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Hearing Loss & Vestibular Dysfunction NATIONALA RESOURCE CENTER GUIDE FOR FOR EARLY HEARING HEARING ASSESSMENT DETECTION & & MANAGEMENT INTERVENTION Chapter 7 Hearing Loss & Vestibular Dysfunction: A Common Comorbidity Violette Lavender, AudD; Gretchen Mueller, DPT; Michael Castiglione, AuD; & Katheryn Bachmann, PhD Early Development of Between the 12th and 24th weeks, neural The vestibular system is connections between the labyrinth the first of the sensory the Vestibular System & and oculomotor nuclei are finalized.1,2 systems to fully develop Associated Milestones Additional connections between the in utero . At birth, cerebellum, reticular formation, and spinal cord also develop.1 The vestibular nerve is the vestibular system is he vestibular system is the first of the first of the cranial nerves to complete complete in structure. the sensory systems to fully develop myelination with full function finalizing Tin utero.1,2 Embryonic ectoderm between the 8th and 9th month.1 develops into otic placodes, and as the otic placodes are surrounded by the embryonic At birth, the vestibular system is complete in mesoderm, they transition into otic pits.1,2 structure.1,2 It continues to mature in function Upon closure of the otic pits, otocysts over the next several weeks—demonstrating are formed, which then develop into the greatest response to environmental the membranous cochlea and vestibular stimuli between the 6th and 12th months of labyrinth.1,2 Beyond this, embryonic life.1,2 Sensory inputs from multiple systems— mesoderm then surrounds the vestibular including the visual system, somatosensory labyrinth to form the bony labyrinth. At system, and peripheral vestibular system— the completion of the 7th week in utero, converge within the vestibular nuclei and the vestibular apparatus is formed.1,2 The cerebellum.1,2 Connections to the muscles of vestibular apparatus consists of three the eyes and neck, as well as the spinal cord, semicircular canals, which are responsible produce nearly immediate responses within for detecting angular acceleration, as the body.1,2 The output from these afferent well as the utricle and saccule, which inputs result in the body’s ability to control are responsible for detecting linear posture, locomotion, spatial orientation, acceleration.1,2 and gaze stabilization.1 eBook Chapter 7 • Hearing Loss & Vestibular Dysfunction: A Common Comorbidity • 7-1 A RESOURCE GUIDE FOR EARLY HEARING DETECTION & INTERVENTION There are multiple developmental reflexes This is the communication between the that are pertinent to the maturation of visual and vestibular systems that results the vestibular system in the first 6 months in the maintenance of clear vision of an of life.2 Within the first 2 weeks of life, a object in front of one’s self during head doll’s eye response can be elicited.1,2 As the movements.1,2 Although the VOR is present infant’s head is rotated, movement of the at birth, maturation of the centrally- eyes opposite of the rotation is observed.1,2 mediated visual pathways is needed for This represents stimulation of the vestibular full functionality.2 This is typically present system. Beginning at 4 months of age, head by 2 months of age, reaching full maturity righting begins to emerge.1,2 As the infant between 6 and 12 months of age.2 It is is tipped horizontally when held, the infant considered abnormal if a VOR has not lifts the head so that the head and eyes align developed by the age of 10 months.2 vertically.1,2 This represents integration of vestibular and visual input.1,2 Head The second of the vestibular-specific righting continues to mature over the next reflexes is the vestibulo-spinal reflex (VSR), 2 months, and emergence of responses from which is responsible for body stabilization.1,2 the lower extremities are observed, as they Afferent signals are received from three lift opposite of the side that they have been main sensory systems, which coordinate tilted.1,2 This indicates additional integration within the central vestibular system to of the body’s proprioceptive response.1,2 maintain postural control.1,2 The eyes There are multiple Lastly, a parachute response can be produced provide visual input, while the vestibular developmental reflexes by 6 months of age. When the infant is held input from the inner ears provides that are pertinent to in ventral suspension and is quickly tipped information regarding the body’s linear forward toward a support surface, extension and angular acceleration through space.1,2 the maturation of the of the arms and splaying of the fingers are In addition, somatosensory receptors in vestibular system in the observed.3 This represents a protective various joints provide proprioceptive input first 6 months of life. response with resultant interaction to the central vestibular system.1,2 The between the visual and vestibular systems.3 resulting output of the central vestibular system is the brain’s ability to send There are three vestibular-specific reflexes signals to anti-gravity muscles activating necessary for functional vestibular balance equilibrium and protective responses to control. The vestibulo-ocular reflex (VOR) maintain balance control. 