Evolved Gas Disorders


Evolved gas disorders (decompression sickness) is defined as the effects produced by nitrogen gas coming out of solution in the tissues and fluids of the body due to exposure to low barometric pressure.

Types of Decompression Sickness

  1. Bends
  2. Skin Manifestations (CREEPS)
  3. Chokes
  4. Central Nervous System (CNS) Manifestations


Is mild to severe aching pain which is caused by nitrogen bubbles coming out of body fluids in or around the major joints of the body such as knees, elbows, and shoulders. [1]

Factors affecting the degree of pain

  • Exercise
  • Time at altitude
  • Increased altitude



  • Deep, dull and penetrating pain in a joint
  • Can increase to agonising intensity

Skin Manifestations

Paresthesia is caused by very small bubbles of nitrogen gas forming superficially under the skin producing a stimulus to specialised nerves.[1][3]



  • Itching, hot and cold feelings, tingling
  • Creepy feeling (like bugs crawling on the skin)
  • Mottled reddish or purplish rash


Although rare, this condition can be potentially very dangerous. It is caused by nitrogen bubbles in the smaller blood vessels of the lungs and in the tissues of the airways.[1]


  • Deep, sharp pain in the breastbone
  • Dry, progressive cough
  • Inability to take a normal breath, suffocation, apprehension, possible shock symptoms such as sweating, pallor, fainting and cyanosis

Central Nervous System (CNS) Manifestations

Neurological manifestations of decompression sickness in the brain and spinal cord are caused by the nitrogen bubbles circulating through fluid surrounding these areas. This reduces the blood flow and causes localised hypoxia of the brain tissue.[1][2]


  • Disturbances in vision (blind spots, flashing/flickering lights)
  • Dull to severe persistent headache
  • Partial paralysis
  • Disturbance in sensation, loss of orientation
  • Loss of ability to speak or hear
  • Delirium and vertigo
  • Unilateral paresthesia

Note: The symptoms are very similar to stroke and can also be easily mistaken for alcohol intoxication.

The factors influencing decompression sickness[1]

1. Rate of ascent, altitude and duration of exposure
The more rapid the rate of ascent or cabin depressurisation, the sooner the symptoms will appear. The longer a person is exposed to high altitude, such as high-altitude-high-opening (HAHO) operations, the greater the risk of decompression sickness.

2. Physical activity
Exercise during or after altitude is comparable to shaking a cola bottle and making the bubbles come out faster. Exercise should be done before flight and not after.

3. Age and body fat
Occurrence of symptoms increase in proportion to age and body fat.

4. Cabin pressurisatio n
Above FL 180 incidence of decompression sickness increases.

5. Previous injury
Scar tissue denitrogenates poorly. Consequently, injured areas are likely to suffer from decompression sickness.

6. Poor Circulation
Any area of restricted circulation denitrogenates poorly and also provides a localised area for poor off gassing of nitrogen. Joints are a naturally restricted area. Seat harnesses, equipment and restrictive clothing reduce circulation. Do stretch or flex muscles during long periods of sitting or inactivity. DO NOT EXERCISE.

7. Extreme temperatures
Areas exposed to extreme heat or cold, such as ankles, are more likely to develop decompression sickness.

8. Diving prior to flight
48 hours between scuba diving and flying.

9. Diving after flight
Recent studies indicate this also increases the chance of decompression sickness.

Protection from decompression sickness

1. Cabin pressurisation is the most common method of preventing decompression sickness as well as hypoxia. In a sense, your body never gets to altitude.

2. Denitrogenation. It is the act of eliminating some nitrogen from your body by breathing 100% oxygen prior to exposure to low barometric pressure. This is more applicable to military pilots.

Treatment of evolved gas disorders[1]

1. 100% oxygen as soon as possible.

2. Immobilise the affected area if possible. Do not try to massage, rub or exercise.

3. Descend immediately.

4. Seek qualified medical help.

5. Flight surgeon or aeromedical examiner will recommend compression therapy if necessary.

VIDEO: Altitude Induced Decompression Sickness

Published by FAAnews on 02 May 2012

1. Federal Aviation Administration. Altitude-Induced Decompression Sickness. AM-400-95/2.
2. Ulf I. Balldin, Andrew A. Pilmanis, and James T. Webb. Central Nervous System Decompression Sickness and Venous Gas Emboli in Hypobaric Conditions. Aviation, Space, and Environmental Medicine Vol. 75, No. 11, November 2004.
3. Tom S Neuman and Stephen R Thom (2008). Physiology and Medicine of Hyperbaric Oxygen Therapy. USA: Saunders Elsevier

Want to know more?

Decompression Illness: What Is It and What Is The Treatment?
Although this is illustrated based on diving context, it is similar to aviation.

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