Hypophosphatemia

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Hypophosphatemia is an electrolyte disturbance in which there is an abnormally low level of phosphate in the blood. The condition has many causes, but is most commonly seen when malnourished patients (especially chronic alcoholics) are given large amounts of carbohydrates, which create a high phosphorus demand by cells, removing phosphate from the blood (refeeding syndrome).

Because a decrease in phosphate in the blood is sometimes associated with an increase in phosphate in the urine, the terms hypophosphatemia and "phosphaturia" are occasionally used interchangeably; however, this is improper since there exist many causes of hypophosphatemia besides overexcretion and phosphaturia, and in fact the most common causes of hypophosphatemia are not associated with phosphaturia.

Pathophysiology

Hypophosphatemia is caused by the following three mechanisms:

Inadequate intake (often unmasked in refeeding after long-term low phosphate intake)
Increased excretion (e.g. in hyperparathyroidism)
Shift from extracellular to intracellular space (seen in treatment of diabetic ketoacidosis, refeeding, short-term increases in cellular demand (e.g., hungry bones syndrome) and acute respiratory alkalosis)

Causes

Common Causes of hypophosphatemia

Refeeding syndrome This causes a demand for phosphate in cells due to the action of phosphofructokinase, an enzyme which attaches phosphate to glucose to begin metabolism of this. Also, production of ATP when cells are fed and recharge their energy supplies, requires phosphate.
Respiratory alkalosis Any alkalemic condition moves phosphate out of the blood into cells. This includes most common respiratory alkalemia (a higher than normal blood pH from low carbon dioxide levels in the blood), which in turn is caused by any hyperventilation (such as sepsis, fever, pain, anxiety, drug withdrawal).
Alcohol abuse Alcohol impairs phosphate absorption, and alcoholics are also after malnurished with regard to minerals. In addition, alcohol treatment is associated with refeeding, and the stress of alcohol withdrawal may create respiratory alkalosis, which exacerbates hypophosphatemia (see above).
Malabsorption This includes GI damage, and also failure to absorb phosphate due to lack of vitamin D, or chronic use of phosphate binders such as sucralfate, aluminum-containing antacids, and (more rarely) calcium-containing antacids.
Hyperexcretion of phosphate in the urine (phosphaturia). This includes excess excretion from renal dysfunction, and also the action of many classes of diuretics. Additionally, both primary and secondary hyperparathyroidism causes hyperexcretion of phosphate in the urine.

Other rarer causes include

Certain blood cancers such as lymphoma or leukemia
hereditary causes
hepatic failure

Diagnosis

Major signs and symptoms

Muscle weakness. This occurs in major muscles, but also may manifest as: diplopia, low cardiac output (including myonecrosis and cardiomyopathy, dysphagia, and respiratory depression due to respiratory muscle weakness.
Mental status changes. This may range from irritability to gross confusion, delirium, and coma.
White cell disfunction, causing worsening of infections
Instability of cell membrates due to low ATP levels: this may cause rhabdomyolysis with increased CPK, and also hemolytic anemia.

Treatment

Standard intravenous preparations of potassium phosphate are available and are routinely used in malnurished patients and alcoholics. Oral supplementation also is useful where no intravenous treatment is available. Historically one of the first demonstrations of this was in concentration camp victims who died soon after being re-fed: it was observed that those given milk (high in phosphate) had a higher survival rate than those who did not get milk.