Disorders of Calcium-phosphate metabolism (paediatrics, learning outcomes)
Z Profiles
Disorders of Calcium-phosphate metabolism (diagnosis)
Calcium and Phosphorus Metabolism Disorders (Learning Outcomes)
1. Explain the principle of calcium regulation
Serum calcium concentration is strictly regulated by a hormonal system acting on the bones, kidneys, and intestines to maintain homeostasis:
- Parathyroid Hormone (PTH): The primary hormone for increasing blood calcium levels.
- Bone: Stimulates osteoclastic resorption (release of Ca²⁺ and phosphates into the blood).
- Kidneys: Increases Ca²⁺ reabsorption while simultaneously increasing phosphate excretion (phosphaturia).
- Vitamin D: Stimulates the conversion of Vitamin D to active calcitriol in the kidneys.
- Vitamin D (Calcitriol): Increases the availability of both calcium and phosphorus.
- Intestine: Significantly increases the resorption of Ca²⁺ and phosphates.
- Kidneys: Promotes their reabsorption.
- Bone: Supports bone tissue mineralization.
- Metabolism: Skin (UVB) / Food → Liver (25-OH) → Kidneys → (1,25-(OH)₂ = calcitriol).
- Calcitonin: Produced by thyroid C-cells; lowers blood calcium levels (inhibits bone resorption and increases renal excretion). Its role in human calcium regulation is minor compared to PTH.
2. Describe the main physiological roles of calcium and phosphorus
- Calcium (Ca): Mineralization of the skeleton and teeth, muscle contraction, neuromuscular transmission, blood coagulation (Factor IV), and intracellular signaling.
- Phosphorus (P): Component of bone mineral (hydroxyapatite), energy metabolism (ATP), and a fundamental part of nucleic acids (DNA/RNA) and cell membranes (phospholipids).
3. List the causes and clinical manifestations of hypocalcemia
Definition: A drop in serum calcium concentration below the age-appropriate reference range.
- Causes: Vitamin D deficiency, malabsorption (e.g., celiac disease), hypoparathyroidism, chronic renal failure, hyperphosphatemia, or hypomagnesemia.
- Clinical Presentation (Neuromuscular Excitability):
- Paresthesia (tingling), muscle spasms, tetany.
- Laryngospasm (acute respiratory distress).
- Chvostek’s sign (tapping the facial nerve → twitch) and Trousseau’s sign (inflating a BP cuff → "obstetrician's hand").
- In newborns: Apnea or seizures.
4. Describe the basic principles of hypocalcemia therapy
- Acute Condition: Intravenous administration of calcium gluconate (slowly!) under continuous ECG monitoring (risk of arrhythmias/cardiac arrest).
- Chronic Condition: Vitamin D substitution, oral calcium supplementation, and treating the underlying cause.
5. List the causes and clinical manifestations of hypercalcemia
Definition: An elevation of serum calcium concentration above the reference range.
- Causes: Primary hyperparathyroidism, Vitamin D intoxication, malignancies (bone destruction), sarcoidosis, or long-term immobilization.
- Clinical Presentation: Muscle weakness, fatigue, constipation, nausea, polyuria and polydipsia (impaired renal concentrating ability), cardiac arrhythmias, and in severe cases, disorders of consciousness.
- Treatment: Aggressive rehydration, loop diuretics (furosemide), bisphosphonates, and calcitonin.
6. State the causes of phosphorus metabolism disorders
- Hyperphosphatemia: Common in chronic renal insufficiency (kidneys fail to excrete) and hypoparathyroidism.
- Hypophosphatemia: Malnutrition, Vitamin D deficiency, or renal losses.
- Refeeding Syndrome: A dangerous condition in malnourished children. Upon restarting nutrition, phosphates shift rapidly into cells, leading to severe hypophosphatemia.
7. Define rickets and describe its underlying causes
Definition: A mineralization disorder of growing bone in the area of the growth plate. It is essential to distinguish between nutritional, genetic, and renal etiologies.
- Rickets vs. Osteoporosis: Rickets involves a growth plate mineralization defect; osteoporosis involves a reduction in bone mass with normal mineralization.
- Nutritional Rickets: Caused by Vitamin D/Calcium deficiency. Risks: lack of sunlight, exclusive breastfeeding without supplementation, malabsorption (Celiac, IBD).
- Labs: ↓ Ca²⁺ (or normal due to secondary hyperparathyroidism), ↓ P , ↑ Alkaline Phosphatase (ALP), ↑ PTH, ↓ 25-OH Vitamin D.
- Genetic Rickets:
- Vitamin D Dependent: Type I (1 alpha-hydroxylase deficiency) or Type II (tissue resistance).
- Hypophosphatemic: Renal phosphate wasting (mostly X-linked).
- Renal Rickets (Renal Osteodystrophy): Occurs in chronic kidney disease due to decreased calcitriol production and phosphate retention.
