January 29, 1998
Decompression sickness (DCS) describes a condition characterized
by a variety of symptoms resulting from exposure to low barometric
pressures that cause inert gases (mainly nitrogen), normally dissolved
in body fluids and tissues, to come out of physical solution and
form bubbles. DCS can occur during exposure to altitude (altitude
DCS) or during ascent from depth (mining or diving). The first
documented cases of DCS (Caisson Disease) were reported in 1841
by a mining engineer who observed the occurrences of pain and
muscle cramps among coal miners exposed to air-pressurized mine
shafts designed to keep water out. The first description of a
case resulting from diving activities while wearing a pressurized
hard hat was reported in 1869.
Altitude-induced decompression sickness
Altitude DCS became a commonly observed problem associated with
high-altitude balloon and aircraft flight in the 1930s. IN present
-day aviation, technology allows civilian aircraft (commercial
and private) to fly higher and faster than ever before. Though
modern aircraft are safer and more reliable, occupants are still
subject to the stresses of high altitude flight--and the unique
problems that go wit these lofty heights. A century and one-half
after the first DCS case was described, our understanding of
DCS has improved, and a body of knowledge has accumulated; however,
this problem is far from being solved. Altitude DCS still represents
a risk to the occupants of modern aircraft.
According to Henry's Law, when the pressure of a gas over a liquid
is decreased, the amount of gas dissolved in that liquid will
also decrease. One of the best practical demonstrations of this
law is offered by opening a soft drink. When the cap is removed
from the bottle, gas is heard escaping, and bubbles can be seen
forming in the soda. This is carbon dioxide gas coming out of
solution as a result of sudden exposure to lower barometric pressure.
Similarly, nitrogen Is an inert gas normally stored throughout
the human body (tissues and fluids) in a physical solution. When
the body is expose to decrease barometric pressures (as in flying
an unpressurized aircraft to altitude or during a rapid decompression),
the nitrogen dissolved in the body comes out solution. If the
nitrogen is forced to leave the solution too rapidly, bubbles
form in different areas of the body, causing a variety of signs
and symptoms. The commonly symptom is joint pain, which is known
as "the bends."
Although bubbles can form anywhere in the body, the most frequently
targeted anatomic locations are the shoulders, elbows, knees,
and ankles. Table 1 list the different DCS types with their corresponding
bubble formation sites and their most common symptoms. "The
Bends" (joint pain) account for about 60 to 70% of all altitude
DCS cases with the shoulder being the most common site. Neurologic
manifestations are present in about 10 to 15% of all DCS cases
with headache and visual disturbance being the mot common symptoms.
"The chokes" are very infrequent and occur in less
than 2% of all DCS cases. Skin manifestation are present in about
10 to 15 % of all DCS cases.
Mild cases of "the bends" and skin bends (excluding
mottled or marbled skin appearance) may disappear during descent
from high altitude, but still require medical evaluation. If
the sign and symptoms persist during descent or reappear at ground
level, it is necessary to provide hyperbaric oxygen treatment
immediately (100% oxygen delivered in a high pressure chamber).
Neurological DCS, "the chokes," and skin bends with
mottled or marbled skin lesion (see table 1) should always be
treated with hyperbaric oxygenation. These conditions are very
serious and potentially fatal if untreated.
Facts about breathing 100% oxygen
One of the most significant breakthroughs in altitude DCS research
was the discovery that breathing 100% oxygen before exposure to
low barometric pressure (oxygen pre-breathing), decreases the
risk of developing altitude DCS. Oxygen pre-breathing promotes
the elimination (washout) of nitrogen from body tissue. Pre-breathing
100% oxygen for 30 minute prior to initiating ascent to altitude
reduces the risk of altitude DCS for short exposures (10-30 minutes
only) to altitudes between 18,000 and 43,000 feet. However, oxygen
pre-breathing has to be continued, without interruption, with
in-flight 100% oxygen breathing to provide effective protecting
against altitude DCS.
