Page 67 - Journal of Special Operations Medicine - Summer 2016

Figure 1 Ear anatomy. Dizziness, nausea, vomiting, gait instability, and hearing
impairment can ensue. IEB can also occur during the
4
ascent phase of diving. Boyle’s gas law describes that at
a constant temperature, the pressure and volume of a gas
are inversely related. As pressure increases, the volume
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of a gas decreases, and vice versa. As a diver ascends, gas
expands. If the gas does not have a route of egress from
the middle ear, overpressurization may occur, resulting in
damage to the structures of the middle and inner ear. In
this case, as the diver travels toward the surface, the gas
enclosed in the middle ear inflates. The path of least resis-
tance for this expansion is down the eustachian tube, to
the nasopharynx, to be expelled with respiration. If the
eustachian tube is not patent, gas in the middle ear has
no path for exit. It will expand, increasing pressure on
Adapted from US Navy Dive Manual. 3
5
the inner ear until damage results. IEB may occur with
tympanic membrane. The middle ear is a gas-filled space or without associated middle ear barotrauma (tympanic
enclosed in the petrous part of the temporal bone. It ex- membrane injury or rupture).
tends laterally from the tympanic membrane, medially
to the round and oval windows. It is connected to the Inner Ear Decompression Sickness
nasopharynx by a small tubular passage called the eusta- Inner ear decompression sickness (IEDCS), also called
chian tube. Middle ear structures include the tympanic inner ear decompression illness, has a clinical presenta-
membrane, auditory ossicles (malleus, incus, stapes), sta- tion similar to IEB. Both conditions involve damage to
pedius and tensor tympani muscles, the chorda tympani the structures of the inner ear and, therefore, both cause
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nerve, and the tympanic plexus of nerves. Sound waves vertigo, hearing impairment, and/or tinnitus. Henry’s
that travel down the external auditory canal bend the gas law states that the solubility of gas in solution is de-
4,6
tympanic membrane, which, in turn, causes vibrations pendent on the partial pressure of the gas. Therefore,
that are conducted through the ossicles. These vibrations as ambient pressure increases, the amount of a gas that
then continue through the oval window to the inner ear. can dissolve in solution increases. Conversely, as ambi-
The inner ear is a complex fluid-filled cavity containing ent pressure decreases, the solubility of a given gas de-
the structures of the vestibulocochlear system respon- creases. In diving, this law is applied to the dissolution of
sible for balance and hearing. The mechanical vibration inert gas (gas not metabolized by cells [i.e., nitrogen and
of the middle ear is converted to fluid waves in the inner helium]) into the tissues of the body. When a diver de-
ear. The structures of the inner ear then convert this fluid scends under water and breathes a gas mixture that con-
wave into nerve impulses transmitted to the brain by CN tains some proportion of inert gas, the amount of that
VIII (vestibulocochlear). Only the external and middle gas dissolved in body tissues (e.g., blood, fat) increases.
ear are gas-filled spaces. The inner ear is fluid filled. 1,2 As the diver ascends, ambient pressure and the partial
pressure of the inert gas decrease, permitting the gas to
Inner Ear Barotrauma come out of solution and form bubbles. This bubble for-
Inner ear barotrauma (IEB) presents with symptoms mation is believed to lead to the classic manifestations
of vertigo, tinnitus, and/or hearing loss on the affected of decompression sickness (DCS). As a diver ascends,
side. When related to diving, this is usually the result if the removal of inert gas from the body via exhala-
4
of inadequate middle ear pressure equilibration during tion cannot keep pace with the increased amount of gas
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the descent phase of diving operations. Several possible coming out of solution, DCS may occur. Specifically,
mechanisms for this injury have been described. Repeated IEDCS is thought to be due to the formation of bubbles
performance of the Valsalva maneuver in an attempt to in the microvasculature and otic fluids of the vestibu-
equalize middle ear pressure with that of surrounding locochlear apparatus. This bubble formation can cause
gas-filled spaces (commonly known as clearing) can lead mechanical obstruction of venous outflow, hemorrhage,
to an implosion effect on the oval or round windows and protein exudation within the inner ear microvascu-
between the middle and inner ear. This transmits energy lature. Likewise, extravascular bubble formation in otic
to the inner ear chamber, resulting in structural dam- fluid may disrupt microvasculature through mechanical
age. The Valsalva maneuver can alternatively cause an compression. 6,9
1
increase in intracranial and cerebrospinal fluid pressure,
which is conducted to the fluid of the inner ear. Inner ear IEB Versus IEDCS
fluid overpressurization subsequently leads to vestibu- Several key clues can help differentiate symptoms caused
locochlear damage and presentation of symptoms. 1,4,5 by IEB and those due to IEDCS (Table 1). Examples

Inner Ear Barotrauma 53