Osmotic demyelination syndrome is a rare entity characterized by noninflammatory demyelination afflicting the central pons, basal ganglia, thalami, peripheral cortex, and hippocampi. Histopathologically, there is a destruction of myelin sheaths sparing the underlying neuronal axons due to the susceptibility of oligodendrocytes to rapid osmotic shifts often encountered in chronically debilitated patients. We present the temporal progression of signal abnormalities on sequential magnetic resonance imaging (MRI) scans in a middle-aged male initially presenting with altered mental status due to severe hyponatremia and subsequently developing rigidity, diagnosed as osmotic demyelination syndrome based on typical imaging findings and supportive laboratory parameters. This case demonstrates the importance of diffusion signal abnormalities as the first indicator of osmotic demyelination, evolution of extrapontine before pontine signal changes and the relative lack of correlation between patient's clinical profile and the extent of MRI signal abnormalities.

Observations on the blood pressure of a group of 660 displaced male Tibetans revealed striking dissimilarities in regard to the mean systolic and diastolic figures as compared to those reported for Europeans and Americans. The average Tibetan figures are lower than those of Western populations; however, a comparable variability after the fourth decade and a parallel spread of values with age and weight were found in the Tibetan data. The socioeconomic status and height bore no relation to the blood pressure levels. The results obtained from this survey on the Tibetans are compared with those of other Mongolian groups. It is suggested that the differences of blood pressure among related Mongolian groups and populations studied in the West are largely determined by environment.

JBmO of the circumstances which depress the rate of clearance of citrate from the plasma are present, by having available blood collected without the use of citrate. Heparin may be used-though this is often undesirable, particularly in vascular surgery-or blood may be collected over an ionexchange resin (Walter, 1950). The use of commercial packs for the latter purpose has been satisfactory in our hands, though they are expensive and the blood may be safely stored for only about 10 days. SummaryHigh plasma citrate levels occurred during the infusion of citrated blood into four patients undergoing surgery under hypothermia. One

We thank Mr. M. P. Curwen for statistical advice and Miss W. Smith for her help in the follow-up study.

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