Renal Osteodystrophy — Laboratory and Radiological Abnormalities and Treatment

Renal osteodystrophy is the spectrum of bone histological abnormalities occurring in chronic kidney disease. It is the skeletal component of the broader syndrome CKD–Mineral and Bone Disorder (CKD–MBD), which also includes biochemical abnormalities and vascular/soft-tissue calcification.

Definitive classification is by bone biopsy using the TMV system — Turnover, Mineralisation, Volume.

High-turnover disease: osteitis fibrosa cystica from secondary hyperparathyroidism. Low-turnover disease: adynamic bone disease (low turnover, normal mineralisation) and osteomalacia (low turnover, defective mineralisation). Mixed uraemic osteodystrophy combines both. Dialysis patients may also develop β2-microglobulin amyloid bone disease.

Falling GFR → phosphate retention → rise in FGF-23 (the earliest abnormality) with Klotho deficiency.

FGF-23 and reduced renal mass suppress 1-α-hydroxylase → low calcitriol → reduced intestinal calcium absorption → hypocalcaemia.

Hypocalcaemia, hyperphosphataemia and calcitriol deficiency all stimulate PTH → secondary hyperparathyroidism, with parathyroid hyperplasia and down-regulation of calcium-sensing and vitamin D receptors; autonomous nodular hyperplasia produces tertiary hyperparathyroidism with hypercalcaemia.

Adynamic bone disease results from over-suppression of PTH — by aggressive calcium and vitamin D loading, high-calcium dialysate, calcimimetics, diabetes, ageing and peritoneal dialysis. It is now the commonest lesion, and is important in the elderly because it causes fractures and drives vascular calcification.

Aluminium (from phosphate binders or dialysate) causes osteomalacia and microcytic anaemia — historical but still examinable.

Supporting tests: serum aluminium (with desferrioxamine test if indicated), β2-microglobulin in long-term dialysis, and bone-specific alkaline phosphatase plus intact PTH as the practical non-invasive discriminators. Bone biopsy with tetracycline double labelling remains the gold standard, reserved for unexplained fractures, hypercalcaemia, suspected aluminium disease, or before parathyroidectomy.

Subperiosteal resorption at the radial border of the middle phalanges of the index and middle fingers — the earliest and most specific sign; also at the distal clavicle, symphysis pubis, sacroiliac joints and medial proximal tibia. Terminal phalangeal acro-osteolysis with tuft resorption.

"Rugger-jersey spine" — alternating sclerotic bands at the vertebral end-plates with a lucent centre; and a "salt-and-pepper" skull with a granular mottled vault.

Brown tumours — expansile lytic lesions (osteoclastomas) in mandible, ribs, pelvis or long bones, which may fracture and can be mistaken for metastases.

Osteomalacic changes: Looser's zones (pseudofractures) at the medial femoral neck, pubic rami, scapula and ribs; coarse trabeculae, biconcave vertebrae.

Extraskeletal calcification: vascular (Monckeberg medial), periarticular tumoral calcinosis, cardiac valve and corneal calcification; calciphylaxis (calcific uraemic arteriolopathy) presents as painful violaceous livedoid skin necrosis with high mortality.

Modern assessment: lateral abdominal radiograph and echocardiography for vascular and valvular calcification (KDIGO-endorsed); DXA is now recommended in CKD G3a–G5D to predict fracture, though it cannot distinguish the type of renal osteodystrophy.

Control phosphate first — dietary phosphate restriction (avoid processed food with inorganic phosphate additives, which is highly absorbable), adequate dialysis, and phosphate binders: calcium carbonate/acetate (limit total elemental calcium, avoid if adynamic bone or vascular calcification), sevelamer, lanthanum carbonate, and iron-based binders (sucroferric oxyhydroxide, ferric citrate). Avoid aluminium binders except short-term.

Correct native vitamin D deficiency with cholecalciferol; then, for persistently high and rising PTH, use active vitamin D analogues (calcitriol, alfacalcidol) or the less calcaemic paricalcitol / doxercalciferol.

Calcimimetics — oral cinacalcet or IV etelcalcetide (dialysis) — lower PTH, calcium and phosphate simultaneously by sensitising the calcium-sensing receptor; the drug of choice when PTH is high with hypercalcaemia.

Target PTH in dialysis patients at approximately 2–9 times the upper limit of normal for the assay — deliberately not normal, because normal PTH in CKD predicts adynamic bone.

Parathyroidectomy for severe refractory secondary or tertiary hyperparathyroidism with hypercalcaemia, intractable pruritus, calciphylaxis, or progressive bone disease. Anticipate hungry bone syndrome with profound post-operative hypocalcaemia.

Adynamic bone disease is managed by doing less: reduce calcium load, lower dialysate calcium, withdraw or reduce active vitamin D and calcimimetics, and allow PTH to rise into the target range.

Osteoporosis therapy in CKD: bisphosphonates are generally avoided below eGFR 30–35 mL/min and can worsen adynamic bone; denosumab is not renally cleared but causes severe hypocalcaemia in advanced CKD and needs close monitoring with calcium and vitamin D loading; teriparatide may help low-turnover disease but is contraindicated where PTH is already high.

General measures: correct metabolic acidosis with bicarbonate, treat hypogonadism, encourage weight-bearing exercise and resistance training, prevent falls, avoid aluminium-containing antacids, and manage calciphylaxis with sodium thiosulfate, wound care and withdrawal of calcium, vitamin D and warfarin.