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(smooth pursuit) and heading direction. Projections to the cortex also go to the frontal eye field as well as Brodmann’s areas 2, 3, and 7 (Furman & Lempert, 2016).

AUDIOLOGY NUGGET

Throughout a vestibular assessment, the peripheral and central vestibular systems are subject to evaluation. Specific patterns of results are indicative of specific pathologies. However, the patient’s history and symptoms are equally, if not sometimes more, important. Appendix 2–E displays common pathologies and their associated symptomology. Differential diagnostic procedures and results are discussed in Chapter 7.

Disorders of the Vestibular System

Obtaining a detailed case history for patients undergoing vestibular evaluation is paramount. Vestibular disorders will vary in description and time course of symptoms, provoking factors, and symptoms related to auditory and other domains. Unfortunately, “dizziness” does not mean the same thing for every patient, so further questioning is required. For example, true vertigo is the sensation of movement, either of the individual or their surroundings, when no movement is occurring. Unsteadiness is what precedes falls, or the sensation that one is going to fall. “Dizziness” is the catch-all term for every other sensation associated with feelings of disorientation, imbalance, lightheadedness, or unsteadiness.

Symptoms can differ between peripheral and central vestibular pathologies. For example,

peripheral origins of vertigo will often be accompanied by severe nausea/vomiting, mild imbalance, mild oscillopsia (unstable vision), and hearing loss and will generally recover with treatment or be compensated for with time. Central disorders rarely include hearing loss, but do have severe imbalance and oscillopsia (unstable vision with movement), as well as neurological symptoms and some nausea and vomiting (not to the extent of peripheral disorders). Compensation for central disorders is usually quite slow.

The goal of a vestibular evaluation is to determine the site of lesion and plan the best course of

rehabilitation. The following provides a brief overview of selected disorders; for an exhaustive list of disorders with associated history and site of lesion, see Appendix 2–E. Chapter 7 also describes the differential diagnoses of vestibular disorders.

Genetics

While a number of causes for hearing loss are documented within the general population, there is speculation that as much as 60% of hearing loss occurring in babies is simply due to genetic traits inherited through transmission by their parents (Korver et al., 2017). Some genetic traits are easily traced through mendelian inheritance (autosomal dominant, autosomal recessive, X-linked recessive, and X-linked dominant). In brief, an individual’s inheritance of traits is controlled by only a single gene with two alleles. One of the alleles may have genetic dominance over the other. Unfortunately, there

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are many occasions when a baby or child is identified with hearing loss that may have a specific genetic trait, but genetic testing is not pursued since none of the family members are known to have hearing loss associated with a genetic trait (i.e., autosomal recessive).

Some of the challenges arise from the vast ranges of hearing loss, especially in onset and degree

occurring coupled with variability of genetic traits. Such differences in penetrance of traits may result in delayed identification for any genetic traits that upon closer investigation might be found in other family members. Many syndromes involve chromosomal abnormalities that could result in firsttrimester miscarriages because of specific gene(s) shared and transmitted through the family lineage. Nearly 20 years ago, there were more than 300 documented forms of syndromic hearing loss that had distinctive features clinically recognized (Morton & Nance, 2006). In fact, there were specific genes associated with hearing loss, such as Connexin 26, impacting the vestibular apparatus and multiple aspects of the hearing mechanism throughout the cochlea, including stereocilia of the fine structure within the cochlea. Further, there are genetic traits attributed to multifactorial inheritance that are influenced by the environment. Though not substantiated, there have been proposed examples of multifactorial traits impacting hearing loss during embryogenesis that can result in other conditions such as neural tube defects, Type 1 insulin-dependent diabetes, cleft lip with or without cleft palate, presbycusis, otitis media, and so on. Genetic disorders and syndromes associated with hearing loss are shown in Appendix 2–F.

Major Embryological Developments

The gestational period is divided into three main stages: preembryonic (fertilization to 3 weeks), embryonic (4–8 weeks), and fetal (9 weeks to birth). A summary of the development of the auditory and vestibular systems is depicted in Table 2–6.

