Pathologic hypercalcemia in the dog

Introduction

Calcium is the fifth most abundant element in the body, existing as an essential cation found within bodily fluids and also stored within cellular organelles. It is responsible for many vital intracellular and extracellular functions, including neuromuscular transmission, enzymatic reactions, blood coagulation, vasomotor tone, hormone secretion and bone metabolism. While calcium is broadly distributed throughout cellular tissues, rapid fluctuations in intracellular calcium serve as a primary regulator of cellular responses following plasma membrane receptor activation, and it also serves as a secondary messenger responsible for perpetuating external signals into cells to orchestrate downstream biologic functions 1.

While intracellular calcium is critical for normal cellular activities, the clinical measurement of calcium is restricted to its presence in body fluids where it exists in three different forms, namely ionized, protein-bound, and complexed with anions or organic acids. Ionized calcium (iCa) is the biologically active form that can pass through plasma membranes by virtue of permeable ion channels, active transporters and cation exchangers 2, and comprises 50% of total serum calcium. The remaining fraction is approximately distributed as 40% protein-bound and 10% complexed. Given its importance in cellular functions, iCa concentrations must be tightly regulated to ensure proper physiologic activities of a myriad of cellular, tissue, and organ systems; this is done through the concerted actions of parathyroid hormone (PTH), 1,25-dihydroxycholecalciferol (active vitamin D₃ or calcitriol) and calcitonin 3. Similar to its importance for intracellular signaling, calcium within extracellular fluid also regulates cellular functions of many vital glandular and epithelial tissues, including the parathyroid gland, thyroid C cells, and kidney.

Calcium homeostasis

The three principal mediators, PTH, calcitonin, and calcitriol, are responsible for balancing whole-body calcium concentrations through the exertion of complementary and/or synergistic biologic activities on three target organs, namely the kidneys, small intestines, and inorganic bone matrix (hydroxyapatite) 3 (Figure 1).

PTH serves as the master regulator, governing minute-to-minute fluctuations of calcium levels within the body. If calcium levels are increased, PTH secretion is downregulated, leading to a net calcium loss through the distal tubules in the kidneys, a reduction in intestinal absorption of calcium, and diminished osteoclastic bone resorption 4. Conversely, if serum calcium levels are decreased, the parathyroid glands secrete PTH that acts on the distal renal tubules to cause calcium reabsorption and phosphorus excretion from the kidney. PTH also indirectly contributes to calcium absorption within the small intestine via the conversion of vitamin D to highly active vitamin D3 (calcitriol) in the proximal renal tubules through the upregulation of 1-alpha-hydroxylase activity 4. Additionally, PTH will act on skeletal tissue to stimulate the proliferation of existing bone-forming cells (osteoblasts), which is an early effect and enhances bone mineral density 5. However, chronic PTH signaling can upregulate osteoblast RANKL expression, which results in osteoclast activation and survival, with subsequent augmented bone resorptive activities 6.

Calcitriol participates in calcium regulation principally through increased synthesis of calbindin-D proteins, which increase small intestinal absorption of dietary calcium with subsequent release into the bloodstream 7. Calcitriol can also operate as a negative feedback regulator of itself by conversion into 24,25-dihydroxycholecalciferiol, which is less active, as well as negatively regulating calcium by decreasing PTH mRNA transcription.

Calcitonin is not a major factor in the minute-to-minute regulation of calcium, but serves as an emergency hormone to reduce serum levels when there is a rapid increase in calcium. Calcitonin is released by C cells of the thyroid gland when stimulated by hypercalcemia and ingestion of high calcium meals, resulting in secretion of enteric hormones (i.e., gastrin and cholecystokinin), and its biologic activities are principally mediated through inhibition of osteoclastic bone resorption 8.

