Case 7: Neil Wartson Pt 1

Ear: Parts

Outer ear

Middle ear

Inner ear

Outer Ear

External portion

Contains:

• Auricle/Pinna

• External auditory meatus

• Tympanic membrane

Outer Ear: Auricle/Pinna

Visible portion

Elastic cartilage

Function: Collect sound waves into auditory canal

Outer Ear: External Auditory Meatus

Auditory canal

Keratinized stratified squamous epithelium

• Contain glands producing cerumen (ear wax)

Function: Transmit sound waves to tympanic membrane

Outer Ear: Tympanic Membrane

Eardrum

Thin cone-shaped membrane

• Cone of Light: Light reflection from otoscope on anterior inferior quadrant

Function: Vibrate from sound waves = Transmit vibrations to ossicles

Middle Ear

Middle portion

• Internal to tympanic membrane + external to oval window

Contains:

• Tympanic cavity

• Ossicles

• Windows

• Skeletal muscles

• Mastoid process

• Eustachian tube

Middle Ear: Tympanic Cavity

Air-filled space

Contain ossicles, muscles, nerves

Middle Ear: Ossicles

Small bones connecting tympanic window → Oval window

• Malleus: Lateral

  • Insert into tympanic membrane

• Incus: Middle

• Stapes: Medial

  • Insert into oval window

Function:

• Receive + amplify vibrations from tympanic membrane

• Transmit vibrations to inner ear through oval window

Middle Ear: Windows

Oval: Opening at cochlea base

• Covered by stapes

• Function: Transmit sound waves to inner ear

Round: Membrane-covered opening below oval window

• Perilymph transmit stapes vibrations from oval window = Vibrate in opposite phase

Middle Ear: Skeletal Muscles

Tensor Tympani: Attach to malleus

• Contract = Pull malleus medially = Tense tympanic membrane = Dampen vibration + sound

Stapedius: Attach to stapes

• Contract = Pull stapes laterally = Dampen stapes vibration = Dampen sound

Middle Ear: Mastoid Process

Temporal bone process behind ear

Middle Ear: Eustachian Tube

Connect tympanic cavity → Nasopharynx

Function: Equalize pressure around tympanic membrane

• Middle ear pressure = External environment pressure

• Open when chewing, swallowing, yawning

Inner Ear

Inner portion

Auditory section

• Labyrinth

• Cochlea

Vestibule section

• Otolith organs

• Semicircular canals

• Vestibular hair cells

Inner Ear Auditory: Labyrinth

Osseous/Bony: Bony wall (temporal bone)

• Filled with perilymph (low K+, high Na+)

• Contain vestibule, semicircular canals, cochlea

Membranous: Inside osseous labyrinth

• Filled with endolymph (high K+, low Na+)

• Contain sensory organs

• Function: Transmit vibrations from oval window to round window

Inner Ear Auditory: Cochlea

Fluid-filled cavity in osseous labyrinth

Layers:

1. Bone

2. Vestibular duct

3. Reissner membrane

4. Cochlear duct

5. Basilar membrane

6. Tympanic duct

7. Bone

Cochlea: Vestibular Duct

Filled with perilymph

Helicotrema (cochlea apex) connect oval window → Tympanic duct

Cochlea: Cochlear Duct

Filled with endolymph

Organ of Corti: Transform vibrations → Auditory neural signals

Cochlea: Basilar Membrane

Support organ of Corti

Covered in auditory hair cells with stereocilia

• High-Frequency: Stimulate hair cells at cochlea base = Thin + rigid basilar membrane

• Low-Frequency: Stimulate hair cells at helicotrema = Wide + flexible basilar membrane

Cochlea: Tympanic Duct

Filled with perilymph

Connect to round window

Inner Ear Vestibule: Otolith Organs

Fluid-filled pouches with sensory epithelium

Utricle: Sense motion in horizontal plane

• Forward-backward

• Left-right

Saccule: Sense motion in sagittal plane

• Up-down

Inner Ear Vestibule: Semicircular Canals

Sense rotary motion (angular acceleration)

