Session 6: The Ear Flashcards
The temporal bone constitutes a large portion of the lateral area of the skull. It has 4 components: squamous part, petromastoid part, tympanic plate and styloid process. Describe the tympanic plate
[*] The external acoustic meatus consists mostly of the tympanic plate. Its free outer border provides attachment for the cartilage of the external ear. Medially, it fuses with the petrous part of the temporal bone.
What parts of the ear does the petrous part contain?
[*] The petromastoid part contains the middle and inner ears. Its upper surface forms part of the floor of the middle and posterior cranial fossae.
- The part forming the front of the posterior cranial fossa is pierced by the internal acoustic meatus transmitting the facial and vestibulocochlear (auditory) cranial nerves.
- The inferior surface is irregular and contains the carotid canal for the internal ceratoid artery.
- The mastoid process is a large palpable landmark to which several muscles are attached.
- The cavity of the mastoid antrum (a prolongation of the cavity of the middle ear) is prolonged into the process by intercommunicating air cells.
- Middle ear disease spreads by this to cause mastoiditis.
Describe the parts of the external ear
[*] The external ear consists of the auricle and external acoustic meatus (external auditory canal)
- The auricle (pinna) collects sound whilst the external acoustic meatus leads inward through the tympanic part of the temporal bone and terminates at the fibrous tympanic membrane (the ear drum).
- The auricle is an irregularly shaped plate of elastic cartilage and covered with thick skin. The elastic cartilages are arranged in a number of curved ridges – several depressions and elevations - that include the outer rim (helix) and a small flap (the tragus) guarding the external acoustic meatus.
- The earlobe (lobule) is non-cartilaginous – consists of fibrous tissue, fat and blood vessels.
Describe the neurovasculature supply to the external ear
- Sensory innervation anterior to the external acoustic meatus is the Auriculotemporal nerve, a branch of the Mandibular Nerve (CN V3). Sensory innervation for the rest of the auricle is from the Greater Auricular Nerve (C2 & C3).
- Blood supply to the Pinna is from the Posterior Auricular Arteries, Superficial Temporal Arteries and Occipital Arteries which are all branches of the External Carotid Artery plus their corresponding veins
- The auricle (outer ear) functions to collect sounds and funnel into external auditory meatus. It has some intrinsic and extrinsic which are supplied by the facial nerve.
- Innervation of skin is derived from
- Lesser Occipital Nerve (C2)
- Trigeminal Nerve (CN V) – auriculotemporal nerve (CN V3)
- Vagus Nerve (CN X) – auricular branch
- Facial Nerve (this is variable between individuals, absent in some people
Describe the external acoustic meatus
- The external acoustic meatus (canal) is an elastic cartilaginous tube laterally (1/3rd) and a bony canal medially (2/3rd) – tympanic plate of temporal bone. It extends from concha to the outer plate of the tympanic membrane and is ~3cm in straight line length. It is sinuous in profile and narrows about 5mm from tympanic membrane. This narrowing is called the isthmus.
- The meatus is lined by skin throughout secreting cerumen (which is modified sebum) that affords protection for the delicate meatal skin. The skin is adherent and non-moveable. Lining of the skin extends to the outer surface of the tympanic membrane.
- The skin lining the outer 1/3 of canal has hairs (sometimes called cilia), sebaceous glands, ceruminous glands (modified sweat glands, secrete yellowish brown wax which is bactericidal).
- The discarded cells of the skin together with cerumen form the wax.
- The course of the external acoustic meatus is sigmoid-shape and thus the auricle is pulled upwards and backwards during ear examination in order to achieve a good internal view.
Describe the tympanic membrane
- The fibrous tympanic membrane (ear drum) is arranged as a shallow cone with its apex pointing medially. It separates the external ear from the middle ear.
- It is a thin, oval semi-transparent, pearly grey membrane, allowing visualisation of some structures within the middle ear, most notably, the malleus to which the apex of the eardrum is attached. It faces downwards, forwards and laterally. It is concave laterally.
- ~1cm diameter
- It comprises of 3 layers of tissues: the outer plate/lateral layer (keratinised stratified squamous cells), the middle plate layer (fibrous layer) and the inner plate/medial layer (respiratory epithelium – low columnar)
- Blood vessels visible around the periphery
- Partition between external and middle ear
- External surface is supplied by Auriculotemporal Nerve Branch of CN V3 and Auricular Branch of the Vagus Nerve (CN X)
- Internal surface is supplied by the Glossopharyngeal Nerve (CN IX)
Describe the Arnold Cough’s Reflex
(Ear Cough Reflex)
- Stimulation of the auricular branch of the vagus nerve e.g. insertion of cotton bud
- Cough reflex (some even vomit)
Describe the 6 walls and components of the middle ear
[*] The middle ear lies in the petrous temporal bone and includes the tympanic cavity (lying immediately medial to the tympanic membrane) and the epitympanic recess (a space superior to the membrane, where the temporal bone is hollowed out). It is a biconcave compartment and approx. 15-mm in diameter.
