Hyponatremia associated with overhydration in U.S. Army trainees

Hyponatremia associated with overhydration in U.S. Army trainees

O’Brien, Karen K

This report describes a series of hyponatremia hospitalizations associated with heat-related injuries and apparent overhydration. Data from the U.S. Army Inpatient Data System were used to identify all hospitalizations for hyposmolality/ hyponatremia from 1996 and 1997. Admissions were considered as probable cases of overhydration hyponatremia if this was the only, or primary, diagnosis or if it was associated with any heat-related diagnosis. Seventeen medical records were identified, and the events leading to hospitalization were analyzed. The average serum sodium level was 122 +/- 5 mmol/L (range, 115-130 mmol/L). All 17 patients were soldiers attending training schools. Seventy-seven percent of hyponatremia cases occurred in the first 4 weeks of training. Nine patients had water intake rates equal to or exceeding 2 quarts per hour. Most patients were in good health before developing hyponatremia. The most common symptoms were mental status changes (88%), emesis (65%), nausea (53%), and seizures (31%). In 5 of 6 cases in which extensive history was known, soldiers drank excess amounts of water before developing symptoms and as part of field treatment. The authors conclude that hyponatremia resulted from too aggressive fluid replacement practices for soldiers in training status. The fluid replacement policy was revised with consideration given to both climatic heat stress and physical activity levels. Field medical policy should recognize the possibility of overhydration. Specific evacuation criteria should be established for exertional illness.


Hydration status is an important factor influencing the physical performance and health of soldiers during hot weather operations.1,2 Body weight loss during exertion is directly proportional to loss of total body water. Body water deficits as minimal as 2% of body weight can impair physical performance, and greater water deficits can result in exertional heat illness.3 In addition, moderate dehydration (4-5% of body weight) negates the thermoregulatory advantages conferred by high aerobic fitness and heat acclimation.3 The military threat of dehydration was demonstrated by the estimated 20,000 Egyptian deaths attributed to water deficit and heat illness during the 1967 Six-Day War.4 In contrast to the Egyptians, Israeli troops, who stressed rehydration, had minimal heat casualties. Emphasis by military commanders on “forced rehydration” is believed to be associated with the high operational effectiveness and low rates of serious heat injuries during Operation Desert Storm.

During hot weather operations, an individual soldier’s daily water requirements to sustain full hydration can range from 0.2 to 2 L/h (for short periods) to 3 to 12 L/d.5 Sweating rate is dependent on physical activity level, weather conditions, clothing worn, and physical sizes6-8 Although voluntary intake of 3 L/d is easily achieved in both military garrison environments and during deployment, 12 L/d requires emphasis by leaders, because individuals will not drink such large volumes voluntarily.5

The term hyponatremia (serum sodium

In July 1997, a large U.S. Army training post in the southeast reported five cases of hyponatremia during the month, one case ending in death.12 This death prompted the review of this and other cases of hyponatremia in soldiers. The purpose of this paper is to describe a large case series of hyponatremia hospitalizations associated with heat injuries occurring at U.S. Army training installations during 1996 and 1997 and the behaviors leading to overhydration.


This study involved a retrospective review of medical records. Data from the U.S. Army Inpatient Data System were used to identify all hospitalizations for hyposmolality/hyponatremia from 1996 and 1997. A case of suspected overhydration hyponatremia was defined as a hospitalization with the International Classification of Diseases, 9th Revision (ICD-9) code 276.1: “hyposmolality and/or hyponatremia.” Admissions were considered as probable cases of overhydration hyponatremia if this was the only or the primary diagnosis, if it was associated with any heat-associated diagnosis (ICD-9 code 92.x), or if it was associated with a diagnosis of fluid overload (ICD-9 code 276.6). All available inpatient records of overhydration hyponatremia cases from Fort Benning, Georgia, plus one case from Fort Jackson, South Carolina, and one case from Parris Island, South Carolina, were then analyzed. When information was not available in the inpatient record, the individual was contacted and questioned. Final criteria for a diagnosis of overhydration hyponatremia were a symptomatic, hospitalized soldier with a sodium level

The data were analyzed using frequency tables to examine patterns and trends in terms of person, place, and time. A total of 17 hospital records for hospitalized soldiers met the criteria, including six detailed case studies from Fort Benning, Fort Jackson, and Parris Island.


