FAA Brochure - Altitude-Induced Decompression Sickness

ALTITUDE-INDUCED

DECOMPRESSION

SICKNESS

OK-24-1154 9/18/2024Age

There are some reports indicating a higher risk of

altitude DCS with increasing age.

Previous Injury

There is some indication that recent joint or limb

injuries may predispose individuals to developing

“the bends.”

Ambient Temperature

There is some evidence suggesting that individual

exposure to very cold ambient temperatures may

increase the risk of altitude DCS.

Body Type

Typically, a person who has a high body fat content 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.

Exercise

When a person is physically active while flying at

altitudes above 18,000 ft., there is greater risk of

altitude DCS.

Alcohol Consumption

The after-effects of alcohol consumption increase the

susceptibility to DCS.

Scuba Diving Before Flying

SCUBA diving requires breathing air under high

pressure. Under these conditions, 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 ft. 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 breathe oxygen.

• If one of your symptoms is joint pain, keep the

affected area still; do 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 hypobaric medicine may not be familiar

with this type of medical problem. Therefore, be your

own advocate.

• 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 flight.

THINGS TO REMEMBER

• Altitude DCS is a risk every time you fly in an

unpressurized aircraft above 18,000 feet (or at lower

altitude if you SCUBA dive prior to the flight).

• Be familiar with the signs and symptoms of altitude

DCS (see Table 1) and monitor all aircraft occupants,

including yourself, any time you fly an unpressurized

aircraft above 18,000 ft.

• Avoid unnecessary strenuous physical activity prior to

flying an unpressurized aircraft above 18,000 ft. and

for 24 hrs. after the flight.

• Even if you are flying a pressurized aircraft, altitude

DCS can occur as a result of sudden loss of cabin

pressure (inflight rapid decompression).

• Following exposure to an inflight rapid decompressions

do not fly for at least 24 hrs. 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

(ascent, enroute, descent) without oxygen prebreathing

prior to take off does not prevent the occurrence of

altitude DCS.

• Do not ignore any symptoms or signs 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 experienced

altitude DCS, do not fly again until you are cleared to do

so by an FAA medical officer, an aviation medical examiner,

a military flight surgeon, or a hyperbaric medicine

specialist.

• Allow at least 24 hrs. to elapse between SCUBA diving

and flying.

• Be prepared for a future emergency by familiarizing

yourself with the availability of hyperbaric chambers in

your area of operations. However, keep in mind that not all

of the available hyperbaric treatment facilities have

personnel qualified to handle altitude DCS emergencies.

To obtain information on the locations of hyperbaric

treatment facilities capable of handling altitude DCS

emergencies, call the Diver’s Alert Network

at (919) 684-9111.

Provided by

Aerospace Medical Education Division, AAM-400

To obtain copies of this brochure online:

https://www.faa.gov/pilots/safety/

pilotsafetybrochures/

or contact:

Federal Aviation Administration

Civil Aerospace Medical Institute

AAM-400

P.O. Box 25082

Oklahoma City, OK 73125

(405) 954-4831

DCS Type Bubble Location Signs & Symptoms (Clinical Manifestations)

BENDS Mostly large joints of the

body (elbows, shoulders,

hip, wrists, knees, ankles)• 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 joint aggravates the pain.

• Pain can occur at altitude, during the descent, or many hours later.

NEUROLOGIC

ManifestationsBrain • 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 unconsciousness may occur

Spinal Cord • 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

Peripheral Nerves • Urinary and rectal incontinence

• Abnormal sensations, such as numbness, burning, stinging and tingling (paresthesia)

• Muscle weakness or twitching

CHOKES Lungs • Burning deep chest pain (under the sternum)

• Pain is aggravated by breathing

• Shortness of breath (dyspnea)

• Dry constant cough

SKIN BENDS Skin • Itching usually around the ears, face, neck arms, and upper torso

• Sensation of tiny insects crawling over the skin

• Mottled or marbled skin usually around the shoulders, upper chest and abdomen, accompanied

by itching

• Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema)Tiny Bubbles, BIG Troubles

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 occurrence 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 flights 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 with 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.

Tiny Bubbles

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 physical solution. When the body is exposed to decreased

barometric pressures (as in flying an unpressurized aircraft

to altitude, or during a rapid decompression), the nitrogen

dissolved in the body comes out of 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 most common symptom is joint pain, which is

known as “the bends.” Trouble Sites

Although bubbles can form anywhere in the body, the most

frequently targeted anatomic locations are the shoulders,

elbows, knees, and ankles.

Table 1, lists 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 disturbances

being the most common symptoms. “The chokes” are very

infrequent and occur in less than 2% of all DCS cases. Skin

manifestations are present in about 10 to 15% of all

DCS cases.

Medical Treatment

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 signs

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 lesions (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 a low barometric pressure (oxygen prebreathing),

decreases the risk of developing altitude DCS. Oxygen

prebreathing promotes the elimination (washout) of nitrogen

from body tissues. Prebreathing 100% oxygen for 30 minutes

prior to initiating ascent to altitude reduces the risk of altitude

DCS for short exposures (10-30 min. only) to altitudes between

18,000 and 43,000 ft. However, oxygen prebreathing has to

be continued, without interruption, with inflight 100% oxygen

breathing to provide effective protection against altitude DCS.

Furthermore, it is very important to understand that breathing

100% oxygen only during flight (ascent, enroute, descent) does

not decrease the risk of altitude DCS and should not be used in

lieu of oxygen prebreathing.

Although 100% oxygen prebreathing is an effective method

to provide individual protection against altitude DCS, it is

not a logistically simple or 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.

PREDISPOSING FACTORS

Altitude

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 altitude DCS.

However, there is very little evidence of altitude DCS

occurring among healthy individuals at altitudes below

18,000 ft. who have not been SCUBA (Self Contained

Underwater Breathing Apparatus) diving. Individual

exposures to altitudes between 18,000 ft. and 25,000

ft. have shown a low occurrence of altitude DCS. Most

cases of altitude DCS occur among individuals exposed

to altitudes of 25,000 ft. or higher. A US Air Force study

of altitude DCS cases reported that only 13% occurred

below 25,000 ft. The higher the altitude of exposure, the

greater the risk of developing altitude DCS. It is important

to clarify that although exposures to incremental altitudes

above 18,000 ft. 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 exposures to altitudes above 18,000 ft.

within a short period of time (a few hrs.) also increase

the risk of developing altitude DCS.

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) above

18,000 ft. has a greater risk of altitude DCS than

being exposed to the same altitude but at a lower

rate of ascent.

Time at Altitude

The longer the duration of the exposure to altitudes

of 18,000 ft. and above, the greater the risk of

altitude DCS.Table 1. Signs and symptoms of Altitude Decompression Sickness