Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4385_Библиотеки_им_академика_М_И_Перельмана
.pdf
ANATOMY AND PHYSIOLOGY OF BALANCE
Arcuate eminence
t
Scala tympani
scala vestibuli
canal
Endolymphati
sac
saccule). e three semicircular canals (superior, posterior, and lateral) detect angular
acceleration in orthogonal planes. e utricle and saccule detect linear acceleration in
primarily horizontal and vertical planes, respectively, as well as head position in relation
to grav it y.
Anatomy
Bony Labyrinthine Anatomy
e bony labyrinth lies within the petrous temporal bone, which is the densest bone in the
body. e bony labyrinth (otic capsule) has three main components: the cochlea (anteriorly),
vestibule (centrally), and semicircular canals (posteriorly) (Figure 2.1).
On the medial surface of the vestibule there is a further bony extension that projects posteriorly called the vestibular aqueduct. e vestibule has two openings on its lateral surface, the
oval window superiorly and round window inferiorly.
e posterior limb of the superior canal and medial limb of the posterior canal join to
become the common crus as they enter the vestibule. Each semicircular canal has a bony
dilatation, called the ampulla, which houses its sensory organ. e semicircular canals are
arranged in orthogonal planes (see Figures 2.1 and 2.2). e superior and posterior canals
both lie 45° from the sagittal plane. erefore, the superior canal of the right ear lies parallel
to the posterior canal of the le ear, and vice versa.
tegmen
Superior SCC
Vestibular aqueduct
Endolymphatic duct
Utricle
Posterior
limb
Ampulla
Saccule
Endolymph
Oval
window
Common
cruz
Medial
limb
Lateral
Perilymph
SCC
Round
window
Ductus
reuniens
Vestibule Cochlear
Cochlear
aqueduct
Cochlear duc
Posterior
c
SCC
Semicircular
Lateral
limb
Figure 2.1 Detailed scheme of a right otic capsule’s medial surface demonstrating the bony
covering and internal ducts. Locations of oval and round windows are demonstrated. SCC, semicircular canal.
8 e Ear
ANATOMY AND PHYSIOLOGY OF BALANCE
Le
SS
LL
PP
Right SC
Right PC
Left PC
ft SC
45° 45°
Figure 2.2 This gure demonstrates the orthogonal relationship of the semicircular canals. The
superior and posterior canals (SC and PC) are 45° from the sagittal plane. Additionally the left
superior semicircular canal is in a parallel plane to the right posterior semicircular canal. L, lateral;
P, posterior; S, superior.
e superior semicircular canal abuts the mastoid tegmen, with a corresponding intracranial projection called the arcuate eminence. e thin bone of the tegmen that separates the
canal from the dura at this point can be less than 1 mm in diameter.
Ductal Anatomy
e cochlear, utricle, and saccule semicircular and endolymphatic ducts are the membranous ducts of the four bony coverings. e ducts are lled with endolymphatic uid that is
potassium rich (similar to intracellular uid, see Chapter 1). Anatomically, there appear to be
two distinct membranous systems with implications on function and disease. e cochlear
duct joins the saccule via the ductus reuniens, whilst the semicircular ducts branch directly
o the utricle. Intense acoustic stimulation can inadvertently activate the saccule causing the
sensation of movement (vertigo) in some people, called the Tullio’s phenomenon, particularly if uid pressure wave pathways are abnormal, for instance, in inner ear ‘third window’
syndromes, such as superior semicircular canal dehiscence.
e saccule lies in a spherical recess, anterior and inferior in the vestibule, ~0.6 mm deep to
the anterior surface of the stapes footplate. is relationship is important for stapes surgery
where an overlong piston inserted beyond the footplate can stimulate the saccule. e saccule is united with the cochlear duct from its anterior surface and endolymphatic duct on
its posteromedial surface. e saccular macula contains the sensory hair cells, present in a
cellular layer on its anterior surface.
e utricle is more oblong in shape, lies posterosuperior in the vestibule, and is ~0.8 mm
deep to the posterior edge of the stapes footplate. e ve semicircular ducts are projections
of the utricle. e endolymphatic duct exits from its anteromedial surface. e utricular
macula, containing the sensory cells, lies in its oor and is orientated primarily in a horizontal plane. e saccular macula lies on the medial surface of the saccule and is orientated
primarily in a vertical plane.
e Ear 9
ANATOMY AND PHYSIOLOGY OF BALANCE
e endolymphatic duct is formed from smaller ducts leading o the medial surfaces of the
utricle and saccule and travels in the vestibular aqueduct. It courses behind the arcs of the
posterior semicircular canal and narrows at its isthmus before it leads to the endolymphatic
sac, which is a highly complex structure of interconnecting tubules, cisterns, and crypts.
e distal, extraosseous portion of the sac rests on the posterior wall of the petrous bone,
between layers of dura.
