The contribution of peripheral components (ear and nerve) are distinguished from the central components (brain stem and cortex), making separation of central and/or peripheral lesions possible. In other words, does a patient has a good cochlea and a bad brain stem, or does the patient has normal brain components, but a bad inner ear?
a) The auditory and vestibular systems provide two very important sensory functions to the human body. The auditory system¹s main function is to detect sound, and the function of the vestibular system is to detect linear and angular acceleration. The auditory system functioning is minimally influenced by other sensory modalities directly, except for the conduction of sound through bone which can affect hearing.
b) The auditory function resides peripherally in the cochlea, which is a coiled duct with nerve fibers and specialized cells. In humans the length of the cochlea is approximately 30 mm, and cells along its length are specialized to detect different sound levels and frequencies. High frequency sounds like those of a siren are detected at the base of the coil, mid-frequency sounds in the middle and low frequency sounds at the end or apex. During the first four years of life children learn to mimic sounds for developing speech. Hearing problems must be detected early.
c) The auditory cochlea by itself is of little use in the detection of sound without an intact and functional cranial nerve VIII, nuclei of the brain stem and neurons of the cortex. Failure of any one of these components results in a damage to the auditory function. Failure can lead to sensorineural hearing loss (neuronal) or conductive hearing loss (via bone).
Children with persistent middle ear infection (otitis) develop conductive hearing loss. Hearing problems (1) can be acquired (presbycusis) or genetic (Otosclerosis, Amalric Syndrome, Barany Syndrome, Cockayne Syndrome, etc.). Conductive hearing loss due to infection of the middle ear is very important. In children serous otitis media usually produces thick mucoid fluids, but thin in adults. Obstruction of the eustachian orifice must be determined because it may cause collapse of the ear drum. Myringotomy (pressure equilizing tubes) may be needed for avoiding collapse of the drum. Chronic otitis media with cholesteotoma usually need surgical intervention. Otosclerosis may cause conductive hearing loss leading patients to claim that the speech of others is mumbled. When the hearing loss is bilateral (conductive) the tendency is to lower one¹s voice that seems too loud, gradually leading to an increasingly withdrawn behavior.
d) The perception of sound Hearing (audition), the processing of sound and the understanding of sound each takes place in different regions of the auditory systems. For this reason clinical tests were designed for testing parts of the system that are suspected to be involved in various aspects of function. For instance, sound detection (inner ear) sound processing (inner ear, afferent cranial nerve VIII and brain stem) and analysis (brain stem and cortex). Most tests used are non invasive (tests do not permanently alter the system) and are repeatable. Loudness of sound is measured in decibels (dB) which is a logarithmic comparison of two intensities. Hearing loss >20dB is considered significant (2).
e) Common clinical auditory problems that physicians encounter consist of accidental damage to the inner ear and/or brain stem and cortex. Foreign objects in the middle ear, puncture of the ear drum (tympanic membrane), fracture of the temporal bones, blows to the head, etc. can produce inner ear problems. Other common complaints at the clinical setting is partial hearing loss due to second hand effect of drugs. Among the most common offenders are aminoglycosides antibiotics (e.g., streptomycin, kanamycin, neomycin, etc.) which produce selective destruction of either auditory and vestibular, or both hair cells. The effects of these drugs is potentiated by drugs that affect the Na+-K+ ATPase pump and the Cl- co-transporter system of the plasma membrane (e.g., furosemide, ethacrinic acid, etc.). Fortunately monitoring the blood level of these drugs aids in preventing irreversible damage, but the thresholds for administering these drugs safely are sometimes difficult to establish. A possible consequence of the malfunctiong of the pump is increase endolymphatic fluid accumulation, which leads to endolymphatic hydrops. Meniere¹s disease induces hydrops, and may be associated with ringing in the ear (tinnitus) among other problems. However, acoustic neuromas also induce tinnitus, can compress the vestibular nerve and decrease its function. If no residual hearing is left in either case, surgery may be indicated.
