Clinical disorders of calcium and phosphate

Renal stone disease, elevated iPTH level and normocalcemia

BACKGROUND

There is a well established relationship between primary hyperparathyroidism and recurrent calcium-containing calculi. Traditionally, the diagnosis is confirmed by the presence of elevated intact parathyroid hormone (iPTH) and serum ionised calcium levels. The prevalence and role of elevated iPTH in the presence of normocalcemia in patients with renal stone disease is poorly understood. The aim of the present study was to describe the findings in patients who had renal stone disease, an elevated iPTH level and normocalcemia.

METHODS

During the last decade, 414 patients, who had normal renal function, were investigated and treated for renal calculi in the Renal Stone Clinic of the Toronto Western Hospital. Of these 414 patients, 40 (9.6%) had an elevated intact iPTH level and normal serum calcium (total and ionised) on repeated measurements. In all these patients we performed detailed clinical and laboratory investigations to determine risk factors for stone formation. Correlation analysis was done using Pearson test and the weights of factors influencing iPTH level were compared using multiple regression analysis.

RESULTS

The average duration of a history of stone disease was 12.0 +/- 10.5 years. Most of these patients had passed their stones spontaneously, 15 underwent lithotripsy, in six the stones were removed by basket catheters and one patient each had partial nephrectomy, nepholithotomy or uretero-lithotomy. Twelve had a positive family history, two had history of intestinal malabsorption and one patients had a history of immobilisation. All of these patients had elevated serum parathyroid hormone in the range of 3% to 134% (median 20.5%) above upper limit of normal (F = M); all had normal serum total and ionised calcium and normal urine excretion of calcium (except in one). Additional risk factor for stone formation included: low level of 25-hydroxyvitamin D in four patients, low output and high urine osmolality in four patients, high urine sodium in nine and high oxalate excretion in eight patients. Citrate excretion was low in seven, magnesium excretion in six patients and tubular reabsorption of phosphate in 22 patients. Urine hydroxyprolin was increased in two and decreased in four patients. Combined abnormalities were found in 14 while 17 patients did not have any abnormality apart from elevated iPTH level. Multiple regression analysis did not suggest that any of the selected predictors had a significant influence on PTH release.

CONCLUSIONS

9.6% of patients with recurrent kidney stones had normocalcemia and elevated iPTH level in the presence of normal renal function. The study did not show any distinct pattern of abnormalities that would suggest a mechanism of stone disease in these patients. Further investigations are necessary to determine the significance of elevated iPTH in normocalcemic patients with renal stone disease and establish whether we should consider neck exploration for parathyroidectomy in these patients.

Hypophosphataemia in a hospital population and the incidence of concomitant hypokalaemia

There have been few recent studies describing the incidence and causes of hypophosphataemia in a hospital population and the frequency of concomitant hypokalaemia. In this study a majority (53%) of patients with hypophosphataemia had been on a glucose infusion at the time of their low plasma phosphate. The largest patient group found to show hypophosphataemia (less than 0.8 mmol/L) were in the Intensive Therapy Unit (ITU), and this was also the largest patient group manifesting severe hypophosphataemia (less than 0.3 mmol/L). Approximately, 11% of patients with hypophosphataemia (less than 0.8 mmol/L) had a plasma potassium equal to or less than 3.5 mmol/L, compared with 5.7% of hospital patients without hypophosphataemia. Furthermore, 20% of patients with severe hypophosphataemia (less than 0.3 mmol/L) had hypokalaemia as similarly defined.

Fluid and electrolyte balance in critical patients

Recognition and appropriate management of fluid and electrolyte disorders in critical patients is extremely important. In many cases, these secondary problems are more complicated and more serious than the initiating disease process. A severely ill diabetic patient, for example, is more likely to die from dehydration, hyperosmolality, metabolic acidosis, hypokalemia or hypophosphatemia than from hyperglycemia or lack of insulin therapy. Proper fluid therapy and treatment of electrolyte abnormalities make a major difference in the survival rate of critically ill animals.

Hypophosphatemia. Causes and clinical consequences

Severe hypophosphatemia (i.e., serum phosphorus concentration below 1 mg/dl) occurs infrequently in veterinary patients. It is most often associated with diabetic ketoacidosis in small animals. Phosphate is necessary for the production of 2,3 diphosphoglycerate (2,3-DPG) and adenosine triphosphate (ATP); both are important for normal cellular metabolism. Consequences of severe hypophosphatemia may include hemolytic anemia, seizures, altered mentation, cardiomyopathy, and skeletal muscle weakness. Parenteral phosphate therapy is necessary in most cases of severe hypophosphatemia.

Rib fractures and anorexia nervosa

A twenty-five year old patient with long standing anorexia nervosa and recent forced vomiting presented with a pulling sensation in her lower anterior thorax. Seven months later, callus formation over the left tenth rib was documented. Chest x-ray revealed healing fractures of the seventh, ninth, and tenth ribs. Since her oral intake of calcium was low, she was started on a regimen of milk and sun bathing. We postulate her rib fractures were the result of forced vomiting in an individual with fragile bones. The osteoporotic state may have been induced by her malnourished low-estrogen state.