24th June 2023

Renal bone disease

Mechanism

The overall term for this condition and its complications is CKD mineral and bone disorder (CKD-MBD). The pathophysiology centres around levels of calcium and phosphate – this is affected by factors including parathyroid hormone (PTH), 1,25(OH)₂D (calcitriol) and FGF-23 (fibroblast growth factor-23).

Clinical consequences

• Bone – renal osteodystrophy encompasses a range of skeletal abnormalities, including osteomalacia, osteoporosis, osteosclerosis, adynamic bone disease (low turnover state) and osteitis fibrosis cystica (high turnover state).
• Skin/soft tissue – calcinosis cutis, calcium phosphate deposition
• Cardiovascular – vascular calcification/stiffness and left ventricular hypertrophy

Management

The aim of management strategies in CKD-MBD is to maintain normal serum levels of calcium and phosphate and prevent very raised or low PTH levels (aim 2-9 times the upper limit of normal) to maintain normal bone metabolism and prevent parathyroid gland hyperplasia.

• Dietary phosphate restriction – foods high in phosphate include dairy products and eggs.
• Phosphate binders – must be taken with meals to allow the binding of phosphate in the gut. Examples include calcium carbonate (note concerns about the calcium load in patients at risk of vascular calcification) and non-calcium agents such as sevelamer or lanthanum carbonate.
• Sodium bicarbonate – acidosis exacerbates renal bone disease; it can be improved with oral replacement.
• Renal replacement therapy – treatments such as regular dialysis can reduce phosphate levels
• Vitamin D analogues – 1α-hydroxylated analogues such as alfacalcidol are often used. It is important that phosphate levels are corrected first and to monitor the patient for side effects of hypercalcaemia or hyperphosphataemia.
• Cinacalcet – used in tertiary hyperparathyroidism, it is a calcimimetic that stimulates the calcium-sensing receptors of the parathyroid gland, reducing PTH secretion.
• Parathyroidectomy – surgical intervention is reserved for severe cases where medical management has failed.