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Blood glucose awareness training as a tool for the return of insulin treated aviators to the cockpit
Alon Grossman M.D1, Erez Barenboim M.D1, Ayal Romem M.D2, Bella Azaria M.D1, Yaniv Sherer M.D1, Liav Goldstein M.D2, Ohad Cohen M.D3.
1. The Israel air force aeromedical center, Tel Hashomer, Israel
2. The Israel air force surgeon general’s headquarters, Tel Hashomer, Israel
3. Institute of endocrinology and metabolism, Sheba Medical Center, Tel Hashomer, Israel
Corresponding author:
Running title: Glucose awareness training in aviators
Word count in abstract: 166
Word count in manuscript: 1866
Abstract
Insulin treated diabetes mellitus has traditionally been considered disqualifying for aviation duties, the major concern being the risk of hypoglycemia. This phenomenon may lead to impaired judgment and even loss of consciousness, potentially leading to a mishap. Blood glucose awareness training has been advanced as a strategy to avoid hypoglycemia by teaching individuals to use the appearance of autonomic and neuroglycopenic symptoms as indicators of decreasing blood glucose levels. We present two military aviators with newly diagnosed type 1 diabetes mellitus, who were returned to flying duty in a multi-crew aircraft. Blood glucose awareness training was used as a tool for the education of these aviators regarding the early signs of hypoglycemia in order to avoid development of more significant hypoglycemia. These cases attest to the importance of blood glucose awareness training in the return of diabetic patients to the cockpit.
Key words: diabetes mellitus, aviators, aerospace medicine, hypoglycemia
Introduction
Diabetes mellitus (DM) was found to be one of the major 5 causes for permanent disqualification from flying duties in a study performed among USAF pilots and navigators that were permanently disqualified from flying duties in the years 1995-19991. The incidence of this disease has been estimated at 0.47 cases per 1000 aviator-years per year and a study published in 1995 found that 78% of diabetic aviators were removed from flying duties2. Our growing understanding of type 1 diabetes mellitus has permitted the identification of subgroups of diabetics with an acceptable aeromedical risk and therefore a greater proportion of aviators were returned to restricted flying duties in recent years3. Yet, fear of hypoglycemia in insulin treated diabetics has lead to a general reluctance for the return of such aviators to the cockpit, as this phenomenon has the potential to cause variable symptoms that may lead to in-flight mishaps. In this regard, hypoglycemia is very similar to hypoxia, as both conditions may develop in flight, lead to variable symptoms with great individual variability and may result in in-flight incapacitation. Altitude chamber training has been devised in order to expose aviators to controlled hypoxia and teach them its early signs, thus avoiding more ominous complications of this dreaded phenomenon. By using the insulin clamp study to induce controlled hypoglycemia in diabetics, we attempted to teach the early signs of hypoglycemia to diabetic aviators who were returned to the cockpit. Two such cases are presented and discussed in detail.
Hyperinsulinemic clamp technique
After an overnight fast, insulin treatment was withheld. Subjects were admitted in the morning and two intravenous catheters were placed using a local anesthetic cream. The first catheter, for infusion of glucose and insulin, was inserted in a large vein in the non-dominant hand. The second, for sampling of arterialized venous blood, was inserted retrograde into the distal wrist or a hand vein of the same arm and kept patent with a slow saline infusion. At the beginning of the study, a primed continuous infusion of regular insulin (Human Actrapid, Novo Nordisk) was started at a maintenance rate of 1.5mU/kg/min, and infused for 180 minutes. Glucose was maintained at 90 mg% for 10 minutes, and then reduced in successive steps to 70, 60 and 50 mg%, before being restored to 90 mg% after 140 minutes. Each glucose plateau lasted 25 minutes in total. At the end of each study, subjects received lunch. Glucose monitoring was continued until stable euglycemia was maintained, after which all intravenous lines were withdrawn.
