An increased serum calcium is typically first noted when total calcium (tCa) is measured as part of a biochemistry profile.
An increased serum calcium is typically first noted when total calcium (tCa) is measured as part of a biochemistry profile. Abnormalities in tCa warrant further diagnostic investigation. First it should be verified that the abnormality is repeatable. If the abnormality is repeatable, ionized calcium (iCa) should be measured for an accurate assessment of calcium status. Total or adjusted tCa are not reliable measurements of calcium status as noted by a high degree of diagnostic discordance between total, adjusted, and ionized calcium measurements.
Small increases in ionized serum calcium concentration can have adverse consequences in some animals whereas others with a similar or greater degree of hypercalcemia may not manifest obvious clinical signs. A mild degree of hypercalcemia may not be immediately dangerous and there is time to establish a definitive diagnosis before starting treatment. In those with severe clinical signs associated with hypercalcemia, diagnostic and therapeutic efforts may need to proceed concurrently. Interaction with serum phosphorus is important, as those with a tCa (mg/dL) times phosphorus concentration product greater than 70 are most likely to have severe tissue changes associated with mineralization. Hypercalcemia can be toxic to all body tissues, but major deleterious effects are on the kidneys, nervous system, and cardiovascular system. Most animals with tCa greater than 15.0 mg/dL will show systemic signs, and those with tCa concentrations greater than 18.0 mg/dL are critically ill.
Polydipsia, polyuria, and anorexia are the most common clinical signs attributed to hypercalcemia, though depression, weakness, vomiting, and constipation can also occur. Uncommonly, cardiac arrhythmias, seizures, and muscle twitching are observed. Severe hypercalcemia that has developed rapidly (hypervitaminosis D) can result in death. Cats with hypercalcemia do not display polyuria, polydipsia or vomiting as commonly as do dogs with a similar degree of hypercalcemia. Cats with idiopathic hypercalcemia may have no obvious clinical signs.
Hypercalcemia is initially defined on results of serum total calcium from obviously sick animals, but also fortuitously during wellness examinations, pre-anesthesia screenings, evaluation of urolithiasis, and from those evaluated for vague GI signs. The initial finding of a mild increase in serum tCa should be repeated to see if the hypercalcemia is persistent. A transient increase in serum tCa is documented in some cats with minor increases in serum tCa; further workup is not indicated in these instances in which the serum tCa concentration is normal on subsequent analysis. Measurement of serum iCa is the next step in the diagnostic evaluation of those with persistent or more substantial increases of serum tCa. Prediction of iCa status from tCa measurement is not accurate, and iCa needs to be specifically measured. Increased iCa concentration is documented in all cats with IHC, but may be normal or low in other conditions associated with increased serum tCa, especially chronic renal failure (CRF). Serum iCa can be measured alone, or preferably at the same time that parathyroid hormone (PTH) concentration is measured.
Table 1. Anticipated changes in calcemic hormones and serum biochemistry associated with disorders of hypercalcemia
There are many potential causes of hypercalcemia (See HARDIONS Eponym). Though cancer-associated hypercalcemia has traditionally been noted to be the primary cause of elevated serum in both dogs and cats, IHC appears to be the most prominent cause in cats followed by renal failure, and then malignancy in primary care practice. In some cases with persistent hypercalcemia, the diagnosis of the cause of the hypercalcemia will be obvious after analysis of history (vitamin D exposure, drugs, ingestion of houseplants), and findings from physical examination (masses, organomegaly, cancer or granulomatous disease). In other cases, the cause will not be obvious and information from hematology, serum biochemistry, body cavity imaging, cytology, and histopathology will be necessary.
Hypercalcemias can be classified as parathyroid-dependent (primary hyperparathyroidism), or parathyroid-independent (normal parathyroid gland). In hypercalcemic dogs, neoplasia is the most common diagnosis, followed by hypoadrenocorticism, primary hyperparathyroidism, and chronic renal failure. Approximately 70% of hypercalcemic dogs are also azotemic, with azotemia uncommon only in dogs with hyperparathyroidism. In hypercalcemic cats, neoplasia is second to renal failure or idiopathic hypercalcemia.