• Horizontal

• Superior

• Posterior

Inner Ear Vestibule: Vestibular Hair Cells

Transform displacement → Neural signals

In otolith organs + semicircular canals

Auditory Pathway

1. Sound waves collected into outer ear by pinna → Pass through external auditory meatus

2. Sound waves vibrate tympanic membrane = Transmit vibrations to ossicles (malleus → incus → stapes)

• Ossicles amplify vibrations

3. Stapes transmit vibrations to oval window

4. Vibrations enter osseous labyrinth → Cochlea → Organ of Corti

5. Hair cells in organ of Corti vibrate = Transform into auditory neural signals

6. Signals transmitted to bipolar neurons (spiral ganglion)

• 1st order neuron

7. Bipolar neurons form cochlear nerve (part of vestibulocochlear nerve)

• Synpase with ventral + dorsal cochlear nuclei (brainstem)

• 2nd order neuron

8. 2 pathways:

• Direct: Dorsal cochlear nucleus neurons join contralateral lateral lemniscus → Inferior colliculus

  • 3rd order neuron

• Indirect:

  • Ventral cochlear nucleus neurons synapse with ipsilateral + contralateral superior olivary nucleus

    • 3rd order neuron

  • Superior olivary nucleus → Lateral lemniscus + synapse on inferior colliculus

    • 4th order neuron

9. Inferior colliculus neurons synapse with medial geniculate body cells

• 4th (direct) + 5th (indirect) order neuron

10. Medial geniculate body neurons → Primary auditory cortex

Vestibular Pathway

1. Head movement = Move vestibular hair cells in otolith organs + semicircular canals = Transform into vestibular neural signals

2. Signals transmitted to bipolar neurons (vestibular ganglion)

• 1st order neuron

3. Bipolar neurons form vestibular nerve (part of vestibulocochlear nerve)

• Synapse with vestibular nuclei complex (brainstem)

• 2nd order neuron

4. Vestibular nuclei complex transmit signal to cerebellum, CN III/IV/VI, reticular formation, spinal cord, thalamus → Vestibular cortex + reflexes

• 3rd order neuron

Hearing Loss: Description

Decreased auditory function

Hearing Loss: Types

Conductive: From impaired sound wave transmission to inner ear

• Outer + middle ear dysfunction

Sensorineural: From impaired neuronal transmission to brain

• Inner ear + cochlear nerve dysfunction

Mixed: Conductive + sensorineural components

Hearing Loss: Location

Central: Impaired brainstem + brain processing

• No understanding of sound

Peripheral: Impaired ear (outer, middle, inner) + cochlear nerve

• Decreased sound sensitivity

Hearing Loss: Epidemiology

20% in children

Conductive = Most common

Hearing Loss: Etiology

Pediatric:

• Genetics

• Perinatal complications

  • Premature birth

  • Increased brith weight

  • Hyperbilirubinemia

• Trauma

• Infection

  • Measles + mumps

  • Meningitis

• Chronic otitis media

Adults:

• Conductive:

  • Otosclerosis: Stapes overgrowth = Bone fixation = Decrease sound conduction

  • Otitis Media: Middle ear inflammation

  • Cerumen impaction

• Sensorineural:

  • Idiopathic

    • Meniere disease: Impaired endolymph reabsorption

  • Older age

    • Presbycusis: Damage hair cells in organ of Corti (base first) = Lose high-frequency hearing

  • Trauma (noise-induced)

  • Neurologic

    • Acoustic neuroma: Schwannoma in vestibular nerve

  • Infection

Hearing Loss: Conductive Pathophysiology

Blockage (cerumen, inflammation) or dampening pathology (stapes fixing) = Decrease sound wave vibrations = Decrease sound transmission = Hearing loss

Hearing Loss: Sensorineural Pathophysiology

Decreased hair cells/movement or cochlear nerve pathology = Decrease auditory signal transmission = Hearing loss