Differential diagnosis

Hypercalcemia has been associated with a variety of physiologic and pathologic conditions in both dogs and cats, and these can be broadly categorized as parathyroid-dependent and parathyroid-independent. Parathyroid-dependent causes include primary hyperparathyroidism and (uncommonly) secondary nutritional or renal hyperparathyroidism. All other causes of hypercalcemia are considered parathyroid-independent and include diverse pathologies such as cancer-associated (Figures 2 and 3), toxic, idiopathic, metabolic, skeletal, and granulomatous diseases 9. In recent studies evaluating large numbers of companion animals, the most common pathologic causes of ionized hypercalcemia in dogs were neoplasia, primary hyperparathyroidism, and hypoadrenocorticism (in comparison, the most common causes in cats are neoplasia (e.g., oral squamous cell carcinoma), chronic kidney disease, idiopathic hypercalcemia (secondary to derangements in calcium-sensing apparatus) and (less commonly) hypervitaminosis D 10,11,12,13,14. Overall, the most common cause in companion animals is cancer, with approximately 60% of dogs (and 30% of cats) diagnosed with cancer-related hypercalcemia 11,12,13 (Figures 4 and 5). There are various mnemonics or acronyms for recalling different clinical scenarios associated with hypercalcemia. Given that it can often foreshadow the identification of significant underlying disease pathologies, one very appropriate mnemonic is “GOSH DARN IT” (Table 1).

Although elevations in calcium secondary to cancer are common, several discrete mechanisms can be responsible 15,16. The first and most common cause for hypercalcemia is the production of soluble factors by tumor cells, which results in bone resorption, known as humoral hypercalcemia of malignancy (HHM). A second mechanism is direct invasion of tumor cells into bone, leading to osteolysis, commonly seen with carcinomas or hematopoietic bone marrow malignancies such as leukemias, lymphoma, and multiple myeloma. Lastly, and least commonly, production of the active form of vitamin D by 1-α-hydroxylase-expressing cells will enhance intestinal absorption of calcium.

Table 1. GOSH DARN IT – A useful mnemonic and acronym for causes of hypercalcemia.

Humoral hypercalcemia of malignancy (HHM)

Humoral hypercalcemia of malignancy may involve malignant secretion of parathyroid hormone-related peptide (PTHrp), which is structurally similar to PTH 17, and cytokines such as IL-1, IL-6, or tumor necrosis factor 15. These humoral factors lead to generalized and diffuse osteoclastic resorption without visible radiographic bone lesions. Because PTHrp is a secreted protein, any cell type which is secretory in nature and undergoing malignant transformation can potentially liberate excessive amounts of the hormone.

Lymphoma (LSA) is the most common cause of HHM, particularly mediastinal lymphoma, however, other tumors responsible for hypercalcemia in dogs (and cats) include apocrine gland anal sac adenocarcinoma (AGASACA), thyroid carcinoma, multiple myeloma, multiosseous bone tumors, thymoma, squamous cell carcinoma, mammary gland carcinoma/adenocarcinoma, melanoma, primary lung tumor, chronic lymphocytic leukemia, renal angiomyxoma, and parathyroid gland tumors. As a general rule of thumb, hypercalcemia can be seen in 10-35% of dogs with LSA, ≥ 25% of dogs with AGASACA, and approximately 20% of dogs with multiple myeloma.

Clinical signs

Given the wide range of physiological functions of calcium ions, both hypercalcemia and hypocalcemia will cause multisystemic effects 10,11. Increased serum calcium concentration causes decreased cellular function by altering the cell membrane permeability and the cell membrane pump activities. An increase in intracellular iCa can cause deranged cellular function and reduced energy production, which can result in cell death and lead to dystrophic and/or metastatic mineralization. While many tissues can be affected by hypercalcemia, the effects on the central nervous system, gastrointestinal tract, heart and kidneys are of greatest clinical importance. Regardless of the inciting cause, compromised renal function is a significant clinical feature of hypercalcemia, especially in the setting of neoplasia 18. Clinical signs associated with hypercalcemia can be nonspecific, insidious in nature, and vary in severity, but common signs might include primary polyuria with compensatory polydipsia (PU/PD), anorexia, lethargy, weakness, vomiting, depression, muscle twitching, cardiac arrhythmias and seizures 9. This compares to hypercalcemia in cats, where gastrointestinal signs, particularly anorexia and vomiting, are among the most frequent signs 19.

Diagnostic approach to the hypercalcemic patient

Measurement of iCa is more accurate than total calcium in companion animals. Interestingly, the magnitude of calcium concentration elevation tends to be higher in cases of hypercalcemia of malignancy than other causes 12,13. When hypercalcemia is suspected in a patient, obtaining a thorough history and physical examination, which includes careful assessment of peripheral lymph nodes and rectal palpation (to check for AGASACA) (Figures 6-8), should be the first clinical step. Guided by these findings, additional diagnostics (including a complete blood count, chemistry panel, urinalysis, thoracic radiographs, and abdominal ultrasound) can be performed to identify potential findings that might further corroborate clinical suspicions or conversely uncover pathologies not overtly identified during physical examination.