It has 6 walls:
- Lateral wall: inner plate of tympanic membrane
- Medial wall: outer plate of oval window
- Roof: Tegmen Tympanum
- Floor: Jugular wall
- Posterior wall: Mastoid wall
- Anterior wall: carotid wall
Describe the Tympanic Cavity
The Cavity of the Middle Ear, or Tympanic Cavity, is the narrow air-filled chamber in the petrous part of the temporal bone. The cavity has 2 parts:
- Tympanic Cavity Proper: the space directly internal (immediately medial) to the tympanic membrane, connected anteriomedially with the nasopharynx via the Pharyngotympanic (Eustachian) Tube and connected posterolaterally with the mastoid air cells via the mastoid antrum and epitympanic recess
- Epitympanic Recess: space superior to the membrane
The tympanic cavity is lined with mucous membranes that is continuous with the lining of the pharyngotympanic tube, mastoid air cells and mastoid antrum.
Describe the significance of the Tympanic Cavity Proper’s connection with the Nasopharynx via the Eustachian (auditory) tube
- The nasopharynx is part of the upper respiratory tract and thus is liable to respiratory tract disease.
- The communication via the Eustachian tube with the nasopharynx allows for equalisation of air pressure between the middle ear and the atmosphere. This is necessary for efficient transfer of sound energy to the internal ear. Therefore the function of the middle ear is dependant on the function of the Eustachian tube.
- The Eustachian tube is usually closed, being intermittently opened by the pull of attached palate muscles when swallowing.
- The posterolateral 1/3 of the auditory tube is bony, the rest is cartilaginous. Walls of the cartilaginous part are normally in apposition – opened by action of 2 muscles of palate levator and tensor veli palate.
Describe the 3 auditory ossicles. What else does the tympanic cavity contain?
The middle ear contains the auditory ossicles (3 bones that conduct sound way to the inner (sensory) part of the ear): malleus, incus and stapes. They lie in the upper part of the tympanic cavity.
- The handle of the malleus is attached to the tympanic membrane while its body articulates with the body of the incus. The incus articulates with the stapes. The stapes articulates with the bony labyrinth of the inner ear at the Oval Window.
- The articulations are by synovial joints that serve to relay the vibrations encountered by the tympanic membrane to the internal ear.
- The ossicles amplify and concentrate sound energy from the vibration eardrum in turn, converting sound into a mechanical form, to the oval window.
- Middle ear function depends on ventilation
As well as containing the auditory ossicles, the middle ear also contains the tympanic membrane, auditory tube (Eustachian), muscles of the ossicles, branches of the facial and glossopharyngeal nerves, epitympanic recess, mastoid air cells and respiratory epithelium.
Describe the muscles of the ossicles
2 tympanic muscles, stapedius and tensor tympani are concerned with dampening large movements of the tympanic membrane.
- The tensor tympani inserts into the handle of the malleus – pulls handle medially, tenses the tympanic membrane, reducing the amplitude of its oscillations (vibrations of the malleus). Thus it prevents damage to the inner ear when exposed to loud sounds.
The tensor tympani is supplied by branch of mandibular division of CN V
- The stapedius pulls the stapes posteriorly and tilts its base in the oval window. It tightens the anular ligament and reduces the oscillatory range. This prevents excessive movement of the stapes and also offers protection from loud noise.
Nerve to Stapedius arises from the **Facial Nerve (CN VII) **
Describe the relationship of the Middle Ear to CN VII
The single most important relationship of the middle ear is the facial nerve; the facial nerve lies in the facial canal separated from the middle ear cavity by a very thin bony partition. Because of this proximity, a middle ear infection may cause a lesion of the facial nerve.
As well as containing the auditory ossicles, the stapedius and tensor tympani muscles, the middle ear also contains the Chorda Tympani Nerve (Branch of the Facial Nerve) and Tympanic Plexus of nerves. The Chorda Tympani joins the lingual nerve (branch of mandibular) => supply special sensation to the anterior 2/3rds of the tongue
Describe the Inner Ear
[*] The inner ear, also called the labyrinth, contains the end organs responsible for the perception of sound and the maintenance of balance. The inner ear consists of 2 parts, the bony and membranous labyrinths. The inner is buried in petrous temporal bone.
