Middle Ear

The Middle Ear refers to a collection of bones (ossicles) and muscles contained within a chamber (tympanic cavity) that sit between the Outer Ear and the Inner Ear, bounded by the tympanic membrane (eardrum) and the oval window respectively. The eardrum transforms air pressure waves into physical vibrations – the middle ear amplifies these vibrations – the oval window allows the amplified vibrations to “flow” into the fluid-filled cochlea. Therefore, the primary function of the middle ear is to act as a mechanical amplifier. Without the middle ear 99.9% of the sound energy entering the outer ear, and vibrating the eardrum, would be reflected by the fluid-filled inner ear. This is because fluid has a much higher impedance to sound waves than air, which in contrast is a very efficient medium.

The Ossicles

The eardrum is attached at the umbo to the ossicular chain, which consists of three tiny bones:

1. malleus (hammer) which adjoins the eardrum
2. incus (anvil) which links the two other bones
3. stapes (stirrup) which is attached to the oval window.

It is these tiny bones that create the mechanical amplification. This amplification is variable, depending on the frequencies; higher frequencies need less amplification for transmission to the cochlea.

Normal Operation of the Middle Ear

Efficient hearing requires an intact tympanic membrane (eardrum), a normal ossicular chain, and a well-ventilated tympanic cavity. The tympanic cavity is air-filled and carved out of the temporal bone. For the eardrum to have maximal mobility, the air pressure on either side must be equal. This means the air pressure within the middle ear must equal that of the external environment. This is achieved via the Eustachian tube which connects the tympanic cavity to the throat/nasopharynx and acts as a pressure release valve to equalise the middle ear pressure to that outside.

Normally the walls of the Eustachian tube are collapsed, and jaw-moving actions such as swallowing, talking, yawning and chewing open the tube to allow air in or out as needed for equalisation.

Sinuses

Sinuses are cavities within a bone or other tissue, and humans have many around their bodies.

For flight, we are concerned mostly with the “paranasal sinuses”, which are a group of four paired air-filled spaces that surround the nasal cavity and they are named after the facial bones in which they are located:

• the maxillary sinuses, the largest of the paranasal sinuses, are under the eyes, in the maxillary (cheek) bones
• the frontal sinuses, superior to the eyes, in the frontal bone, which forms the hard part of the forehead
• the ethmoid sinuses, which are formed from several discrete air cells within the ethmoid bone between the nose and the eyes
• the sphenoid sinuses, in the sphenoid bone at the centre of the skull base under the pituitary gland.

The paranasal sinuses are lined with respiratory epithelium; which is a moist tissue whose purpose is protection from potential damage caused by mucous movement, and also to act as a barrier to pathogens and particles.

The paranasal sinuses are joined to the nasal cavity via small orifices called ostia; these are normally clear and allow pressure to equalise between the sinus cavities and the external environment.

There are several competing hypotheses regarding the biological function and purpose of the paranasal sinuses, but it is possible that they serve no biological function at all.

Pressure Changes in Flight

Whether flying in an unpressurised or pressurised aircraft, passengers and crew will experience a decrease in ambient (environmental) pressure whilst climbing, and an increase in pressure during descents. Some of these changes can be quite rapid. Such changes in pressure, and the speed of change, are not normally experienced “on the ground”; a rare example concerns travelling on a train as it enters or exits a tunnel at great speed. Divers will also experience similar pressure changes as they descend and ascend.

In most people, the ability of the middle ear to equalise pressure via the Eustachian tubes is adequate to prevent discomfort and retain good hearing during all stages and profiles of flight. This is mostly achieved automatically, and any “lag” between the middle ear cavity and the aircraft cabin can usually be eliminated by gentle yawning or chewing. Sometimes, if pressure is felt within the ear, it is possible to force the issue by clasping the nose and blowing into a closed mouth (Valsalva method); what may be known as “popping” the ears. Even when the Eustachian tubes are infected (see below) it can be relatively easy to equalise the ears when adjusting to lower cabin pressure (i.e. when climbing). However, in the descent, even the Valsalva method may not “pop” the ears and the increased external pressure will cause great discomfort

Similarly, the ostia usually remain clear enough to allow equalisation between the paranasal sinus cavities and the external air pressure. There is no natural mechanism we can employ to hasten the equalisation of the sinuses as there is for the middle ear.

If the Eustachian tube(s) and/or the ostia become blocked, or partially blocked, then a pressure differential will exist across the eardrum(s) and parts of the face and skull respectively; this can lead to pain, which can become severe.

