CHAPTER 8: Ultrasound Imaging of the Lumbar Spine for Central Neuraxial Blocks

INTRODUCTION

Central neuraxial blocks (CNBs), which include spinal, epidural, and combined spinal epidural (CSE) injections, are frequently performed in the lumbar region for anesthesia and analgesia and for managing chronic pain. ¹ Traditionally, they are performed using a combination of surface anatomic landmarks, the operator's tactile perception of “loss of resistance” during needle advancement through the ligamentum flavum, and/or visualizing the efflux of cerebrospinal fluid. Anatomic landmarks (eg, the spinous processes) are useful but they are not always easily palpable in patients with edema, obesity, ² underlying spinal deformity, or previous back surgery. The “Tuffier's line,” which is a line joining the highest points of the iliac crests, is another surface anatomical landmark that is widely used to estimate the location of the L4 to L5 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. ^4–7 This inaccuracy is exaggerated in the obese and in the upper spinal levels. ^4,6,8 Furthermore, using surface anatomical landmarks alone, it is not possible to 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 prevalent in clinical practice. ^9,10

Recently, ultrasound imaging of the spine ^11–13 has emerged as a useful tool to overcome many of the shortcomings of the traditional approach to CNBs, and it has been used with great success. Ultrasound is most frequently used as a preprocedural tool, ¹¹ but can also be used to guide the epidural or spinal needle in real time during CNBs. ¹⁴ Advantages of the preprocedural scan include being able to accurately locate the midline, ¹⁵ identify a given lumbar interspace, predict the depth to the epidural space, detect any vertebral rotational defects (eg, in scoliosis), and identify patients with a potentially difficult CNB. ^11,16 In expert hands the use of ultrasound for epidural needle insertion reduces the number of puncture attempts, ^17–22 improves the success rate of epidural access on the first attempt, ¹⁸ reduces the need to puncture multiple levels, ^18–20 and improves patient comfort during the procedure. ¹⁹ This chapter briefly outlines the anatomy, the technique of ultrasound imaging, and sonoanatomy relevant for CNBs in the lumbar region.

BASIC LUMBAR SPINE ANATOMY

The lumbar spine makes up the lower back and is made up of five vertebra, numbered L1 to L5 (Figs. 8–1 and 8–2). It connects with the thoracic spine above and with the sacrum below at the lumbosacral joint. L1 to L4 are typical lumbar vertebrae because they share common characteristics, but L5 is atypical because it has certain peculiarities. The lumbar ...