Hypocalcemia and Hypoparathyroidism

Mechanisms of hypocalcemia development

A decrease in the concentration of ionized calcium ($Ca^{2+}$) in the blood is caused by a failure in one or more links of phosphorus-calcium metabolism. The pathological process can affect various physiological systems responsible for maintaining the homeostasis of this trace element.

Fundamental causes (mechanisms) of hypocalcemia include:

• Impaired absorption in the GI tract: full absorption of $Ca^{2+}$ ions in the intestinal lumen is blocked.
• Blockade of the bone depot: the processes of $Ca^{2+}$ mobilization from bone tissue, which normally serves as the main calcium reservoir in the body, are inhibited.
• Renal losses: a pathological decrease in the reabsorption (tubular reuptake) of $Ca^{2+}$ ions in the renal tubules is observed.
• Vitamin deficiency: lack of the active form of vitamin $D_3$, known as cholecalciferol, which directly exacerbates problems with calcium assimilation and distribution.

Increased neuromuscular excitability

The key and most striking manifestation of hypocalcemia is a radical increase in neuromuscular excitability. This phenomenon is not associated with hormonal or acid-base imbalance, but has a purely electrolyte nature. Excitability is manifested in the form of two main syndromes: tetany and seizures.

1. Tetany

This condition is diagnosed in the vast majority of patients (about 90% of cases).

• Definition: Tetany is a state of prolonged, continuous muscle contraction reaching its maximum physiological tension.
• Localization: The pathological process usually involves symmetrical muscle groups. Most often, the flexors of the extremities are the first to be involved in the process. In the most severe and advanced cases, the spasm spreads to the facial muscles.
• Pathogenesis: Tetany is based on an abnormally high frequency of excitation impulses. These signals are continuously delivered to the muscle via nerve fibers, causing the natural relaxation phase of muscle fibers to be completely absent.

2. Seizures

Convulsive syndrome is also extremely common, occurring in more than 50% of patients with hypocalcemia.

• Definition: These are involuntary, uncontrolled contractions of various muscle groups.
• Specific feature: Any seizures in this condition are always accompanied by a pronounced pain syndrome (severe pain).
• Typology:
• Clonic seizures — characterized by rhythmicity, where the muscle contraction phase is sequentially replaced by a relaxation phase.
• Tonic seizures — differ in that muscle contraction continues for a long time without periods of relaxation.

Systemic organ function disorders

Calcium deficiency is not limited solely to damage to the muscular system. The pathology leads to large-scale dysfunction of tissues and entire physiological systems. It is important to understand that the root cause of these organ lesions is precisely the electrolyte imbalance, rather than a water or acid-base shift.

The most vulnerable to changes in calcium and phosphorus levels are the following organs:

• Kidneys (the tubular apparatus suffers, reabsorption processes are disrupted).
• Cardiovascular system, primarily the heart itself, for the normal contraction of which a strict balance of calcium ions is critically important.
• Other internal organs whose work depends on the correct transmission of nerve impulses and electrolyte balance at the cellular level.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• The relationship between calcium ion levels and the development of hyperphosphatemia
• The role of cholecalciferol (the active form of vitamin D3) in phosphorus-calcium metabolism
• Pathogenesis of impaired calcium reabsorption in renal tubules
• The effect of electrolyte imbalance on cardiovascular function
• Mechanisms of formation of severe pain syndrome during seizures

Other topics in "Chapter 27. Typical Forms of Endocrine System Pathology"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.