Vestibular & Auditory Systems: Exam Prep Notes

Vestibular System and Nystagmus

• The viscosity of the fluid in the vestibular system can change, potentially leading to crystal formation, etching, and altered fluid flow, causing dizziness.
• Nystagmus: An involuntary eye movement that can be observed during a test where a person is laid down. It indicates an issue within the vestibular system.
• Personal experience with positional vertigo:
  • Sudden onset of severe dizziness upon standing, leading to falls.
  • Diagnosis involved specific head positioning to stop the nystagmus.
• VNG (Videonystagmography) or ENG (Electronystagmography): A test used to induce vertigo and measure its origin.
  • VNG is now more common due to advanced technology like specialized glasses for tracking eye movements.
  • The test helps determine if the vertigo is peripheral (inner ear) or central (brain).
• Observation of eye movements (nystagmus) during the test is crucial for diagnosis.
• The vestibular system and nystagmus are interconnected, with vestibular issues often leading to nystagmus.

Auditory System: Cochlea and Hair Cells

• Vestibule: The central area where different parts of the inner ear connect, similar to a building's vestibule.
• Cochlea: A snail-shaped, bony structure connected to the vestibule.
  • Promontory: The large base of the cochlea.
  • Helicotrema: The opposite end of the cochlea when unrolled.
• Stapes: The smallest bone in the middle ear, which sits in the membrane window and pushes on the fluid within the cochlea, initiating the hearing process.
• Unrolled Cochlea:
  • Consists of a bony outside with cartilaginous separations.
  • Contains three parts: scala vestibuli, scala media, and scala tympani but there is no need to know them.
  • Fluid movement occurs as the stapes pushes in on one side, causing displacement.
  • The round window at the bottom moves in conjunction with the fluid motion.
  • FluidDisplacement = InputForce/Area
  • Helicotrema: Plays a role in fluid dynamics within the cochlea.
• Factors affecting the cochlea: Dehydration can impact the fluid balance within the cochlea.
• Cross-section of the cochlea: Membranes separate the fluid, maintaining balance.
• Hair Cells:
  • The sensory organ for hearing, located on the basilar membrane.
  • Stereocilia: Small, hair-like structures on top of the hair cells that move with fluid, stimulating the nerve.
  • Vasomabembrane: The membrane where the hair cells are located.
• Hair Cell Types:
  • Outer hair cells: Primarily responsible for hearing sensitivity and are highly vulnerable.
  • Inner hair cells: More durable and have more nerve connections (20 nerves per hair cell compared to 1 nerve per outer hair cell).
  • Damage to outer hair cells results in a loss of signal due to the one-to-one connection.
  • Even with some damage, inner hair cells can still transmit signals due to the multiple nerve connections.
• Tonal Organization (Tonotopic Organization):
  • Different locations along the cochlea stimulate different frequencies.
  • Base of the cochlea: Higher frequencies are processed.
  • Tip (helicotrema): Lower frequencies are processed.
• Noise-Induced Hearing Loss:
  • Typically affects the high-frequency end of the cochlea due to its location at the entryway.
  • Explosions, loud music, and factory noise can damage the high-frequency hair cells first.
  • Low-frequency hearing loss is often associated with genetic or congenital factors.
• Cochlear Stimulation Simulation: High frequencies (e.g., 512 Hz and 4000 Hz) stimulate the fatter end of the cochlea, while lower frequencies (e.g., 250 Hz) stimulate closer to the tip.

Cochlear Mechanics and Neural Pathways

• Cochlear Ducts: The cochlear duct contains endolymph, while the vestibular duct contains perilymph.
• Vestibular Membrane: A thin membrane that separates the cochlear duct from the vestibular duct, allowing pressure waves to pass through.
• Stereocilia Movement: Pressure waves in the vestibular duct affect the fluid in the cochlear duct, causing the stereocilia to move and initiate the hearing process.
• Frequency and Wavelength: Lower frequencies have longer wavelengths, while higher frequencies have shorter wavelengths, impacting how they are processed in the cochlea.
• Sound as Pressure Waves: Sounds enter the ear as pressure waves, causing the eardrum to vibrate.
• Ossicles Amplification: Vibrations set a series of bones (ossicles) in motion, ending with the stapes, which transfers the vibrations to the inner ear.
• Frequency Sensors: The inner ear contains sensors that pick out different parts of the sound and send them to the brain.
• Auditory Neurons: Attached to hair cells, auditory neurons send impulses through the auditory nerve (eighth nerve) to the brain.
• Executive Functioning: Higher-level brain functions, including attention, auditory memory, and impulsivity, play a role in auditory processing.
• Auditory vs. Language Processing:
  • Auditory processing: Purely sound-based, without linguistic content.
  • Language processing: Involves linguistic material and understanding directions.
• Binaural Integration and Separation:
  • Binaural integration: Combining messages from both ears.
  • Binaural separation: Focusing on one sound while ignoring others.
• Bottom-Up and Top-Down Approaches: Auditory processing involves both bottom-up (sound-based) and top-down (language-based) processes.
• Types of Processing Disorders:
  • Auditory processing
  • Language processing
  • Visual processing
  • Sensory processing