The cases occurred predominantly during training in hot weather. Sixteen of 17 cases occurred from June to October. For 11 of 13 cases for which daily maximal WBGT temperatures were known, the WBGT exceeded 78 deg F (Fig. 1). All cases occurred in the trainee population. For 10 of 14 cases for which time in training was known, hyponatremia occurred during the first 4 weeks of training. Most soldiers appeared to have been in good health before developing hyponatremia. Six individuals, however, reported illness on the day of or the days preceding the onset of symptoms, which included nausea (N = 4), vomiting and/or diarrhea (N = 3), and upper respiratory illness (N = 1).

Serum sodium levels at presentation averaged 122 +/- 5 mmol/L (range, 115 to 130 mmol/L) and are presented in Figure 2. Serum potassium and chloride averaged 3.6 +/- 0.5 and 90.6 +/5.6 mmol/L, respectively. Plasma creative kinase was increased in all cases for which this value was available (N = 14), averaging 2,019 +/- 2,594 units/L. Serum creatinine values averaged 94.6 +/- 26.5 (mu)mol/L (1.07 +/- 0.3 mg/dL).

Table I presents water intake, WBGT, and physical activities for the individual soldiers. The soldiers’ self-reported hourly water intake exceeded 2 Qt/h in 9 of 13 cases. Seven of the soldiers were performing moderate to hard work during the hours preceding the onset of symptoms, and five were performing light work (e.g., marksmanship training, infantry skill training).

Figure 3 summarizes the presenting symptoms. Fifteen of 17 patients (88%) presented with central nervous system (CNS) dysfunction. Of those, 47% were characterized as disoriented and 13% as confused. Other CNS symptoms included dizziness, combativeness, uncooperativeness, incoherent speech, and obtundedness. Emesis and nausea were the next most common symptoms, occurring in 65% and 53% of cases, respectively. Seizures occurred in 35% of cases. Muscle fatigue, muscle cramps, and headache occurred in less than 30% of cases. Sixteen of 17 soldiers recovered without sequelae after hospital treatment. One soldier died secondary to cerebral edema.

Case reports were available for six soldiers. The cases described here provide insight into the mitigating factors possibly contributing to overhydration hyponatremia.

Case 1

In early July, a male trainee was admitted to the hospital with acute onset of rapidly progressing weakness that led to unresponsiveness. On the morning of admission, he moved with his unit to the rifle range. As the day progressed, the heat category increased to level 5 (WBGT 119 deg F). At mid morning, he complained of light-headedness and nausea, and he vomited. His supervisors suspected that he was suffering a heat injury. They moved him to the shade, loosened his clothing, and instructed him to drink 1 Qt of water every 30 minutes, as specified in the regulations. He began to vomit repeatedly. Unit members continued to encourage oral rehydration. The soldier was placed in the shade, and during the next 2 hours he attempted to drink 10 Qt of water while vomiting repeatedly. By mid afternoon, he was obtunded and was transported to the hospital. Medics administered normal saline intravenously during transport. The soldier arrived at the emergency department in respiratory distress. He did not have hyperpyrexia. Initial laboratory tests revealed a serum sodium of 121 mmol/L. A chest radiograph revealed diffuse pulmonary edema. Computed tomography revealed dilated lateral and third ventricles and edematous changes in the pons, with obliteration of the pons cistern. Despite intensive medical care, the soldier did not regain consciousness. A postmortem examination documented severe cerebral and brainstem edema and hydrocephalus.

Case 2

During mid July, a male trainee was hospitalized with hyponatremia and generalized seizures. On the day of admission, he was engaged in vigorous physical activity while consuming approximately 2 Qt of water per hour. In the afternoon, he complained of nausea and was instructed to drink more water. He consumed 7 Qt of water in a short period. His nausea worsened, and he vomited. The soldier returned to the barracks, where he developed seizures. He remained in status epilepticus during transport to the hospital. Initial laboratory tests indicated a serum sodium of 124 mmol/L. After 4 days of hospitalization, he was discharged.