In Utero Development

Syngamy: Preembryonic Stage

At the time of syngamy (fertilization), gametes are formed from the female egg (ovum) and male sperm, each contributing 22 chromosomes and 1 sex chromosomes (46 total) forming a zygote (fertilized egg) (Northern & Downs, 2014), thereby resulting in a fusion of one (or more) “embryo” (Greek term: “to swell”). Embryogenesis is an important process in which an embryo is formed by a collection of cells undergoing mitosis and myosis, resulting in an accelerated growth of cells. By about the third day postfertilization, the zygote (i.e., blastocyst) has three main elements:

Embryoblast: inner layer of cells that forms the embryo

A fluid-filled sac connected to the embryoblast

Rudimentary placenta: a thin layer of cells surrounding the first two components

The blastocyst implants into the uterine lining about a week after fertilization and it will either

be unsuccessful and result in spontaneous abortion or more firmly implanted by the second week of gestation. When successful implantation occurs, gastrulation is the next critical embryogenesis stage in which the embryonic disk develops from the embryoblast and contains three (germ cell) layers of tissue divided into ectoderm, mesoderm, and endoderm, which lead to the development of the organs and tissues of the fetus (Figure 2–7). The ectoderm is most relevant for the auditory and vestibular systems

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TABLE 2–6. Embryological Development of the Ear

GESTATIONAL

WEEK EXTERNAL EAR MIDDLE EAR INNER EAR

3 Development of

Otic placode and pit

tubotympanic recess

4 Thickening of tissue begins Otocyst forms

Division of cochlear and vestibular portions

5 External auditory meatus

begins to form

6 Six auricular hillocks are

present

Begin formation of cartilage

Presence of utricle and saccule

Semicircular canals begin formation

7 Pinnae move dorsally and

laterally

Sensory cells in utricle and saccule

Presence of one cochlear turn

8 Outer 1/3 of external

auditory canal formed (cartilaginous)

Cartilaginous malleus and incus

Lower half of tympanic

Sensory cells in semicircular canals

Ductus reuniens present

cavity

9 Membranous layers of TM

present

11 Innervation by CN VIII

Completed cochlear turns

12 Cochlear sensory cells

Ossification of otic capsule Completed membranous

labyrinth

15 Stapes formed

(cartilaginous)

16 Beginning of ossification of

malleus and incus

18 Beginning of ossification of

stapes

20 Adult-shaped pinna (not

Adult-size inner ear

size)

21 TM exposed due to meatal

plug disintegration

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TABLE 2–6. continued

GESTATIONAL

WEEK EXTERNAL EAR MIDDLE EAR INNER EAR

30 Maturation of external

auditory canal (until 7 years)

32 Malleus and incus ossified

34 Development of mastoid

35 Aeration of aditus ad

37 Change in tympanic

Aeration of epitympanum

air cells

antrum

membrane position (until 2 years)

Aeration of epitympanum Stapes continues

development until adulthood;

Ectoderm

Mesoderm

Endoderm

FIGURE 2–7. The three primary germ cell layers present during the third week of gestation: the endoderm in gray, mesoderm in black, and ectoderm in white.

as it gives rise to the inner ear, sensory epithelia, epidermis, and central nervous system. However, the epithelial lining of the Eustachian tube and middle ear cavity are from the endoderm (Northern & Downs, 2014).

At about the 14th to 17th day postfertilization, the process of neurulation ensues, in which

the neural tube, brain, and spinal cord begin to develop. The notochord (primitive nervous system;

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develops into the axial skeleton) develops during this period of gestation (Northern & Downs, 2014). Ultimately, the ectoderm further develops a thickened layer of cells, which becomes the neural plate, folding into a neural groove that then become neural folds. At or about the fourth week of gestation, the neural tube is closed, giving rise to the central nervous system. An overview of the preembryonic and embryonic stages is displayed in Tables 2–7 and 2–8 (Northern & Downs, 2014).

