What are the different types of hearing loss, their pathophysiology, age of onset, and prognosis?

Types of Hearing Loss: Pathophysiology, Onset, and Prognosis

Hearing loss can be classified into three main types: conductive, sensorineural, and mixed hearing loss, each with distinct pathophysiology, age of onset, progression patterns, and prognosis.

Conductive Hearing Loss (CHL)

CHL occurs when pathology in the external auditory canal or middle ear blocks sound conduction to the inner ear.

• Pathophysiology: Results from blockage or damage to structures that conduct sound waves to the inner ear 1
• Common causes:
  • External ear canal: Cerumen impaction, foreign bodies, stenosis, atresia 1
  • Middle ear: Tympanic membrane perforation, middle ear effusion, cholesteatoma, ossicular discontinuity/fixation, otosclerosis 1
  • Rare causes: Inner ear anomalies like superior canal dehiscence 1
• Onset: Can occur at any age; may be congenital (e.g., atresia) or acquired 1, 2
• Progression: Often gradual but can be sudden depending on etiology 2
• Association with vertigo: Uncommon except in specific conditions like superior canal dehiscence 1
• Prognosis: Generally good with appropriate intervention; many causes are medically or surgically correctable 1, 2

Sensorineural Hearing Loss (SNHL)

SNHL involves dysfunction of the inner ear, cochlear nerve, or central auditory pathways.

• Pathophysiology: Results from damage to hair cells in the cochlea, auditory nerve, or central auditory centers 1
• Major subtypes:
  • Age-related (presbycusis): Most common type in adults 1
    • Sensory: Degeneration of hair cells starting at the basal turn 1
    • Neural: Neuronal loss affecting speech discrimination 1
    • Strial/metabolic: Atrophy of stria vascularis changing endolymphatic potential 1
    • Conductive presbycusis: Hypothesized alterations in cochlear aqueduct 1
  • Noise-induced hearing loss 1
  • Sudden sensorineural hearing loss (SSNHL): Occurs rapidly within 72 hours 1, 3
  • Genetic/congenital SNHL 1, 4
• Onset:
  • Presbycusis: Typically begins after age 50, increasing in prevalence with age (affects 1 in 3 adults aged 65-74 and nearly 50% of those over 75) 1
  • SSNHL: Acute onset within 72 hours at any age 1, 3
  • Congenital: Present at birth or develops in early childhood 1
• Progression:
  • Presbycusis: Typically bilateral, symmetric, and gradually progressive 1
  • SSNHL: Acute onset with potential for partial recovery 1, 3
• Association with vertigo:
  • Present in some forms, particularly when vestibular structures are affected 1, 3
  • Presence of vertigo in SSNHL is an adverse prognostic factor 3
• Prognosis:
  • Presbycusis: Permanent and progressive; management focuses on amplification and rehabilitation 1
  • SSNHL: Better prognosis when treated early (within 2 weeks), with mild hearing loss (<90 dB), and upward-sloping audiograms; poor prognosis with severe loss (>90 dB), flat/downward-sloping audiograms, advanced age, and presence of vertigo 3

Mixed Hearing Loss

Mixed hearing loss combines elements of both conductive and sensorineural hearing loss.

• Pathophysiology: Involves dysfunction in both the sound conduction pathway and the sensorineural components 1, 4
• Common causes: Otosclerosis, trauma affecting multiple ear structures, chronic otitis media with sensorineural component 4
• Onset and progression: Varies depending on underlying causes 4
• Prognosis: Depends on the relative contribution of conductive and sensorineural components; conductive component may be correctable 1, 4

Special Considerations for Asymmetric Hearing Loss

• Asymmetric sensorineural hearing loss requires further evaluation as it may indicate serious underlying pathology 1
• Defined as audiometric results with >15 dB difference in pure tone average between ears or >15% difference in word recognition scores 1
• May indicate retrocochlear pathology such as vestibular schwannoma or meningioma 1
• Evaluation typically includes MRI of the internal auditory canals 1, 4

Emerging Treatments

• Current management of permanent hearing loss relies heavily on devices (hearing aids, cochlear implants) 2, 5
• Emerging therapies include:
  • Gene therapy approaches targeting specific genetic causes 5, 6
  • Stem cell-based therapies aiming to restore auditory function 5, 6
  • Drug-based therapies targeting molecular pathways involved in hearing 5, 6

Clinical Implications

• Early identification and appropriate referral are crucial for optimal outcomes 1
• Patients with significant asymmetric hearing loss, conductive/mixed hearing loss, or poor word recognition should be referred for specialized evaluation 1
• SSNHL requires prompt otolaryngology referral as it is considered an otologic emergency 1, 3
• Regular monitoring (at least every 3 years) is recommended for patients with known hearing loss 1
• Patient education about the impact of hearing loss on communication, safety, cognition, and quality of life is essential 1