Case 3

In mid July, a male soldier attending a leadership course was hospitalized after suffering a seizure. On the day of hospitalization, he consumed large quantities of water to prevent heat symptoms. As the day progressed, he developed light-headedness and weakness. He presented to the troop clinic and was transported to the hospital. Upon arrival, he was alert and oriented, but soon thereafter he had a generalized seizure. Laboratory results revealed a serum sodium of 127 mmol/L, potassium of 4.3 mmol/L, and magnesium of 0.53 mmol/L (1.3 mg/ dL). Glucose, blood urea nitrogen, and creatinine were within normal ranges. Arterial blood gases were unremarkable. Computed tomography of the head and a chest radiograph were normal. Magnetic resonance imaging was normal, and electroencephalographic results were consistent with metabolic encephalopathy. After several days, the patients electrolytes gradually normalized, and he was discharged without residual neurological deficits.

Case 4

During late July, a female trainee was hospitalized with headache, nausea, vomiting, and fatigue. On the afternoon of admission, she began to vomit and was instructed to drink more water. She consumed approximately 10 to 12 Qt of water during the next 1.5 hours. It was estimated that she consumed 18 to 20 Qt of water during the 8 hours before hospitalization. Initial laboratory tests indicated a serum sodium of 121 mmol/L. She was discharged on the third hospital day.

Case 5

During mid August, a male trainee was hospitalized for nausea, dizziness, and generalized seizures. On the day of admission, his unit trained at the rifle range as the heat index increased to category 4 (WBGT 88 to 89 deg F). After lunch, he consumed approximately 10 Qt of water during a 4-hour period. Later in the afternoon, he became tired, disoriented, and nauseated. After he vomited, he was taken to the hospital. The soldier experienced a generalized seizure. Initial laboratory tests indicated a serum sodium of 123 mmol/L. After several days of hospitalization, he was discharged.

Case 6

During early January, a male trainee was hospitalized after developing weakness and blurred vision during a road march. The patient stated that he felt fine until that morning. On the day of admission, he marched 11.2 km in 56 deg F, 46% relative humidity while drinking approximately 1 Qt of water per hour. He reported that his uniform was saturated with sweat. After discontinuing exercise because of weakness, he drank 3.7 Qt of water in 30 minutes and developed blurred vision and a bloated feeling. The patient’s initial serum sodium was 128 mmol/L.


Soldiers, like civilian agriculture workers and mine workers, must perform extended strenuous work in oppressively hot conditions. One potential outcome is the development of exertional heat injury resulting from extended hard work and/or from not following guidelines for working in hot weather. In the Army training environment, personnel currently have guidance’3-15 for work time and water replacement that should minimize the likelihood of heat injuries. These recommendations are summarized in Table II. The soldier is also trained to know the common symptoms of heat exhaustion, heat cramps, and heat stroke. If a heat injury is suspected, supervisors are responsible for immediate first aid and notification of emergency medical personnel. When large numbers of persons are routinely exposed to these conditions, there may be a potential to misdiagnose a case of hyponatremia as heat injury.

Previous reports on physically active populations indicate that hyponatremia can arise from overconsumption of fluids in response to fluid replacement needs,10-12,16-18 and this appears to have been the case with the patients studied here. Four factors appeared to precipitate the development of hyponatremia in these patients: (a) fluid replacement practices that were too aggressive for the training situations; (b) lack of leadership awareness that overhydration can have medical consequences; (c) supervisory personnel treating all heat-related illnesses as being mediated by dehydration; and (d) lack of standardized evacuation criteria for persons with suspected heat illnesses.