Embryonic Stage

Appearance of the embryo becoming more recognizable human shape occurs around the end of Week4. There are several small structures that develop and move dorsolaterally from the neck region eventually much closer to the final placement near the lower mandible/jaw. The external auditory meatus is formed from the pharyngeal/branchial groove, which is the counterpart of the pharyngeal pouch on the endodermal side. Four pairs each of pharyngeal (branchial) arches and pharyngeal pouches are present, which are destined to evolve into bone, cartilage, muscles, nerves, glands, and connective tissue of the head and neck:

1. Pharyngeal/Branchial Arch 1 (mandibular arch) — becomes malleus, incus, upper (maxilla) and lower (mandible) jaws, as well as other aspects of the face, muscles of mastication, tensor tympani, tensor veli palatini; tympanic membrane will arise from the ectoderm and mesoderm; innervated by CN V (trigeminal)

2. Pharyngeal/Branchial Arch 2 (hyoid arch) — becomes stapes, part of the malleus and incus, stapedius, stapedial artery, parts of the hyoid, styloid process, and muscles of facial expression; innervated by CN VII (facial)

TABLE 2–7. Preembryonic Development

TIME MAJOR DEVELOPMENTS RESULTS OF INTERRUPTION

Fertilization Zygote Termination

Day 3 Blastocyst: embryolast; fluid-filled sac;

rudimentary placenta

Days 6–7 Implants into uterine wall Termination

Week 2 Firm implantation into uterine wall

Bilayer embryonic disk

Week 3 Trilayer embryonic disk (germ cell layers):

ectoderm, mesoderm, endoderm Notochord Neural plate/groove/folds from ectoderm Primitive cardiovascular system

Termination

Severe abnormalities of spinal cord and brain

Week 4 Fully closed neural tube

Early development of the inner ear Heart begins to beat with primitive blood

Severe abnormalities of spinal cord and brain

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3. Pharyngeal/Branchial Arch 3 — becomes parts of the hyoid, common and proximal internal carotid arteries, stylopharyngeus; innervated by CN XI (glossopharyngeal)

4. Pharyngeal/Branchial Arch 4 — becomes muscles of the soft palate, larynx, and pharynx, cartilages of the larynx; innervated by CN X (vagus; superior laryngeal branch)

Other structures are present during the fourth week as well:

Otic pit — develops into the inner ear

Forming the otocyst, which folds, deepens, and elongates to become the vestibular and

cochlear structures (accomplished Weeks 5–11) Cochlear turns develop through Week 16 Membranous and bony labyrinths become fully developed by Week 20 (marking anatomic

and mechanical structure is in place, but neuroelectrical system and outer hearing

mechanisms are not yet primed to establish functional hearing)

TABLE 2–8. Embryonic Development

TIME MAJOR DEVELOPMENTS RESULTS OF INTERRUPTION

Week 4 4 pharyngeal arches

Otic pits Lens placodes Limb buds Organ systems

Week 5 Enlarged head secondary to increased

brain size Facial features Limb differentiation Organ systems

Week 6 Auricular hillocks

Limb definition Growing head size Organ systems

Week 7 Limb definition

Gonadal development Growing head size Organ systems

Termination or severe congenital abnormalities in multiple organ systems

Ventricular septal defect

Tracheoesophageal fistula

Week 8 Distinctive limbs and facial features,

including pinna and external auditory canals

All major organ systems at least in primitive state

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Cochlear bony and membranous structures are at full size by Week 34; developing base to

apex (i.e., high frequencies first)

Lens placode — develops in the eye

Limb buds — develop into the arms and legs

In the fifth week, facial features become more distinct, as do the limbs. Due to increased brain size, the head also becomes larger. Auricular hillocks (which fuse and become the external ear) emerge during the sixth week, and the first branchial groove will deepen to become the external auditory canal. Limbs further differentiate as the head grows larger. These developments continue into the seventh and eighth weeks; at the end of the eighth week, the embryo has an identifiably human-like appearance with a large head, distinctive limbs, and facial features. Throughout the embryonic stage, major organ systems appear and are differentiated, including the central nervous system (neural tube), peripheral nervous system (neural crest), cardiovascular system, respiratory system (laryngotracheal tube), urinary system, and the fore-, mid- (forming the umbilical cord), and hindgut. A summary of the development during the embryonic stage is shown in Table 2–8.

