Rhythmic oscillations with a wavelength of 0.5-2 min in transcranial Doppler recordings

We have studied intracranial pressure (ICP) B-waves and their association with rhythmic changes in blood flow velocity (B-wave equivalents) by transcranial Doppler sonography (TCD) monitoring. In overnight TCD recordings in 10 normal young adults, these rhythmic changes in blood flow velocity were higher and more frequent during REM sleep and sleep stage 1 than during other sleep stages. B-wave equivalents also had a longer wavelength during REM sleep. Their relative frequency in these normal subjects over one night ranged from 35 to 73%. Peripheral resistance (assessed by the Pourcelot index) was lower and heart rate was higher at the peak of these oscillations. These results support the hypothesis that ICP B-waves are caused by vasodilation. A non-linear relationship between ICP and blood flow velocity was found during B-waves in 9 of 11 patients with suspected NPH. Our results throw doubt on the suggestion that a relative frequency of less than 80% B-wave activity can be a valid indicator for shunt responsiveness in patients with suspected normal pressure hydrocephalus (NPH). ICP recordings in suspected NPH should be accompanied by polysomnography to avoid misleading results due to variability of B-wave appearance dependent on sleep pattern.