Decompression Sickness in HA Reconnaissance Aircraft

Pilots flying high altitude (HA) reconnaissance sorties are vulnerable to decompression sickness (DCS) due to, exposure to “pressure equivalent up to 29,500 ft (8992 m) of altitude for over 8 h” [1]. Cruise altitude of such HA reconnaissance aircraft viz. U-2 [2] and MiG 25 [3] is 70,000 ft and 74,000 ft, respectively. There are two life support measures practiced to prevent DCS due to sudden decompression during HA missions [1]: pilots preoxygenate with 100% oxygen for a minimum of an hour prior to take-off, and don a pressure suit (Full pressure suit and partial pressure suit in U-2 and MiG-25, respectively). Jersey et. al., found that operationally there may be cases of joint pain reported but the severe form of DCS – pulmonary or neurological symptoms, were not found. They stated that 途etrospective surveys of U-2 pilots revealed widespread under-reporting…60-80% of respondents experienced DCS symptoms at some point in their careers, with 16-18% of these neurologic” [1]. 

Following the reporting of two cases of severe CNS (Central Nervous System) DCS in 2009, Jersey et. al. undertook a study to evaluate the potential causes of CNS DCS in U-2 operations from 2002 to 2009. Their findings are summarised here [1]:-

  • 67 incidents of DCS were reported between 1991-2009.
  • Between 2002 to 2009, there were 45 incidents of DCS reported by 37 U-2 pilots.
    • 16 of these were incidents of CNS DCS, with 3 pilots reporting 2 incidents each, and 10 had solitary incidents;
    • 5 of the 16 incidents had severe neurological and pulmonary symptoms, with pilot incapacitation in 3 cases, and the remaining two reporting “sudden-onset waves of vertigo, confusion and depressed consciousness”.
  • In comparison, there were 24 incidents of DCS reported by 13 pilots between 1991 to 2001, as per pre-existing data.
    • Three of these incidents were confirmed cases of CNS DCS and two of pulmonary type.
  • There were 9 pilots with prolonged or permanent neurological symptoms in the studied cohort. The symptoms were limited to the brain, with no spinal symptoms.
    • “Visual problems, vertigo, memory problems, confusion, headache, weakness, altered consciousness and inappropriate fatigue” were reported.
  • Descent to lower altitudes and hyperbaric oxygen therapy (HBOT), once on ground, markedly relieved the symptoms.
  • Symptoms were found to be long-lasting amongst 9 of 20 pilots, despite of HBOT, in the 2002-09 cohort.
    • Most severe were “permanent visual deficits, short-term memory difficulties, and personality changes” [4].
    • Other symptoms were “recurrent headaches, central fatigue, memory difficulties, “foggy thinking”, and personality changes”.
    • Post-traumatic stress disorder in one pilot was successfully treated.
  • Unlike diving related DCS, neurological symptoms in sub-atmospheric DCS is not so often found amongst pilots [5, 6].
    • The evident neurological symptomatology and course of illness is akin to that seen in mild traumatic brain injury.

The operational factors, as brought out by Jersey et. al., are as following [1]:-

  • Increased physical activity inside the cockpit due to mission requirements and the need to operate latest on-board sensors.
  • Increased number of sorties and hours spent at HA, as compared to earlier mission requirements (U-2 Programme – Average decadal hours flown: 1979-89 = 10,140 h; 1998-2009 = 13,195 h, with 3457 sorties flown annually)
  • Reduction in available number of pilots for HA missions – 37 as compared to 49, in 1998-2009 and 1994-97, respectively. Thus, the pilots logged 353 h from 92 sorties annually from 1998 to 2009, as compared to earlier 207 h from 42 sorties annually.
  • Increase in exposure to HA due to “longer sorties”, lasting almost 8 to 9 hours.

The determining but unalterable (due to cost) factor increasing the risk of DCS in HA missions is the aircraft pressurisation system. However, there are four major risk mitigating factors against DCS [7]:-

  • Level of denitrogenation (prebreathe)
  • Altitude
  • Duration of exposure, and
  • Exercise at altitude

Defining the countermeasures to reduce the incidence of DCS amongst HA pilots, Jersey et. al. advocate certain approaches. This includes use of exercise-enhanced prebreathe (EEPB). This entails “10 min of moderate upper and lower body exercise at the beginning of a 60-min total prebreathe period [1]. In addition, during the sorties unless the mission requires them to be flying at the aircraft service ceiling , pilots can fly at (relatively) lower altitudes, with partially inflating their full pressure suit to 0.5 psi. Considering the 菟rotean naturey et. al. proffer a word of advice to the flight surgeons to 杜aintain a high degree of clinical suspicion for at least 72 h fol1].


1. Jersey SL, Hundemer GL, Stuart RP, West KN, Michaelson RS, Pilmanis AA. Neurological altitude decompression sickness among U-2 pilots: 2002-2009. Aviat Space Environ Med 2011; 82:673-82
2. Lockheed U-2
3. MiG-25
4. Jersey SL, Baril RT, McCarty RD, Millhouse CM. Severe neurological decompression sickness in a U-2 pilot. Aviat Space Environ Med 2010; 81:64-8
5.  Operational and Medical Issues in Hypo- and Hyperbaric Conditions
6. Weien RW, Baumgartner N. Altitude decompression sickness: hyperbaric therapy results in 528 cases. Aviat Space Environ Med 1990; 61: 833-6
7. Pilmanis AA, Petropoulos LJ, Kannan N, Webb JT. Decompression sickness risk model: development and validation by 150 prospective hypobaric exposures. Aviat Space Environ Med 2004; 75:749–759.

Acknowledgement  Image courtesy Wikimedia Commons

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