Fetal Stage

With the onset of organ systems formation marking the embryonic stage, the fetal stage is marked by great growth and maturation (Northern & Downs, 2014).

Weeks 9–16: The body and limbs of the fetus grow so that their relative sizes approximate a

newborn’s. The external ears (recognizable pinnae) migrate to the final newborn position.

Weeks 17–20: Fetal skin tissue begins to approximate that of a newborn, hair grows, and

fetal movements continue to increase. The external auditory canal is blocked with a meatal plug (which becomes the epithelial layer of the tympanic membrane) until the 21st week of gestation.

Week 21: External auditory canal becomes filled with mesenchymal tissue and amniotic fluid.

Weeks 22–25: Weight gain significantly increases and alveoli develop in the fetal lungs.

Weeks 26–29: Full development of hair and the beginning of subcutaneous fat.

The respiratory system reaches maturity in the last 9 to 10 weeks of development; weight and subcutaneous fat increases are noted as well. Aside from overall typical physical development, there are many ways typical auditory development can be significantly slowed, disrupted, or ceased by pharmacologic or natural teratogens creating any degree or type of hearing loss. Some are included in Table2–9 (Northern & Downs, 2014).

Postnatal Development

Postnatally, typically the auditory and vestibular systems continue to develop.

Pinna: develops to adult size by 9 years of age

EAC: infant (0.5 cm) grows to about 2.2 cm into adulthood

Eustachian tube: elongates and changes orientation (from more horizontal to more angled)

during childhood

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TABLE 2–9. Developmental Disorders and Syndromes

DEVELOPMENTAL STAGE

Week 4 First arch

DISORDER/ SYNDROME AFFECTED AREAS

Microtia, atresia, ossicles/

syndrome

middle ear, maxilla,

HEARING LOSS TYPE ETIOLOGY

Conductive Cellular migration

disruption

mandible, palate, eyes

Treacher Collins syndrome

Microtia, atresia, middle & inner ear, eyelids, underdeveloped

Conductive, mixed, sensorineural

zygomatic bone

Pierre Robin sequence

Weeks 4–10 CNS, PNS, organ

Outer ear, small lower jaw, palate

systems, limbs

Conductive

Sensorineural Teratogens, e.g.,

CMV, TORCH infections, rubella virus

Weeks 7–8 Mondini aplasia Cochlea Sensorineural Genetic, syndromes,

teratogens (e.g., CMV)

Note: CMV: cytomegalovirus; TORCH: toxoplasmosis, other bacterial or viral infection (e.g., syphilis), rubella, CMV, herpes simplex virus.

Source:

This is important because younger children can be prone to middle ear infections

secondary to Eustachian tube dysfunction from a more horizontal orientation, resulting in pathological fluid buildup within the middle ear space.

Cochlea: development (e.g., tuning curve sharpening, especially at the apex) continues until

age 8

CN VIII: myelination continues through the first several months of infancy (up to 6 months

to full maturation)

Auditory brainstem: pons and midbrain continue to grow until the ages of 8 and 6 years,

respectively; myelination in the auditory brainstem is typically complete by approximately 1year of age

Auditory cortex: fully developed at about 20 years of age due to the caudal-to-rostral

maturation of the central nervous system

Other aspects of the brain, such as the corpus callosum, also have a protracted maturation course (Musiek & Baran, 2020).