Furthermore, it is very important to understand that breathing
100% oxygen only during flight (ascent, en route, descent) does
not decrease the risk of altitude DCS, and should not be used
in lieu of oxygen pre-breathing. Although 100% oxygen pre-breathing
is an affective method to provide individual protection against
altitude DCS, it is not a logistically simple nor an inexpensive
approach for the protection of civil aviation flyers (commercial
or private). Therefore, at the present time it is only used by
military flight crews and astronauts for their protection during
high altitude and space operations.
Table 1 -- Types of Decompression Sickness
- Bubble Location: mostly large joints of the body (elbows,
shoulders, hip, wrist, knees, ankles)
- Signs & Symptoms: localized deep pain, ranging from mild
(a "niggle") to excruciating. Sometimes a dull ache,
but rarely a sharp pain; active and passive motion of the join
aggravates the pain; pain can occur at altitude, during the descent,
or many hours later.
- Bubble Location: brain
- Signs & Symptoms: confusion or memory loss; headache;
spots in visual field (scotoma), tunnel vision, double vision(
diplopia), or blurry vision; Unexplained extreme fatigue or behavior
changes; seizures dizziness, vertigo, nausea, vomiting, and unconscious
- Bubble Location: spinal cord
- Signs & Symptoms: abnormal sensations such as burning,
stinging, and tingling around the lower chest and back; symptoms
may spread from the feet up and may be accompanied by ascending
weakness or paralysis; girdling abdominal or chest pain.
- Bubble Location: peripheral nerves
- Signs & Symptoms: urinary and rectal incontinence, abnormal
sensations, such as numbness, burning , stinging, and tingling
(paraesthesia); muscle weakness or twitching.
- Bubble Location: lungs
- Signs & Symptoms: burning deep chest pain (under the sternum);
pain is aggravated by breathing; shortness of breath (dyspnea);
dry constant cough
- Skin bends
- Bubble Location: skin
- Signs & Symptoms: itching usually around the ears, face,
neck , arms, and upper torso; sensation of tiny insects crawling
over the skin (formication); mottled or marbles skin usually around
the shoulders, upper chest and abdomen, accompanied by itching;
swelling of the skin, accompanied by tiny scar-like skin depressions
There is no specific altitude that can be considered an absolute
altitude exposure threshold below which it can be assured that
no one will develop attitude DCS. However, there is very little
evidence of altitude DCS occurring among healthy individuals at
altitudes below 18,000 feet who have not been SCUBA (Self Contained
Underwater Breathing Apparatus) diving. individual exposures
to altitude between 18,000 and 25,000 have show a low occurrence
of altitude DCS. Most cases of altitude DCS occur among individuals
exposed to altitudes of 25,000 feet or higher. A U.S. Air Force
study of altitude DCS cases reported that only 13% occurred below
The higher the altitude of exposure, the greater the risk of developing
altitude DCS. It is important to clarify that although exposure
to incremental altitude about 18,00 feet show an incremental risk
of altitude DCS, they do not show a direct relationship with the
severity of the various types of DCS (see table 1).
- Repetitive Exposures
Repetitive exposure to altitudes above 18000 feet within a
short period of time (a few hours) also increase the risk of developing
- Rate of Ascent
The faster the rate of ascent to altitude, the greater the
risk of developing altitude DCS. An individual exposed to a rapid
decompression (high rate of ascent) about 18,000 feet has a grater
risk of altitude DCS than being exposed to the same altitude but
as a lower rate of ascent.
- Time at Altitude
The longer the duration of the expose to altitudes of 18,000
feet and above, the greater the risk of altitude DCS.
There are some reports indicating a higher risk of altitude
DCS with increase age.
- Previous Injury
There is some indication that recent join of limb injuries
may predispose individuals to developing "the bends".
- Ambient Temperature
There is some evidence suggesting that individual exposure
to very cold ambient temperatures any increase the risk of altitude
- Body Type
Typically, a person who has a high body fat contest is at
greater risk of altitude DCS. Due to poor blood supply, nitrogen
is stored in greater amounts in fat tissues. Although fat represents
only 15% of an adult normal body, it stores over half of the total
amount of nitrogen (about 1 liter) normally dissolved in the body.