f) The Vestibular system (equilibrium & balance) system is a multisensorial system. It keeps us erect, it balances the body when we move, it maintains images fixed in the retina and is constantly adjusting and rectifying signals from the environment so that organisms can interact with it. Vertigo is probably the most common of symptoms that bring patients to the clinic. Vertigo can be part of Meniere¹s, but so can tinnitus, deafness and other neurologic symptoms. Attacks of vertigo usually produce nystagmus (eye beating). Beating of the eyes can be induced by irrigating the middle ear (and the wall of the semicircular canal) with cool or warm water. Beating is usually first toward the affected side, but later may be erratic needing accurate measures.
g) Contrary to the auditory system, which has little or not input from other sensory modalities, the vestibular system is a combination of many inputs that are closely rectified into a well controlled and closed loop. Any minor alteration to one part of the loop alters the vestibular function. The damage can be temporary only requiring a prompt rectification of signals in the brain stem, cerebellum or cortex, or a permanent damage as those that cause permanent change of gate and induce ataxia and motion sickness.
h) Like the auditory system the vestibular system is also affected by aminoglycoside antibiotics and drugs that inhibit the plasma membrane ionic pumping function. It is believed that the preferential effect these drugs induce to the inner ear resides in the lower than normal turnover rate of molecules in endolymph and the perilymph.
i) The vestibular function is directly amenable to clinical testing. However, the measures are only indirect. Among the most common analyses are the test for the vestibular ocular reflex (VOR) and nystagmus. The VOR consists in a three arc reflex discovered over 50 years ago, and known to control the movement of the eyes when tracking an object. Nystagmus is an uncontrollable palpitation of the eyes (disturbed VOR) due to damage of the vestibular system. Computerized techniques allow the detection of small variance of the VOR, the principle of its function and detection remains unchanged.
j) Like the auditory system the vestibular system also has peripheral and central components, and they can be separated in clinical testing. Knowing which portion is affected is important for determining the proper course of treatment. For instance, if the patient has a tumor in the brain stem the symptoms are usually different than if the tumor is located in the vestibular division of the cranial nerve VIII. Prolonged Meniere¹s attacks are so devastating that patient often opt for neurectomy (cutting the nerve) on the affected side. This is done only if hearing on the affected side is totally absent.
For instance, if an engineer was asked to duplicate the function of the inner ear, he would have to fit into 16 cubic centimeters a sound system that is capable of equilizing (impedence) a wide range of inputs, a mechanical analyzer, a mobile relay and amplification unit, a multi-channel transducer to convert mechanical energy into neural energy, a system to maintain a delicate hydraulic balance and an internal two-way feedback system. When the extreme sensitivity of the auditory system is analyzed, one has only but to wonder about its functions. In fact, if the inner ear were as ³vissible² as the eye are it is likely that subjects would be more impressed and appreciative of what they can hear. And in fact blind subjects corroborate this. For example, if the eyes were exposed to the above extremes it will be the equivalent as if looking directly into the bright sun, it is likely after such an experience the individual would have to wait a few minutes before focusing the eyes completely. Nonetheless, the ear can distinguish between the whisper and the cannon sound with little or no effort. Adaptation and masking are important properties of the auditory system. Someone can learn to live near a railroad and not be bothered by the sound of a passing train, but at the same time can wake up by the sound of an alarm clock. Similarly, during a cocktail party one can effectively ignore the noise around; and center ones attention on the conversation of the person across the table.
Before we can appreciate sound, waves in the air reach the outer ear or auricle (pinna) which in lower animals contributes to the localization of the origin of the sound. The sound waves then reach the tympanic membrane. The membrane vibrate and the vibration is transmitted to the inner ear by mean of the three small bones, malleus (hammer), anvil and stirrup. The vibration causes the stirrup to act as a piston which by displacement of a small and thin membrane on the oval window of the cochlea displaces the endolymphatic content of the cochlea duct. Displacement makes the portion of the membrane where hair cells rest to undulate in conjunction with another membrane on top of the hair cells, making the hair cells transduce the mechanical energy into neural stimulation. From the hair cells of the inner ear, the neural stimulus is transmitted by the afferent cochlear nerve fibers to the brain stem; and from there to the various stations along the brain center up to the cortex where speech and sound are finally decoded (3, 4, 5, 6) .
