Because adequate hearing is essential to the safe operation of any aircraft or spacecraft, it is of the utmost importance that existing hearing be preserved, and that any treatable hearing loss be considered for treatment. For any sound to be heard normally, a complex chain of events must happen, and any interruption in this chain can result in diminished or absent hearing.

Perception of sound involves both physical and electrochemical processes. It begins with the collection of sound pressure waves by the external ears, which provide a small amount of amplification, and more importantly, localization of sounds in space. Localization is possible because sound waves reach each ear at slightly different times, at slightly different volumes, and with slightly different resonances, depending on the position of the head. Tiny movements of the head help in this localization. Much of this localization is lost when wearing headsets, although sophisticated acoustic engineering can restore some of this localization. Once the sound waves have entered the gently curved external ear canal, they impinge on the tympanic membrane (TM) causing physical vibration of the TM and its three attached middle ear ossicles. The malleus is embedded within the midportion of the TM, the incus provides a bridge from the malleus to the stapes, and the stapes inserts into the oval window, which is the middle ear’s interface with the inner ear. The area of the TM is approximately 20 times the area of the oval window, providing mechanical amplification of the sound waves. In addition, the ossicles themselves are levered in such a way as to provide further mechanical advantage to the system. Absence of the TM and/or ossicles invariably leads to a rather large conductive-type hearing loss. Although this may occur congenitally, in the trained aircrew it is usually the result of trauma, infection, or surgery.

At the oval window, the stapes footplate is in direct contact with the perilymph of the inner ear and sound vibrations are transmitted through this fluid to the neurosensory hair cells of the cochlea. Each of these cells is ciliated, and the cilia deform in response to the propagating vibratory wave. The amplitude of this wave is interpreted as volume, whereas the frequency is sensed as pitch. Different pitches are sensed by hair cells in different areas of the cochlea. As the cilia deform, neurotransmitters are released and action potentials are generated. These action potentials are carried to the CNS by the eigth cranial nerve through the brainstem to the temporal cortex, where they are interpreted as sound.

The sense of balance is achieved through a complex interaction between the eyes, the cerebellum, skin, muscle and joint proprioceptors, and the vestibular portion of the inner ear. When any one of these components is compromised, it is possible to compensate for its loss. For instance, if the eyes are closed but all other balance systems are normal, we can usually remain upright and even walk fairly normally. However, when two or more systems are impaired, the sense of balance can be seriously degraded. This discussion will only focus on the balance function of the inner ear.

Each inner ear has two types of “accelerometers,” the ampullae of the semicircular canals, which sense angular acceleration and the maculae of the utricle and saccule that sense linear acceleration. There are three semicircular canals in each ear, oriented at right angles to each other, allowing them to sense acceleration in three different axes. These axes are analogous to the three basic axes of aircraft maneuvering, yaw, pitch, and roll. The opposite ear senses the same motions in a complementary manner, and together they can sense complex combinations of these three basic motions. There are ciliated cells within the ampullae, and they deform in response to the motion of fluid within the canals, thereby generating action potentials that are then sensed as motion. However, the canals have a rotational threshold of approximately 3 degrees/s. Motion below this threshold is not sensed, which has important implications for aviation. For example, it is possible for an aviator to have the aircraft slowly roll and descend without him or her being aware of any sensation whatsoever. If this occurs in instrument meteorologic conditions and the gauges have not been monitored carefully, control of the aircraft may be lost due to spatial disorientation and controlled flight into terrain.

Linear acceleration, such as felt during a catapult launch or when extending the speed brake, is sensed by the utricle and saccule. Gravity is also a linear acceleration, although we are generally unaware of it unless its forces on the body are altered or eliminated such as during aerobatics or space flight.

Inner ear inputs to the vestibular system help control the many activities in which we engage, from getting out of bed in the morning to performing competitive gymnastics. Through these inputs, the semicircular canals help in keeping our eyes focused on a target when our head is moving, and they help prevent falling to the ground when we stumble. However, the aerospace environment can challenge these inner ear functions, so it is important to be aware of their limitations.

