Environmental temperature and the risk of subarachnoid hemorrhage.

Stroke welcomes Letters to the Editor and will publish them, if suitable, as space permits. They should not exceed 1000 words (excluding references) and may be subject to editing or abridgment. Please submit letters in duplicate, typed double-spaced. Include a fax number for the corresponding author. To the Editor. Like Lejeune and associates, 1 we have found an association between low environmental temperature and the risk of subarach-noid hemorrhage. 2 In our study, which was carried out in King County, Wash, the meteorologic variable with the strongest relation was dew point. Dew point is the temperature to which a given parcel of air must be cooled at a constant pressure and constant water vapor content for saturation to occur. 3 We hypothesize that exposure to cold was the actual risk factor, probably because of changes in blood pressure, but that people were more likely to expose themselves to low temperatures on clear days when the dew point was low than on rainy or overcast days when the temperature was the same but the dew point was higher. Thus, low dew point would be associated more strongly than temperature alone with an increased risk of subarachnoid hemorrhage. We wonder whether the investigators from France also found an association of sub-arachnoid hemorrhage with dew point. Response We are very pleased to learn that other researchers corroborate our findings on the influence of weather conditions on aneurysmal bleeding. 1 The relation of dew point to subarachnoid hemorrhage was not evaluated in our study; in our opinion, the humidity level, to which the dew point is closely related, is a more independent variable. The publication of more studies on this subject will perhaps help us better understand these yet mysterious correlations. Association of occurrence of aneurysmal bleeding with meteo-rologic variations in the north of France. To the Editor We read with great interest the recent article by Bunegin et al 1 presenting a transcranial Doppler (TCD) technique for detecting and sizing air emboli by spectral analysis and signal power measurements. Their article raises a number of issues on which we would like to comment. 1. It is interesting that Bunegin et al 1 find no significant power spectrum feature above 500 Hz, for either intrinsic blood flow (expected peak Doppler shifts, 700 to 2000 Hz diastolic/systolic) or air bubbles, for which they expected frequencies of over 2500 Hz. It would be very helpful if …