The ampullae in the semicircular canals act as true accelerometers detecting angular acceleration while the maculae of the utricle and the saccule detect linear acceleration (gravity). The trajectory nerve fibers take from the ear, the eye, the trunk, and the neck take, as well as the limbs to reach the vestibular nuclei is well known in frogs, birds, and mammals thanks to the HRP technique, fluorescent chromogens, and silver stains. The superior, medial, lateral, and inferior in each side of the brain stem constitutes what is known as the central vestribular nuclei complex. Most of the nuclei from one side (ipsilateral) communicate with the nuclei from the other side (contralateral) via nerve fibers that are known as commissural fibers. Such connections between both sides of the brain stem are very important for maintaining a symmetrical and rectified vestibular function.
The connections are also essential for the process of vestibular compensation following damage to one side of the ear. The visual contribution to the vestibular system gives origin to the so-called vestibular ocular reflex (VOR). The VOR is the vestibular response that is more amenable to clinical examination and manipulation. Another source of important input to the vestibular system is the lower limb input, which is essential for maintenance of posture. In addition to the four major vestibular nuclei mentioned above, there are also small and disperse nuclei which probably receives different names in different species. Of the minor vestibular nuclei, the best known is the Y nucleus, which receives many connections from the saccule. Minor contributions to these classical works has been added in the last few decades, but our knowledge of how these connections contribute to the final control, equilibrium, and posture remains open for new research.
Vestibular evoked responses. The vestibular function can be measured using these responses, which consists on a registry of electrical potentials via electrodes fixed to the skin of the subject¹s head (9). The response consist of positive and negative peaks that correspond to the position of the electrical wave as they travel from the nerve to the brain¹s cerebral cortex. Rectification in the brain stem of all the inputs can be interrupted by damaging only one of the inputs mentioned above and, when such is the case, posture and equilibrium is impaired.
Vestibular compensation (10, 11). The vestibular system can repair itself and reorganize the contact of input reaching the brain stem supposedly through the formation of new synaptic contacts. In the brain stem, synapses between the ipsilateral and contralateral neurons are modified or created in an effort to preserve the symmetry that is required for the maintenance of posture and equilibrium. The regainment of vestibular function following damage to one of the inputs mentioned above, constitutes the so-called ³vestibular compensation². Such compensation involves repair of the system by numerous metabolic reactions of the neurons, including production of peptides, expression (up regulation or down regulation) of certain proto-oncogenes, etc.
2. H. Silverstein, R. Wolfson, S. Roasemberg, CIBA-Geigy Clinical Symposia 44, 2-31 (1992).
3. J. P. Kelly, in Principles of Neural Science E. R. Kandel, J. H. Schwartz, Eds. (Elsevier, 1981) pp. 258-268.
4. J. O. Pickles, An Introduction to the Physiology of Hearing (Academic Press, London, 1982).
5. C. Tsuchitani, in Bases of Auditory Brain-Stem Evoked Responses E. J. Moore, Ed. (Grune & Stratton, Inc., NY, 1983) pp. pp67-108.
6. W. A. Yost, D. W. Nielsen, Fundamentals of Hearing (Holt, Rinehart and Winston, New York, ed. 2, 1985).
7. R. Lorente De Nó, Laryngoscope 42, 233-238 (1932).
8. R. Lorente De Nó, Arch Neurol Psych 30, 245-291 (1933).
9. T. A. Jones, R. C. Nelson, Hear Res 62, 181-186 (1992).
10. C. Fermin, M. Igarashi, G. Martin, H. Jenkins, Acta Anat. 135, 62-70 (1989).
11. M. Igarashi, in The Biology of Change in Otolaryngology R. W. Ruben, et al, Ed. (Elsevier Science Publishers B.V., 1986) pp. 339-352.
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