When a person is physically active while flying at altitude
above 18,000 feet, there is grater risk of altitude DCS.
- Alcohol Consumption
The after-effects of alcohol consumption increases the susceptibility
- SCUBA Diving Before Flying
SCUBA diving requires breathing air under high pressure. Under
there is a significant increase in the amount of nitrogen dissolved
in the body (body nitrogen saturation). The deeper the SCUBA
dive, the greater the rate of body nitrogen saturation. Furthermore,
SCUBA diving in high elevations (mountain lakes), at any given
depth, results in greater body nitrogen saturation when compared
to SCUBA diving at sea level at the same depth. Following SCUBA
diving, if not enough time is allowed to eliminate the excess
nitrogen stored in the body, altitude DCS can occur during exposure
to altitudes as low as 5,000 feet or less.
What to do when altitude DCS occurs
- Put on your oxygen mask immediately and switch the regulator
to 100% oxygen.
- Begin an emergency descent and land as soon as possible.
Even if the symptoms disappear during descent, you should still
land and seek medical evaluation while continuing to breath oxygen.
- If one of your symptoms is join pain, keep the affected area
still; don not try to work pain out by moving the joint around.
- Upon landing, seek medical assistance from an FAA medical
officer, aviation medical examiner (AME), military flight surgeon,
or a hyperbaric medicine specialist. be aware that a physician
not specialized in aviation or hyperbaric medicine may not be
family with this type of medical problem. Therefore, be your
- Definitive medical treatment may involve the use of a hyperbaric
chamber operated by specially trained personnel.
- Delayed signs and symptoms of altitude DCS can occur after
return to ground level whether or not they were present during
Things to Remember
- Altitude DCS is a potential risk every time you fly in an
unpressurized aircraft above 18,000 feet (or at lower altitudes
if you SCUBA dive prior to flight).
- Be familiar with the signs and symptoms of altitude DCS (See
Table 1) and monitor all aircraft occupant, including yourself,
any time you fly an unpressurized aircraft above 18,000 feet.
- Avoid unnecessary strenuous physical activity prior to flying
an unpressurized aircraft above 18,000 feet and for 24 hours after
- Even if you are flying a pressurized aircraft, altitude DCS
can occur as a result of sudden loss of cabin pressure (in-flight
- Following exposure to an in-flight rapid decompression, do
not fly for at least 24 hours. In the meantime, remain vigilant
for the possible onset of delayed symptoms or signs of altitude
DCS. If you present delayed symptoms or signs of altitude DCS,
seek medical attention immediately.
- Keep in mind that breathing 100% oxygen during flight without
oxygen pre-breathing prior to take off, does not prevent the occurrence
of altitude DCS.
- Do not ignore any symptom of sign that go away during the
descent. In fact, this could confirm that you are actually suffering
altitude DCS. You should be medically evaluated as soon as possible.
- If there is any indication that you may have experience altitude
DCS, do not fly again until you are cleared to do so by an FAA
medical officer, AME, military flight surgeon, or a hyperbaric
- Allow at least 24 hours to elapse between SCUBA diving and
- Be prepared for a future emergency by familiarizing yourself
with the availability of hyperbaric chamber in your area of operations.
However, keep in mind that not all of the available hyperbaric
treatment facility have personnel qualified to handle altitude
DCS emergencies. To obtain information on locations of hyperbaric
treatment facilities capable of handling altitude DCS emergencies,
call the Diver's Alert Network at 919-684-8111.
If you are interested in learning more about altitude DCS, as
well as the other stressors that may affect your performance and/or
you health during flight, we encourage you to enroll in the physiologic
training course offered by the Aeromedical Education Division
(Airman Education Programs) at the FAA Civil Aeromedical Institute
in Oklahoma City. A similar course is also available to you at
the U.S. military physiologic training facilities around the country
through an FAA/DOD training agreement. For more information about
any of these courses, call us at 405-954-4837.