The U.S. Army guidelines in place when these hospitalizations occurred were Technical Bulletin Medical 507, “Prevention, Treatment and Control of Heat Injury,” and Field Manual 21-10, “Field Hygiene and Sanitation Manual.”13-14 These manuals recommend fluid replacement based on ambient weather conditions, ranging from 0.5 Qt/h in mild heat stress (WBGT 78– 81.9 deg F) to more than 2 Qt/h during periods of heat category 5 conditions (WBGT > 90 deg F). These guidelines were designed to emphasize the importance of adequate fluid intake, with the assumption that overdrinking would be balanced by increased urine output. However, little consideration was given to possible overhydration and the development of water intoxication.

Soldiers training in hot weather can consume large volumes of water over many hours in the absence of eating and still maintain electrolyte balance.7,8 Dilutional hyponatremia can occur only if the individual persistently and extensively overdrinks relative to sweat and urine losses. Assuming a 70-kg soldier with an extracellular fluid (ECF) volume of 14 L (20% of body weight) and a serum sodium concentration of 140 mmol/L, the total ECF sodium content would be 1,960 mmol (140 mmol x 14 L). To dilute serum sodium from 140 to 125 mmol/L, the ECF would need to increase to approximately 15.7 L (1,960 mmol 125 mmol/L). Assuming that ECF and total body water (TBW) increase in proportion as a result of osmotic equilibrium, TBW would need to increase approximately 5.1 L (X/42 L = 15.7 L/ 14 L). However, because soldiers exercising in hot climates lose sodium in sweat, less overhydration is actually required to reduce plasma sodium to 125 mmol/L. If sweat losses total 6 L and sweat sodium concentration is 25 mmol/L^sup 2^, this 150-mmol deficit would reduce sodium to 1,810 mmol (1960 mmol – 150 mmol). If the 6-L water loss is replaced by water consumption, the deficit is spread equally in the body to maintain osmotic balance. The 150-mmol total loss results in a loss of 3.6 mmol/L (150 mmol/L / 42 L). The new serum sodium thus becomes 136 mmol/L. To dilute serum sodium from 136 to 125 mmol/L while maintaining osmotic balance, the TBW would have to increase to 47.5 L (X/42 L = 135/125 mmol/L), which would be achieved by 3.7 L of water intake. These calculated values agree with changes in serum sodium measured during water hyperhydration experiments.18 In addition, because urine output averages approximately 12 mL/min at rest and approximately 3 mL/min during exercise,19 considerable and persistent overdrinking must have occurred to increase TBW sufficiently to induce hyponatremia.

Examination of individual cases indicates that high fluid replacement rates contributed to the hyponatremia hospitalizations. At least 9 of the 17 patients were drinking 2 or more Qt of fluid per hour before the development of symptoms. This rate of water consumption was approximately twice the predicted hourly sweating rate for the physical activity and weather conditions.6,7 This amount of excess fluid would be expected to reduce sodium to less than 125 mmol/L during a 4- to 6-hour training period if urine production did not increase substantially to compensate for the excess water intake. Both heat stress and physical exercise reduce renal blood flow and urine output.2

Another possibility is that some of these persons had excessive sweat sodium losses. Unacclimated persons2 and individuals with cystic fibrosis24 can have very high sweat sodium concentrations (50-70 mmol/L) and would incur a greater hyponatremia for a given hyperhydration level. These individuals had been in training for only a few weeks, and it is possible that they were not fully acclimated. It is possible that some people may have a genetic predisposition that prevents the dilution of sweat that occurs with acclimatization. Although excessive salt loss in sweat and urine may have contributed to the onset of hyponatremia symptoms, the excessive amount of water consumed could have produced the effect without such losses.

Excessive water intake as part of field medical management also appeared to contribute to the development of symptomatic hyponatremia. In three of the six case studies available, the presenting symptoms were interpreted as dehydration-mediated heat exhaustion and treated by administration of additional water. Records obtained suggest that the affected soldiers consumed 7 to 20 Qt between the onset of symptoms and arrival at a medical facility. In the case that eventually proved fatal, this treatment regimen continued despite the persistence of nausea and emesis. In addition, if excessive water consumption induced either nausea or emesis (as in more than half of the cases), plasma vasopressin levels would increase25 and reduce urine output.

