Mechanisms of Hypocalcemia Development
A decrease in the concentration of ionized calcium ($Ca^{2+}$) in the blood is caused by a disruption in one or more links of calcium-phosphate 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 gastrointestinal absorption: the normal uptake of $Ca^{2+}$ ions in the intestinal lumen is blocked.
- Blockade of the bone reservoir: mobilization of $Ca^{2+}$ from bone tissue, which normally serves as the main calcium reservoir in the body, is inhibited.
- Renal losses: there is a pathological decrease in the reabsorption of $Ca^{2+}$ ions in the renal tubules.
- Vitamin deficiency: a lack of the active form of vitamin $D_3$, known as cholecalciferol, which directly worsens problems with calcium assimilation and distribution.
Increased Neuromuscular Excitability
The key and most prominent manifestation of hypocalcemia is a radical increase in neuromuscular excitability. This phenomenon is not related to a hormonal or acid-base imbalance, but is purely electrolytic in nature. Excitability manifests as 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 typically involves symmetrical muscle groups. Flexors of the extremities are usually affected first. In the most severe and advanced cases, the spasm spreads to facial muscles.
- Pathogenesis: Tetany is based on an abnormally high frequency of excitation impulses. These signals continuously travel to the muscle via nerve fibers, causing the natural relaxation phase of the muscle fibers to be entirely omitted.
2. Seizures
Seizure disorders are also extremely common, occurring in over 50% of patients with hypocalcemia.
- Definition: These are involuntary, uncontrolled contractions of various muscle groups.
- Specific Feature: Any seizure in this condition is invariably accompanied by significant pain syndrome (severe pain).
- Typology:
- Clonic seizures — characterized by rhythmicity, where a phase of muscle contraction alternates sequentially with a relaxation phase.
- Tonic seizures — distinguished by a prolonged muscle contraction lasting for an extended period without periods of relaxation.
Systemic Organ Dysfunction
Calcium deficiency is not limited to the muscular system alone. The pathology leads to widespread dysfunction of tissues and entire physiological systems. It is important to understand that the primary cause of these organ damages is precisely the electrolyte imbalance, rather than fluid or acid-base shifts.
The organs most vulnerable to changes in calcium and phosphorus levels include:
- Kidneys (the tubular apparatus suffers, and reabsorption processes are disrupted).
- Cardiovascular system, primarily the heart itself, which critically requires a strict balance of calcium ions for normal contraction.
- Other internal organs whose function depends on correct nerve impulse transmission and cellular-level electrolyte equilibrium.