Hypophosphatemia is characterized by a serum inorganic phosphate concentration of less than 0.8 mmol/L.
Etiology
Decreased Intestinal Phosphate Absorption
This can occur due to starvation, prolonged fasting, and repeated episodes of vomiting or diarrhea.
Increased Urinary Phosphate Excretion
Conditions such as acute alcohol intoxication, hyperparathyroidism, long-term use of glucocorticoids or diuretics, metabolic acidosis, and diabetes can lead to increased urinary phosphate loss.
Intracellular Phosphate Shift
Factors such as insulin administration, androgen therapy, or large-volume intravenous glucose infusion can promote the movement of phosphate into cells.
Inadequate Phosphate in Parenteral Nutrition
Long-term parenteral nutrition without appropriate supplementation of phosphate is another cause.
Clinical Manifestations
Mild hypophosphatemia usually presents without specific clinical symptoms. Metabolic encephalopathy may develop in cases of hypophosphatemia, leading to increased irritability and altered mental status, with severe cases progressing to stupor or coma.
Neuromuscular symptoms include muscle weakness, which can involve respiratory muscles and result in difficulty breathing or respiratory failure. Gastrointestinal symptoms may include loss of appetite, nausea, vomiting, diarrhea, or constipation. Severe hypophosphatemia can lead to arrhythmias, acute heart failure, cardiac arrest, hypotension, and shock.
Diagnosis
Diagnosis is typically made based on the medical history, clinical manifestations, and laboratory findings. Urinary phosphate and serum phosphate measurements are helpful for diagnosis. Hypophosphatemia is confirmed when the serum inorganic phosphate concentration is less than 0.8 mmol/L.
Treatment
Management of hypophosphatemia primarily involves addressing the underlying cause. Mild and asymptomatic hypophosphatemia typically does not require specific treatment, and oral phosphate supplementation of 1 to 2 grams per day in divided doses is often sufficient.
In cases of severe hypophosphatemia or when symptoms are significant, intravenous phosphate supplementation is required. For serum phosphate concentrations less than 0.3 mmol/L, the recommended daily intravenous phosphate dose is 0.3 mmol/kg, administered over 24 hours. When serum phosphate levels are between 0.3 and 0.6 mmol/L, an intravenous phosphate dose of 50 to 60 mmol per day is generally safe and effective.
During phosphate supplementation, monitoring is necessary to address potential complications such as hypocalcemia, seizures, hypotension, and diarrhea. Treatment should also address coexisting conditions such as hypokalemia, hypomagnesemia, and disturbances in fluid or acid-base balance. Maintaining the function of critical organs such as the heart and lungs is essential.