Microanatomy
Hair cells are the main receptors of the vestibular apparatus and occur in both the maculae
(Figure 2.3) and crista ampullaris (Figure 2.4).
e apical surface of a hair cell is covered by large, actin-rich, rod-like microvillar projections called stereocilia, grouped in a bundle (the ‘hair bundle’), wherein stereocilia
progressively increase in length, like a staircase, with one large microtubule-based true
cilium called the kinocilium positioned behind the longest row of stereocilia. e hair
bundle staircase and position of the kinocilium dene the axis of sensitivity or ‘polarity’
of the hair bundle (note the kinocilium, although present in early embryonic development,
is absent in mature cochlear hair cell bundles). e tips of each shorter stereocilium is
linked to the sha of its neighbouring taller stereocilium by a thin proteinaceous lament,
the tip link. Tip links are composed of two linear proteins joined end to end: cadherin 23
and protocadherin 15. Defects in one of these proteins are associated with dierent forms
of Usher’s syndrome. All of the hair cells in a crista are orientated in a single direction,
along the plane of the semicircular duct, whereas hair cells in the maculae are arranged in
a multiplanar orientation.
e hair bundles, which are bathed in endolymph, are covered by acellular membrane structures. In the maculae, the otolithic membrane is covered with otoconia, which are white
crystalline particles composed predominantly of calcium carbonate. In semicircular canals,
the hair cells of cristae protrude into a barrel-shaped gelatinous cap (termed the cupula) that
lacks otoconia.
Sensory aerents of the saccule and the posterior semicircular canal are formed into the
inferior vestibular nerve and those of the utricle; lateral and superior semicircular canals are
formed into the superior vestibular nerve. e vestibular nerves travel medially in the internal auditory canal to the vestibular nuclei at the medullary-pontine junction. ere is cross
communication of nuclei on each side. In addition, each nucleus sends bres to the nuclei of
the ipsilateral and contralateral abducens, trochlear and oculomotor nerves, cerebellum, and
descending bres to the vestibulo-spinal tracts.
Physiology
e otolith organs sense gravity and linear translation of the head, and the semicircular
canals sense angular acceleration of the head. Standing upright, in the absence of movement,
the semicircular canals, utricles, and saccules of both the le and right ears cause ring of the
right and le vestibular nerves at an equivalent baseline rate. Perception of movement and
spatial position are a consequence of how these organs change the ring rate in the vestibular
nerves from their basal rate and in relation to each other.
Movements of the head are synonymous with movements of the embedded bony labyrinth,
e endolymphatic uid lags behind owing to inertial forces and the viscous drag between
the uid and the duct wall. e ‘lagging’ endolymph deects the cupula, which in turn
deects the stereocilia of the hair cell. If the stereocilia are pushed towards the kinocilia, then
the tip links at the tips of stereocilia are stretched and thus ‘gate open’ mechanotransducer
ion channels allow cations (mostly potassium) from the endolymph to enter the hair cell.
e resulting depolarisation of the hair cell triggers pre-synaptic calcium inux and neurotransmitter release at the basal end of the cell and a post-synaptic depolarisation and thus
increased ring rate of aerent nerve bres. If the head movement is in the opposite direction
10 e Ear
ANATOMY AND PHYSIOLOGY OF BALANCE
t
(a)
Utricular
(b)
(c)
component
Striola
Saccular
maculae
Medial
Lateral
componen
Otoconia
Striola
maculae
Striola
Figure 2.3 (A) Right utricular macula orientated in horizontal plane in oor of utricle. (B) A
striola is a strip with no hair cells that separates the polarity of the hair cells on the medial and
lateral components. The otoliths sit on an otolithic membrane which the hair cells project into. (C)
Tilting head to the right will cause the otoconia to fall to the right, causing stereocilia to deect to
the right. The lateral component of the macula will thus become hyperpolarised and the medial
component depolarised.
e Ear 11
ANATOMY AND PHYSIOLOGY OF BALANCE
a
(a)
Turn head to the left
activity
e
Ner
(b)
Nerve bre activity
(c)
Gelatinous cupul
Stereocilia
Kinocilium
Hair cell
Supporting cell
Nerve bres
Excitation
ve bre
activity
Direction of
endolymph
Kinocilium
Resting – before head turn
Nerve bre
Figure 2.4 (A) Crista ampullaris of a left lateral semicircular duct. This shows the hair cells envel-
Inhibition
Nerve br
activity
oped within the gelatinous cupula. Endolymphatic uid surrounds the cupula. (B) Blow up demonstrates movement of head to the left. Inertia of the endolymph pushes cupula to the right; the
stereocilia move towards the kinocilia and thus the cell becomes active on the left, increasing the
ring rate of associated nerve bres. (C) The stereocilia move away from the kinocilia on the right
and thus ring rate decreases on the right. The differential ring between right and left semicircular canals provides an accurate awareness of head movement.