In the case of the eardrum, the tympanic membrane will be distorted inward or outward, depending on the direction of the pressure differential. The ensuing pain is known as otic barotrauma, and will be felt acutely in the ears, also affecting one’s hearing. Sinus barotrauma will be felt above the eyes, behind the nose and in the cheeks, and possibly with a sensation that the teeth and gums are affected.

Causes of Blockage to the Middle Ear & Sinuses

Apart from physiological reasons, the anatomic causes of blocked Eustachian tubes and ostia are practically identical and often the two events will occur together.

Most commonly, these causes are both bacterial and viral infections such as colds and flu; however, blockages can also occur as a reaction to allergies and various diseases. Because, in both the middle ear and the sinuses, a blockage means that mucous cannot be removed, it is possible for bacteria to grow in this mucous, causing further infection, such as sinusitis, and therefore prolonging the problem.

Safety Issues

• Pain - Even moderate amounts of pain will distract an aircrew member from being able to conduct their tasks – they can become, effectively, incapacitated. In severe cases of barotrauma an aircrew member can be grounded for a long period, and may suffer a burst eardrum.
• Vertigo - If one ear equalises, but the other doesn’t, then a form of pressure vertigo can be experienced as dizziness, which is not helpful for aircrew, especially pilots.
• Disorientation - Blocked Eustachian tubes may also give rise to infection in the inner ear and therefore lead to risk of dizziness and spatial disorientation – both very serious flight hazards. The inner ear contains semi-circular canals which automatically detect accelerations in three dimensions and give us our “sense” of balance – where is up, down, left and right, and which way are we moving.
• Headache - Sinusitis may develop giving rise to a constant throbbing headache felt in the face (often under the eyes or in the upper teeth). The pain gets worse when you move your head, strain or bend down, and when you experience extreme changes in temperature. Your face will also feel tender to the touch, and you may have other general symptoms such as a runny nose and sore throat. The symptoms of a sinus headache can easily be confused with both migraine and tension headaches.
• Hearing - Unequalised ears result in distortions of the eardrum and therefore, hearing will be impaired, in terms of attenuation and also clarity. This can lead to pilots and cabin crew not hearing, or mishearing, critical instructions.
• Workload - Finally, but by no means least, it can be quite traumatic (and a distraction) having a colleague or passenger on-board suffering from blocked Eustachian tubes while descending. Pilots will need to balance the desire to get the patient to a medical facility as soon as possible against the need for a slow descent. A delayed and/or slowed descent will alter the flight profile, and will affect fuel burn, landing weights, power settings, perhaps arrival and approach path, and even management of the pressurisation system. All of these are additional to the pilots’ normal workload and concerns, and can potentially introduce hazards.

Treatment & Mitigating Measures

If symptoms of otic or sinus barotrauma, or discomfort, occur in-flight then action is required, depending on the stage of the flight profile (climb, cruise, or descent), the severity of the trauma, and also depending on whether the patient is a crew member or passenger. Solutions may include: an immediate return to the departure aerodrome; a reduced cruising altitude; pressurisation management; and, continuation to destination with the application of first aid. Most critical for severe cases will be the need to control the descent in stages, allowing the patient time to equalise their ears frequently. The most crucial stage (i.e. with the greatest pressure change) is the last few thousand feet of altitude.

An badly affected crewmember should be removed from duties. There can be some leeway if one of only two pilots is affected, depending on the severity of pain; but discussions will need to cover the potential symptoms of degraded hearing, performance and possible dizziness and imbalance.

All cabin and flight crew are regularly trained in first aid and the use of the first aid equipment on-board. This will include the use of nasal decongestants to assist in freeing up the Eustachian tubes and ostia, usually in connection with the Valsalva method of ear “popping”. They will also be familiar with the use of analgesics to relieve some of the associated pain.

The simplest measure to take is to never fly with a cold, flu, or any other similar illness and infection. Similarly, do not fly if you cannot equalise the pressure in your middle ears using the Valsalva method; perhaps making it a habit to check this before every flight is a good habit to adopt?

The common causes of blocked ostia and Eustachian tubes usually clear up within a period of a few weeks, and usually no long-term damage occurs. Symptoms, including sinusitis headaches, can be alleviated by an array of different prescribed and over the counter drugs and products. However, flight and cabin crew should not self-medicate and should consult their Company surgeon or private Doctor. For some persistent bacterial infections antibiotics may need to be prescribed, which may preclude any flying for the duration of the course (often 10-days).

Blockages resulting from allergic reactions may be relieved through decongestants and antihistamine, but ultimately the source of the allergy will need to be addressed.

Further Reading

• Hearing and Noise (OGHFA BN)
• The Common Cold
• Vestibular System and Illusions (OGHFA BN)