely and may be disrupted significantly in retinopathy of prematurity.
Examination of the Eye
The eyelids meet and adhere by the tenth week of gestation. They remain adherent until approximately 26 weeks’ gestation. Although uncommon, babies born vaginally with a face presentation may have everted eyelids, which readily reduce with few complications and normal eyes otherwise ( Fig. 1 ). An eyelid coloboma (notched lid) is a rare defect limited to the upper eyelid that requires surgery to protect the cornea and conjunctiva.
Conjunctival hemorrhage, often associated with a difficult delivery, is absorbed within several weeks. The sclera may be discolored yellow with significant jaundice and may appear bluish in inherited collagen vascular diseases because of scleral thinning and visualization of the underlying retina. Newborn eye prophylaxis to prevent bacterial infection often produces a transient chemical conjunctivitis. Conjunctival discharge may be caused by an infection, with gonorrhea and chlamydia being the most serious infections ( Fig. 2 ). Obstruction of the nasolacrimal duct results in excessive tearing. Cloudy or protruding cornea indicates glaucoma ( Fig. 3 ). The increased pressure of the aqueous humor in the anterior chamber is an emergency requiring immediate consultation and intervention by a pediatric ophthalmologist.
The iris color at birth is bluish in most infants. Pigmentation often progresses to a darker color, with the final iris color achieved by 4 months. Lack of pigmentation with a pink iris is a primary feature of albinism. Aniridia, complete lack of irises, is caused by an arrest of development of the rim of the optic cup at the eighth week. A failure of the ventral groove to fuse in early development leads to an iris coloboma, seen as a keyhole defect of the iris, which may extend into the ventral retina. The ciliary body is similarly affected, resulting in the inability to constrict the pupil and subsequent photophobia.
The classic newborn eye test is the red reflex, elicited by shining a light into the eye, and the reflecting light off the highly vascular retina appears red. Any color but red may indicate anterior chamber disease (glaucoma), cataract, or retinal disorder such as detached retina or retinoblastoma ( Fig. 4 ). Premature babies with immature retinas at birth may develop retinal scarring and detachment, a condition termed retinopathy of prematurity, which can also cause abnormal red reflex.
Examination of the extraocular muscles is difficult at birth. It is common for newborns to have discordant muscle movement because their ability to focus and the resultant conjugate gaze take several months to mature. In addition, unusually long eyelashes can be an indication of a genetic syndrome, the foremost being Cornelia de Lange syndrome.
Early Vision
Neonatal vision is limited, such that term infants can only focus approximately 25 cm (10 inches) shortly after birth. At less than 34 weeks’ gestation, neonates do not have sufficient cone development to see color and can discriminate between dark and light only at a limited distance. The initial color humans see is red, presumably because of low light exposure from transillumination of the red color of maternal oxygenated hemoglobin into the uterus. With continued exposure to various wavelengths of light, the retinal cones of other colors develop. The progression of eye function development is summarized in Table 1 .
| Characteristic | Gestational Age (wk) |
|---|---|
| Blink/squint in response to a bright light | 26 |
| Pupils constrict to light | 30 |
| Ability to fixate vision on a large object in close proximity | 32 |
| Track large moving objects | 34 |
| Color perception; red at first | 34 |
Hearing
Normal Development of Hearing
The most important aspects of the auditory system development take place during the second half of gestation. Babies born prematurely and exposed during this period to multiple potential adverse effects of life-sustaining therapies are at great risk for hearing deficits and secondarily speech delay. Among neonatal intensive care unit (NICU) graduates, the incidence of hearing impairment is estimated to be at least 10-fold greater than in their term counterparts.
Structure and Function
The auditory system comprises 3 related sets of structures: the peripheral components, including the outer, middle, and inner ears; the auditory nerves (cranial nerve VIII); and the auditory regions of the brain located primarily in the brainstem and left temporal lobe.
The outer ear of neonates features a narrow canal with thin cartilage, which is readily blocked and compressed. The shape, position, and peripheral tissue of the ear may provide clues to dysfunctional development. The classic low-set or malformed ear is found in more than 120 well-characterized syndromes. For the ear to be considered low set, the entire ear must be below an extended line drawn from the inner canthus of the eye to the outer canthus ( Fig. 5 ). A second criterion for low-set ear is the ear canal below an imaginary line drawn from the outer canthus to the base of the occiput. Posteriorly rotated ears or preauricular skin tags are more commonly seen in babies with syndromes.
The fluid-filled middle ear reaches adult size by 20 weeks’ gestation, but the middle ear ossicles remain cartilaginous until 32 weeks’ gestation. Cochlear structures, including inner and outer hair cells, are fully developed by 25 weeks’ gestation. This process extends to myelination developing from the brainstem to higher level auditory pathways. The cochlea transduces acoustic wave energy into electrical impulses, which occurs in the inner hair cells. Outer hair cells adjust reflexively to sound input by producing frequency-specific echo sounds called otoacoustic emissions (OAE).
Hearing is the first sense exposed to stimulation that promotes development of the neural pathways. Functional hearing in human fetuses develops at 25 to 27 weeks’ gestation. Low-frequency sounds, such as the mother’s heartbeat and speech, elicit physiologic responses that are consistently detectable. Maturing fetuses respond to a wider range of sound frequencies progressing through the third trimester and shortly after birth. The functional maturation of hearing in the newborn is caused by structural changes in the outer and middle ears. Progressive myelination of auditory axons results in a maturing brainstem evoked response (auditory brainstem response [ABR]) test because of increased conduction velocities and wave amplitudes.