Changes in Training Guidance

The practice of overdrinking has likely evolved from the emphasis by commanders on prevention of dehydration without considering that sufficient overhydration might occur and have potentially severe medical consequences. Since the outbreak occurred, several actions have been taken to reduce the likelihood of additional cases. First, water replacement guidelines were revised, and water guidance based on both physical activity and weather conditions (rather than on weather conditions alone) were implemented throughout the Army.15 Second, Fort Benning modified its evacuation criteria to expedite the removal of suspected heat casualties from the training environment. Finally, we recommend that military personnel be educated regarding the adverse consequences of overdrinking.

The authors developed fluid replacement guidelines designed to reduce the incidence of overdrinking and underdrinking during hot weather training. The procedures used to produce these guidelines and subsequent validation are presented elsewhere. 15 Briefly, the revised guidelines set the hourly rate of fluid intake based on the heat category and the level of physical activity. Because sweating rate increases with increasing intensity of exercise as well as by heat category, such recommendations provide more reasonable guidance than recommendations based on the heat category alone.

Fort Benning Modified Field Evaluation/Evacuation Procedures

Our experience with hundreds of simple heat exhaustion cases is that patients show rapid improvement after being removed from heat stress and given 1 to 2 Qt to rehydrate. If marked improvement does not occur within 1 hour and with 2 quarts of fluid, a soldier is evacuated for clinical evaluation. Because many of the cases involved repeated vomiting, soldiers who experience more than one episode of emesis are likewise evacuated.

Given the command emphasis on fluid replacement and the consistent overconsumption of water during training in these cases, an unanswered question is why there are so few cases of symptomatic hyponatremia among trainees. Are these soldiers drinking greater volumes than their fellow trainees, or are their peers able to tolerate the excess fluid intake without developing symptoms? It is clear from the water intake values presented that the individuals presenting with symptoms had the capacity to absorb fluids at high rates for sustained periods. Whether they were able to produce free water and diurese as effectively as their peers cannot be determined from the data. It is also possible that renal function was temporarily compromised. In each of the cases, plasma creatine phosphokinase was increased, suggesting ongoing protein degradation. If kidney function was altered as a result of handling the protein load, it may have produced a situation in which less sodium was reabsorbed and less free water was produced compared with what would occur normally with the same volume overload.

Distinguishing between persons suffering from heat exhaustion with or without dehydration and persons suffering from symptomatic hyponatremia is difficult in the field because of the common symptoms of the conditions, i.e., nausea, emesis, fatigue, headache, and CNS dysfunction. One presenting symptom that stands out from the data set examined, however, is the high incidence of emesis in persons with hyponatremia versus persons with heat exhaustion from other causes. When combined with their rehydration history, urine output, presence of abdominal distention, and speed of recovery, it should be possible for personnel to make a more accurate diagnosis. Better educated supervisory personnel and standardized evacuation procedures likely would have prevented the soldier’s death reported here (case 1) and eliminated most hospitalizations for hyponatremia.

In summary, we have described a recent outbreak of hyponatremia cases within the U.S. Army. Evaluation of the individual cases revealed that the hyponatremia arose consequent to overhydration during military training exercises. To remedy this situation, revised fluid replacement recommendations were implemented, as were more defined criteria for medical evacuation. These findings emphasize the potential danger of excessive water consumption to prevent or treat heat injuries.


The authors acknowledge the invaluable assistance given by MAJ Thomas Garigan, LTC John V. Barson, LTC Rose Marie Hendrix, COL Howard Cushner, and Dr. John Brundage, MD.


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Guarantor: MAJ Karen K. O’Brien, MC USA

Contributors: MAJ Karen K. O’Brien, MC USA; Scott J. Montain, PhD; MAJ William P. Corr, MC USA; Michael N. Sawka, PhD; Joseph J. Knapik, ScD; COL Stephen C. Craig, MC USA

Department of Family Practice and Community Medicine, Martin Army Community Hospital, Fort Benning, GA 31905.

This manuscript was received for review in January 2000. The revised manuscript was accepted for publication in September 2000.

Copyright Association of Military Surgeons of the United States May 2001

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