1,2. These muscles is responsible for gaze stabilization.1,2 counteract the downward pull of gravity, activating equilibrium and protective responses to maintain balance control with an upright posture and prevent a fall. In addition to the VOR and VSR, the vestibulo-collic reflex (VCR) assists with both visual and body stabilization.1,2 The VCR utilizes co-contraction of the cervical muscles in the neck to stabilize the head while the body is moving.1,2 This is most notable during predictable movements, as is demonstrated while one is ambulating.1,2 However, the VCR is also functional during movements that are unpredictable.1,2 The functionality of the vestibular system in a growing child is dependent on the integration of the physiologic performance of the above vestibular reflexes.1,2 Balance control develops in a cephalo-caudal Photo courtesy of NCHAM fashion, with head control maturing eBook Chapter 7 • Hearing Loss & Vestibular Dysfunction: A Common Comorbidity • 7-2 NATIONAL CENTER FOR HEARING ASSESSMENT & MANAGEMENT first followed by the ability to sit, stand, independently, the interpretation of Multiple studies have and walk independently.2 Head control somatosensory input proves to be very looked at the integration progresses throughout the first 4 months valuable. Even slight somatosensory input of the sensory systems of life. This is demonstrated as the infant to the infant’s hand on a support surface by age utilizing both progresses from holding the head in line improves upright posture by decreasing with the body, to holding the head above postural sway, which is why infants rely computerized dynamic the plane of the body, and finally holding on holding onto furniture prior to hands- posturography as well as the head steady while rotating the neck to free standing.5 The ability to stand while foam posturography. visualize the environment around him.1,2 having hand(s) on a support surface develops between 10 to 12 months of The ability to sit independently is age.5 Independent walking is attained in normally achieved by 6 months of age the typically-developing child between in the typically developing child. It is 12 to 15 months of age. It is during these most definitely a building block for the months that the child’s balance transitions attainment of more advanced gross motor from static to dynamic, which molds the skills, including standing and eventually functional balance control of the child.5 walking. It is also of utmost importance in Responses to sensory input build upon that it allows the infant to reach and play, past experiences. This allows the infant to which is necessary for the development of transition from being reactive to proactive fine motor and cognitive skills.4 In order to with balance control while ambulating.5 maintain balance while sitting, the infant must demonstrate the ability to integrate Multiple studies have looked at the sensory input from all three systems integration of the sensory systems (visual, vestibular, and somatosensory) by age utilizing both computerized for proper execution of protective and dynamic posturography as well as foam equilibrium responses when stimulated. posturography.6-8 As stated earlier, the The ability to maintain this upright visual system is the primary sensory posture is challenged by the: system utilized by infants, which continues through the toddler and preschool ages • Surrounding environment. up to age 5.5-8 Per posturography testing, • Infant’s own biomechanics. it appears that the visual system is fully • Task the infant is trying to complete integrated for balance at age 5, as the (i.e., play)4. child can adapt to changes in the visual surround at this age.7 At 7 years of age, The infant who has developed the skill of the child has fully integrated individual independent sitting is constantly weight visual and somatosensory input; however, shifting and utilizing degrees of freedom continues to have difficulty maintaining to maintain an upright posture.4 Studies balance during conditions that create a show that infants are most dependent on sensory-conflict for the vestibular system. visual input for maintenance of postural This occurs when vision is occluded control during this time.4,2 In fact, a (eyes closed) and somatosensory cues change in somatosensory input (sitting are inaccurate (surface is responsive to on compliant foam vs. even ground)
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