What causes sensorineural hearing loss?

Causes of Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL) is caused by damage to the inner ear structures (cochlea), auditory nerve, or central auditory pathways, with the most common etiologies being age-related degeneration, noise exposure, ototoxic medications, genetic factors, and infections. 1, 2

Major Categories of SNHL Causes

Congenital/Genetic Causes

• Family history of permanent childhood hearing loss 1
• Syndromes associated with hearing loss:
  • Usher syndrome, Waardenburg syndrome, Alport syndrome, Pendred syndrome, and Jervell and Lange-Nielsen syndrome 1
  • Neurofibromatosis and osteopetrosis 1
• Craniofacial anomalies involving the pinna, ear canal, ear tags, ear pits, and temporal bone 1
• Neurodegenerative disorders such as Hunter syndrome, Friedreich ataxia, and Charcot-Marie-Tooth syndrome 1

Acquired Causes

Infectious

• In utero infections: CMV, herpes, rubella, syphilis, and toxoplasmosis 1
• Postnatal infections: Bacterial and viral meningitis 1
• Other infections: Lyme disease, syphilis, and HIV 1, 3

Trauma-Related

• Head trauma, especially basal skull/temporal bone fractures 1
• Acoustic trauma from sudden loud noise exposure 1

Noise-Induced

• Occupational noise exposure exceeding 75-80 dBA 1, 4
• Impulse noise with peak SPL exceeding 145-155 dB 4
• Hidden hearing loss - cochlear synaptopathy from moderate noise exposure that may not initially affect hearing thresholds but causes neural degeneration 1

Medication/Toxin-Related

• Ototoxic medications:
  • Aminoglycoside antibiotics (gentamicin, tobramycin) - causing irreversible hearing loss through damage to vestibular and auditory branches of the eighth nerve 5, 6
  • Chemotherapeutic agents (particularly cisplatin) 1, 6
  • Loop diuretics (furosemide) 5, 6
  • Other drugs: Phosphodiesterase-5 inhibitors, antimalarials, NSAIDs 7, 6

Age-Related

• Presbycusis (age-related hearing loss) - progressive bilateral SNHL in persons ≥50 years old 1
  • Subtypes: Sensory (hair cell degeneration), neural (neuronal loss), strial/metabolic (stria vascularis atrophy), and conductive 1

Vascular/Metabolic

• Sudden sensorineural hearing loss - often idiopathic but may be due to vascular mechanisms 1, 3
• Stroke - patients with SSNHL have 1.64 times higher stroke risk at 5 years 3
• Metabolic disorders including MELAS syndrome 3

Other Causes

• Autoimmune inner ear disease and Cogan syndrome 3
• Menière's disease - fluctuating hearing loss with vertigo and tinnitus 3
• Vestibular schwannoma (acoustic neuroma) - typically causes asymmetric SNHL 1, 3
• Mitochondrial dysfunction and oxidative stress - implicated in various forms of SNHL 8

Pathophysiological Mechanisms

The primary pathophysiological mechanisms of SNHL include:

1. Hair cell damage/degeneration - particularly outer hair cells in the basal turn of the cochlea 1, 8
2. Neuronal loss affecting speech discrimination 1
3. Stria vascularis atrophy changing the endolymphatic potential 1
4. Mitochondrial dysfunction leading to oxidative stress in cochlear cells 8
5. Cochlear synaptopathy - damage to synaptic connections between inner hair cells and auditory nerve terminals 1

Clinical Presentation and Diagnosis

SNHL typically presents with:

• Difficulty hearing, especially in noisy environments
• Poor speech discrimination
• Tinnitus
• Ear fullness or pressure
• Sometimes vertigo 1

Warning Signs of Non-Idiopathic Causes

• Bilateral sudden hearing loss
• Recurrent episodes of sudden hearing loss
• Focal neurologic findings
• Fluctuating hearing (suggesting Menière's disease, autoimmune inner ear disease) 1, 3

Prevention Strategies

• Noise protection in occupational and recreational settings 1
• Monitoring for ototoxicity when using potentially ototoxic medications 7, 6
• Early identification of genetic risk factors 1
• Regular hearing assessments for those with known risk factors 1

Important Clinical Considerations

• SNHL is typically irreversible as human inner ear hair cells have no capacity for regeneration 7, 2
• Asymmetric SNHL warrants further evaluation, including MRI to exclude retrocochlear pathology 1
• Multiple factors often interact to cause hearing loss (e.g., aging plus noise exposure, or ototoxic drugs plus renal impairment) 5, 4
• Early identification and intervention are crucial for optimizing outcomes, especially in children 1

Understanding these causes is essential for proper diagnosis, management, and potential prevention of further hearing deterioration.