Evaluation of bone mineral density with dual x-ray absorptiometry for osteoporosis in children with bladder augmentation

Abstract

Background/purpose: Osteoporosis can appear as a result of metabolic acidosis in patients with bladder augmentation and total bladder replacement. These patients must be monitored for early diagnosis and osteoporosis to avoid related complications such as fracture. The current method for diagnosis of osteoporosis mainly involves bone densitometry. Dual x-ray absorptiometry (DXA) provides bone mineral content and bone mineral density (BMD). The purpose of this report is to determine the value of BMD measurement with DXA in the diagnosis and follow-up of osteoporosis and evaluation of response to treatment in the patients with bladder augmentation and total bladder replacement.

Methods: Six patients with bladder extrophy and neurogenic bladder underwent colocystoplasty, ureterocystoplasty, and total bladder replacement. The Sigmoid colon segment was used for colocystoplasty. DXA (Hologic 2000 DXA) was used for measuring bone mineral content and density. Results for lumbar spinal BMD were expressed as the average of L1 through L4 values. Each patient's BMD was compared with the mean BMD in the young normal population (T score) and in the age- and sex-matched group (Z score). But because our patients were children, BMD was evaluated according to Z score. Arterial blood gas analysis was obtained periodically with BMD measurement. Arterial blood pH and HCO(3) levels of the patients were compared with normal values, which ranged from 7.35 to 7.45 for pH and from 22 to 27 mmol/L for HCO3.

Results: Arterial blood pH of 5 of 6 patients (83.3%) and HCO3 levels of all patients were low. Ten BMD measurements were obtained for 6 children. One patient underwent BMD measurement 3 times, 2 patients underwent twice, and the others once. Z scores of 4 of 6 patients (66.6%) were decreased. Blood pH, HCO3 level, and BMD of 2 patients increased after oral bicarbonate intake. Two patients had normal BMD. One of these patients had ureterocystoplasty. The other had undergone colocystoplasty but was incontinent.

Conclusions: Intestinal segments have been used for bladder augmentation and total bladder replacement. Exposure of highly absorptive intestinal mucosa to urine has been associated with electrolyte abnormalities and metabolic acidosis. Hyperchloremic acidosis leads to bone demineralization. DXA provides bone mineral content and BMD for diagnosis of osteoporosis. DXA is preferred because the low radiation dose (average, 2 to 4 mrem), accuracy, low price, and short examination time. Not only can BMD confirm the diagnosis of osteoporosis, but it also can be used to monitor the course of the disease and the effectiveness of treatment.