- The bony labyrinth consists of the cochlea (concerned with the perception of sound), the vestibule and the semicircular canals. Both the vestibule and the semicircular canals are involved in the balance. The vestibule and semicircular canals are suspended in perilymph within the bony labyrinth.
- The bony labyrinth is a series of channels hollowed out of the petrous temporal bone, surrounding the membranous labyrinth. The walls of the bony labyrinth are made of very thick bone, the otic capsule.
- The membranous labyrinth is formed by a series of communicating sacs and ducts and contains endolymphs. It consists of the vestibular and cochlear labyrinths.
Balance involves an interplay between vestibular end organ, vision and sensation
Describe the first part of the cochlea and the round window and oval window
The cochlea is shaped like a shell and contains the cochlea duct and is concerned with hearing.
[*] The spiral canal of the cochlea begins at the vestibule and makes 2.5 turns around a central bony core called the modiolus – the spiral canal is a spiral-shaped cavity in the bony labyrinth.
The cochlea lies deep to medial wall of tympanic cavity and communicates with it via fenestra cochlea (or round window). The vestibule is crossed by the facial canal and communicates with tympanic cavity by fenestra vestibule (oval window)
Describe the Cochlear Duct including the spiral organ of Corti
[*] The cochlear duct accommodates the spiral organ of Corti, which contains the receptors of the auditory meatus. The Organ of Corti consists of specialised hair cells resting on supporting cells, which in turn are attached to the basilar membrane. Hair cells are arranged as one row of inner hair cells and three rows of outer hair cells. Afferent fibres of cochlear nerve coil around base of hair cells, which lie between the basilar membrane and overlying tectorial membrane.
- Sound waves vibrate tympanic membrane
- Vibrations transmitted to oval window by ossicles
- Pressure waves transmitted to perilymph of vestibular cavity causing vesticular and basilar membranes to vibrate and round window to move in opposite direction to oval window.
- Tectorial membrane relatively rigid so movement of basilar and vestibular membranes causes relative movement of hair cell stereocilia.
- Results in depolarisation of receptor cells and activity in fibres of cochlear nerve.
- Fibres of basilar membrane are of different length – shortest at base and longest at apex. Fibres at different parts of the cochlea are therefore tuned to vibrate at different frequencies. Depending on frequency of sound, different afferent fibres are stimulated.
What happens at the base of the shell?
[*] At the base of the shell the bony labyrinth communicates with the subarachnoid space via the cochlear aqueduct.
Describle the Vestibule
The vestibule, a small oval chamber, contains the utricle and saccule and is concerned with balance. Semicircular canals + Vestibule = Vestibular apparatus.
[*] The utricle and saccule each contain one receptor (called a macula) that respond to linear acceleration and the static pull of gravity. Each macula consists of hair cells resting on supporting cells. Hair cells have many stereocilia and a long kinocilium embedded in overlying jellylike otolithic membrane. Otolithic membrane contains tiny CaCO3 crystals called otoliths. In the utricle macula is horizontal with hairs oriented vertically when head is upright. In saccule macula is nearly vertical and hairs protrude horizontally.
[*] On the vestibule’s lateral wall, the oval window is found; the stapes are attached to the middle ear aspect of this membrane.
[*] The vestibule is continuous with the cochlea anteriorly and the semicircular canals posteriorly, and the posterior cranial fosse via the aqueduct which opens posterolateral to the internal auditory meatus.
Describe the Semicircular Canals
[*] The semicircular canals are 3 in number and communicate with the vestibule. They are set at right angles (perpendicular) (anterior, posterior and lateral/horizontal) to each other. Each canal forms about two thirds of a circle and its end exhibits an expanded ampulla.
- Within the canals are the semicircular ducts.
- The semicircular ducts contain receptors that respond to rotational acceleration in 3 different planes including detecting acceleration and position of head. The receptors for dynamic equilibrium, crista ampullaris, are located in the ampullae of the semicircular canals.
- Each crista ampullaris consist of supporting cells and hair cells which project into a gel-like mass, the cupola
- Hair cells carry number of stereocilia and one long kinocilium and make synaptic connections with vestibular nerve fibres
- Cristae respond to velocity of rotational movement of head.
List some Congenital Pinna deformities
- Antihelix deformity
- Pinna malformation
- Pre-auricular pit
- Pre-auricular skin tag
Describe Tympanosclerosis and Perforations of the Tympanic Membrane
[*] Tympanosclerosis aka Myringosclerosis is a condition in which there is calcification of tissue in the eardrum and middle ear – if extensive it may affect hearing.