If an underlying cause for hypercalcemia cannot be identified following these diagnostics, further tests can be performed, including measuring the circulating concentrations of serum PTH, PTHrp and calcitriol. In patients with hypercalcemia of malignancy, serum PTH concentrations should be low or undetectable, while serum PTHrp levels can be measurable and/or elevated. Serum calcitriol is usually normal, but can be increased or decreased. Beyond these conventional diagnostics, additional tests such as bone survey radiographs, bone scans (nuclear scintigraphy), bone marrow aspiration and computer tomography (CT), can be considered if an underlying cause remains elusive (Figures 9-11).

Calcium disturbances can be encountered in serious and life-threatening conditions and regardless of the underlying cause, hypercalcemia can lead to life-limiting complications if left untreated 20. Measuring iCa requires specialized analyzers, which are not always readily available to veterinary healthcare professionals. In attempts to mitigate this limitation, the clinician must often rely on deciphering nuances in the total calcium (tCa), which includes all three serum calcium fractions. Unfortunately, accurate interpretation can be difficult because total calcium is not a true reflection of the iCa status in any particular patient. While correction formulas for total calcium using albumin and total protein have been proposed, these should not be considered reliable, and have failed to validate and improve upon the observed diagnostic discordance between total calcium and iCa 20,21. Because of this, it is recommended that any abnormality in the total calcium measurement should be further investigated with additional diagnostics to directly quantify true iCa concentrations.

Treatment

Given that hypercalcemia can arise from a wide range of disease processes, the proper treatment, severity of clinical signs, and the overall prognosis are based on the underlying etiopathogenesis. There is no single treatment protocol that is consistently effective for all causes, but induction of calciuresis can provide immediate clinical benefit in most patients. Definitive and optimal management of hypercalcemia is achieved by identifying and treating the underlying cause, although precise identification of some disease processes can be more complex and elusive. The clinical status of the patient will ultimately dictate how aggressive the treatment needs to be.

The most effective treatment for hypercalcemia of malignancy is removal of the underlying neoplasm by surgery if anatomically feasible, induction of clinical remission with chemotherapy (most applicable for hypercalcemia associated with LSA), or radiation therapy. Empirically, serum calcium concentration of 16 mg/dL (4 mmol/L) or greater has been recommended as the basis for initiating aggressive therapy; however, the intensity of patient management should be individualized and guided by real-time assessment and diagnostic findings. Unfavorable prognosis should be anticipated in particularly compromised patients with any of the following characteristics:

• clinically ill patients from a hypercalcemia in excess of 16 mg/dL (4 mmol/L),
• patients with severe renal azotemia,
• patients where the calcium and phosphorous product is greater than 60 with resultant metastatic mineralization,
• patients suffering from hypercalcemia of malignancy associated with solid tumors that are not amenable to surgical removal.

Initial intervention for treatment should consist of aggressive fluid therapy with isotonic (0.9%) sodium chloride to correct existing dehydration, a common sequalae to hypercalcemia-induced primary polyuria (termed secondary nephrogenic diabetes insipidus). Hemoconcentration through decreased glomerular filtration leads to additional calcium retention as the kidneys attempt to conserve sodium, which decreases urinary calcium excretion 9. Judicious administration of IV saline will not only restore hydration, but also has the added benefit of promoting volume expansion with increased glomerular filtration rate and consequent enhanced calciuresis. Saline does not contain supplemental calcium, and the high sodium content competes with calcium for renal tubular absorption, which further helps promote calcium excretion 22. Once adequate rehydration has been achieved, the use of loop diuretics (e.g., furosemide at 2-4 mg/kg BID or TID IV/SC/PO), in conjunction with continued saline fluid therapy, is recommended to further promote urinary calcium excretion and lessen the likelihood of iatrogenic hypervolemia. However, care is required to avert dehydration in these patients, as hemoconcentration can negate desired calciuresis.