The nose and the paranasal sinuses may be considered as a unit because normal sinuses are aerated, and they communicate directly with the air in the nasal cavity. Both structures are lined with ciliated mucosa that normally produces a thin mucus blanket that is slowly transported by ciliary action toward the nasopharynx where it is eventually swallowed. Although there is no obvious reason why sinuses exist, the nose itself has four clearly important functions in addition to allowing the passage of air: humidification, warming, cleansing of inspired air, and olfaction. In addition, the nose and sinuses together lend a characteristic resonance to speech that can change temporarily during the course of a sinonasal infection. Within the nasal cavity, there are three pairs of turbinates: inferior, middle, and superior. The inferior turbinates are highly vascular and can readily engorge with blood in response to inflammatory or autonomic stimuli. The middle turbinates are smaller and not quite as vascular, but are also capable of some swelling, and can develop polypoid degeneration in response to assorted irritants such as nasal allergens or chronic purulent sinus drainage. Because of their location near the ostia of the maxillary, frontal, and anterior ethmoid sinuses, swelling of the middle turbinates has a greater effect on sinus aeration and drainage than does swelling of the inferior turbinates, which contribute more to symptomatic congestion. The superior turbinates are small, difficult to see on routine examination, and rarely play a role in nasal or sinus obstruction. All of the turbinates taken together create a large surface area, allowing for inspired air to interact more thoroughly with the mucosa.

As air enters the nose and reaches the turbinates, its flow becomes somewhat turbulent. This allows for maximum contact between air and mucosa, enhancing the above-mentioned nasal functions. The normal rhythmic back-andforth nasal airflow also creates a small amount of airflow into and out of the sinuses, thereby maintaining normal oxygen partial pressures within. Well-aerated sinuses with functioning ciliated mucosa are not prone to infection. In contrast, interruption of this airflow coupled with compromise of ciliary action can lead to mucus stagnation and an increased likelihood of infection. Conditions that cause injury to the mucosa and/or blockage of the ostia and nasal cavity favor the development of bacterial sinusitis. These include viral upper respiratory infections (URIs), significant allergic rhinitis (particularly, when accompanied by nasal polyps), overuse of topical decongestant sprays, and even inhalation of excessively dry air. Although decongestants and dry air do not by themselves cause sinusitis, they can lead to thickening and crusting of nasal mucus secretions, which can hinder the migration of the mucus blanket, thereby increasing the chances of stagnation and bacterial growth. It is important to maintain adequate hydration when exposed to very dry air, and the additional use of saline nasal sprays or gels can be helpful.

The Eustachian tube is the only route for air to enter and leave the middle ear when the TM is intact. The Eustachian tube also provides a drainage route for mucus secreted by the middle ear mucosa, and because it is normally closed at rest it protects the middle ear from pharyngeal secretions and dampens the perception of one’s own voice. Normal transmission of sound from the TM and ossicles to the oval window depends on a middle ear filled with air at ambient pressure, and any change from that state can result in conductive hearing loss. The air cells within the mastoid bone communicate with the middle ear and provide an additional volume of air that can briefly act as a buffer against potentially painful changes in ambient pressure, but the Eustachian tube is always the primary route for pressure equalization. Middle ear ventilation usually happens without our awareness, because the Eustachian tube opens chiefly by contraction of the tensor veli palatini muscle during swallowing and yawning; this opening allows a small volume
of air to pass. This ventilation must be done periodically, for if the Eustachian tube does not open, gases in the middle ear will equilibrate with lower partial-pressure gases in the mucosa, causing a slow decrease in middle ear pressure, and possibly leading to an effusion. As the ambient pressure decreases during ascent in an aircraft, the middle ear pressure becomes relatively higher, and the Eustachian tube in almost every case will vent passively. This venting can be facilitated by a quick swallow or yawn, but it is generally not necessary. However, during descent, when increasing ambient air pressure begins to push the TM inward, the Eustachian tube must be actively opened by performing some type of maneuver. If nothing is done, significant pain invariably results, and if the pressure differential is high enough a middle ear effusion will occur. This will be discussed further in the section Barotitis Media.

The oral and nasal airways are equipped with structures that can increase resistance to inhalation and exhalation, thereby helping to maintain lung compliance. The primary resistive structures during normal respiration in the absence of adenotonsillar hypertrophy are the turbinates, with the soft palate playing a lesser role. Autonomic nervous system control of the vessels within the turbinates produces alternating engorgement and shrinkage of the turbinates, shifting nasal resistance from side to side every 1 to 5 hours. We are rarely aware of this cycling because total nasal resistance remains fairly constant. However if there is unilateral nasal obstruction from a neoplasm, polyp, or septal deviation, the total resistance will increase when the turbinates on the “good” side are engorged, leading to symptomatic obstruction and even mouth breathing. Such upper airway obstruction can contribute to obstructive sleep apnea, a condition with great implications for aviation safety that will be discussed in a subsequent section.

