We know that hearing loss is caused by death or dysfunction of cochlear hair cells, but the ability to hear and the reasons for hearing loss may not be so simple. New evidence illustrates the essential role of spiral ganglion neurons, the nerves that connect hair cells to the brain, in the ability to hear. A study from a Harvard Medical School research team described a subtype of age-related hearing loss caused by spiral ganglion degeneration rather than hair cell loss.
Age-related Primary Cochlear Neuronal Degeneration in Human Temporal Bones
Makary CA, Shin J, et al J Assoc Res Otolaryngol 2011;12:711
The authors examined hearing thresholds and the anatomical changes in cochleas obtained from deceased patients ranging in age from newborn to 100 years who donated their temporal bones to the NIDCD National Temporal Bone, Hearing and Balance Pathology Resource Registry. (See FastLinks.) Age-related changes were investigated in spiral ganglion density in 100 cochleas where there was no evident loss or damage to inner or outer hair cells.
The authors found a steady decrease in the number of spiral ganglion cells as age increased. Patients up to age 10 possessed approximately 34,000 spiral ganglion neurons, but all patients experienced a loss of approximately 1,000 spiral ganglion cells per decade as they aged. The authors also examined the most recent hearing thresholds obtained before death, and found that spiral ganglion loss preceded hearing loss, which became significant only after age 60.
This paper demonstrated several noteworthy findings. The authors found that age-related hearing loss can occur in those with normal hair cells, which was exhibited by a high-frequency sensorineural hearing loss that increased with age and is consistent with presbycusis. These cases of hearing loss, however, were caused by spiral ganglion loss, which is a type of auditory neuropathy, rather than hair cell loss. This suggests that auditory neuropathy may be more prevalent as a person ages but may be underreported as presbycusis.
Findings showed that spiral ganglion cells may play a discerning role in the ability to hear complex sounds. Ten to 30 spiral ganglion cells innervate each inner hair cell in the newborn cochlea. This study, along with previous studies, has shown that hearing loss does not become evident on a pure-tone audiogram until 50 percent to 90 percent of the spiral ganglion cells have degenerated.
Why are there so many redundant spiral ganglion connections to the hair cells? The authors suggested that redundant innervation of the hair cells plays a role in speech comprehension in complex listening situations, including speech discrimination in noise. A patient's response of “I can hear, but can't understand” may be attributed to a decrease in spiral ganglion cell innervation rather than hair cell loss.
This study confirmed the adage that the more we know, the less we understand. Data published in the 1950s alluded to the crucial role of spiral ganglion cells in hearing loss. (Laryngoscope 1953;63:441.) The discovery of otoacoustic emissions in the 1970s and the subsequent discovery of outer hair cell motility in the 1980s, however, have led to the conventional wisdom that speech discrimination is dependent on proper outer hair cell function. (J Acoust Soc Am 1978;64:1386; Science 1985;227:194.)
Which cell type is more important for speech discrimination: outer hair cell or spiral ganglion neuron? The authors unfortunately did not attempt to correlate speech discrimination scores with spiral ganglion degeneration. It would be interesting to discover whether the loss of spiral ganglion neurons or the loss of outer hair cells would correlate more closely with poor speech discrimination.
The study fortunately highlighted the essential role of spiral ganglion neurons in the ability to hear and also highlighted the complexity of the auditory system. The obvious explanation is that the hair cells, spiral ganglion, and the various supporting and secretory cells of the cochlea are essential to the organic function of audition. Identifying the specific roles of these cell types, however, is required for a complete understanding of the ability to hear.
Why is it important to know the contributions of a particular cell type to normal hearing function? It is a clinician's responsibility to describe accurately the reason why a patient can hear but cannot understand his loved ones. It is important to understand the evolving and underlying principles of normative hearing anatomy and physiology to become an expert, as well as the mechanisms of otopathology to apply effective treatments.
The closest clinical population to this condition would be auditory neuropathy, also called auditory dyssynchrony, which is often considered a subtype of central auditory processing disorder (CAPD). The American Speech-Language-Hearing Association's current treatment recommendations for CAPD include a multidisciplinary team approach that uses a combination of bottom-up (auditory training, hearing aids, frequency modulation systems, and cochlear implants) and top-down compensatory treatment approaches that strengthen higher order central processing (e.g., language, memory, attention). (ASHA 2005. [Central] Auditory Processing Disorders; see FastLinks.)
Appropriate identification of this population is required before treatment options can be applied. An increase in research is expected into this newly described clinical population between the design of new audiological evaluation and new treatment options. Stay tuned for more developments.
© 2012 Lippincott Williams & Wilkins, Inc.
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