Hearing Loss: Clinical Preentation

Conductive:

• Hearing improves in noisy environments

• Normal voice

• No sound distortion

Sensorineural:

• Hearing worsens in noisy environments

• Loud voice

• Sound distortion (lose high frequency)

• Tinnitus

Pediatric:

• Abnormal development

  • Delayed language + communication

• Inappropriate response to sounds

• Inattention

• Symptoms of underyling cause

  • Facial asymmetry

  • Ear malformations

  • Microcephaly

  • Goiter

Hearing Loss: Investigations

Physical exam

Audiometry

Imaging

Hearing Loss Investigations: Physical Exam

Whispered voice test

Weber’s Exam: Tuning fork on forehead

• Conductive: Louder in impaired ear

  • Detect vibration

• Sensorineural: Louder in unimpaired ear

  • No sound transmission by damaged inner ear or auditory nerve

Rinne Exam: Tuning fork on mastoid bone until sound gone → In front of ear

• Normal: Air conduction > bone conduction

• Conductive: Bone > air

  • Vibrations bypass blockage in bone

• Sensorineural: Air > bone (both decreased)

  • Inner ear + cochlear nerve cannot transmit sound to brain

Otoscopy:

• Visualize tympanic membrane

• Pneumatic: Apply pressure with pneumatic bulb = Tympanic membrane movement

Hearing Loss Investigations: Audiometry

Subjective:

• Audiogram (pure tone testing)

• Speech audiometry

Objective:

• Impedance tympanometry

• Otoacoustic emissions (OAE)

Audiometry: Audiogram

1. Pt listen to frequencies with headphones (air conduction) and bone oscillator (bone conduction) on mastoid bone

2. Determine threshold frequencies heard

Conduction: Threshold increase in air conduction + decrease in bone conduction

Sensorineural: Threshold increase in air + bone conduction

Audiometry: Speech Audiometry

1. Increasingly loud words played + pt repeat

2. Determine threshold level

Conductive: Louder = Increased comprehension

Sensorineural: Louder ≠ Increased comprehension

Audiometry: Impedance Tympanometry

1. Seal external auditory canal + emit frequencies inwards

2. Measure sound reflected from tympanic membrane as pressure

3. Record on tympanogram

Determine middle ear pathology

• Eustachian tube dysfunction

• Secretory otitis media

Audiometry: OAE

Measure sound emission (OAE) from cochlea in response to acoustic stimuli

Determine hair cell pathology

• Normal cochlear function = Detectable

• Cochlear hearing loss = Not detectable

Hearing Loss Investigations: Imaging

MRI/CT

Indication:

• Unilateral

• Gradual sensorineural

Acoustic neuroma

Hearing Loss: Investigations in Children

Hearing test

Otoscopy

Tympanometry

Hearing Loss Investigations in Children: Hearing Test

≤ 6 Months:

• Automatic auditor brainstem response: Gold standard

  • Measure electrical activity along cochlear nerve + brainstem in response to sound

• OAE

7 Months - 3 Years: OAE

≥ 4 Yeats:

• Audiogram

• OAE

Hearing Loss: Treatment

Treat underlying cause

Irreversible Causes:

• Hearing aids: Bone/air-conduction aids

• Cochlear implants

• Hearing assistive technology

Hearing Loss Treatment: Hearing Aids

Amplify sound

Indications:

• Conductive + sensorineural hearing loss

• Mild to severe

Hearing Loss Treatment: Cochlear Implants

Surgically-implanted device electrically stimulate vestibulocochlear nerve

Improve speech discrimination

Indications:

• Intact cochlea + cochlear nerve

• Moderate to severe sensorineural hearing loss

• Decreased speech recognition

• Unsuccessful hearing aid treatment

Hearing Loss: Complications

Developmental delays

• Language

• Inattention

Cochlear implants: Increase meningitis risk

• Prevention: Vaccinations