# Chronic Kidney Disease-Mineral Bone Disorder in Growing and Aging Skeletons

## Introduction

**Chronic kidney disease-mineral bone disorder (CKD-MBD)** is a systemic disorder of mineral and bone metabolism arising from CKD. It encompasses abnormalities in **calcium, phosphorus, PTH, and vitamin D metabolism**, along with bone disease and vascular calcification. In children, CKD-MBD disrupts skeletal growth, linear growth, and development. In adults, it accelerates cardiovascular disease and fracture risk. Med-peds physicians must understand the pathophysiology, monitoring, and age-appropriate management of this condition.

## Pathophysiology

### The CKD-MBD Axis

Declining GFR leads to **phosphorus retention**, reduced **1,25-dihydroxyvitamin D** synthesis, and decreased calcium absorption. **FGF23** rises early (CKD stage 2-3), promoting phosphaturia and suppressing 1-alpha-hydroxylase. **Secondary hyperparathyroidism** develops as PTH increases to maintain calcium and phosphorus homeostasis. Elevated FGF23 and PTH contribute to **left ventricular hypertrophy** and cardiovascular mortality. Progressive disease leads to **tertiary hyperparathyroidism** with autonomous PTH secretion.

### Bone Disease Classification (Renal Osteodystrophy)

**High-turnover disease (osteitis fibrosa cystica)**: Driven by elevated PTH; increased bone resorption and formation. **Low-turnover disease (adynamic bone disease)**: Over-suppression of PTH; increased fracture risk; more common in adults, particularly diabetics. **Osteomalacia**: Defective mineralization due to vitamin D deficiency or aluminum accumulation. **Mixed uremic osteodystrophy**: Features of both high and low turnover. Gold standard diagnosis: **Bone biopsy** with tetracycline labeling (rarely performed in practice)

![Pathophysiology of CKD-MBD with the FGF23-PTH-vitamin D axis](illustration-ckd-mbd-pathophysiology.jpg)

## Pediatric-Specific Considerations

### Growth Impairment

CKD-MBD is a major contributor to **short stature** in children with CKD. Growth failure results from metabolic acidosis, nutritional deficiency, GH resistance, and bone disease. **Growth hormone therapy** is indicated for children with CKD and persistent growth failure despite optimized nutrition and metabolic control. **Rickets-like changes**: Widened, frayed, cupped metaphyses on radiographs; genu valgum or varum deformities.

### Bone Development

The growing skeleton is uniquely vulnerable; growth plate disturbances lead to permanent deformity. **Slipped capital femoral epiphysis (SCFE)** is a recognized complication of renal osteodystrophy in children. Skeletal deformities may require orthopedic intervention if medical management is insufficient. Peak bone mass acquisition is impaired, increasing lifelong fracture risk.

### Monitoring in Children

**Calcium, phosphorus, alkaline phosphatase**: Starting at CKD stage 2; frequency increases with advancing CKD. **PTH**: Measured starting at CKD stage 3; target levels are stage-dependent (KDIGO guidelines) **25-hydroxyvitamin D**: Maintain >30 ng/mL; supplement with cholecalciferol or ergocalciferol. **Radiographs of wrists and knees**: Assess for rickets-like changes and growth plate abnormalities. Growth velocity monitoring with attention to height velocity Z-scores.

## Adult-Specific Considerations

### Cardiovascular Implications

**Vascular calcification**: Both FGF23 and hyperphosphatemia are independent risk factors for cardiovascular mortality. Coronary artery calcification is prevalent even in young adults with CKD. **Calciphylaxis**: Rare but devastating condition with calcification of small dermal vessels leading to ischemic skin necrosis; associated with warfarin use, obesity, and high calcium-phosphorus product. CKD-MBD management is integral to cardiovascular risk reduction in CKD.

### Fracture Risk

CKD patients have 2-4 times higher fracture risk than the general population. **DXA** has limited utility in CKD stages 4-5 due to inability to distinguish renal osteodystrophy subtypes. Fractures in CKD patients are associated with higher morbidity and mortality. Bisphosphonates are generally avoided in CKD stage 4-5 due to adynamic bone disease risk; denosumab may cause severe hypocalcemia in advanced CKD.

