Hypercalcaemia
Hypercalcaemia is the most common metabolic complication of malignancy. It can occur in up to 40% of patients with cancer and is an indicator of poor prognosis. It is particularly common in multiple myeloma, cancers of the breast, lung, kidney, head and neck, and lymphoma. In 80% of cancer-associated cases it is due to parathyroid hormone-related peptide (PTHrP) production by the tumour. PThRP acts on the bone, kidney and gastrointestinal system to increase serum calcium levels. The remaining 20% are due to local resorption of bone by osteoclasts in areas of marrow space with malignant cells.
Clinical presentation is often non-specific, and may mimic deterioration due to progressive disease (Figure 12.1). Serum calcium values should be corrected:
The mainstay of treatment is rehydration using large volumes of intravenous fluids, followed by a bisphosphonate such as pamidronate. Normalisation of calcium occurs in 80% of patients but may take up to 3 days. In the remaining 20%, alternative treatments include somatostatin analogues such as octreotide, calcitonin and mithramycin. Discontinue any medication that may elevate calcium, such as thiazide diuretics.
Syndrome of inappropriate antidiuresis
Hyponatraemia is common in advanced cancer; however, the finding of concentrated urine in conjunction with hypo-osmolar plasma suggests abnormal renal free water excretion and the diagnosis of the syndrome of inappropriate antidiuresis (SIAD). This is a preferred term (to SIADH) as no ADH secretion occurs in 15% of cases. Significant symptoms occur when the serum sodium is below 125 mmol/L, and can progress to stupor, coma and seizures.
Essential criteria to establish this diagnosis are:
- plasma osmolality <275 mosmol/kg H2O;
- plasma sodium <135 mmol/L;
- urine osmolality >100 mosmol/kg H2
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