Otoacoustic Emissions and Hearing Loss

• Otoacoustic Emissions (OAEs): Measure the neural activity generated in the cochlea in response to a stimulus, indicating hearing integrity.
• Auditory Brainstem Response (ABR): Another specialized test for assessing hearing function.
• Sensorineural Hearing Loss: The most common type, caused by damage to the hair cells.
• Neural Loss: Damage to the auditory nerve.
• Common Complaint: Misunderstanding similar-sounding words is a common issue for those with hearing loss.
• Types of Hearing Loss:
  • Congenital or prenatal: Occurs during gestation/development before birth.
  • Perinatal: Occurs during birth (e.g., lack of oxygen, meconium aspiration).
  • Acquired: Occurs after birth, often in adults.

Causes of Hearing Loss

• Genetic Factors: Hearing loss can be hereditary and may skip generations.
• Maternal Infections: Rubella (German measles) during pregnancy can cause hearing loss in the child.
• Embryonic Malformations: Issues during embryonic development can lead to hearing loss.
• Anoxia: Lack of oxygen can occur before or during birth, leading to hearing loss.
• Thalidomide: A drug formerly used to treat morning sickness but found to cause birth defects, including hearing loss.
• Perinatal Causes: Anoxia, exposure to viruses, and trauma during birth can result in hearing loss.
• Acquired Postnatal Causes: Syndromes or genetic components can cause progressive hearing loss.
• Otitis Media: Chronic, untreated otitis media can cause permanent hearing loss, while temporary infections can impact language development. Otitis media will gives temporary conductive hearing losses.
• Meningitis: Can damage hair cells or result from the drugs used to treat it.
• Barotrauma: Atmospheric pressure changes can damage the ears (e.g., diving).
• Ototoxic Drugs: Medications that are toxic to the ears (e.g., certain antibiotics, chemotherapy drugs).
• Noise-Induced Hearing Loss: Results from prolonged exposure to loud noise, commonly causing high-frequency hearing loss.
  • Noise Notch: A typical pattern in audiograms for noise-induced hearing loss.
  • Head Baffle: How the head affects sound wavelengths, potentially causing differences in hearing loss between ears.
• Hearing Protection: The use of AirPods and headphones can increase the risk of hearing loss. The use of them should done with common sense.

Presbycusis and Meniere's Disease

• Presbycusis: Age-related hearing loss, often affecting higher frequencies first.
• Meniere's Disease:
  • Characterized by vertigo, fullness in the ear, and fluctuating hearing loss.
  • Related to fluid changes in the inner ear.
  • Endolymphatic Hydrops: Fluid changes that affect the flow and viscosity of fluids in the inner ear.
  • If the fluid changes and it's not moving or flowing this is what is referring to endolymphatic high drop.
  • Can be triggered by hormonal or fluid changes in the body, such as during pregnancy.
  • The 3 characteristics are : vertigo, fullness, and fluctuating hearing loss.
• Sudden Sensorineural Hearing Loss: Can result from a virus or sickness that attacks the inner ear.

Audiograms and Test Preparation

• Audiograms: Used to graph hearing thresholds and patterns of hearing loss.
• Test Preparation: Focus on the structures and functions of the outer, middle, and inner ear.
• The primary functions of the pinna or oracle, Canal, tympanic membrane.
• The disorders such Otitis media, swimmers ear.
• The ossicles and disorders of the middle ear. Just know general difference between cholesteatoma, otosclerosis, mastoiditis, and otitis media.
• The main parts and functions of inner ear
  • vestibular
  • cochlear
• Types causes of different hearing losses such congenital vs peri vs acquired.
• Disorder that cause hearing loss
  • Ototoxic drugs
  • presbycusis
  • menieres
  • noise induced.