Case reports
Patient A
A 29-year-old C-130 navigator presented to the IAF aeromedical center on April 1999 with complaints of polyuria, nocturia and polydipsia. Blood glucose was 323mg% and glycosylated hemoglobin (HgbA1C) was 8.5%. Type 1 DM was diagnosed and treatment with insulin Lispro (Humalog) and NPH was started. Insulin Lispro was later replaced with Glargine insulin in an attempt to further minimize the chances of hypoglycemia. Upon follow-up, his diabetes was well controlled with HgbA1c values in the range of 6-7.9%. Annual ophthalmologic examination and microalbumin were within normal limits. Two hypoglycemic insulin clamp studies were performed (figure 1). In the first study, paresthesias appeared after 70 minutes, when glucose level was measured at 94mg/dL. The patient reported a feeling of fatigue after 145 minutes, when plasma glucose measured 71 mg/dL. During a second study, he reported a ‘funny feeling’ at a glucose level of 115 mg/dL. He learned that the earliest symptoms of his hypoglycemia were paresthesias and a ’funny feeling’. He was returned to flight in a multi-crew aircraft (C-130 as a navigator) under limiting conditions. He was required to measure his blood glucose levels prior to every flight and during flight on long distance missions and to be equipped during flight with a glucometer, a glucagon vial and glucose oral tablets. A diabetic control team including an endocrinologist and his attending flight surgeon has followed him. No hypoglycemic episodes that required exogenous treatment, and no in-flight episodes.
Patient B
A 28-year-old Apache AH pilot presented to the IAF aeromedical center on March 2001 with complaints of polyuria, polydipsia and weight loss. Glycosylated hemoglobin values were 14.48% and fasting glucose was 330 mg%. Anti GAD antibodies were positive and he was diagnosed with type 1 DM. Insulin reserve was evaluated through C-peptide measurement following glucagon administration; C-peptide rose to 1.8 micromole/liter, attesting to a low insulin reserve. Treatment with insulin lispro (Humalog) and NPH was begun. Insulin Lispro was later replaced with Glargine insulin in an attempt to further minimize the chances of hypoglycemia. Statin therapy was instituted due to dyslipidemia. During follow-up, his glycosylated hemoglobin values were in the range of 6-8.5%. Annual ophthalmologic examination and microalbumin testing were within normal limits. Hypoglycemic insulin clamp study was performed (figure 2). He reported hunger after 100 minutes, when his plasma glucose level was 91.2 mg/dL. After 160 minutes, at a plasma glucose level of 57 mg/dL, he reported dizziness, after which his blood sugar dropped to 35 mg/dL. He learned that hunger was the preliminary symptom of impending hypoglycemia. He was returned to flight in a multi-crew aircraft (Apache AH which is operated by two crewmen) under limiting conditions. He was required to measure his blood glucose levels prior to every flight and to be equipped during the flight with a glucometer, a glucagon vial and glucose oral tablets. He has been followed for 3 years with no hypoglycemic episodes requiring exogenous treatment, and no episodes occurring in flight.
Discussion
One of the major aeromedical concerns in insulin treated diabetics is the potential risk of hypoglycemia. Hypoglycemia results in cognitive impairment that may manifest as emotional changes, visual impairment or auditory dysfunction4-6. A study performed in diabetic subjects revealed that these patients were more prone to motor vehicle accidents, depending on the method of insulin treatment (higher among those on intensive insulin therapy as compared with pump therapy), the frequency of blood glucose monitoring before driving and the rate of hypoglycemic episodes 7. Another study revealed that a narrow window exists between a patient's decision to treat self-detected hypoglycemia and the appearance of neuroglycopenia that may lead to driving impairment8. It is thus not surprising that the decision to return insulin treated diabetics to the cockpit relies to a large part on the risk of hypoglycemia. Features that characterize a low risk of hypoglycemia include an educated awareness of the disease with an established pattern of stable glycemic control based on sustained compliance to a diabetic diet and exercise program, awareness of hypoglycemia and lack of previous hypoglycemic episodes. Intensive insulin therapy in type 1 diabetes decreases the rate of long-term diabetes-related complications, but increases the risk of hypoglycemia. Yet, preservation of endogenous insulin secretion as measured by C-peptide levels, which may be achieved through intensive insulin therapy, decreases the risk of future severe hypoglycemia9. In aviators, where the consequences of hypoglycemia may be particularly hazardous, it is important to balance between the need for strict blood glucose control through intensive insulin therapy and the need to avoid hypoglycemic episodes.