8. Explain the clinical picture, basic diagnostics, and therapy of rickets
- Clinical Features: Delayed fontanelle closure, craniotabes (softening of skull bones), rachitic rosary (swelling at the costochondral junctions), chest deformities (Harrison’s groove), genu varum/valgum (bow-legs or knock-knees), and growth retardation.
- Diagnostics: Laboratory tests (Ca, P, ALP, PTH, 25-OH Vitamin D) and X-ray of long bones (widening/cupping/irregularity of metaphyses).
- Therapy: High-dose Vitamin D supplementation, calcium intake, and addressing the primary cause.
| Laboratory parameter | Nutritional rickets
(vitamin D deficiency) |
Vitamin D dependent rickets | Hypophosphatemic rickets (genetic) | Renal osteodystrophy |
|---|---|---|---|---|
| Calcium (Ca) | ↓ or N | ↓ | N | ↓ or N |
| Phosphorus (P) | ↓ | ↓ | ↓ | ↑ |
| Alkaline phosphatase (ALP) | ↑ | ↑ | ↑ | ↑ |
| PTH | ↑ (secondary hyperparathyroidism) | ↑ | N or slightly ↑ | ↑ |
| 25-OH vitamin D | ↓ | N | N | N or ↓ |
| 1,25-(OH)₂ vitamin D (calcitriol) | ↓ or N | ↓ (type I) / ↑ (type II) | N or ↓ | ↓ |
| Phosphaturia | N | N | ↑ | N or ↑ |
| Kidney function (creatinine) | N | N | N | ↑ |
8. Imaging Benefits and Treatment of Rickets
Benefits of Long Bone X-rays:
X-rays are the gold standard for confirming the diagnosis of rickets and assessing the severity of bone involvement. Typical radiographic changes include:
- Widening and irregularity of the metaphyses: The "growth zones" appear fuzzy and frayed.
- Cup-shaped (cupping) deformation of the metaphyses: Instead of a straight line, the end of the bone looks concave like a saucer.
- Decreased bone mineralization: The bones appear more transparent (osteopenia) with thinning of the cortex.
Therapy:
- Vitamin D substitution: High doses (stoss therapy or daily drops) to replenish stores.
- Calcium supplementation: To ensure enough mineral is available for new bone formation (especially during the "hungry bone" phase of recovery).
- Dietary and lifestyle changes: Increasing dairy intake and ensuring adequate (but safe) sunlight exposure.
9. Define Osteoporosis in Children
Definition: A systemic skeletal disorder characterized by low bone mass and microarchitectural deterioration of bone tissue, leading to increased bone fragility and risk of fractures.
- Types: Primary (genetic, e.g., Osteogenesis Imperfecta) and Secondary (acquired).
Most common causes of Secondary Osteoporosis (at least 3 examples):
- Chronic glucocorticoid treatment: Steroids inhibit osteoblasts and decrease intestinal calcium absorption.
- Chronic inflammatory diseases: (e.g., JIA, IBD) where inflammatory cytokines stimulate bone resorption.
- Immobilization: Lack of mechanical loading on bones (e.g., in children with cerebral palsy).
- Malabsorption syndromes: (e.g., Celiac disease) leading to Vitamin D and Calcium deficiency.
Diagnostics:
- Dual-energy X-ray Absorptiometry (DXA): The primary tool to assess Bone Mineral Density (BMD). In children, results are interpreted using Z-scores (comparing the child to peers of the same age and sex).
10. Propose a Diagnostic Procedure for Phosphocalcic Disorders
Basic Biochemical Screening:
- Total Calcium & Ionized Calcium: Ionized is the biologically active fraction.
- Phosphorus ($P$): To check for deficiency or renal retention.
- Alkaline Phosphatase (ALP): A marker of bone turnover.
- PTH & 25-OH Vitamin D: To assess hormonal regulation and nutritional stores.
Significance of Additional Tests:
- Albumin: Necessary to calculate "corrected calcium" if total calcium is used.
- Blood pH: Acidosis increases ionized calcium; Alkalosis decreases it (potentially causing tetany).
- Urea and Creatinine: To evaluate kidney function (critical for Vitamin D activation and phosphate excretion).
- Magnesium: Low magnesium can cause functional hypoparathyroidism.
- Urinary Calcium and Phosphorus: To determine if the kidneys are wasting minerals.
Imaging:
- Long bone X-rays (for rickets) and DXA (for bone density).
11. Acute and Long-term Treatment
Acute Treatment of Severe Hypercalcemia:
- Rehydration: Aggressive I.V. saline to increase renal calcium clearance.
- Loop diuretics (Furosemide): To further promote calcium excretion.
- Bisphosphonates: To stop bone resorption.
- Calcitonin: For a rapid (though temporary) decrease in serum calcium.
Acute Treatment of Severe Hypocalcemia:
- Intravenous Calcium (Calcium Gluconate): Administered slowly via a large vein.
- ECG Monitoring: Mandatory to detect QT interval changes or arrhythmias.
Long-term Treatment:
- Maintenance Vitamin D and Calcium supplementation.
- Treatment of the underlying disease (e.g., gluten-free diet for Celiac, treating underlying kidney disease).
