Premature infants undergo intensive growth during the postnatal period. Adequate mineralization is dependent on sufficient intake of calcium (Ca) and phosphorus (P). However, Ca and P supplementation can be associated with some risks, for example development of nephrocalcinosis. We investigated pathophysiological risk factors in premature very low birth weight (VLBW) infants associated with the development of nephrocalcinosis. From June 1994 to September 1995 all preterm neonates with a birth weight below 1,500 g were screened prospectively. At regular intervals of 2 weeks, ultrasonography (US) of the kidneys was performed and parameters of mineral metabolism were assessed in blood and spot urine samples. For analysis, premature infants with nephrocalcinosis (group N) were compared with infants without nephrocalcinosis (group R) and with a retrospectively pair-matched subgroup of premature infants without nephrocalcinosis (control group C) taken from the same study. Nephrocalcinosis was detected in 20 of 114 preterm neonates (group N, 17.5%). Of these 20 infants with nephrocalcinosis, 16 presented with a tendency towards systemic acidosis (pH<7.25) on day 2-7, compared with only 4 of 20 premature infants of the control group. Premature infants of group N had a lower serum P at 2 weeks of life and 5 (versus 0 patients of the control group C) had transient hypophosphatemia (serum P<1.6 mmol/l). Moreover, the Ca/creatinine ratio in spot urine specimens tended to be higher (P<0.1) in patients developing nephrocalcinosis. There were no significant differences in the duration of ventilation, the length of stay in the intensive care unit, and duration and frequency of furosemide and steroid treatment between the groups N and C. VLBW premature infants developing nephrocalcinosis frequently presented with slightly impaired acid-base homoeostasis within the 1st week, followed by signs of impaired mineralization (and immature or impaired renal function) within 2 weeks. In VLBW premature infants, close observation of acid-base status and regular analysis of spot urine specimens (Ca, P, creatinine) during the first weeks of life may help to identify those premature infants at risk for nephrocalcinosis.

The results of our study show that, despite effective patient immobilisation, an increase in needle positioning accuracy is limited by the method of stereotactic 3-D navigation. Effects such as modification of body shape caused by the needle application morphologic alterations, and inherent inaccuracies within the navigation system have an important influence upon accuracy, which it is not yet possible to calculate.

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