### Monitoring in Adults

**Calcium, phosphorus**: Every 6-12 months in CKD 3; every 3-6 months in CKD 4-5 and dialysis. **PTH**: Every 6-12 months in CKD 3-5; every 3-6 months on dialysis. Target PTH on dialysis: 2-9 times the upper limit of normal (KDIGO) **Alkaline phosphatase**: Trends help assess bone turnover activity.

## Management

### Phosphorus Control

| Agent | Type | Advantages | Disadvantages |
|-------|------|-----------|---------------|
| Calcium carbonate | Calcium-based binder | Inexpensive; corrects hypocalcemia | Vascular calcification risk; hypercalcemia |
| Calcium acetate | Calcium-based binder | Better phosphorus binding per calcium load | Same calcification concerns |
| Sevelamer | Non-calcium polymer | No calcium load; may lower LDL | Pill burden; GI side effects; cost |
| Lanthanum carbonate | Non-calcium binder | Effective; chewable | Long-term tissue deposition concerns |
| Sucroferric oxyhydroxide | Iron-based binder | Low pill burden; effective | Dark stools; GI side effects |

**Dietary phosphorus restriction**: Limit to 800-1000 mg/day; prioritize reducing processed food and additive phosphorus. **Phosphate binders**: Calcium-based (calcium carbonate, calcium acetate) -- limit use due to vascular calcification risk; non-calcium-based (sevelamer, lanthanum, sucroferric oxyhydroxide) preferred in adults. In children, palatability and pill burden affect adherence; liquid formulations and binder choice are important. Dialysis adequacy contributes to phosphorus removal.

### Vitamin D Therapy

**Nutritional vitamin D** (cholecalciferol/ergocalciferol): Replete deficiency in all CKD stages. **Active vitamin D analogs** (calcitriol, paricalcitol, doxercalciferol): For secondary hyperparathyroidism in CKD stages 3-5. Monitor calcium closely during active vitamin D therapy to avoid hypercalcemia. Paricalcitol and doxercalciferol may have less calcemic effect than calcitriol.

### Calcimimetics

**Cinacalcet**: Allosteric activator of the calcium-sensing receptor; lowers PTH, calcium, and phosphorus. Used in dialysis patients with secondary hyperparathyroidism not responsive to other therapies. **Etelcalcetide**: IV calcimimetic administered at dialysis sessions; improves adherence. Not approved for pediatric use, though off-label use occurs in refractory cases.

### Parathyroidectomy

Indicated for refractory hyperparathyroidism unresponsive to medical therapy. Subtotal or total parathyroidectomy with autotransplantation. Post-surgical **hungry bone syndrome**: Severe hypocalcemia requiring aggressive calcium and vitamin D supplementation.

![Management algorithm for CKD-MBD across CKD stages](illustration-ckd-mbd-management-algorithm.jpg)

![Radiographic comparison of renal osteodystrophy in pediatric and adult patients](illustration-renal-osteodystrophy-xray.jpg)

## Clinical Pearls

FGF23 elevation is the earliest detectable abnormality in CKD-MBD, preceding changes in calcium, phosphorus, or PTH. In children, growth failure and rickets-like skeletal changes are hallmarks of CKD-MBD and require aggressive management. Vascular calcification in adults with CKD is an active, regulated process driven by hyperphosphatemia and is not simply passive calcium deposition. Over-suppression of PTH leads to adynamic bone disease with increased fracture risk; the target is a range, not the lowest possible value. Transition of pediatric CKD patients to adult care must include transfer of CKD-MBD management plans, growth history, and skeletal monitoring.

## References

1. KDIGO. Clinical practice guideline update for the diagnosis, evaluation, prevention, and treatment of CKD-MBD. *Kidney Int Suppl*. 2017;7(1):1-59.
2. Wesseling-Perry K, Salusky IB. Chronic kidney disease: mineral and bone disorder in children. *Semin Nephrol*. 2013;33(2):169-179.
3. Block GA, Klassen PS, Lazarus JM, et al. Mineral metabolism, mortality, and morbidity in maintenance hemodialysis. *J Am Soc Nephrol*. 2004;15(8):2208-2218.
4. Bacchetta J, Schmitt CP, Ariceta G, et al. Recommendations for the management of CKD-MBD in children with CKD stages 4-5 and on dialysis. *Pediatr Nephrol*. 2023;38(11):3539-3557.