What is Sickle Cell Anemia?
This is an inherited condition of people of mostly African heritage in which there are abnormal configurations of the red blood cells that appear as sickles instead of doughnuts. They tend to clump in vessels causing tissue infarcts and anemia, and are triggered by hypoxic conditions causing what is called ‘a crisis’, during which the person has severe pain and incapacity. The condition can complicate the management of serious decompression illness and can be another cause of aseptic necrosis of bone.
What is sickle cell trait?
Sickle cell trait is a person who carries one sickle hemoglobin producing gene inherited from their parents and one normal hemoglobin gene. Normal hemoglobin is called type A. Sickle hemoglobin called S. Sickle cell trait is the presence of hemoglobin AS on the hemoglobin electrophoresis. This will NOT cause sickle cell disease.
Recommendations to divers and doctors certifying prospective divers concerning sickle cell illness have been under close scrutiny and these recommendations are changing. Whether or not to require mandatory testing for prospective divers is a question that arises in in the area of sports diving. Dr. Chris Edge, chair of a committee in the UK that writes the rules states emphatically that “The BSAC medical protocol does NOT require a mandatory sickle cell test. The only time that BSAC would require a mandatory sickle cell test would be when the subject was of Afro-Caribbean origin or there was a history of sickle cell disease in the family”.
Dr. Nick McIver has summarized this debate:
“The latest HSE ( UK) guideline in the current Consultative Document being circulated. still mentions initial candidate Sickle cell test but does not indicate whether or not this will be mandatory. It was being debated at the last revision of HSE MA1 medical standards and guidelines in 1986. The advice then was that it would be most inadvisable for individuals with Sickle cell trait to dive. At that time the controversy centred around 3 morbidity factors:
1. The risks of Sickle cell anemia tended to be extended unreasonably to those with Sickle Cell trait.
2. The classification of those of those with HbS did not always include Hb electrophoresis and was “haphazard”.
3. There was a variable HbS/HbA ratio in those with Sickle cell trait, and the arterial pO2 level below which sickling will occur was not constant. There then followed a number of statements regarding electrophoretically proven sickle cell trait individuals:
- Visceral infarcts had been shown to occur, e.g., renal medulla, the spleen and the lung.
- Unquantified physical stress could cause significant sickling.
- Some individuals might be more susceptible to irreversible hypoxic collapse.
- Most sickle cell trait individuals could expect to have splenic infarcts at altitude of 15000ft.
- Sickle cell trait individuals were probably at greater risk of experiencing visceral infarcts in hypoxic conditions and where bubbles were forming in the vascular bed; probably not at greater risk than normal under normoxic conditions at sea level.
- In hypoxic conditions the HbS trait individual was at greater risk of experiencing thrombotic lesions.
- Neurological abnormalities secondary to thrombotic episodes had been reported.
- Pulmonary steady state diffusing capacity had been shown to be reduced in HbS trait individuals.
- Avascular necrosis of bone had been associated with the HbS trait. As diving could include posssibility of hypoxia, cold, acidosis, heavy exertion, and bubble formation with or without clinical DCI, it seemed prudent to exclude personnel with sickle cell trait from Navy diving.
1. Ref: Risberg J: Hematology in Ed. DH Elliott. Medical Assessment of Fttness to dive. Biomedical Seminars 1994. pp 190-196. He spoke at the Edinburgh Conference in 1994 which was published. He cites 2 reports which had reviewed the available literature and examined the effects on exercise capacity:
A. D Gozal et al 1992. Effect of different modalities of exercise and recovery on exercise performance in subjects with Sickle cell trait. Med. Sci. Sports Exerc. 1992; 24:1325-1331.
B. VM Voge et al 1991. Sickle cell anemia trait in the military aircrew population: A report from the military aviation safety sub-committee of the Aviation Safety Committee, AsMA. Aviat. Space Environ. Med 1991: 62:1099-1102. Dr Risberg differentiates between the obvious frank Sickle cell anemia homozygoteHb-S disorder which is incompatabile with diving due to multi-organ malfunction and crises. He cites the 2 studies as clearly indicating there is no reason to reject a diver candidate with Sickle cell trait from diving. There is no scientific reason for putting 40% Hb-S level as a cut-off level. He states the US Air Force currently (in 1994) allowed pilots with sickle cell trait without any restriction.
There is one further relevant paper which has come to light recently: CW Caldwell. The Sickle Cell Trait: A rebuttal. Military Medicine: Vol 149; March 1984: 125-129.
In conclusion for commercial divers: It would seem logical to suggest that testing should be left to the discretion of the examining doctor for commercial diving candidates to be performed at initial examination only if the subject were of Afro-Caribbean origin or there were a history of Sickle cell disease in the family . The level of Hb-S as well as the ratio of Hb-S to Hb-A should be recorded also as an on-going audit , for example, to be monitored by the HSE(UK), and any event whatever recorded.