Progressive hearing loss due to occupational noise exposure is a widespread and serious problem, resulting in individual impairment and costing the government and industry hundreds of millions of dollars a year in compensation payments; much of it is preventable. The U.S. military services mandate enrollment in a hearing conservation program for all members who are expected to be exposed to high levels of noise during their careers. The program involves periodic audiograms, education, workplace sound level measurements, and provision of personal hearing protection. Continuous noise is more harmful than intermittent noise, and over time is more likely to impair hearing in the lower speech frequencies than is intermittent noise. Fortunately, neither type of noise causes a profound hearing loss, but they both cause early damage in the higher frequencies. Loss in these frequencies impairs speech discrimination due to muffling of consonants, and is most noticeable when there is background noise. When it is impossible to escape noise, and when engineering solutions have been maximized, hearing protection is all that is left. Those workers who are exposed to persistent loud noise, such as flight-line personnel, will usually wear double protection in the form of insert earplugs and over-the-ear muffs. Together, when worn properly, they can be fully protective during a normal working day provided ambient noise levels remain less than 125 dB, but while wearing them communication is difficult at best. Aircrew must be able to communicate in the face of high levels of noise, and when double protection is worn, maximum radio volumes may still not be adequate. Active noise reduction headsets can be quite effective at attenuating unwanted background noise, but some are bulky and all require additional electronic components. A cheaper, lighter, and equally effective solution is to use communication earplugs, which occlude the ear canal like standard soft earplugs, but they also function like music player “ear buds.” Radio communications bypass the occlusive effects of the earplug and allow for clear reception at comfortable volumes, while the plug itself provides background noise attenuation.

This term refers to a sudden hearing loss caused by an extremely loud noise. It is not a progressive loss as seen in
chronic occupational noise exposure. A gun fired close to the ear or a powerful firecracker going off nearby typically causes it. There may be a small degree of hearing recovery over a few days, but most of the loss is immediate and often permanent.

This is a medical condition and is not caused by noise. It may develop over minutes or days. It is often noticed upon awakening, and about half of patients experience some dizziness or vertigo. Viral, vascular, and autoimmune causes, as well as internal inner ear membrane disruptions have been postulated and investigated without definitive answers. Patients are usually middle-aged or younger, and many recover most or all of the lost hearing within weeks. The only treatment that has yielded data showing a positive effect is an oral steroid taper at sufficient doses over 10 days (1,2). Factors working against recovery are older age, severe initial loss, vertigo, and delay in seeking treatment. As long as no underlying disqualifying condition is diagnosed, a history of sudden hearing loss should not by itself preclude aviation duties.

Most cases of external ear infection are acute and easily treated. Chronic cases do occur, and can be quite stubborn. The responsible microbes are more likely to be bacterial than fungal, with the common culprits being Pseudomonas aeruginosa and Staphylococcus aureus. Susceptibility is increased by warm, humid weather, water immersion (particularly, in fresh-water pools), and use of cotton-tipped applicators that can abrade the skin and remove protective cerumen. Optimal treatment includes cleaning of the infected canal, irrigation with an acidifying solution, instillation of antibiotic drops, and, if the canal is too swollen to allow drops to enter, placement of a temporary wick. During the acute phase, the ear may be too tender to wear a flight helmet or a headset, and hearing may be down due to canal occlusion. Once the infection has cleared, all flying duties can resume.

Cerumen accumulation is common, but it is almost always harmless because it rarely results in total impaction. A large collection of hard cerumen in the ear canal can cause pain by contacting the sensitive inner bony part, and may even rest against the TM. The pain can be quite noticeable when chewing, yawning, performing the Valsalva maneuver, or trying to insert an earplug. Cerumen should be removed if the TM must be seen, or if it is causing pain or hearing loss. Removal techniques are many, and with the exception of simple instillation of dilute hydrogen peroxide, all entail some risk of canal injury or TM perforation. Irrigation should not be used if there is a history of perforation, but otherwise it is fairly safe if done gently. Use only warm water, with or without peroxide. Hot or cold water can cause a nauseating caloric reaction. Softening drops may be useful in improving the results, but difficult removals should be referred.