Pathogenesis and etiological factors
The development of the pathological process may be associated with genetic defects in the polypeptide chains responsible for the normal conversion of the vitamin in the body.
The cascade of main pathogenetic links is formed in the following sequence:
- Onset of vitamin D deficiency.
- A drop in the production of its active metabolite, calcitriol (D-hormone).
- Decreased intestinal calcium absorption.
- Development of hypocalcemia followed by the activation of compensatory mechanisms.
The role of vitamin D in calcium-phosphorus metabolism
The active metabolite of vitamin D performs key regulatory functions in the body:
- Stimulation of parathyroid hormone secretion: enhances bone tissue resorption, which ensures the maintenance of physiological calcium concentration in the bloodstream.
- Renal effects: increases calcium reabsorption and stimulates phosphate excretion, preserving an optimized Ca/P balance.
- Extraskeletal action: via specific calcitriol receptors located in numerous organs, the hormone regulates cell division, growth, and differentiation processes, and participates in hormonal synthesis.
Clinical manifestations and bone deformations in rickets
Rickets is the primary consequence of vitamin D deficiency. Disturbances in calcium-phosphorus metabolism lead to a whole group of structural skeletal pathologies.
| Type of disorder | Pathogenetic essence and clinical picture |
|---|---|
| Impaired mineralization | Impaired mineralization manifests as osteoporosis and an increased frequency of fractures. |
| Osteomalacia | The process of bone element resorption, leading to skeletal softening. |
| Bone deformations | Formation of "rachitic rosary" (thickenings at the cartilage-bone junction), curvature of the spine and limbs, as well as softening of the flat bones of the skull. |
Dental and muscular disorders
D-hormone deficiency affects not only the skeletal system but also other tissues:
- Dental status: there is a distinct delay in the eruption of primary teeth, as well as dentin formation defects.
- Muscle hypotonia: a drop in muscle tone is associated with impaired innervation, metabolic disorders in muscle fibers, and disturbed assimilation of key micronutrients—Ca, P, and Mg.
Briefly: answers to topical questions
What types of hereditary defects in vitamin D metabolism exist?
The following types of hereditary defects in vitamin D metabolism are described in literature:
- Recessive renal α1-hydroxylase deficiency, leading to delayed development and rachitic skeletal changes (treated with high doses of vitamin D).
- Defect in cellular calcitriol receptors, where the clinical picture is similar to α1-hydroxylase deficiency, but is additionally accompanied by alopecia, epidermal cysts, and muscle weakness.
How do blood biochemical parameters (calcium, phosphorus, alkaline phosphatase) change in established rickets?
The degree of change in blood biochemical parameters depends on the severity of rickets, reflecting the depth of vitamin D deficiency:
- Grade I severity: calcium and phosphorus levels are within normal limits or slightly decreased, alkaline phosphatase (ALP) is elevated.
- Grade II severity: phosphorus is steadily decreased (hypophosphatemia), calcium is at the lower limit of normal or decreased, ALP continues to rise.
- Grade III severity: failure of compensation, pronounced drop in both phosphorus and calcium (hypocalcemia), maximum ALP values are observed.
Deficiency pathogenesis chain: Deficiency -> Calcitriol ↓ -> Absorption of Ca ↓ -> Hypocalcemia (D-C-A-H).
Class and exam questions
What reduction does vitamin D deficiency lead to at the initial stage of pathogenesis?
The primary key link is a drop in the synthesis of the active metabolite—calcitriol (D-hormone), which subsequently leads to a decrease in intestinal calcium absorption.
What effect does vitamin D have on renal function?
In the renal tubules, it promotes increased calcium reabsorption and enhances phosphate excretion to maintain the Ca/P ratio.
What explains the development of muscle hypotonia in rickets?
Muscle hypotonia is caused by disorders of their innervation, disturbances in local metabolic processes, and impaired assimilation of calcium, magnesium, and phosphorus.
What is the "rachitic rosary"?
These are characteristic bone deformations in the form of thickenings that form at the border of bone and cartilage tissues.
What else to study on this topic
- Genetic defects in vitamin D metabolic polypeptides
- Molecular mechanisms of calcitriol action on cell division and differentiation
- Pathophysiology of osteomalacia and osteoporosis in calcium-phosphorus disorders
- The role of parathyroid hormone and bone resorption in maintaining calcium levels
- Mechanisms of impaired dentin development in vitamin D deficiency
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