Nerve injury during ultrasound guided regional anaesthesia (UGRA) can be attributed to local anaesthetic toxicity,1 needle trauma,2 ischaemia,3 pressure4 and nerve haematoma.5 the key intervention that dictates the presence and extent of nerve injury is the position of the needle tip.
Histology from the anaesthetised rat model shows that most damage occurs following subperineural injection of local anaesthetic.1 In contrast, two laboratory studies6 7 and several clinical studies,8 9–11 have not demonstrated functional impairment following intraneural injection of local anaesthetic. Failure to show functional side effects has been interpreted by some that intraneural injection is safe, as long as the needle tip lies subepineural and that subperineural injection is avoided.10 11
The feature that may explain this dichotomy, and links animal and human studies, is inadequate visibility of the interaction between the needle tip and target tissue.
Precise needle tip location is difficult to visualise during in-vivo animal research, even after dissection of skin, muscle and fascia. In rats, for example, needles were inserted under the guidance of a dissecting microscope and the subepineural and subperineural position of the needle tip subjectively judged by the mode of nerve expansion in response to fluid injection.1 12 In larger animals such as pigs intraneural injection was seen as uniform nerve expansion on ultrasound images,13 14 but differentiation between subepineural and subperineural injection14 was not possible.
Visualisation in clinical practice is further impaired by the physical nature and interaction of needles, tissue and ultrasound. Needle tip visibility reduces with increased angle and depth.15 Nerve, muscle and adipose tissue have similar acoustic impedance and ultrasound reflections and ultrasound images may be difficult to interpret.16–18 Ultrasound energy attenuates with depth and transducer frequency and reduces image resolution, thus reducing fine anatomical detail.19 For example using a 10MHz ultrasound transducer, resolution is limited to a maximum of 300 μm.
The clinical impact of inadequate needle tip visibility and inaccurate local anaesthetic injection20 is that the incidence of nerve damage has not decreased compared to peripheral nerve stimulation techniques despite at least a decade of routine ultrasound use.21 Surveys show an incidence of nerve damage between 4 in 10,000 and 9 in 10,000 patients.22 23
There is a clear need for technology that improves the safety of ultrasound guided regional anaesthesia and provides accurate and reliable high resolution, real time images of needle/tissue interaction for clinical practice and research. Microultrasound, defined as transducer frequencies >30 MHz, offers the potential to enhance visibility and improve patient safety. For example a 30 MHz transducer is capable of resolutions of 100 μm and a 40 MHz transducer capable of resolution down to 75 μm.
Microultrasound applied to the skin surface, however, does not improve nerve resolution because high frequency energy attenuates with increased depth and frequency and optimal imaging is limited to 1 to 1.5 cm depths.
Logic dictates that ultrasound is taken as close to the target as possible in order to optimise image resolution. That requires positioning of ultrasound elements at the distal end of the needle rather than on a transducer applied to the skin.
Likely images derived from ultrasound at the end of a needle are presented. High resolution images of nerves may be obtained by dissection of tissue and application of single element needles or as a commercialmultiple array transducers used for small animal and biological research (Visualsonics, Eglinton, Toronto). Alternatively OCT or spectroscopy may also be incorporated into epidural needles. table 1 summarises the literature currently available.
Both Chiang et al and our group used A-Mode ultrasound to image the epidural space in pigs. Characteristic reflections are seen from the ligaments and dura in figure 1A. Our group also developed a neurosurgical biopsy needle that operated using M-Mode ultrasound. In a pig back model, we inserted a single element 40MHz ultrasound needle in the midline. Reflections from tissue interfaces were observed with movement of the needle (figure 1B). the deeper the needle, the closer the reflections to the tip of the needle (figure 1C). Application of a 15MHz ultrasound probe to a dissected pig back gave a high resolution image of the epidural space in figure 1D. Application of microultrasound to imaging of fresh and soft embalmed Thiel cadaver nerves using a single element 40MHz transducer showed that the distribution and size of poorly echogenic regions on ultrasound images corresponded to fascicles on histology slides (figure 1E).
figure 1F shows a 21g regional anaesthesia needle close to the epineurium of the axillary median nerve in an anaesthetised pig model. the image was obtained by dissecting through tissue and exposing the brachial plexus. a 40MHz transducer (Visualsonics, Toronto, Canada) was placed over the target nerve, separated by a thin muscle or gelatin bridge and the ultrasound image optimised by engineers.
Good images of needle-nerve interface were consistently obtained by all operators and in all pigs.
Alignment of the shaft with the needle was more difficult than normal because the microultrasound beam was relatively narrow.Reflection from the needle shaft sometimes obscured deeper tissue, but did not interfere with tip visibility.Nerves rotated in response to tangential needle alignment, whereas epineurium indented in response to perpendicular force. We observed four different responses to intraneural injection. They included: uniform nerve swelling; peripheral, crescent-like nerve swelling; epineural rupture comprising intraneural and extraneural swelling; and uniform swelling with failure to return to pre-injection morphology on needle withdrawal.
Needles encountered high resistance on the perineurium and rotated laterally to lie between fascicles. on needle withdrawal, neural tissue demonstrated elasticity. Relaxation to pre-injection morphology occurred in 85% of cases.
Thus, the magnitude and extent of nerve trauma may be attributed to the position of the needle tip. We observed cleaving of epineurium when the needle tip was positioned peripherally in the nerve, and global expansion of the nerve when the needle tip was placed centrally. Nerve rupture tended to occur with higher pressures generated by second 0.5ml increments. the principal safety mechanism that prevented nerve damage was rapid rotation of nerves and fascicular bundles in response to needle contact.
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