Neither adults nor children are particularly adept at identifying hypoglycemia-associated symptoms or estimating blood glucose values reliably10. One early study noted that 76% of adults with type 1 diabetes overestimated their blood glucose values during experimentally induced hypoglycemia11. Gold et al found that insulin-treated diabetics with hypoglycemia unawareness exhibited more profound cognitive dysfunction during acute hypoglycemia that persisted for longer periods following blood glucose recovery than non-diabetics12. These studies indicate that many diabetic subjects fail to estimate their glucose levels accurately and are thus at risk for severe complications from this phenomenon. Patients who develop hypoglycemia unawareness are also more prone to develop recurrent episodes of hypoglycemia, being increasingly more severe. Avoidance of hypoglycemia restores hypoglycemia unawareness even in those with established unawareness, primarily by increasing beta-adrenergic sensitivity13. In addition, military aviation is a task that certainly increases metabolic demands and increases the chances of hypoglycemia, as has been demonstrated in drivers14. It is thus clear that insulin treated diabetics face a significant risk of in-flight hypoglycemia, especially those who are well controlled. This phenomenon may affect the aviator’s cognitive performance and may lead to in-flight incapacitation. Every effort should thus be made to recognize those patients at high risk of developing hypoglycemia and to educate those returning to the cockpit of the early signs of this phenomenon in order to minimize the chances of in-flight incapacitation.
The insulin infusion test is a safe and reproducible method and can reliably predict which patients with type 1 diabetes are at risk of developing severe hypoglycemia during intensive insulin therapy15. This test also exposes the patient to controlled hypoglycemia, thus educating him to recognize its early symptoms and can aid in the decision to return a certain aviator to the cockpit. Although this test cannot eliminate the risk of hypoglycemia, it is capable of decreasing the frequency and severity of such episodes. Such a program exists for the early recognition of hypoxia, another dreaded complication of modern aviation16. This training program exposes the aviators to gradual hypoxia in an altitude cabin and teaches them to recognize the early signs of this phenomenon in order to avoid persistent hypoxia that may lead to loss of consciousness.
Blood glucose awareness training has been advanced as a strategy to avoid hypoglycemia by teaching individuals to use the appearance of autonomic and neuroglycopenic symptoms as indicators of decreasing blood glucose levels17. The insulin-glucose clamp study is certainly a major tool in this aspect, as it enables the clinician to gradually lower the subject’s blood glucose levels and monitor the appearance of symptoms at various glucose levels. The patient will learn to recognize the earliest signs and symptoms of hypoglycemia and respond appropriately, so preventing a further decrement in blood glucose values. The competence of an individual to decrease his risk of severe hypoglycemia after participation in a formal blood glucose awareness-training program is variable, depending partially on psychological factors18. These patients may be returned to the cockpit with greater assurance, as their ability to recognize early signs of hypoglycemia may aid them in avoiding in-flight incapacitation. We believe that these aviators should be permitted to flight with a co-pilot as the risk of hypoglycemia is significantly lower than in untrained individuals, but is still certainly higher than in the general population. We also believe that since the use of a blood glucose awareness program does not eliminate the risk of in-flight hypoglycemia, all those returned to the cockpit should measure their blood glucose levels prior to flight and during flight, particularly during long flights.
Conclusions
Insulin treated diabetics may be returned to the cockpit in a multi-crew aircraft provided they are aware of the risks of hypoglycemia and are able to recognize this complication at an early stage, so as to avoid in flight incapacitation. This may be achieved through a formal blood glucose awareness-training program, such as an insulin glucose clamp study. This program follows similar principles as an altitude chamber-training program performed in most aviators worldwide. Future developments such as continuous blood glucose monitoring and closed loop insulin delivery systems may enable aeromedical examiners to further eliminate present restrictions placed on diabetic aviators.
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