Quantitative ultrasound of bone and markers of bone turnover in Cushing's syndrome

Quantitative ultrasound (QUS) of bone is a valuable tool in the assessment of postmenopausal osteoporosis. QUS and new markers of bone turnover have been poorly assessed in Cushing's syndrome, however. Twenty-five patients with Cushing's syndrome (20 women, 3 men; mean age +/- SEM: 38 +/- 2 years) were studied and compared with 35 age- and sex-matched control patients (mean age +/- SEM: 38 +/- 2 years). The following variables were measured in both groups: QUS parameters at the heel (BUA; SOS; Stiffness index, SI); bone mineral density (BMD) at both the lumbar spine (LS) and femoral neck (FN) by dual-energy X-ray absorptiometry; and serum markers of bone turnover (osteocalcin, procollagen type I N- and C-terminal propeptides (PINP and PICP), bone alkaline phosphatase (BAP), procollagen type I C-terminal telopeptide (ICTP) and urinary type I collagen C-telopepetide breakdown products (CTX)). Both BUA and SI were decreased in patients with Cushing's syndrome (p<0.01) but not SOS (p=0.08). BMD was also strongly decreased in Cushing's syndrome, at both the LS and FN (p<0.005). The two markers of bone turnover statistically significantly different between the two groups were osteocalcin (mean +/- SEM: 3.5 +/- 0.7 ng/ml (Cushing's syndrome) vs 6.4 +/- 0.5 ng/ml (controls, p<0.01)) and CTX (mean <plus/minus> SEM: 148.7 +/- 17.1 mug/mmol Cr (Cushing's syndrome) vs 220.8 +/- 22.9 mug/mmol Cr (controls), p<0.05). The areas under the receiver operating characteristic curve (AUC) were 0.72 (BUA), 0.73 (SI), 0.90 (BMDLS), 0.81 (BMDFN), 0.83 (osteocalcin) and 0.64 (CTX) respectively. AUC was significantly higher for BMDLS than for both BUA and SI (p<0.05). Conversely AUC was not statistically significantly different for BMDFN as compared with either BUA or SI. AUC was also higher for osteocalcin than for other markers of bone turnover. In conclusion, QUS of bone seems to be a relevant tool for assessing bone involvement in Cushing's syndrome. QUS does have a lower sensitivity compared with DXA, however, and the relevance of QUS cannot be ascertained until some longitudinal data are forthcoming. Except for CTX, the other new markers of bone turnover assessed in this study (PINP, PICP, BAP and ICTP) do not seem of interest in Cushing's syndrome.