By Patricia E. Connelly, PhD, CCCA
Schuknecht HF. Further observations on the pathology of presbycusis. Archives of Otolaryngology 1964;80:369—382.
As a result of reading the 1964 Schuknecht article, the reader should be able to:
I stumbled across this 1964 article while researching another topic. In it Schuknecht describes four different pathological origins of presbycusis and their audiometric profiles. I found the article to be an oldie but a goodie, with excellent potential as a teaching piece.
Despite the fact that Dr. Schuknecht’s article appeared almost 40 years ago, his findings have been supported by and cited in many other studies since its 1964 publication. Some discoveries and findings in science and medicine have little practical applicability, but his research is directly relevant to interpreting the audiometric results we find on many of our more senior patients who have hearing loss.
Presbycusis is defined as hearing loss associated with the degenerative processes of aging. The term comes from the Greek “presbys” meaning “old” and “(a)kousis” meaning “hearing.” A similar use of “presbys” is found in ophthalmology with “presbyopia” or “vision of the elderly.” Dr. Schuknecht found that presbycusis is not a singular physiological entity. Rather, it is a term that represents four distinct degenerative conditions and their corresponding distinct audiometric patterns in their purest manifestation.
Sensory presbycusis is caused by atrophy of the organ of Corti and involves the distal-most segments of the auditory neurons located at the basal turn of the cochlea. The structural changes in the organ of Corti that lead to sensory presbycusis may begin in early childhood and typically progress very slowly. As a result, the audiometric pattern remains as a precipitous high frequency sensorineural hearing loss with good threshold preservation of the speech frequencies. Hence, good speech understanding is preserved. What causes sensory presbycusis? Schuknecht postulated in 1974 that it’s due to “wear-and-tear,” that is, degeneration from aging that results in decreased enzyme activity leading to death of the cells that comprise the organ of Corti.1
Figure 1. Sensory Presbycusis. Sensory presbycusis is typically seen as a bilateral precipitous high frequency sensorineural hearing loss with good to excellent speech discrimination ability.
Schuknecht described neural presbycusis as a diminished population of auditory neurons all throughout the cochlea and auditory pathways. Obviously, the transmission of an auditory signal through the central auditory pathways and the decoding of this signal at the level of the auditory cortex depend on a minimum number of neurons that are 1) healthy and fire in a normal pattern at the periphery, and 2) adequately and without distortion transmit auditory information through the higher central auditory pathways to the temporal cortex for processing. Neural presbycusis is not manifested until the number of normally functioning neurons falls below a critical minimum. So, although the process of neural degeneration may begin early and continue throughout an individual’s life, it might have no appreciable effect on auditory function until the number of healthy neurons falls below that minimum. Neural degeneration is variable in its age of onset and its severity, both of which are under genetic control. Audiometrically, neural presbycusis is manifested as a loss for speech understanding. In other words, monaural word recognition scores are far worse than would be predicted from that ear’s puretone findings. This phenomenon was described in 1948 by Gaeth as “phonemic regression” in his unpublished doctoral dissertation from Northwestern University.
The third type described by Schuknecht is metabolic presbycusis and “is due to defects in the physical and chemical processes by which energy is produced and made available for use by the sense organs” (p. 375) with degeneration of the stria vascularis as the cause. Other authors have referred to this as strial presbycusis. In this article he described the three physiological functions of the stria vascularis as: 1) the source for the endocochlear potential, 2) the producer of endolymph and 3) the producer of the energy that the cochlea runs on. Since the stria vascularis maintains the cochlea’s bioelectric and biochemical properties, any malfunction that affects those properties will naturally result in auditory dysfunction. Patients with metabolic presbycusis typically have a flat sensorineural hearing loss with excellent word recognition ability at levels where speech is audible. This type of presbycusic hearing loss is typically first noted later in life, slowly progresses and exhibits a flat puretone pattern.
