Hyponatremia occurs when the blood contains too little sodium. Common causes of hyponatremia are a sodium-deficient diet, excessive water intake, or purging. Urine sodium-to-chloride ratios of >1.6 in combination with hypokalemia may indicate vomiting, while ratios <0.7 often indicate laxative use. Mild cases are often treatable through cessation of purging and oral hydration, but in serum sodium levels are <125mmol/L, the patient should be hospitalized and given saline, on a slow drip of 4-6mmol/L or less per 24hr to prevent central pontine myelinolysis. If serum sodium levels are <118mmol/L, intake into the intensive care unit with renal consultation should occur to determine if desmopressin, an antidiuretic, is required to restore sodium levels. 

Hyponatremia in Eating Disorder Patients

Hypokalemia occurs when the blood contains too little potassium. Common causes of hypokalemia are a potassium-deficient diet or purging of ingested food. Urine potassium-to-creatinine ratios of <13 are often indicative that hypokalemia is resultant from eating disorder behaviors. If serum potassium levels are 2.5 mmol/L or more and no symptoms of hypokalemia are present, it can be treated with oral potassium supplements and purging cessation in bulimia nervosa patients. However, if serum potassium levels are <2.5mmol/L, potassium supplementation must be provided both orally and intravenously in a saline solution drip at no greater than 50-75mL/hr to restore intravascular volume while preventing hyperkalemia. Severe hypokalemia can lead to QT prolongation, TdP, other arrhythmias, and rarely, acute renal failure via hypokalemic nephropathy. 

Hypokalemia in Eating Disorder Patients

Metabolic alkalosis is an acid-base disorder in which the blood becomes too alkaline. In bulimia nervosa patients, it often results from purging, which depletes both potassium and intravascular volume, and is worsened by hypokalemia. In turn, this causes aldosterone levels to rise, which makes the kidneys absorb more sodium and chloride while excreting more potassium through urine. Purging also causes hydrogen ions to be lost, raising the body's pH. While normally bicarbonate is excreted to correct this alkalemia, bulimia nervosa patients may not excrete as much bicarbonate as necessary to restore the blood's pH as a result of the rapid decrease in sodium chloride from purging. Furthermore, among those who abuse laxatives, metabolic alkalosis can be indicative of an extended duration of abuse. If spot urine chloride tests show <10mmol/L, metabolic alkalosis can be treated with slow administrations of intravenous saline.

Metabolic Alkalosis in Bulimia Nervosa Patients

Those who self-induce vomiting to purge are at risk of multiple potentially fatal conditions:

- Hypokalemia
- Hypochloremia
- Hyponatremia
- Metabolic alkalosis

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Numerous metabolic and electrolyte-related complications can occur due to bulimia nervosa, contributing to its elevated mortality rates as such imbalances can result in potentially fatal cardiac issues. The electrolyte and metabolic complications that may arise are largely dictated by the method of purging:

- Self-induced vomiting metabolic/electrolyte complications
- Laxative abuse metabolic/electrolyte complications
- Diuretic abuse metabolic/electrolyte complications
- Pseudo-Bartter syndrome

Bulimia Nervosa Metabolic/Electrolyte Complications

Nitsch, A., Dlugosz, H., Gibson, D., & Mehler, P. S. (2021). Medical complications of bulimia nervosa. Cleveland Clinic Journal of Medicine, 88(6), 333-343.

Medical Complications of Bulimia Nervosa

Self-Induced Vomiting Metabolic/Electrolyte Complications

Diuretic abuse can lead to numerous electrolyte imbalances and dangerous metabolic alkalosis, although some complications can be avoided with the use of diuretics that do not affect potassium, such as spironolactone, in replacement of other diuretics. Sodium chloride is lost in urine, causing intravascular volume depletion which in turn leads to elevated aldosterone levels. High serum aldosterone then causes hydrogen and potassium to be excreted via urination, which can lead to metabolic alkalosis.

Diuretic Abuse Metabolic/Electrolyte Complications

Laxative abuse can result in several electrolyte imbalances and acid-base disorders. The resulting acid-base disorder may be a means of indicating the duration of laxative abuse:

- Hypokalemia
- Hypochloremia
- Hyponatremia
- Non-anion gap metabolic acidosis
- Metabolic alkalosis

Laxative Abuse  Metabolic/Electrolyte Complications 

Pseudo-Bartter syndrome results from chronic dehydration and thus sometimes presents in eating disorder patients, especially those who purge. It may cause hypokalemia and other electrolyte imbalances, and swelling when purging stops as a result of elevated aldosterone levels causing retention of water and salt. This edema, which can lead to weight gain and resulting emotional distress, can be avoided by less aggressive methods of fluid replacement such as a slow saline infusion of 50mL/hr in conjunction with 50-400mg low-dose spironolactone. These medications should then be gradually tapered off.
	


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