Editor,—We were interested to read the findings of Voss and Bailey concerning diurnal variation in the stature of children.1 De Puky was the first to suggest that changes in stature, which lengthens during sleep and shortens during daytime, were due to fluctuations in the water content of the intervertebral discs.2 Adams et al postulated that increased fluid within the discs resulted in the spine being more resistant to flexion immediately after a period of rest.3 Magnetic resonance imaging (MRI) has demonstrated diurnal variation in the water content of the discs.4 5 We have applied the Cavalieri method of modern design stereology in combination with MRI to obtain unbiased estimates of the volume of the lumbar intervertebral discs immediately at the end of a day of normal activity and again after a night’s rest, in seven women of average age 21 years (range 19–23 years).6 In addition, pixel-by-pixel mapping of the relaxation time has been used to assess the water content and its distribution within the intervertebral discs in this group. The mean increase in height of the subjects, measured with a stadiometer, was 19.3 mm (range 8–26 mm) overnight. Image analysis showed that the mean overnight increase in volume of lumbar discs was 1300 mm3. The increase in disc volume was accompanied by an increase in the T2 relaxation time of the nucleus pulposus. This suggests that the change in disc volume is probably caused by a preferential increase in the water content of the nucleus pulposus. Voss and Bailey observed a mean loss in height of 31 mm between the morning and afternoon.

This difference in diurnal variation in height between children and adults could be attributed to greater concentrations of purified proteoglycan macromolecules in the nucleus pulposus of children compared with adults,7 allowing more drawing in of water into the intervertebral discs during periods of rest.