Neuro-Ophthalmology & Neuro-Otology
Aug. 22, 2022
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In this article, the author explains the clinical presentation, pathophysiology, prevention, diagnostic workup, and management of sensorineural hearing loss. Sensorineural hearing loss is most often caused by abnormalities in the hair cells of the organ of Corti in the cochlea. The hair cells may be abnormal at birth or damaged during the lifetime of an individual. There are both external causes of damage (eg, noise, trauma, and infection) and intrinsic abnormalities (eg, some genetic disorders). Less commonly, there may be dysfunction of the auditory nerve as a result of mass lesions, trauma, demyelination, infectious or inflammatory disorders, autoimmune processes, and nutritional disorders.
• With sensorineural hearing loss, hearing is impaired for both air- and bone-conducted sounds.
• There are different audiogram patterns for different causes of sensorineural hearing loss: presbycusis is typically associated with a downward-sloping, high-frequency loss pattern; noise-induced hearing loss is associated typically with a notched pattern (generally at 4 kHz); and Meniere disease is associated with a low-frequency trough pattern.
• More than 50% of prelingual deafness is genetic, most often autosomal recessive and nonsyndromic. Genetic bases have also been identified for some postlingual deafness.
• Functionally significant hearing loss is very common in the elderly, affecting about a third of those age 70 years or older, adversely impacting quality of life and on ability to carry out routine activities and interact socially and, thereby, contributing to isolation, frustration, disappointment, and depression
• Situations where prevention is most likely to impact hearing outcomes are the prevention of noise-induced hearing loss and the prevention of drug ototoxicity.
• Cochlear implantation may improve audiologic performance and quality of life in elderly patients, even into their eighties
In the second century, Galen of Pergamon (c130-c200) believed that hearing loss or deafness could be caused by dysfunction of the ear, acoustic nerve, or brainstem (104).
Nevertheless, relatively little was known about the anatomy and physiology of the ear until the discovery of 2 of the middle ear ossicles (malleus and incus) in the early 16th century by Jacopo Beregario da Carpi (ca. 1460-ca. 1530) in the Provence of Modena, Italy (104).
In 1543, in his De humani corporis fabrica (On the fabric of the human body), Andreas Vesalius (1514-1564) of Brussels later named the 2 then-known ossicles the “malleus” and the “incus”; he was the first to illustrate these structures (140), and he may have discovered the tensor tympani muscle. Nevertheless, despite the great advancements in anatomy that Vesalius brought about, otology and, particularly, the anatomy of the inner ear were relatively neglected by Vesalius (104; 70).
Gabrielle Falloppio of Modena (Fallopius; 1523-1562) described all 3 auditory ossicles, the oval and round windows, the chorda tympani (although he was unclear whether this was a nerve), and the eponymous fallopian aqueduct or canal through which the facial nerve passes in the temporal bone (104).
Fallopius recognized the 2 parts of the inner ear: the “cochlea” and the “labyrinth,” the latter being composed of the vestibule and the semicircular canals (104).
Bartolomeo Eustachi (1524-1574) apparently independently described the stapes, presented cross sections of the temporal bone, and also provided a more complete description of the eponymous Eustachian tube that links the nasopharynx to the middle ear than was known from vague descriptions by Aristotle, Celsus, Vesalius, and others (104).
Cross section of the temporal bone through the external auditory meatus, the tympanic cavity surrounding the bones of the middle ear, the superior and horizontal semicircular canals, and the cochlea. Illustration from Tabula 45...
Little progress was made in understanding the anatomy and physiology of hearing during the 17th century, particularly because after English physician William Harvey (1578-1657) published De Motu Cordis (On the Motion of the Heart and Blood) in 1628, anatomists and physiologists focused heavily on the cardiovascular system and ignored the sensory organs. English physician Thomas Willis (1621-1675) almost a half century after Harvey summarized the contemporary understanding of hearing (147).
He claimed that the auricle collected and channeled particles of sound toward the tympanic membrane, which facilitated or prepared the sound for reception in the inner ear. The internal auditory muscles and the ossicles served to sort the sounds. Sounds are then transmitted through the oval window to reverberate in the semicircular canals before reaching the cochlea and then the acoustic nerve. Willis was the first to realize that the cochlea is the auditory sense organ, ie, the site of sensory transduction (104).
The most important otological research in the 18th century was that of Italian physician-scientist Antonio Scarpa (1747-1832). In his Disquisitiones Anatomicae de Audiu et Olfactu (1789), Scarpa was the first to describe the membranous labyrinth (104). Among later 18th-century otological researchers was Dutch physician Fredrik Ruysch (1638-1731), whose meticulous drawings and skill in the technique of anatomical injection showed details as fine as the injected vessels in the mucous membrane of the complete ossicular chair.
