Chapter 40: Spinal Sonography and Applications of Ultrasound for Central Neuraxial Blocks

INTRODUCTION

Central neuraxial blocks (CNBs), which include spinal, epidural, combined spinal epidural (CSE), and caudal epidural injections, are commonly practiced regional anesthesia techniques and frequently used in the perioperative period for anesthesia and analgesia and for managing chronic pain.¹ Traditionally, CNBs are performed using a combination of surface anatomical landmarks, the operator’s perception of tactile sensation (loss of resistance) during needle advancement, and/or visualizing the free flow of cerebrospinal fluid. Although the spinous processes are relatively reliable surface anatomical landmarks in many patients, they are not always easily recognizable in patients with obesity,² edema, underlying spinal deformity, or previous back surgery. Tuffier’s line, which connects the highest points of the iliac crests, is another surface anatomical landmark that is widely used to estimate the location of the L3–L4 interspace; however, the correlation is inconsistent.³ Even in the absence of spine abnormalities, estimation of a specific intervertebral level may not be accurate in many patients^4,5 and may result in needle placement one or two spinal levels higher than intended.^4,6,7 The difficulty in identifying the correct spinal level is exaggerated in patients with obesity and in the upper spinal levels.^4,6,8 This inaccuracy has been implicated in cases of injury to the conus medullaris after spinal anesthesia.^6,8 Moreover, surface landmarks alone do not allow the operator to reliably predict the ease or difficulty of needle placement prior to skin puncture. Unanticipated technical difficulty, multiple attempts at needle placement, and failure of CNB are therefore not uncommon.^9,10 Recently, however, ultrasound (US) imaging of the spine has emerged as a useful method of overcoming many of these shortcomings of the surface landmark–guided approach to CNBs.

US imaging offers several advantages when used to guide needle placement during CNBs. It is noninvasive, safe, simple to use, can be performed expeditiously at the point of care, provides real-time images, is devoid of significant adverse effects, and is particularly helpful in delineating abnormal or variant spinal anatomy. When used for chronic pain interventions of the spine, US can reduce or eliminate exposure to ionizing radiation. Presently, US is most frequently used as a preprocedural tool,¹¹ but it can also be used for real-time needle guidance during CNBs.¹² During the preprocedural scan, one can accurately locate the midline,¹³ identify a given lumbar interspace, predict the depth to the epidural space, and identify patients in whom a CNB may be difficult.^11,14 In expert hands, the use of US for epidural needle insertion reduces the number of puncture attempts,^15-20 improves the success rate of epidural access on the first attempt,¹⁶ reduces the need to puncture multiple levels,^16,17,18 and improves patient comfort during the procedure.¹⁷ However, despite its advantages, the integration of US into clinical practice for CNBs is still in its infancy. ...