Sensorineural hearing loss

Sensorineural hearing loss (SNHL) is a type of hearing loss caused by damage to the sound-receptive apparatus of the ear. The pathological process may affect the structures of the inner ear (hair cells of the cochlea), the vestibulocochlear nerve (8th pair of cranial nerves) or the central parts of the auditory analyzer in the brain.

Unlike conductive hearing loss, which is associated with impaired conduction of sound, SNHL affects the process of converting mechanical vibrations into a nerve impulse and its subsequent transmission to the brain. This is the most common type of hearing loss, and in most cases the damage is permanent.

Aetiology and Pathophysiology

At the root of most cases of SNHL is damage to or death of the sensitive hair cells in the cochlea of the inner ear. These cells have an extremely low ability to regenerate, so their loss results in permanent hearing loss. There are numerous causes leading to this damage.

The main reasons for SNHL:

• Congenital:
  • Genetic and hereditary factors: cause more than half of cases of congenital deafness.
  • Intrauterine infections: rubella, cytomegalovirus, toxoplasmosis.
  • Perinatal factors: extreme prematurity, hypoxia in labor, nuclear jaundice.
• Acquired:
  • Presbycusis: age-related hearing loss, the most common cause of SNHL in adults.
  • Acoustic injury: damage from exposure to loud noise (acute or chronic).
  • Ototoxic drugs: some antibiotics (aminoglycosides), chemotherapeutic agents (cisplatin), loop diuretics.
  • Viral infections: measles, rubella, influenza, mumps, herpes, cytomegalovirus (CMV), human immunodeficiency virus (HIV).
  • Vascular disorders: disturbance of blood supply to the inner ear.
  • Injuries: temporal bone fractures.
  • Meniere’s disease.
  • Tumors: vestibular schwannoma (auditory neuroma).

Clinical Significance

The main manifestation of SNHL is not only a decrease in hearing thresholds, but also an impairment in speech intelligibility. Patients often complain that they “hear but can’t make out words,” especially in noisy environments.

Primary symptoms include:

• Hearing loss: may be uni- or bilateral, of varying severity.
• Subjective ear noise (tinnitus): constant or intermittent ringing, squeaking, humming in the ears that often accompanies SNHL.
• Dizziness: may be present if vestibular structures are involved in the pathologic process.

Diagnosis is based on audiological examination data. Tone threshold audiometry is the “gold standard”: it detects an increase in both air and bone conduction hearing thresholds without a significant bone-air interval.

Treatment in most cases is aimed at rehabilitation, as it is impossible to restore dead nerve cells. The exception is acute sensorineural hearing loss, which is a medical emergency requiring urgent hospitalization and high-dose glucocorticoids. In chronic SNHL, the main method of rehabilitation is hearing aids, and, in severe and profound hearing loss, cochlear implantation.

Differential Diagnosis

The primary task is to distinguish sensorineural hearing loss from conductive or mixed hearing loss. This is accomplished with the use of valvular sampling and audiometry. Once the diagnosis of SNHL is confirmed, the diagnostic search is aimed at establishing its cause. Any unilateral or asymmetric SNHL is an “alarm symptom” and requires a brain MRI with contrast to rule out retrocochlear pathology, primarily vestibular schwannoma. Medical history data (noise exposure, medications, or injuries) and audiogram patterns are often helpful in establishing the etiology of chronic bilateral SNHL.