Sodium depletion and hemoconcentration: overlooked complications in patients with anorexia nervosa?

Objective: This study evaluated sodium and fluid balance disturbances in anorexia nervosa.

Methods: Serum urea, creatinine, urea nitrogen/creatinine ratio, protein concentration, osmolality, electrolytes, hemoglobin, and hematocrit were evaluated on admission, after intravenous fluid supplementation, and at discharge in 14 patients who had anorexia nervosa and were admitted to a medical unit for severe malnutrition and/or medical complications. Diet history and physical signs of salt and water depletion were also evaluated on admission. The diagnosis of plasma volume depletion was considered confirmed if serum urea nitrogen/creatinine ratio was higher than 25.

Results: Mean body mass indexes were 12.9 +/- 2.4 kg/m2 on admission and 14.2 +/- 2.0 kg/m2 at discharge. Mean duration of hospitalization was 24.2 +/- 11.9 d. Plasma volume depletion was found on admission in nine patients (64%), and hyponatremia was associated with hypovolemia (hypovolemic hyponatremia) in three of them. Most of the commonly used indexes of hemoconcentration (hematocrit, hemoglobin, serum urea, and creatinine) were normal on admission but decreased significantly after intravenous fluid supplementation and were maintained at lower levels at discharge (P < 0.001, P < 0.0004, P < 0.009, and P < 0.001, respectively, compared with admission values). Hemoconcentration masked an underlying anemia (hemoglobin < 12 g/dL) in six patients. Only two of the nine patients with plasma volume depletion had a history of diuretic abuse, and the other seven patients reported long-term sodium restriction as a method to control body weight.

Conclusions: Plasma volume depletion and hypovolemic hyponatremia are common in the most severely malnourished patients with anorexia nervosa. Clinicians should be aware that, because of malnutrition, the common indexes of hemoconcentration may be within normal range and that hemoconcentration may mask anemia. Further studies are needed to assess the role of neglected behaviors such as sodium and fluid restriction.