In 1821, French physician Jean-Marc Gaspard Itard (1774-1838) published Traité des Maladies de l'Oreille et de l'Audition, a monograph that laid the foundation of modern otology with the results of Itard's studies of ear diseases in over 170 cases (71). Itard was also later recognized as an educator of deaf-mutes.
French otologist Prosper Ménière (1799-1862) devoted much of his career on diseases of the ear. Meniere studies at the Imperial Institute for Deaf Mutes in Paris (Institut Royal des Sourds et Muets à Paris) helped him recognize a group of patients with episodic and progressive auditory and vestibular symptoms, which he attributed to inner ear pathology (72).
Ménière established that vertigo may be a symptom of labyrinthine disease and specifically recognized a fairly homogenous group of patients with auditory and vestibular dysfunction that he linked to inner ear pathology. Although largely discounted during his lifetime, this work received increased recognition after his death when in 1874 French neurologist Jean-Martin Charcot (1825-1893) labeled the disease “Maladie de Ménière.” Charcot also noted that episodic symptoms in affected patients ceased when the deafness became complete.
German physician, physiologist, and physicist Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was a major contributor to sensory physiology (73).
While serving as professor of physiology at the University of Heidelberg, Helmholtz wrote Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik (The Doctrine of the Sensations of Tone as a Physiological Basis for the Theory of Music, 1863), in which he introduced his place theory of frequency discrimination (53; 70). Helmholtz suggested that the ear could be understood as a sensory organ whose purpose is to decompose complex sounds (resulting from an admixture of various tones of different frequencies) into their component frequencies--a spectrum analyzer. Helmholtz proposed that each section of the tapering basilar membrane of the cochlea operated as an independent resonator with a natural period of vibration, or resonance, that responded only to sounds that vibrated at that period, like the tensioned strings on a harp. Based on the anatomy of the basilar membrane, which is 3 to 4 times wider at the apex than the base, Helmholtz postulated that high-frequency tones are perceived near the base of the cochlea whereas lower frequencies are perceived toward the apex, just as short harp or piano strings produce high-frequency notes whereas long strings produce low-frequency notes. Helmholtz’s theory of hearing was a place theory, according to which the pitch of a sound is determined by the place where the membrane vibrates, in contrast to temporal or timing theories of hearing in which the frequency of neural firing determines the perception of pitch (70).
As early as 1885, an “auditory chart” was developed that graphically presented auditory acuity as a function of frequency, with responses based on testing with a set of standard tuning forks (142). Later means of graphically presenting auditory acuity as a function of frequency were developed in the early 20th century (142). In 1899, American psychologist Carl Emil Seashore (1866-1949) invented an audiometer, which was marketed initially around 1900 (113; 122; 87; 142).
In the 1920s, Western Electric developed a commercially successful electronic audiometer (142); this went through a series of technological improvements, including the incorporation of bone-conduction testing capability by 1928.
In the 1940s and 1950s, Hungarian-born biophysicist and physiologist Georg (György) von Bekesy (1899-1972) studied experimentally how sound is analyzed and communicated in the cochlea and extended research that had begun in the 19th century in psychoacoustics by Helmholtz (11; 12; 70). Bekesy invented an early type of automated audiometer, which was released in 1946 and continued in use until the 1990s. Bekesy went on to develop a modern place theory of hearing to explain how the cochlea functions as a frequency analyzer, work that overthrew Helmholtz’s earlier resonance theory. Bekesy’s theory was based on his discovery that sound vibrations travel along the basilar membrane in waves, peaking at different places, where nerve receptors determine pitch and loudness. Bekesy found that the entire basilar membrane vibrates, although the point of maxim amplitude varies in a systematic way with the fundamental frequency of the vibration because of the tapering shape of the membrane as it stretches along the length of the cochlea. This is the underlying basis for tonotopic or place coding, in which the inner ear hair cells with the greatest response code for the fundamental frequency of the sound. As Helmholtz had postulated for different reasons, Bekesy found that high-frequency tones are perceived near the base of the cochlea whereas lower frequencies are perceived toward the apex. For his “discoveries of the physical mechanism of stimulation within the cochlea,” Bekesy was awarded the 1961 Nobel Prize in Physiology or Medicine (12; 72).
Other studies, however, have indicated that pitch is coded by a combination of rate information for lower frequencies and place information for higher frequencies; in consequence, a number of rate-place schemes have been proposed (22). Moreover, since the 1970s we have learned that the ear does not act passively, as both Helmholtz and Bekesy had thought, but instead is an active detector (76; 38; 143; 59; 45; 64).
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