The fourth type of hearing loss associated with aging is mechanical presbycusis that Schuknecht later termed cochlear conductive presbycusis.1 He ascribed it to an unfavorable alteration of basilar membrane micromechanics that affects the membrane’s motion. Although it was not a proven pathological entity in 1974 when Dr. Schuknecht’s book was published, he nonetheless developed the theory based on the audiometric data archived in his laboratory on patients whose temporal bones later became available post-mortem and on the histopathological research data available at the time. The puretone and speech audiometric findings of those patients—typically a slowly progressing sloping high frequency sensorineural hearing loss—could not be explained based on the loss of sensory or neural cells as found from the post-mortem analyses. However, other studies completed from animal preparations have revealed calcification and deposits of calcium salts at the basal turn that resulted in stiffening and a pathological change in motion of the basilar membrane, thereby elevating threshold.
The cellular and structural changes noted above can occur in isolation or in combination. Occurrence in isolation leads to the typical audiometric pattern, whereas a combination of effects can lead to additive audiometric results that appear to have mixed pathological causes. Vascular and neurological degenerative processes, other chronic medical conditions associated with aging, and some medications used by elderly patients can also lead to cognitive changes that mimic hearing loss or appear to exaggerate existing hearing impairment.
Not only do the sensory and neural mechanisms of the peripheral auditory system age, but the central auditory pathways degenerate, as well. In 1985 Welsh, Welsh and Healy published a study entitled, “Central Presbycusis.”2 They defined it as “a hearing disability which is not related to the cochlea and auditory nerve but rather due to a dysfunction in the central auditory connections, nuclei, and auditory cortex” (p. 128) that results in a combined “receptive and perceptual disability.” The goal of their study was to determine if the peripheral and central components of presbycusis could be separated functionally. They studied 72 individuals between the ages of 60 and 80 years of age with normal hearing and speech discrimination scores of >80% under monaural syllabic challenge. All patients were given a comprehensive battery of tests of central auditory function including those that present stimuli under diotic as well as dichotic stimulation and required the subjects to either fuse or separate the stimuli into combined or distinct percepts. Their data demonstrated that central presbycusis exists as a function of age. They concluded that despite what monaural word recognition scores might show, a perceptual dysfunction will only be evident under binaural and dichotic challenge. They stated further, “From a therapeutic standpoint the classical audiogram of the elderly depicting a falling curve in the higher frequency range can be compensated by a selective amplification unit (hearing aid) which, within the limitation of output and feedback, can improve the situation. Not so for the defect within the CNS, since amplification alone does not resolve the deficit. The problem of background competition over the signal (S/N ratio) is more influential in perceptual depression in the elderly. The factors of diminished central suppression or overwhelming central competition increasingly impairs function, as our data indicate” (p. 134).
Figure 2. Neural Presbycusis. Neural presbycusis is characterized as a degeneration of neurons and results in hearing loss similar to sensory presbycusis. However, speech understanding is far worse than would be anticipated from the audiogram, in this example, perhaps only 40% to 60% in each ear.
Stach, Spretnjak and Jerger also described central presbycusis in terms of age.3 They found that 17% of their younger subjects (50 to 54 years of age) and 90% to 95% of their older subjects (> 80 years old) demonstrated evidence of central auditory processing problems. Further, Jerger, Jerger, Oliver and Pirizzolo determined that the higher prevalence of central presbycusis in older persons was neither a reflection of hearing loss nor principally due to cognitive changes.4
Figure 3. Metabolic Presbycusis. Metabolic or strial presbycusis is
seen as a flat sensorineural hearing loss with good preservation of speech understanding.
Figure 4. Mechanical Presbycusis. Mechanical or cochlear conductive presbycusis shows the typical bilateral sloping high frequency sensorineural pattern of hearing loss, but with good preservation of cochlear elements (sensory and neural cells) speech understanding is generally good.
