Knock signals in ischemic stroke.

Stroke welcomes Letters to the Editor and will publish them, if suitable, as space permits. They should not exceed 750 words (including 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 and a completed copyright transfer agreement form (available online at To the Editor: I have described a new Transcranial doppler ultrasonogra-phy finding " small vessel knock (SVK) " and have proposed that this is an obstructive arterial signal. 1,2 SVK is an asymmetrical high-intensity low-velocity signal found at the baseline occurring at aortic valve opening and closure with the signals moving in opposite directions. SVK has a characteristic appearance on single-gated and M-mode spectra and can be distinguished from wall movement, which is symmetrical. 3 In the February edition of Stroke, Chung et al 4 report the finding of an asymmetrical SVK-like intraluminal signal in vitro, at the site of obstruction, in both silastic and glass tubing. They found that the intensity of this signal increased as the concentration of ultrasound scatterers in the lumen increased. This shows that asymmetrical SVK-like signals are intraluminal obstructive ultrasound signals. Symmetrical " knock signals " were also found because of wall movement, and these increased in intensity toward the site of obstruction. The authors stated that the lumen diameter used in these models (4 mm main and 3 mm branch for silastic tubes, 3 mm main and 2 mm branch for glass tubes) does not represent the expected size of small arterial bifurcations found in vivo. However, SVK-like signals have recently been found in association with vascular occlusion ranging from likely small vessel occlusion up to internal carotid artery occlusion. 5 In the same edition of Stroke, Tsivgoulis et al 6 reported findings in 327 patients with stroke or TIA who were examined for " knock signals. " No SVK-like signals were identified or found at depths likely to be responsible for MRI abnormalities found in these patients and only symmetrical wall movement knock signals were identified in 14% of patients. The authors did not use DWL–Compumedics TCD machines, which have been used to detect SVK in all previously published cases. A filter of Ͼ100 Hz was used, which unfortunately in most cases is too high for SVK detection. This is illustrated in Figure 1, where the SVK signal is visible at a precise depth of 74 …