To the Editor,

With improvements in ultrasound (US) technology, there is significant momentum to develop and apply a clinically viable real-time ultrasound-guided neuraxial technique.1,2 Previously described methods using in-plane approaches have been hindered by poor needle visibility due to the steep needle insertion angles required for neuraxial blockade, the acoustic shadows of the bony spinal structures, and the fine gauge of spinal needles. We recently used a cadaver model to develop a new technique involving single-operator real-time US-guided needle-through-needle spinal injection.

Two unembalmed cadavers, one female and one male, were obtained in accordance with the University of British Columbia Clinical Research Ethics Board. A SonixTablet™ ultrasound system (Ultrasonix, Richmond, BC, Canada) was used in an out-of-plane approach with either a 5-14 MHz SonixGPS™ enabled linear transducer or a 2-5 MHz convex transducer, depending on the depth to the neuraxial space.

The SonixGPS™ system (Ultrasonix, Richmond, BC, Canada) uses sensors in the needle and ultrasound probe to track the needle position relative to the ultrasound image. Predicted trajectory and location of the needle tip are displayed digitally in real-time and overlaid on the ultrasound image.

Two anesthesiologists with experience using the SonixGPS™ system for regional anesthesia used the technology for the first time for a neuraxial technique. With the cadavers in a prone position, the anesthesiologists performed 16 injections between T2/3 and L4/5. For each simulated spinal block, they obtained an optimal sonographic view of the laminae and neuraxial structures in a paramedian oblique sagittal plane. A SonixGPS™ enabled 8-cm 19G introducer needle was inserted out-of-plane and directed with the needle guidance technology towards the ligamentum flavum. Once the tip was positioned near the ligamentum, the inner stylet containing the needle sensor was removed, and a 12-cm 22G spinal needle was inserted through the introducer in a needle-through-needle technique. The spinal needle was then advanced to the neuraxial space and coloured acrylic dye was injected through the needle (see Figure). The spinal needles were left in situ to guide subsequent dissection by a blinded anatomist.

Figure
figure 1

The needle is inserted into the neuraxial space of the cadaver with ultrasound guidance and dye is injected

Full size image

All 16 spinal needles and injections were found to be at the target upon dissection. The anatomist confirmed presence of dye in the neuraxial space in 100% of attempts, and there was no dye found in the tissues outside the neuraxial space. There was excellent visualization of spinal sonoanatomy in 63% of attempts and good visualization in the remainder. The SonixGPS™ system allowed prediction of the appropriate needle direction to target the neuraxial space in 100% of attempts. Mean (standard deviation) depth to the neuraxial space on US was 3.21 (0.71) cm and 4.74 (0.59) cm for the female and male cadavers, respectively. Importantly, all 16 attempts required only a single skin puncture with minimal requirement for redirection.

SonixGPS™ needle guidance technology facilitated real-time ultrasound-guided neuraxial injection in cadavers with accuracy and reproducibility. Our out-of-plane approach enabled the needle to be inserted in a traditional paramedian trajectory, and SonixGPS™ needle guidance technology overcame problems of poor visibility of the needle tip at steep insertion angles. Limitations of the cadaver included the immobile spine and a lack of cerebrospinal fluid. Given that the proceduralists were experienced with the new technology and our study occurred in a non-clinical setting, we accept that the generalizability of this technique may be limited at the present time. Nevertheless, our preclinical series in cadavers using the new SonixGPS™ needle guidance system has provided valuable insights, and a clinical phase is now underway.