The incidence of permanent hearing loss in neonates ranges between 1.4 and 3 per 1000 births in the United States. With progressive or new-onset hearing loss, the prevalence of permanent sensorineural hearing loss increases during childhood to estimated rates of about 2.7 per 1000 in 4-year-old children and 3.5 per 1000 in adolescents.
Types and Causes of Hearing Impairment
Causes of hearing impairment
Based on the anatomic location of the hearing dysfunction, hearing loss can be classified as conductive, sensorineural, or neural :
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Conductive hearing loss: blockage of sound transmission in the outer or middle ear caused by permanent conditions like anatomic malformations or transient problems such as fluid or debris.
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Sensorineural hearing loss: failure of sound transduction in the inner and outer hair cells of the cochlea, and of transmission through the auditory nerve.
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Neural hearing loss, also known as auditory neuropathy: dysfunction of the inner hair cells and auditory nerve, but OAE from the outer hair cells remain intact.
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Mixed hearing loss: combination of conductive and sensorineural hearing deficits.
In addition to the neurophysiologic classification described earlier, hearing loss can be further categorized according to severity (ie, mild, moderate, severe, or profound), based on the sound pressure level of the individual’s hearing threshold. In addition, hearing loss can be unilateral or bilateral.
About two-thirds of congenital hearing loss has an underlying genetic cause. Mutations in the connexin 26 gene ( GJB2 ), predominantly the 35delG point mutation, account for 20% of congenital deafness. An additional 44% of congenital deafness has other genetic causes; one-third of these being related to recognizable syndromes and two-thirds being nonsyndromic. Most nonsyndromic hearing loss cases follow an autosomal recessive inheritance pattern ( DFNB ), whereas a minority are autosomal dominant ( DFNA ); X-linked and mitochondrial inheritance is rare. Although many gene mutations have been associated with hearing loss, about 95% of congenitally deaf infants are born to parents with normal hearing, so a negative family history of deafness does not exclude the possibility of hereditary hearing loss. A newly diagnosed infant may serve as the index case to prompt genetic evaluation for the family.
The underlying pathophysiology of hearing dysfunction is complex. The 2007 Position Statement of the American Academy of Pediatrics Joint Committee on Infant Hearing outlined causes of hearing loss that can be congenital, delayed onset, and/or progressive. These categories include:
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Infections
Fetal: cytomegalovirus (CMV), varicella, syphilis, rubella, toxoplasmosis, and others
Postnatal infections: meningitis, otitis media, encephalitis
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Environmental and therapeutic toxicity:
Perinatal asphyxia, anoxia
Ototoxic medications (aminoglycosides, loop diuretics)
Mechanical ventilation, extracorporeal membrane oxygenation, sustained metabolic or respiratory acidosis
Severe hyperbilirubinemia requiring exchange transfusion
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Trauma: perinatal, child abuse, temporal bone fracture
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Familial hearing loss
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Craniofacial anomalies/syndromes:
Malformations of craniofacial structures derived from the first and second branchial arches, even without genetic associations, are embryologically related to the development of the inner ear, and are thus a risk factor for hearing loss. The many syndromes with craniofacial involvement include Waardenburg type I and II (white forelock), neurofibromatosis, and Alport.
Familial syndromes associated with progressive hearing loss include:
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Pendred syndrome accounts for only 3% of deafness diagnosed from birth, but comprises 12% of the cases of deafness in the preschool population. Although deafness occurs early, the other clinically obvious component of the syndrome is goiter, which does not present until late childhood.
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Mitochondrial mutations or other neurodegenerative disorders, such as Friedrich ataxia, may first manifest beyond early infancy.
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Usher syndrome is a familial disorder characterized by progressive hearing loss and progressive retinitis pigmentosa leading to blindness.
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Jervell and Lange-Nielsen syndrome presents with cardiac dysrhythmias caused by a prolonged QT interval and should prompt reevaluation of hearing function.
Even if the newborn hearing screen is normal, a family history of hearing loss should trigger continued monitoring of the infant and formal audiologic assessment should be repeated by 24 to 30 months of age.
Medical screening systems cannot detect hearing loss in many children in a timely manner. Concerns by the family members regarding hearing, speech, language, or developmental delay must be addressed to improve early detection of hearing loss. Despite early hearing screening, more than two-thirds of children with subsequent hearing loss were diagnosed following parental concern and school hearing screens.
Conditions with combined auditory and visual impairment, such as Usher syndrome, are particularly devastating to development of communication and psychosocial function. Increased association exists with autism spectrum disorder, which occurs in about 7% of 8-year-old children with visual or auditory impairment. The clinical diagnosis of hearing or visual deficits resulted in a substantial delay in the diagnosis of the coexisting autism spectrum disorder.
Functional Consequences
Congenital or neonatally acquired permanent hearing loss adversely affects expressive and receptive language development, resulting in diminished academic achievement and social development. These sequelae can be mitigated by diagnosis and appropriate therapeutic intervention within the first 6 months of life. Therefore, the age of 6 months represents a critical target for initial interventions in infants with hearing loss to optimize functional outcomes.
The functional consequences of hearing loss depend on the age of onset and the specific subcategory of the hearing loss described earlier. Although bilateral deafness is most incapacitating, even unilateral hearing loss may affect language and educational performance. Minimal information is available regarding the persistent effects of milder transient or reversible hearing dysfunction, such as that related to external ear debris in newborns, persistent otitis media with effusion, or auditory neuropathy in severe hyperbilirubinemia.
Screening in Newborns and Young Children
Intervention for hearing loss is most effective when initiated early to salvage speech and language development. Because of vigorous advocacy by the American Academy of Pediatrics, newborn hearing screening is performed in most individual birthing hospitals in the United States. There is great variability in accuracy because of multiple testers. Table 2 summarizes the most useful hearing tests.
