Statural growth and serum-P in hypophosphataemic rickets: 41

Abstract

Top of pageAbstract In cross-sectional studies the average height of 37 untreated boys and girls with X-linked hypophosphataemic rickets (X-LH) between 1 and 13 years of age was 2 standard deviations below the normal mean, but there was no relation between height and serum-P in single determinations (Steendijk and Latham, Helv Paed Acta 26: 179, 1971). Sustained increased growth rates have only rarely been observed during therapy with high doses of vitamin D or dihydrotachysterol, and it is not known whether those occurred following a sustained rise in serum-P. Catch-up growth during treatment with oral phosphate supplements, causing a rise in serum-P, has more often been seen, but only in short-term studies. To establish a relation between the level of serum-P during childhood and adolescence, and adult height in a long-term longitudinal study 8 patients with X-LH were followed for periods of 12 to 17 years until adult height had been reached. The patients were treated with dihydrotachysterol in high doses but did not receive phosphate supplements. Since serum-P was not constant during the years of treatment it was necessary in every patient to derive an average value from the serum-P determinations. This was done in the following manner: serum-P was determined at least 4 times per year in each patient, and the average serum-P value per year of age was calculated. From these mean values, the average serum-P (av. serum-P) for the whole period of observation was determined. This final value was correlated with adult height, expressed as standard deviation score (SDS). The coefficient of correlation (r) was 0.789 (p < 0.05) and the regression equation was: Height (SDS) = 7.64 × av.serum-P - 24.51 Av. serum-P ranged from 2.77 to 3.17 mg/100ml and adult height ranged from 0.04 to -3.34 SDS. Within this range of values a reduction of av.serum-P by as little as 0.1 mg/100ml would entail a reduction of adult height by 0.77 SDS (1 SDS equals 6.8 cm in men and 6.2 cm in women). This relatively large effect of a small change in serum-P may help to explain why a relation between serum-P and height has not been found before.