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AUDIOLOGY NUGGET

The infant EAC creates some interesting clinical effects related to diagnostic testing and the fitting of amplification. Their canal is less stiff and has a smaller

volume than adults (0.4–1.0 cm affects resonant properties. Otoacoustic emissions (OAEs) may have a greater range of amplitudes in infants than in older patients because of the potentially increased intensity resulting from a smaller resonating cavity. It is important to be mindful of the physics of sound waveforms and the linear interaction of waveforms unpredictably resulting in constructive or destructive interference within the external auditory canal. Consequently, when undertaking hearing aid fittings with infants and children with personal amplification systems, programming modifications would account for the increased or decreased sound intensity delivered to the TM (depending upon EAC shape, length, width, and also frequency of the stimulus). When completing verification measures, clinicians can somewhat account for such differences by measuring real ear to coupler differences (RECD) or utilizing appropriately sized couplers to appropriately match the actual hearing aid output delivered within the ear canal of infants and children. In addition, this smaller chamber can ultimately result in a higher resonant frequency. Ultimately, these differences can lead to more variability in OAE amplitudes, especially in the higher frequencies, as well as an increase in highfrequency amplification. Finally, it is also imperative that clinicians are mindful of the canal size differences when inserting materials (specula, inserts, probe tips) prior to testing so that the TM is not punctured (Musiek & Baran, 2020).

vs. 0.6–1.5 cm3, respectively), which directly

Lifetime Changes of the Auditory and Vestibular Systems

Auditory System

With age, the peripheral auditory system is subject to degradation, especially the OHCs. The middle ear can also be subject to aging, such as arthritic changes and the formation of sclerotic tissues. The pinna and EAC become less elastic, with the pinna lengthening with age. From infancy through young adulthood, the CANS is largely in a plasticity and development mode. Once adulthood is achieved, there is considerable evidence in various animal species that age-related changes occur within the CANS structures. The few studies with humans about CANS changes have implicated a reduction in density of nuclei of the anatomical structural lateral lemniscus, inferior colliculus, superior temporal gyrus, superior temporal gyrus, and lobe (e.g., Ferraro & Minckler, 1977; Thompson et al., 2003). Though the changes were seen in humans with advancing ages, there is also unpredictable variability in degree of anatomical changes as well as ages of the adult humans.

A variety of studies on the aging central auditory system have demonstrated an expected

deterioration of CANS function resulting from the natural aging process: reduced myelination at the corpus collosum (Musiek & Weihing, 2011); loss of vascular density that leaves fewer viable vessels (Kalaria, 1996); atrophy of vessels along with reduced metabolism, resulting in

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reduced optimal neural function (Clinkard et al., 2013); reduction of cerebral spinal fluid and blood circulation in the brain (Kalaria, 2010); and physiological consequences of reduced blood supply (Thore et al., 2007). With all of these changes, it is expected that auditory processing abilities become altered with advanced aging in adults.

Vestibular System

The vestibular system is fully formed at birth, developed by the 12th to 14th week of gestation and adult-like by the 24th week of gestation (Jacobson et al., 2021). Like the auditory system, the vestibular organs arise from the otic placode and begin development in the third to fourth week of gestation: Disruptive events occurring in this time period can have catastrophic results on the vestibular system. By Week 5, the utricle is formed, and the saccule is formed by Week 9. The sensory epithelium is differentiated between the maculae, cristae, and cochlea in Weeks 8 through 10. Hair cells are fully formed by Weeks 12 to 15 and reach adult-like structure by Weeks 20 to 23. The peripheral system may be developed at birth, but the integration of information and processing of vestibular information is not completely developed until adolescence/early teenage years. Childhood years yield significant changes and maturation in the various postural and motoric responses to vestibular input, including alterations in the VOR (Jacobson et al., 2021).

With advancing age, there are several factors that result in an increased incidence of dizziness,

vertigo, imbalance, and falls (e.g., polypharmacy, disordered vision or proprioception, neuromuscular changes, and medical comorbidities). One of these is the changes that occur in the vestibular system, both peripheral and central.

In the peripheral vestibular system (semicircular canals and otolithic organs), sensory

cells degenerate with age, which leads to degradation of the nerve fibers in the vestibular portion of cranial nerve VIII. The number of otoconia has been observed to decrease with age.

Alterations have been noted in the connectivity of the central vestibular system as well. On the

positive side, vestibular rehabilitation may decrease the fall risk in older adults.