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The Clarus Video System as a useful diagnostic tool

to the thyroid gland or vertebral artery [4], Narouze believes that the inferior thyroid artery is the main culprit. Peng and Narouze have outlined the differences in endpoints between the blind stellate block and those performed under fluoroscopic and ultrasound guidance [2]. Using fluoroscopy, a needle may be accurately directed towards Chassaignac’s tubercle. However, this bony landmark is a surrogate marker for the stellate ganglion, generally being closer to the middle cervical ganglion. The ultrasound method uses the prevertebral fascia as its endpoint [2]. This fascia lies over the vertebral bodies, their anterior transverse processes, the longus colli, capitis and anterior scalene muscles. Peng and Narouze direct the tip of the needle to the prevertebral fascia superficial to the longus colli and inject a total of 5 ml local anaesthetic using real-time scanning to ensure that the spread of injectate is visible during injection. We agree with Dr Narouze that it is advisable to perform a prescan before performing a stellate ganglion block to familiarise oneself with the relevant anatomy, using colour flow Doppler to detect any vessels and plan a safe path for the needle whether inserted in-plane or out-ofplane [2]. We wish to thank the authors of this letter for highlighting that ultrasound guidance alone does not prevent this life-threatening complication of a stellate ganglion block.