Adv AuD: Pathways of Hearing, Types of Hearing Loss, Disorders of the Ear

Auditory pathway (peripheral vs central)

The route from the outer ear through the middle and inner ear to the auditory cortex, including peripheral structures and central auditory pathways (CANS).

Outer ear

The external portion of the ear: pinna, concha, and external auditory canal; collects sound.

Pinna

External, visible part of the ear that helps capture sound.

Concha

Bowl-shaped outer ear region that helps collect and direct sound into the canal.

External auditory canal

Channel leading from the pinna to the tympanic membrane; part of the outer ear.

Tympanic membrane

The eardrum; vibrates in response to sound and transmits vibration to the middle ear.

Middle ear

Air-filled cavity containing the ossicles and tympanic membrane and the eustacian tube; transmits from mechanical vibrations to the inner ear.

Ossicles

Malleus, incus, and stapes; three small bones that amplify sound vibrations.

Malleus

Hammer-shaped bone attached to the tympanic membrane; first ossicle.

Incus

Anvil-shaped middle ear bone between malleus and stapes.

Stapes

Stirrup-shaped middle ear bone that interfaces with the oval window of the cochlea.

Round window

Membrane-covered opening at the cochlea that allows fluid movement in the inner ear.

Eustachian tube

Channel between the middle ear and nasopharynx that equalizes pressure.

Inner ear

Cochlea and vestibular system; converts mechanical vibrations into neural signals.

Cochlea

Spiral-shaped organ in the inner ear containing hair cells responsible for hearing.

Cochlear nerve (VIII)

Auditory component of the vestibulocochlear nerve carrying signals from the cochlea.

Auditory nerve

VIIIth cranial nerve carrying auditory information from the inner ear to the brainstem.

Cochlear nucleus

First brainstem nucleus in the central auditory pathway.

Superior olivary complex

Brainstem nucleus involved in binaural processing and sound localization.

Inferior colliculus

Midbrain auditory center integrating auditory information.

Medial geniculate body

Thalamic relay nucleus to the primary auditory cortex.

Auditory cortex

Cortical region in the temporal lobe that processes hearing.

Temporal lobe

Brain region housing the auditory cortex and higher-level processing.

Efferent

The minority of the auditory system is inhibitory

Afferent

The majority of auditory functions are perceptual

Air conduction pathway

Sound reaches the inner ear through air via OE → ME → IE → VIII nerve → CANS.

Bone conduction pathway

Vibration of bone directly stimulates the inner ear; bypasses outer and middle ear. IE→ 8th nerve → CANS

How does bone conduction stimulation work in hearing?

Bone conduction stimulates both cochleas. If one ear has better hearing, the sound is heard in that ear. If hearing is equal, the sound is perceived in the midline.

Peripheral pathways

Structures from the outer ear through the VIII nerve before brainstem.

Central pathways

Brainstem to auditory cortex; part of the central auditory nervous system.

Conductive hearing loss (CHL) location

Loss due to problems in the outer or middle ear

Sensorineural hearing loss (SNHL)

Loss due to the inner ear, VIII nerve, or central pathways. Dynamic range shrinks for hearing, both AC and BC show loss (equal).

Mixed hearing loss

Combination of conductive and sensorineural components; both AC and BC show loss, with AC typically worse. May cause auditory distortions.

Presbycusis

Age-related sensorineural hearing loss; high frequencies affected first, often with tinnitus and recruitment. One of the most common causes of HL in adults, and amplification is the treatment.

Noise-induced hearing loss (NIHL)

SNHL from excessive noise exposure (temporary or permanent); notched loss around 4000 Hz; prevent with protection; may need amplification if permanent.

Meniere’s disease

Overproduction of endolymph (hydrops) in the inner ear causes fluctuating low-frequency SNHL, vertigo, tinnitus, and fullness. Treatment is a diet, medication, or surgery.

Otosclerosis

Abnormal bone growth around the oval window/stapes causing CHL; Carhart notch may appear.

Cholesteatoma

Destructive expanding growth in the middle ear linked to chronic otitis media; foul discharge and dizziness, facial weakness, and CHL; requires surgical removal.

Osteoma

Single benign bony growth in the external ear canal; can obstruct the canal; treated surgically.

Exostosis

Multiple little bony growths in the external ear canal, often from cold-water exposure. Benign and can grow large.

Otitis externa (external otitis)

Inflammation/infection of the pinna or ear canal caused by bacteria or fungus; swimmer’s ear; treated with medication.

Cerumen

Earwax; buildup can cause conductive hearing loss; removal fixes the issue.

Foreign body (in ear canal)

Foreign object in the ear canal causing obstruction; removal needed.

Glomus tumor

Vascular middle-ear tumor causing pulsatile tinnitus and CHL; treated surgically.

8th nerve tumor (acoustic neuroma/vestibular schwannoma)

Aquired unilateral benign tumor on the VIIIth nerve; asymmetry, tinnitus, dizziness, disproportionate SNHL; treated surgically. Most often in adults, and is often referred to as retrocochlear pathology.

Auditory neuropathy / dys-synchrony

Disorder where normal outer hair cell function exists, but neural transmission is from IE to the brain is impaired (desynchronized); ABR absent; OAEs present; may benefit from cochlear implant and visual cues, but not really amplification. Speech recognition is often disproportionate to pure tones. Sounds like TV static.

