A 58-year-old woman with Stage 5 CKD on hemodialysis (eGFR 8 mL/min/1.73m²) for 18 months presents with bone pain and muscle weakness. Laboratory investigations show: serum calcium 7.8 mg/dL (low), serum phosphate 6.2 mg/dL (high), iPTH 420 pg/mL (elevated), alkaline phosphatase 95 U/L (normal), 25-OH vitamin D 18 ng/mL (low). Bone biopsy shows increased osteoid volume and delayed mineralization. X-ray of the hand shows subperiosteal resorption. What is the most likely diagnosis?

Explanation

## CKD-Mineral Bone Disorder: Osteitis Fibrosa Cystica **Key Point:** Osteitis fibrosa cystica is the high-turnover bone disease of secondary hyperparathyroidism in CKD, characterized by excessive osteoid formation, increased bone resorption, and fibrosis of the bone marrow. ### Pathophysiology of Secondary Hyperparathyroidism in CKD ```mermaid flowchart TD A["Declining GFR<br/>(Stage 4-5 CKD)"]:::outcome --> B["↓ Phosphate excretion<br/>↑ Serum phosphate"]:::outcome B --> C["↓ Calcitriol production<br/>by failing kidneys"]:::outcome C --> D["↓ Serum calcium"]:::outcome D --> E["↑ PTH secretion<br/>by parathyroid glands"]:::outcome E --> F{"PTH effect on bone"}:::decision F -->|"Excessive<br/>osteoblast activation"| G["↑ Bone turnover<br/>↑ Osteoid formation"]:::action G --> H["Osteitis fibrosa cystica<br/>HIGH-turnover disease"]:::outcome E --> I["↑ FGF23 secretion"]:::outcome I --> J["Further ↓ calcitriol<br/>Perpetuates cycle"]:::outcome ``` ### Distinguishing High-Turnover from Low-Turnover Bone Disease | Feature | Osteitis Fibrosa Cystica (High-Turnover) | Adynamic Bone Disease (Low-Turnover) | |---------|------------------------------------------|--------------------------------------| | **PTH level** | Markedly elevated (>300 pg/mL) | Normal or mildly elevated | | **Bone biopsy: Osteoid volume** | **Increased** | Decreased | | **Bone biopsy: Mineralization** | Normal or slightly delayed | **Delayed** | | **Bone resorption** | **Marked** (subperiosteal, intracortical) | Minimal | | **Bone marrow fibrosis** | **Present** (fibrosa = fibrosis) | Absent | | **Alkaline phosphatase** | Elevated | Normal or low | | **X-ray findings** | **Subperiosteal resorption**, salt-and-pepper osteopenia | Generalized osteopenia | | **Cause** | Severe secondary hyperparathyroidism | Over-suppression of PTH (excessive calcitriol/cinacalcet) | ### Why This Patient Has Osteitis Fibrosa Cystica **Clinical Pearl:** The combination of: - **Markedly elevated iPTH (420 pg/mL)** — driving excessive bone turnover - **Increased osteoid volume on biopsy** — hallmark of high-turnover disease - **Delayed mineralization** — PTH-driven osteoblasts produce osteoid faster than it can mineralize - **Subperiosteal resorption on X-ray** — pathognomonic for secondary hyperparathyroidism - **Low calcium + high phosphate** — classic CKD-MBD picture driving PTH secretion **High-Yield:** "Fibrosa" in the name refers to marrow fibrosis caused by chronic PTH excess and increased bone remodeling. This is a **high-turnover** disease, not low-turnover. ### Management 1. **Phosphate binders** (calcium carbonate, sevelamer, lanthanum carbonate) 2. **Vitamin D analogs** (calcitriol, paricalcitol) — cautiously, to avoid hypercalcemia 3. **Calcimimetics** (cinacalcet) — to suppress PTH 4. **Parathyroidectomy** — if medical management fails and PTH remains >600–800 pg/mL **Warning:** Over-suppression of PTH with excessive vitamin D or cinacalcet can swing the pendulum toward **adynamic bone disease** (low-turnover), which has poor fracture healing. [cite:KDIGO 2017 Clinical Practice Guideline Update for CKD-MBD; Harrison 21e Ch 279]