CHAPTER 5: Ultrasound Imaging of the Spine: Basic Considerations

INTRODUCTION

Ultrasound has revolutionized the practice of regional anesthesia, particularly peripheral nerve blockade, and it has also been used for central neuraxial blocks (spinal and epidural injections). ^1–3 However, the use of ultrasound for central neuraxial blocks is still in its infancy and not as popular ⁴ as that for peripheral nerve blocks. The reasons for this are not clear, but may be related to the high success rate of landmark-based techniques, limited data on ultrasound for neuraxial blocks, perceived difficulty in performing spinal sonography, limited acoustic window for ultrasound imaging, and poor understanding of spinal sonoanatomy. However, recently published data suggest that ultrasound is beneficial for central neuraxial blocks. Identification of a given lumbar intervertebral level for central neuraxial block using surface anatomical landmarks (Tuffier line) is often imprecise, ⁵ and ultrasound is more accurate than clinical assessment. ⁶ It can also be used to accurately measure the depth to the epidural space or thecal sac ^7–9 and predict the ease of performing a neuraxial block. ¹⁰ Ultrasound also offers technical advantage by reducing the number of puncture attempts, ^11–14 improves the success rate of epidural access on the first attempt, ¹² reduces the need to puncture multiple levels, ^12–14 and improves patient comfort during the procedure. ¹³ Ultrasound may also be beneficial for central neuraxial blocks in patients with difficult (ie, abnormal or variant) spinal anatomy. ^15,16 Therefore, it is envisioned that the use of ultrasound for central neuraxial blocks will grow in the near future. A sound knowledge of the anatomy of the spine is a prerequisite for understanding the sonoanatomy of the spine. In this chapter, we describe general details of spine anatomy and basic considerations relevant for spinal sonography and central neuraxial blocks.

BASICS OF SPINE ANATOMY

The human spine or vertebral column is made up of 33 vertebrae—7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal—that are stacked on top of each other (Fig. 5–1). A typical vertebra has unique features (Fig. 5–2), and they differ at different levels (Figs. 5–3 to 5–5). The number of spinal nerves in the thoracic, lumbar, and sacral region corresponds to the number of vertebra, each lying below the corresponding vertebra. In the cervical region there are eight spinal nerves. The first seven spinal nerves lie above the corresponding vertebra, but the eighth cervical nerve lies below the seventh cervical vertebra. In the coccygeal region there is only one coccygeal nerve.