Introduction
Prevalence of metastatic pulmonary calcification (MPC) among patients with end stage renal disease (ESRD) at autopsy was 79% (1). However, it is rarely diagnosed clinically. We report an interesting case of metastatic pulmonary calcification who presented with respiratory failure.
Case
Fifty-eight-year-old Chinese male o and was restarted on haemon haemodialysis since 1984 presented in 2015 with 1-month history of progressive dyspnoea. He had COPD. He underwent renal transplant in 1987. His kidney allograft failed after 20 yearsdialysis. Over that 30-year period, he developed secondary hyperparathyroidism (SHPT). Parathyroidectomy was offered but declined. In 2009, CT thorax showed bilateral upper lung fields patchy ground-glass opacities. These ground-glass opacities demonstrated uptakes on Technetium-99m bone scan in keeping with calcification. In 2009, his corrected serum calcium (cCa) was 2.51 mmol/L, Phosphate (Po4) 1.32 mmol/L, iPTH 39.7 pmol/L, and alkaline phosphatase (ALP) was 120 U/L. On this admission, he presented with increasing dyspnoea. His chest X-ray (CXR) showed extensive areas of pulmonary calcification, progressively increased in the intensity and distribution when compared to previous CXR, sparing only the left lower zone. There was right pleural calcification, mural calcification of the aorta. CT thorax showed dense calcified opacities with lower density-ground glass opacities on both lung fields. When compared to previous CT, the changes have significantly worsened, with the calcified opacities markedly larger/ more extensive than before. There was worsening of right pleural calcification. Bone scan (99Tc-MDP) showed patchy tracer uptakes in both lung fields, consistent with metastatic pulmonary calcification. His cCa in 2015 was 2.64 mmol/L, Po4 1.71 mmol/L, ALP 763U/L and iPTH 273 pmol/L. Spirometry showed a severe obstructive pattern and a rapid decline of FEV1 of 150ml/year, which could not be attributed to COPD alone. He was treated with Cinacalcet, Sodium thiosulfade, Lanthanum, Pamidronate, and alcohol ablation of parathyroid adenoma with overall improvement in his biochemical markers after 3 months of therapy. Throughout his admission, he was oxygen dependent. His symptoms persisted and died from respiratory failure.
Discussions:
Metastatic pulmonary calcification (MPC) was thought to occur as a result of passive calcium salt deposition in the lung interstitium. Its precipitation in the lungs’ parenchyma tends to occur in the upper lobes in which the distribution and was thought to be related to higher ventilation-perfusion ratio resulting in lower partial pressure of carbon dioxide and therefore a higher pH in the upper lung zones (2). Precipitation of calcium in the soft tissue during post dialysis alkalotic state occur mainly in the upper lung zones. However, recent researches suggest the process of calcification occurs in a complex, active, regulated manner in which mesenchymal stem cells transform into distinct “osteoblast-like” cell. Deposition of hydroxyapatite with high degree of crystallization takes place and hence causing calcification in normal tissue. It remains unknown if MPC follows similar pathogenesis (3).
Most patients with MPC remain asymptomatic. There have been reports of MPC resultant respiratory failure, which was seen in our case (4).
The diagnosis of MPC remains a challenge. It is commonly seen in autopsy among the patients with ESRD, however, this is rarely diagnosed clinically. A normal CXR does not exclude MPC (5). MPC may show bilateral airspace opacities, usually without sufficient density to suggest calcification, and often mimics pulmonary oedema and pneumonia. In our case, florid dense calcification changes on CXR suggested late features of MPC. CT thorax may have multiple diffuse calcified nodules; ground-glass opacities or consolidation, or confluent high-attenuation parenchymal consolidation involving mainly the upper lung zones (6,7). Magnetic resonance imaging is relatively insensitive for MPC and not recommended (8). Bone scan (99Tc-MDP) is sensitive, specific and less expensive, in detection of pulmonary calcification (9). It manifests as increase uptake of the radioisotope.
Our patient had long standing secondary hyperparathyroidism with multiple co-morbidities which put him into high risk for total parathyroidectomy. Despite having radiological changes, he remained asymptomatic for 6 years. Aggressive medical therapy to normalize calcium, phosphate, iPTH to slow down bone turnover did not seem to reverse MPC nor reverse his respiratory failure. There was suggestion that parathyroidectomy may improve vascular calcification (10). From our previous publication, four out of five patients with MPC developed after parathyroidectomy. We also showed patients with MPC had relatively low to normal iPTH, and normal ALP (11). Brandeburg et al also reported similar findings of low median iPTH levels among dialysis patients with calciphylaxis in their registry (12).
In conclusion, MPC is a rare complication of CKD-MDB and may result in respiratory failure. More research into its underlying pathophysiology is required to improve the prevention and treatment of MPC.
Take home message
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