H = Hyperparathyroidism (1°,3°, hyperplasia), Humoral Hypercalcemia of Malignancy, Houseplants, Hyperthyroid
A = Addison's Disease, Aluminum Toxicity, Vitamin A, Milk-Alkali
R = Renal Disease, Raisins(Grapes)- dogs
D = Vitamin D Toxicosis (Granulomatous Dz),
Drugs, Dovonex, Dehydration, DMSO (calcinosis cutis), Diet
I = Idiopathic (Cats), Infectious, Inflammatory, Immobilization
O = Osteolytic (osteomyelitis, immobilization, Local Osteolytic Hypercalcemia, bone infarct)
N = Neoplasia (HHM and LOH), Nutritional
S = Spurious, Schistosomiasis, Salts of Calcium, Supplements
Excessive calcium ions are toxic to cells. Although all tissues may be subject to the dangerous effects of hypercalcemia, effects on the central nervous system, gastrointestinal tract, heart, and kidneys are of most importance clinically. Mineralization of soft tissues (especially the heart and kidneys) is an important complication of hypercalcemia. The serum phosphorus concentration at the time hypercalcemia develops is important in determining the extent of soft tissue mineralization. Soft tissue mineralization occurs regardless of the serum phosphorus concentration in severe hypercalcemia.
The impetus to prescribe therapeutic intervention becomes more pressing when the magnitude of ionized hypercalcemia continues to increase or clinical signs become more obvious. Aggressive treatment to decrease iCa concentration is warranted in patients with chronic kidney disease, chronic kidney failure, and or those with calcium-containing urinary stones. Continued ionized hypercalcemia poses a risk for further development of renal lesions and for development of new stones and enlargement of existing stones.
Acute rescue from hypercalcemia related to IHC (idiopathic hypercalcemia) is rarely indicated, as hypercalcemia has been gradual in development and relatively longstanding, and dramatic clinical signs are usually absent. Most cats with IHC will be treated as outpatients with either dietary change alone or in combination with drug therapy.
A syndrome in young to middle-aged cats has emerged since the early 1990's, where hypercalcemia occurs without obvious explanation in cats. Serum total calcium is increased for months to more than one year, often without obvious clinical signs. Ionized calcium is increased, sometimes out of proportion to the increase in total serum calcium. Longhaired cats may be over represented in this syndrome. Vomiting and weight loss are the most common clinical signs. Most are nonazotemic, but may develop azotemia at a later date. Nephrocalcinosis is occasionally observed, as are uroliths in the kidney, ureter, and bladder. There is no evidence of malignancy based on radiography, abdominal ultrasonography, bone marrow evaluation, and in some instances, full necropsy. Serology for FeLV and FIV is negative, and T4 values are normal. PTH levels are within the reference range, PTHrP is not detectable, and 25-(OH)-D and calcitriol levels are within normal limits. Blood gas analysis reveals no major acid-base disturbance.
An increase in dietary fiber has been reported to decrease serum calcium in affected cats in some reports. Challenge with prednisone therapy results in long-term decreases in iCa and tCa in some cats. There is concern that this treatment could increase hypercalciuria, which could subsequently enhance genesis of urinary calculi. However, the declining filtered load of calcium decreases as serum iCa declines, which offsets the enhanced formation of calculi. When dietary modification and challenge treatment with prednisolone have been unsuccessful in resolving hypercalcemia, bisphosphonate treatment should be considered. The cause(s) of idiopathic hypercalcemia in cats remains elusive. The role of dietary acidification, dietary magnesium restriction, and/or contribution of any specific dietary constituents deserve further consideration. It is conceivable that hypercalcemia develops only in a genetically susceptible population of cats.