12 e Ear
ANATOMY AND PHYSIOLOGY OF BALANCE
and the endolymph pushes stereocilia away from the kinocilia, this closes mechanotransducer channels, hyperpolarises the cell, and reduces ring in the aerent bres.
Semicircular Canals
e semicircular canals are orientated so that they provide complementary information
about angular movement. Anti-clockwise head rotation (turning your head to the le)
increases the nerve-bre ring rate of the le lateral semicircular canal and decreases the
ring rate of bres from the right lateral semicircular canal. e perception of head rotation
is the net result of dierential ring rates between these complementary lateral semicircular
canals (see Figure 2.4). e superior and posterior canals of opposing ears lie in the same
plane. erefore, with the head rotated 45° to right, head exion results in an increased bre
ring rate of the right superior semicircular canal and decreased ring rate of the le posterior semicircular canal. With the head rotated 45° to right, head extension causes reversal in
this pattern of activation, between the right superior and le posterior semicircular canals.
e semicircular canals are integral in maintaining visual target xation during movement,
through the very short latency vestibulo-ocular reex (VOR).
Dierential ring of nerve bres from each semicircular canal is communicated through the
vestibular nerves from the vestibular nuclei to the abducens and oculomotor nuclei through
a series of interneurons. When the head is rotated to the right, this fast reex causes the le
lateral rectus and right medial rectus to move both orbits to the le at the same rate and
degree of the head turn, allowing visual xation despite movement. Using dierential information delivered from all six semicircular canals, the VOR allows visual stabilisation despite
movements of the head in all planes. Unilateral vestibular lesions cause conjugate eye movement towards the aected ear (due to unopposed action from the healthy vestibular side)
and a corrective (fast-phase nystagmus) movement back to the centre (away from the lesion).
Utricle and Saccule
e stereocilia within the maculae are embedded in a gelatinous matrix covered by otoconia.
ese crystals have a large specic gravity relative to endolymph, and thus pull the matrix to
align with the gravitational vector, keeping it deected post-movement. In addition, linear
translation causes acceleration either horizontally or vertically, which also results in inertial
deection of the otoconia.
Since each hair cell is specialised to detect movement in one direction (movement that pushes
the stereocilia towards the kinocilium), the hair cells within the maculae are arranged such
that there are individual hair cells oriented to detect virtually any head position or direction
of acceleration. is is accomplished by a complex three-dimensional arrangement of each
macula and of the hair cells within the maculae (see Figure 2.3).
e arrangement of the saccule is such that anteroposterior translation is best detected (e.g.
breaking at a red trac light in a car). In addition, the saccule provides continuous information about vertical body position in relation to gravitational centre (e.g. awaking from sleep,
lying at on back). is allows one to detect that they are tilted backwards o the vertical,
and perhaps about to fall. e saccule is therefore a key sensory input for the vestibulo-colic
reex (VCR) and the vestibulo-spinal reex (VSR). ese reexes activate extensor muscles
to maintain posture.
e orientation of hair cells in the utricle are such that horizontal translation (e.g. standing
face forward on a skateboard and being pulled to the le or right) is best detected. In addition,
the utricle provides continuous information about horizontal body position in relation to
gravitational centre (e.g. awaking from sleep lying on side). is allows one to detect that they
are tilted sideways o the vertical. e utricle provides sensory input for the utriculo-ocular
reex that causes eyes to move in the opposite direction to the side of lateral translation and
maintain visual xation. e ocular vestibular myogenic evoked potential (VEMP) is a test
of the utriculo-ocular reex, and the cervical VEMP is likely more dominated by the saccule.
e Ear 13
CLINICAL EXAMINATION OF THE EAR AND HEARING
KEY POINTS
• Movements that cause hair bundles to move ‘towards’ the kinocilia cause vestibular
hair cells to depolarise and this increases the ring rate of afferent nerve bres.
Movements that cause hair bundles to move ‘away’ from kinocilia cause hair cells to
hyperpolarise and reduce the ring rate of nerve bres.
• Perception of movement is detected by the change in ring rate, and the relative
difference between ring rates from complementary organs on either side.
• Lateral canals of each ear are complementary and detect head rotation (yaw). The
superior canal of the right ear is complementary to posterior canal of the left ear (and
vice versa) and together they detect non-horizontal head movements (pitch and roll).
