Sensorineural Hearing Loss

Sensorineural hearing loss results from damage to the inner ear (the cochlea and its hair cells) or to the auditory nerve, rather than to the sound-conducting apparatus. Because the sensory and neural elements that transduce and carry sound are affected, both air- and bone-conducted hearing are reduced and there is no air-bone gap. It is the most common permanent form of hearing loss and ranges from congenital and genetic causes to age-related, noise-related, and ototoxic damage.

Definition

Sensorineural hearing loss is a reduction in hearing caused by dysfunction of the cochlear hair cells or the auditory nerve, in which both air- and bone-conduction thresholds are reduced together and no air-bone gap is present.

Scope

This entry covers the cochlear and neural structures involved, the principal mechanisms of sensory and neural damage, the audiometric pattern of sensorineural loss, and its major causes across the lifespan. It is a reference description of the pathology and how it is classified, not clinical guidance for evaluating or managing an individual.

Key concepts

• Cochlear hair cells (inner and outer)
• Auditory (cochleovestibular) nerve
• Absence of an air-bone gap
• Cochlear synaptopathy
• Congenital and genetic hearing loss
• Ototoxicity
• Sudden sensorineural hearing loss

Mechanisms

Sensorineural loss arises when the transduction of sound into neural signals is impaired. Outer hair cells provide cochlear amplification and frequency tuning, and inner hair cells transmit the signal to the auditory nerve; loss of either degrades hearing and, for outer hair cells in particular, narrows the dynamic range. Damage can also occur at the synapse between inner hair cells and nerve fibers (cochlear synaptopathy) and along the auditory nerve. Causes include genetic and congenital factors, aging, noise exposure, ototoxic agents, infection, and sudden idiopathic events. Because the cochlea or nerve itself is affected, bone conduction does not bypass the lesion, so both conduction routes show comparable reductions.

Clinical relevance

Sensorineural loss accounts for most permanent hearing impairment, and distinguishing it from conductive loss is fundamental to describing and studying hearing disorders. Understanding its mechanisms also clarifies why it is generally not reversible and why difficulties often involve clarity of speech, not only loudness. This entry is for orientation and evidence appraisal, not individual diagnosis or treatment.

Epidemiology

Sensorineural hearing loss is the predominant type of permanent hearing loss worldwide and a major contributor to the global burden of hearing disability. In children, congenital and early-onset sensorineural loss is a leading reason for newborn hearing screening; in adults, age-related and noise-related sensorineural losses dominate.

History

The recognition that the cochlea and auditory nerve constitute a distinct site of hearing loss followed the development of audiometry that separated air and bone conduction. Later, physiological work on outer-hair-cell amplification reshaped understanding of cochlear function, and the more recent description of cochlear synaptopathy showed that neural connections can be lost even when hair cells and audiometric thresholds appear preserved.

Debates

Can hearing damage occur without a shift in standard audiometric thresholds?

Work on cochlear synaptopathy suggests that synapses between inner hair cells and auditory-nerve fibers can be lost while the pure-tone audiogram remains normal, raising questions about whether the standard audiogram fully captures sensorineural damage.

Key figures

• Richard J. H. Smith
• M. Charles Liberman
• Sharon G. Kujawa
• Lisa L. Cunningham

Seminal works

• cunningham-2017
• smith-2005
• liberman-2017

Frequently asked questions

Why is there no air-bone gap in sensorineural hearing loss?

Because the cochlea or auditory nerve is the site of the problem, sound delivered by bone conduction cannot bypass it, so air- and bone-conduction thresholds are reduced together with no gap between them.

What are the most common causes of sensorineural hearing loss?

Across the lifespan the major causes include genetic and congenital factors in children and aging, noise exposure, and ototoxic agents in adults; the cochlear hair cells or auditory nerve are the structures affected.