[*] Perforations
- May result from otitis media, the insertion of foreign bodies, trauma, excessive pressure (e.g. from scuba diving.
- One of several causes of middle ear deafness
- Minor ruptures of the membrane often heal spontaneously – as long as there is no infection.
- Inflammation of tympanic membrane is known as myringitis.
- Large ruptures require surgical repair.
What is meant by Glue Ear?
Often results as a chronic variant of otitis media with effusion
- Characterised by adherence of tympanic membrane to ear ossicles
- Middle ear negative pressure results in the ossicles shrink-wrapped in thick mucous and lining of tympanic membrane.
- It is treated by improving drainage via Eustachian tube
- Use of grommets to ventilate middle ear. These are ventilation tubes applied through the tympanic membrane. Aeration will not promote anaerobic bugs to thrive and the tubes will equalise the middle ear pressure
- Pharmacotherapy regimens for infections
- Complications
- Infections can then spread into the middle cranial fossa via temporal bone
- Infections can spread into posterior cranial fossa via epitympanic recess and mastoid antrum
- Infections can spread into sigmoid venous sinus via mastoid air cells.
- Compromise to hearing (results in conductive hearing loss). In children, loss of hearing can cause delayed maturation of brain. If it remains untreated, this loss of hearing might lead to mental retardation.
What is a Cholesteatoma?
- Blockage of the Eustachian tube leads to negative middle ear pressure
- Negative pressure leads to retraction pockets
- Dead skin cells accumulate in the pockets
- Necrotic mass of dead skin => Cholesteatoma/Colesteatoma
- Erosion of middle ear structures and bone via lytic enzymes
Describe Mastoiditis
[*] Infections of the mastoid antrum and mastoid air cells
[*] Results from otitis media
[*] Causes inflammation of the mastoid process => swelling behind the ear
[*] Infection may spread superiorly into the middle cranial fossa through the petrosquamous fissure in children => osteomyelitis.
[*] Can lead to intracranial bleeding and death
Why is the facial nerve susceptible in middle ear disease?
[*] The facial nerve runs through the middle ear and is vulnerable to damage from middle ear disease.
[*] The Chorda Tympani can also be affected.
Differentiate between Sensorineural and Conductive Hearing Loss & Balance
[*] Sensorineural hearing loss: results from defects in the pathway from cochlea to brain (vibrations not perceived in cochlear)
- Defects of cochlea (defect may be in the transduction mechanism)
- Defects of cochlear nerve (branch of the vestibulocochlear nerve)
- Defects of brainstem
- Cochlear implants can restore hearing: external microphone transmitting to an implanted receiver that sends electrical impulses to the cochlea, stimulating the cochlear nerve
[*] Conductive Hearing Loss (vibrations do not reach cochlear)
- Results from anything in the external or middle ear that interferes with the conduction of sound or movement of the oval or round windows.
- People with this type of hearing loss often speak with a soft voice – to them, their own voices sound louder than background sounds.
- May be improved surgically or by use of a hearing device.
What is meant by BPPV and Meniere Syndrome?
Benign Positional Paroxysmal Vertigo (BPPV)
[*] Vertigo typically lasts seconds
[*] Due to otolith displacement (crystals in the macula receptors)
Meniere Syndrome
[*] Primary (idiopathic) endolymphatic hydrops – disorder of the vestibular system, thought to stem from abnormal fluctuations in the endolymph => distended endolymphatic space.
[*] Recurrent attacks of tinnitus, hearing loss and vertigo and accompanied by a sense of pressure deep inside the ear (aural fullness). Symptoms and severity vary widely. Other symptoms include distortion of sounds and sensitivity to noise
[*] Typically lasts minutes/hours
Describe Otoscopic Examination
[*] To examine the right ear, the otoscope is held in the right hand.
[*] The external auditory canal is straightened by pulling the auricle up, out and back using the left hand.
[*] The straighter the canal, the easier the visualisation and the more comfortable the examination will be for the patient.
[*] In the child, the canal should be straightened by pulling the auricle down and back.
[*] The patient is asked to turn his/her head to the side slightly, so that the examiner can examine the ear more comfortably.
[*] After examining the right ear, the left ear is examined.
[*] Evidence of redness, swelling or tenderness in the canal indicates inflammation.
[*] The walls of the canal should be free of foreign bodies, scaliness or discharge.
[*] Any wax should be left as it is, unless it interferes with the visualisation of the rest of the canal and tympanic membrane; removal of wax is best left to the experienced examiner because any manipulation may result in trauma or abrasions.
[*] In the presence of a discharge, careful examination is carried out to evaluate its site of origin.