Finally, a review of the aging ear would be incomplete without a discussion of the concomitant changes to the external and middle ears. The external ear canal is subject to structural changes that result in cracking and bleeding of the skin that lines it,5 as well as thick, dry wax and itchy, accumulating debris. Although these changes have no effect on audiometric function and are not considered presbycusis per se, they can cause obvious discomfort to older persons wearing amplification. As for the middle ear structures, changes due to the aging process have been noted in the tympanic membrane, the joints connecting the ossicles and the middle ear muscles.5,6,7 These degenerative changes do not appear to adversely affect threshold or speech understanding ability, nor do they seem to influence tympanometric results.8
As the sizeable generation of baby-boomers ages, these classic studies on presbycusis become more and more relevant to hearing care providers in that they provide scientific evidence of the auditory changes than can be expected to occur with the aging process. This evidence provides a foundation for patient and family counseling. In the near future as the number increases, our aging “boomer’s” adult son or daughter might question why their parent seems to hear with the hearing aids but doesn’t understand the TV too well, or may inquire about the paradox that Mom or Dad can hear a plane in the distance but doesn’t understand conversation around the dinner table. Absent any other contributing medical complications, we can tell them with a reasonable degree of certainty that the degenerative changes in the hearing system associated with aging can involve not only the inner ear which results in hearing loss (that is, a problem with sound reception), but also can affect the central auditory pathways and their parent’s ability to process the very dynamic speech signal (a perceptual dysfunction). Noises in the distance are simply recognized, but speech must go through a complex decoding process. Strategies can then be offered to the patient for more effective listening and to the family for more effective ways to communicate. The evidence demonstrates that this perceptual dysfunction can occur in the absence of cognitive changes and is more prevalent in older adults. Therefore, we must stress that these receptive (hearing loss) and perceptual (central presbycusis) manifestations of aging are normal processes and not necessarily symptoms of dementia. However, should a family express concern about a loved one’s auditory functioning as a symptom of severe cognitive decline, they should be encouraged to seek the expertise of a physician who specializes in geriatrics.
Although the references I’ve cited for this article aren’t very current, they are nonetheless instructive and relevant to the patients we see every day. They are also classics. In this 40-year-old article by one of the most prolific and respected otologic researchers, surgeons and clinicians of all time, Dr. Schuknecht presented anatomical proof of the audiometric entities he had observed. From his patients’ temporal bones and their post-mortem histopathological analyses, he observed each cochlea’s different pathological anatomy and theorized about the resulting physiology that produced the audiometric profile he had documented while the patients were still living. Dr. Schuknecht’s descriptions of the four types of presbycusis remain essentially unchallenged after almost half a century. Dr. Gaeth’s description of phonemic regression is his legacy. Oldies truly can be goodies. THP
For a copy of the Schuknecht article, contact Pat Connelly, PhD, CCCA, at email@example.com.
Dr. Connelly is an assistant professor of surgery at the New Jersey Medical School and director, audiology services at the UMDNJ—University Hospital in Newark, New Jersey.
1. Schuknecht H. Pathology of the Ear. Cambridge, MA: Harvard University Press; 1974.
2. Welsh LW, Welsh JJ, Healy MP. Central presbycusis. Laryngoscope 1985;95:128—136.
3. Stach B, Spretnjak M, Jerger J. The prevalence of central presbycusis in a clinical population. Journal of the American Academy of Audiology 1990;1:109—115.
4. Jerger J, Jerger S, Oliver T, Pirozzolo F. Speech understanding in the elderly. Ear and Hearing 1989;10:79—89.
5. Rosenwasser H. Otitic problems in the aged. Geriatrics 1964;19:11—17.
6. Covell W. Histological changes in the aging cochlea. Journal of Gerontology 1952;7:173—177.
7. Etholm B, Belal A. Senile changes in the middle ear joints. Annals of Otology, Rhinology, Laryngology 1974;23:49—54.
8. Gates G, Cooper J, Kannel W, Miller N. Hearing in the Elderly: the Framingham Cohort, 1983—1985. Part I. Basic audiometric test results. Ear and Hearing 1990;11:247—256.