Glucocorticoids can quickly provide benefit in the treatment of certain causes of hypercalcemia. However, ideally the underlying cause should be identified prior to its administration, as indiscriminate institution of glucocorticoids has the potential to confound definitive diagnosis (i.e., covert underlying hematopoietic neoplasia) or could even be medically contraindicated (i.e., infectious granulomatous disease). Glucocorticoids help lower serum calcium levels by decreasing bone resorption, impede intestinal calcium absorption, and increase renal calcium excretion 22. They are particularly beneficial when treating hypercalcemia associated with malignancy such as lymphoma, AGASACA, multiple myeloma, thymoma, hypoadrenocorticism, or hypervitaminosis D. Most commonly used options are prednisone (1-2.2 mg/kg BID IV/SC/PO), and/or dexamethasone (0.1-0.22 mg/kg BID), both being inexpensive and widely available. These doses should be tapered appropriately, and patients are not meant to be maintained at these high doses long term.

Bisphosphonates are another treatment option for hypercalcemia; these are drugs that were developed to inhibit pathologic bone resorption associated with osteoporosis and skeletal metastases in humans. Zoledronate and pamidronate are the most frequently used in human medicine and can normalize calcium levels in 4-10 days, with the effects lasting for about 1-4 weeks 23. While there is currently more data regarding the use of pamidronate, zoledronate has been shown to be efficacious in the control of acute hypercalcemia in dogs 24. Dosages of individual bisphosphonate drugs vary due to differences in antiresorptive potency and potential side effects (Figure 12). The recommended dosing for zoledronate is 0.1-0.25 mg/kg diluted in saline and given IV as a constant-rate infusion over 15-20 minutes, while pamidronate dosage is 1.0-2.0 mg/kg diluted in saline given over 2-4 hours. An important consideration with the administration of these drugs is that bolus infusion of large dosages in preclinical toxicity studies had the potential to cause acute kidney injury. While the above recommended dosing practices in veterinary patients employ longer infusion duration, and hence dramatically reduces the potential for renal damage, it is still recommended to closely monitor a patient’s kidney function during the course of therapy. It is worth noting that alendronate, an oral bisphosphonate, has been investigated for use in cats with persistent idiopathic hypercalcemia, and appears to be well tolerated 14,25, but further investigation is warranted to evaluate its efficacy, given its extremely poor oral bioavailability, and if it should be recommended above other bisphosphonates in this species.

Mithramycin, calcitonin, and gallium nitrate are other theoretical therapies for medically managing hypercalcemia, but these all have limited utility due to their cost, side effects and administration schedules. Mithramycin (plicamycin) is an antitumor antibiotic which inhibits RNA synthesis in osteoclasts, leading to rapid inhibition of bone resorption 22. This medication has fallen out of favor in both veterinary and human medicine due to the potential to cause thrombocytopenia, renal and hepatic necrosis, and hypocalcemia. Calcitonin is another treatment option, as it attenuates bone resorption by inhibiting the activity and formation of osteoclasts. As a result, it rapidly decreases serum calcium concentrations within a few hours following administration, which is faster than any other treatment strategy, but its effects are relatively short-lived due to compensatory receptor down-regulation. Gallium nitrate is an antineoplastic agent that inhibits osteoclasts and decreases the resorptive solubility of hydroxyapatite by binding to hydroxyapatite crystals. This medication is typically used in cases that are refractory to bisphosphonates, and some studies have shown it to be more effective than bisphosphonates at reducing calcium levels in cases of hypercalcemia of malignancy. However, it is not considered a first-line treatment due to its potential for nephrotoxicity. 

Conclusion

Ionized calcium concentrations are very tightly regulated within the body, and alterations can lead to significant and detrimental systemic multi-organ effects. Paraneoplastic hypercalcemia is a serious and relatively common complication in dogs, with various tumor types that can induce hypercalcemia through mechanisms that alter calcium homeostasis and lead to clinical illness. The most common canine neoplasia to cause hypercalcemia is T-cell lymphoma, but other types of neoplasia, and non-neoplastic diseases, should be considered whenever a patient presents with elevated calcium concentrations. Although clinical signs associated with hypercalcemia are often non-specific, early detection of the underlying cause is important. Once identified, instituting definitive treatment and supportive management will minimize life-threatening complications and maximize the chances for achieving a favorable outcome.