ABR vs OAE

ABR measures neural synchrony along the auditory pathway; OAE assesses outer hair cell function in the cochlea.

Central Auditory Processing Disorder (CAPD)

Disorder of central auditory structures affecting processing of sounds: not due to peripheral hearing loss, but rather an impaired CANS ability to manipulate and use acoustic signals; requires specialized tests. Often have difficulty perceiving language in noise. Occurs mostly in young children and older adults. Use the CAP test battery with subtests that are individualized. Treatment includes improving signal quality and therapeutic techniques. NOT A HL.

Central hearing loss

Sensorineural loss occurring in central pathways with normal cochlear function; may result from tumors, stroke, or trauma.

What is the global and adult prevalence of hearing loss?

Over 5% of the world’s population (466 million people) has disabling hearing loss—432 million are adults. Sensorineural hearing loss (SNHL) is the most common type in adults. Prevalence increases with age:

• 2% of adults (45–54)

• 8.5% (55–64)

• 25% (65–74)

• 50% (75+)

What is the prevalence and nature of hearing loss in children?

• 34 million children have disabling hearing loss worldwide

• 2–3 per 1,000 U.S. children are born with hearing loss in one or both ears

• Over 90% of deaf children are born to hearing parents

• 5 out of 6 children experience otitis media by age 3

• Conductive hearing loss (CHL) is most common in children, often due to otitis media

How does hearing loss impact children?

Impact depends on type, degree, configuration, laterality (unilateral/bilateral), and age of onset, diagnosis, and intervention. It may affect:

• Speech and language development

• Social behavior

• Academic performance

How does hearing loss impact adults?

• Job performance

• Social relationships

• Mental health (e.g., depression, cognitive decline if untreated)
  Impact varies based on type, severity, configuration, and timing of onset and treatment.

Characteristics of Hearing Problems

Hearing detection loss and distortion of the auditory signal; (can co-occur or occur independently)

CHL Charactersistics

• Usually temporary and treatable medically

• AC < BC loss

How do those with CHL perceive sound?

Lowered volume, no distortion, sound attenuator, like ears are plugged.

CHL Causes

Otitis Media, TM Disorders (perforation, tympanosclerosis), Glomus Tumor

SNHL Characteristics

• Possible auditory distortions

• usually permanent and not medically treatable

• AC=BC loss

• Changes in cochlear processing and possible tinnitus

How do those with SNHL perceive sound?

• Possible acoustic distortions

• Trouble hearing in noise

• Recruitment

• May affect suprathreshold hearing

SNHL Diagnosis

Pure tone audiometry, speech audiometry, AC=BC

SNHL Treatment

Amplification

1-Microtia and 2-Atresia

1-Malformation of the pinna 2-Malformation of the ear canal opening; they often co-occur. Treatment is surgical correction and/or a bone conduction hearing aid.

Otitis Media

Inflammation of the middle ear that can have acute/infected or serous/non-infected fluid, creating negative middle ear pressure. It is the most common in children and can cause CHL, leading to possible speech and language delays. Treatment includes medication, tymp tubes, or surgical repair in chronic cases (mucoid or adhesive).

TM Perforation

A tear in the eardrum caused by physical trauma or acute otitis media, treatment is self-healing or surgical repair.

Tympanosclerosis

Calcification within TM layers caused by physical trauma or chronic otitis media, with surgical repair treatment.

Scarred TM

Caused by repeated TM ruptures/healing and can cause TM to be hypermonile/flaccid.

Disorders of the 8th Nerve and CANS

Causes difficulty in processing auditory signals and may or may not include SNHL.

• 8th Nerve

  • Tumors

  • Auditory neuropathy

• CANS

  • CAPD

  • Central Hearing Loss

Central Auditory Deafness

Rare cortical deafness related to bilateral brainstem or temporal lobe lesions. Occurs when both hemispheres are compromised. Behavioral audiolometric measures may be absent and may have difficulty recognizing auditory signs or understanding speech.

Disorders of the Outer Ear

• Congenital

  • Microtia

  • Atresia

• Acquired

  • External otitis (otitis externa)

  • Bony growths

    • Osteoma

    • Exostoses

  • Obstruction of the ear canal

    • Cerumen

    • Foreign bodies

Disorders of Middle Ear

• Congenital

  • Fusion or absence of ossicles

  • Often co-occur with atresia

• Acquired

  • Otitis media

  • TM: performation, tympanosklerosis, scarring

  • Cholesteatoma

  • Otosclerosis

  • Glomus tumor

Disorders of Inner Ear

• Congenital

  • Genetic

  • Malformations of the inner ear

  • in-utero factors

    • infections

    • loss of oxygen

    • etc.

• Acquired

  • infections]trauma

  • ototoxicity

  • presbycusis

  • NIHL

  • Meniers disease

Auditory Neuropathy Risk Factors

• Extreme prematurity

• Anoxia

• Genetics

• Jondus (hyperbilirubinemia)

• Congenital brain anomalies

• Demyelinating conditions like MS

• Syndromes associated with peripheral pathologies