• The otolithic organs, the utricle and saccule, are housed in the vestibule (main body)
of the labyrinth. The saccule is closely related to the cochlear duct and as a result can
be activated by intense acoustic stimuli. The saccule detects vertical translation and
forward-backward tilt. It activates the VCR and VSR.
• The utricle is closely related to the semicircular canal. It detects horizontal translation
and side-to-side tilt. It activates the utriculo-ocular reex. Dislodged otoconia can pass
from the utricle into the semicircular canals and cause an intense sense of vertigo on
head movement (benign paroxysmal positional vertigo).
Further Reading
1. Alan Desmond. Vestibular Function Clinical and Practice Management, 2nd Ed. April
2011, Chapter 2, E-Book ISBN: 9781604063622.
2. Michael Gresty. Clinical Neurophysiology of the Vestibular System, 3rd Ed., April 2002,
New York: Oxford University Press.
3. CLINICAL EXAMINATION OF THE EAR AND HEARING
Examination of the Ear
If ear surgery has been performed, external scars (endaural or postauricular) are looked for.
e external ear skin is assessed for dermatological conditions.
Examination of the external auditory, pars tensa, and accida requires illumination and a
speculum. Microsuction should be available to clear debris. Magnication with a handheld
auriscope or microscope allows pathology to be assessed in greater detail. Pulling the pinna
upward and back slightly will straighten the cartilaginous part of the external canal to enable
the largest speculum to be inserted.
Rod endoscopes can be used to gain a wider view (Figure 3.1A), photographically record
pathology, and allow electronic communication with others. Endoscopes with variable magnication can be tted to a smartphone and have the potential of articial intelligence (AI)
to make a diagnosis. However performed, it is important to record the otoscopic ndings, in
detail, in case notes or letters of communication.
If a structured approach to otoscopy is taught, diagnostic skills improve and disease patterns
are more easily recognised and appropriately dealt with.
1
In most ears, the most recognisable
feature is the handle of malleus (Figure 3.2). e umbo and lateral process should be identied and the adjacent tympanic membrane visualised. e pars tensa and pars accida should
14 e Ear
CLINICAL EXAMINATION OF THE EAR AND HEARING
Attic
Pars flaccida
Ant
canal
Post
bulge
canal
wall
Pars tensa
Malleus
Figure 3.1 Endoscopic view of the tympanic membrane (A) with labeled schematic (B). The pars
accida is located above the malleolar folds and is decient of a middle brous layer.
be inspected (see Figure 3.1B). e examiner should decide if the pars tensa is intact and if
so whether it is in its normal position. Clues as to its normal position should be sought in
identifying the angle of the handle of the malleus. Foreshortening indicates retraction of the
tympanic membrane medially as does lipping around the annulus creating a ‘neo-annulus’
(Figure 3.2.1). Mobility of the pars tensa can be assessed by getting the patient to perform a
Valsalva manoeuvre or use of a pneumatic Siegel closed speculum. Immobility would suggest
middle ear uid.
e tympanic membrane is normally a grey, slightly translucent colour. Hyaline degeneration of the brous layer sometimes associated with calcium deposition occurs as a consequence of previous episodes of middle ear inammation. is increases the whiteness of the
tympanic membrane and can be either a diuse thickening of the tympanic membrane or
e Ear 15
CLINICAL EXAMINATION OF THE EAR AND HEARING
(b)
(a)
Pars flaccida
Chorda tympani
Lenticular process
of incus
Umbo
Round window
Eustachian tube
Fibrous annulus
Figure 3.2 Normal tympanic membrane with labeled middle ear structures.
Figure 3.2.1 Severely retracted position of the malleus handle in otitis media with effusion (left
ear). As retraction develops, a neoannular fold may form (arrows).
16 e Ear
CLINICAL EXAMINATION OF THE EAR AND HEARING
Figure 3.2.2 Chalk patch on the anterior pars tensa (right ear).
occur as isolated, tympanosclerotic plaques (Figure 3.2.2). Previous grommet insertion pro-
duces a rather characteristic crescent-shaped deposition of calcium in about 30% of patients
(Figure 3.2.3). Tympanosclerosis may also xate the ossicles with a resultant conductive
impairment. If the tympanic membrane is perforated posteriorly, the ossicular chain may
be visible (Figure 3.3).
It should be routine to angle the vision to have a full view of the pars accida particularly
if the ear has had surgery (Figure 3.3.1). e extent of any attic defect requires assessment
aer any contents have been removed and deciding whether they are from active mucosal or
squamous disease.
Figure 3.2.3 Extruded ventilating tube with otoscopic recurrence of middle ear uid. Left ear
retracted and yellow.
e Ear 17
Соседние файлы в папке Библиотека им академика М.И. Перельмана