Conference Lectures
Ultrasound for Neuraxial block: CON
Dr Anju Gupta
Specialist (Anesthesiology),
Lok Nayak hospital and associated MAM college, New Delhi
Ultrasound has indispensible role in regional anaesthesia and analgesia for peripheral nerve blockade in modern era. Ultrasound-guided neuraxial blockade is a useful technique that can help practitioners more accurately identify intervertebral levels, estimate depth to the epidural space, and locate an appropriate interlaminar space for needle insertion. However, ultrasound for neuraxial block is associated with many potential limitations.
Incorrect identification of the lumbar intervertebral level has been implicated in conus medullaris injury after dural puncture. The cord and conus medullaris can be identified on ultrasound in the young pediatric population because the outer surface and central canal of the spinal cord are visible as bright hyper-echoic lines. These details are not visible in adults because of the greater depth and narrower acoustic windows into the spinal canal. The spinal cord and surrounding cerebro-spinal fluid have a similar hypoechoic appearance on ultra-sound and thus, the conus medullaris cannot be localized on ultrasound in adults.
Compared with other imaging modalities, such as magnetic resonance imag- ing,computed tomography, and plain radiographs of the lumbar spine, ultrasound accurately identifies a spinous process or intervertebral space only 68–76% of the time. Errors are also more likely in the early stages of learning to perform ultrasonography of the spine, and accuracy rates of 90% or greater probably can be achieved with adequate training and experience. Errors usually result from misidentification of the L5–S1 junction or failure to recognize developmental anomalies of the lumbosacral junction, which occur in approximately 12% of the general population.
Currently, little has been published about the clinical utility of Ultrasound guided technique in thoracic spine. A preprocedural scan may provide information that will facilitate thoracic epidural catheter insertion though currently no published data is available to support or refute this assertion. Grau et al. performed an imaging study in which they demonstrated that though it was feasible to identify the pertinent anatomic landmarks with ultrasound imaging, however visualization of the epidural space was much more difficult than that of the lumbar spine.
Visualization of the deeper structures in the vertebral canal can be difficult in certain patient populations where it might have actual clinical utility. In obese patients, structures are often less distinct because of the attenuation that occurs as ultrasound waves travel a greater distance through soft tissue. A phase aberration effect caused by the varying speed of sound in the irregularly shaped adipose layers also has been described. The problem in elderly patients is narrowing of the interspinous spaces and interlaminar spaces caused by ossification of the interspinous ligaments and hypertrophy of the facet joints, respectively. Prominent spinous processes in a thin patient also can hinder adequate skin-probe contact and contribute to poor visualization. In such patients, obtaining a transverse view of the vertebral canal may be difficult or impossible.
There is an inherent degree of inaccuracy when marking the needle insertion point on the
skin during the preprocedural scan. Currently available curved-array probes do not have markings that precisely indicate from where the ultrasound beam emanates. There is
also an element of tissue distortion when performing the ultrasound scan, particularly in the elderly, who often have loose and mobile skin. Finally, skin marking does not indicate the caudad-to-cephalad angle at which the needle must be advanced in a midline approach. This can be estimated only from the angulation of the probe required to produce an optimal image of the interlaminar space.
Extensive experience with the ultrasound-guided technique may be required before competence is attained. In virtually all published studies to date, ultrasound imaging has been performed by a small number of experienced investigators. Preliminary studies on determining the learning curve for ultrasonography of the lumbar spine suggest that once basic knowledge on has been acquired, experience with 40 or more cases may be required to attain competence in scanning. This needs to be confirmed by larger and more robust studies. Additional research is also needed to determine the learning curve associated with the actual performance of a successful ultrasound-guided neuraxial block and optimal training strategies. Novel spine phantom models have been described that permit scanning and needle insertion to be practiced in a workshop setting. However, no data exist to demonstrate how effective these models are at knowledge and skills translation.
Most studies of ultrasound-guided neuraxial blockade have used preprocedural ultrasound imaging instead of real time ultrasound guided approach. The real-time approach is demanding technically, and more data are required before it can be recommended for routine use. There is also a risk of introducing ultrasound gel into the epidural or intrathecal space, the safety implications of which are unclear.
Conclusion
At this time, we do not believe the technique should supplant the traditional surface landmark-based techniques of spinal and epidural anesthesia which are simple, safe, and effective in most patients.
References
- Prasad GA, Tumber PS, Lupu CM: Ultrasound guided spinal anesthesia. Can J Anaesth 2008; 55:716 –7
- Yeo ST, French R: Combined spinal-epidural in the obstetric patient with Harrington rods assisted by ultrasonography. Br J Anaesth 1999; 83:670 –2
- Karmakar MK: Ultrasound for central neuraxial blocks. Tech-niques in Regional Anesthesia and Pain Management 2009;13:161–70
- Chin KJ, Chan V: Ultrasonography as a preoperative assess-ment tool: Predicting the feasibility of central neuraxial blockade. Anesth Analg 2010; 110:252–3