Physiology and causes of imbalance
Calcium absorption occurs in the duodenum and small intestine. This process requires bile and fatty acids, as well as a specific carrier protein in the epithelium. In the blood, the element circulates in free (ionized) and bound forms. Shifts in its concentration lead to severe systemic disorders.
Hypocalcemia occurs in endocrine pathologies (hypoparathyroidism, Conn's syndrome), renal failure, calcium loss (pregnancy, lactation), or alkalosis (gastric due to vomiting or respiratory due to hyperventilation). Clinically, it manifests as tetanic convulsions, bronchiolospasm, and laryngospasm.
Hypercalcemia is most commonly associated with excess parathyroid hormone. Several variants of hyperparathyroidism are distinguished:
- Primary: due to adenoma or hyperplasia of the parathyroid glands (manifests as Recklinghausen's disease).
- Secondary: a compensatory response to prolonged hypocalcemia (in rickets, malabsorption, chronic renal failure).
- Tertiary: a consequence of long-term hemodialysis.
Hypercalcemia is also caused by paraneoplastic syndromes (when cancer cells, e.g., in bronchogenic carcinoma, secrete parathyroid hormone), bone destruction in metastases, thyrotoxicosis, excess corticosteroids, vitamin $D_3$ hypervitaminosis, and prolonged immobilization. Calcium excess causes dyspepsia, and during a sharp surge — hypercalcemic crisis with severe psychiatric disorders up to coma.
Dystrophic calcification
In this type of calcinosis, blood calcium levels remain normal. Salts are deposited locally — exclusively in areas of necrosis or sclerosis. Typical examples: petrifactions in foci of caseous necrosis in tuberculosis, calcification of atherosclerotic plaques, and heart valve involvement in aging or rheumatic diseases.
Pathogenesis includes two phases:
- Initiation (nucleation). Can occur intracellularly (in the mitochondria of dead cells) or extracellularly. Extracellular initiation occurs in membrane vesicles, where calcium binds to acidic phospholipids, and microcrystals are formed under the action of phosphatases.
- Propagation (growth). Calcium phosphate microcrystals (in the form of apatite) increase in size. This process is accelerated by collagen and regulated by non-collagenous matrix proteins: osteopontin, osteonectin, and osteocalcin.
Metastatic calcification
Develops against the background of marked hypercalcemia. Unlike the dystrophic variant, salts precipitate in completely normal tissues. Causes include hyperparathyroidism, vitamin D intoxication, sarcoidosis, Addison's disease, leukemias, and multiple myeloma.
Most commonly, calcium deposits are localized in the interstitium of the gastric mucosa, kidneys, lungs, myocardium, and arteries. The choice of organs is not accidental: during their functioning, these tissues lose acid. Local alkalization of the environment predisposes to the precipitation of calcium salts. Usually, such deposits do not cause organ dysfunction, except for massive kidney damage (nephrocalcinosis), which leads to renal failure.
Special forms: cutaneous calcinosis and calciphylaxis
- Cutaneous calcinosis proceeds as focal dystrophic calcification characteristic of systemic connective tissue diseases (scleroderma, systemic lupus erythematosus). Nodules form under the skin of the hands and feet. Over time, they become inflamed, ulcerate, and form fistulas from which a mushy mass is discharged.
- Calciphylaxis (calcific uremic arteriolopathy) is a rare life-threatening condition occurring predominantly in chronic renal failure. Notably, blood calcium and phosphate levels are not decisive here. Calcification of the media of small and medium-sized arteries occurs with the development of productive endovasculitis and thrombosis. This leads to acute ischemia and painful skin necroses. The prognosis is often unfavorable due to the risk of gangrene and sepsis.
Briefly: answers to topical questions
What causes hypocalcemia?
Causes of hypocalcemia include endocrine, physiological, gastrointestinal, renal, toxic factors, and acid-base balance disorders.
- Endocrine — hypoparathyroidism, primary aldosteronism (Conn's syndrome).
- Physiological — increased demand during pregnancy and lactation.
- Acid-base disorders — gastric and respiratory alkalosis.
- Gastrointestinal — malabsorption in enteritis, vitamin D deficiency.
- Renal — chronic renal failure.
- Toxic and iatrogenic — oxalic acid poisoning, citrated blood transfusion.
Hypocalcemia is also caused by magnesium deficiency and hereditary forms of rickets.
What staining methods are used to detect calcium salts in histological specimens?
Sources indicate the use of hematoxylin and eosin staining in the detection of calcification, particularly in metastatic myocardial calcification. Psammoma bodies containing calcium salts stain intensely with hematoxylin. Von Kossa staining is used in osteomalacia to detect a wide zone of unmineralized osteoid.
What outcomes are characteristic of areas of dystrophic calcification?
A characteristic result of dystrophic calcification is the formation of petrifactions: foci of calcium salt deposition acquiring stony hardness. Calcification can cause organ dysfunction, such as stenosis in heart valve calcification and complications of atherosclerosis.
In which systemic connective tissue diseases does cutaneous calcinosis develop?
Cutaneous calcinosis develops against the background of a number of systemic connective tissue diseases. These pathologies include:
- Systemic sclerosis — including its specific form, CREST syndrome, which is characterized by soft tissue calcinosis.
- Dermatomyositis — accompanied by the formation of stony hard areas in muscles and skin lesions.
- Systemic lupus erythematosus (SLE).
In these diseases, focal calcium salt deposition is considered dystrophic calcification.
To remember the difference between types of calcification, use the "DM" rule: Dystrophic — Defective tissue (necrosis, normal blood calcium). Metastatic — Much calcium (hypercalcemia, healthy tissues).
Class and exam questions
How does dystrophic calcification differ from metastatic?
In dystrophic calcification, blood calcium is normal, and salts are deposited in damaged tissues (necrosis, sclerosis). In metastatic calcification, hypercalcemia is always present, and salts precipitate in healthy tissues.
Why do the stomach, kidneys, and lungs specifically get affected in metastatic calcification?
These organs secrete acidic products during their physiological function. As a result, their stroma becomes locally alkaline, creating ideal conditions for the precipitation of calcium salts.
What is calciphylaxis and why is it dangerous?
This is a rare complication of renal failure in which the media of small arteries calcifies. Vessels thrombose, causing acute ischemia, skin necrosis, and gangrene with a high risk of mortality.
What else to study on this topic
- Mechanisms of intestinal calcium absorption and the role of the carrier protein
- Pathogenesis of hypercalcemic crisis and its effect on the CNS
- The role of osteopontin and osteocalcin in regulating microcrystal growth
- Recklinghausen's disease (osteitis fibrosa cystica) in primary hyperparathyroidism
- Morphological picture of petrifactions in tuberculosis
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