Ultrasound-guided lumbar subarachnoid block is utilized in a variety of clinical scenarios as a diagnostic, prognostic, and therapeutic maneuver as well as to provide surgical anesthesia for pelvic and lower extremity surgeries. As a diagnostic tool, ultrasound-guided lumbar subarachnoid block allows accurate placement of the needle tip within the subarachnoid space to perform differential spinal block on a pharmacologic basis to determine if the patient’s lower abdominal, back, groin, pelvic, bladder, perineal, genital, rectal, anal, and lower extremity pain are somatic, sympathetic, or central in origin. As a prognostic tool, ultrasound-guided lumbar subarachnoid block can be utilized as a prognostic indicator of the degree of motor and sensory impairment that the patient may experience if neurodestructive procedures of the spinal cord are being contemplated in an effort to palliate intractable pain in patients too sick to undergo neurosurgical destructive procedures. This technique also provides important prognostic information regarding the side effects of drugs administered into the subarachnoid space for the treatment of pain or spasticity when an implantable drug delivery system is being considered.

In the acute pain setting, ultrasound-guided lumbar subarachnoid block with local anesthetics and/or opioids may be used to palliate acute pain emergencies while waiting for pharmacologic, surgical, and/or antiblastic methods to become effective. This technique has great clinical utility in both children and adults when managing acute postoperative and posttrauma pain when the local anesthetics and/or opioids are administered via a catheter placed into the subarachnoid space. This paramedian oblique approach to the subarachnoid space has an advantage over the midline approach to the subarachnoid space, as the paramedian oblique approach allows the catheter to enter the subarachnoid space at a less acute angle than with the midline approach. This results in less catheter kinking and breakage. Lumbar subarachnoid block is used primarily for surgical and obstetric anesthesia. It is unique among regional anesthesia techniques in that the small amounts of drugs used to perform a successful lumbar subarachnoid nerve block exert essentially no systemic pharmacologic effects.

The lumbar subarachnoid administration of local anesthetic in combination with opioids is useful in the palliation of cancer-related lower abdominal, groin, back, pelvic, perineal, and rectal pain. The long-term subarachnoid administration of opioids via implantable drug delivery systems has become a mainstay in the palliation of cancer-related pain. The role of chronic subarachnoid opioid administration in the management of chronic benign pain syndromes remains controversial.

The spinal cord ends at approximately L2 in the majority of adults and at approximately L4 in most infants (Fig. 109.1). Therefore, in most settings, lumbar subarachnoid nerve block should be performed below these levels to avoid the potential for trauma to the spinal cord. The spinal cord is surrounded by three layers of protective connective tissue: the dura, the arachnoid, and the pia mater (Fig. 109.2). The dura is the outermost layer and is composed of tough fibroelastic fibers that form a mechanical barrier to protect the spinal cord. The next layer is the arachnoid. The arachnoid is separated from the dura by only a small potential space, which is filled with serous fluid. The arachnoid is a barrier to the diffusion of substances and effectively serves to limit the spread of drugs administered into the epidural space from diffusing into the spinal fluid. The innermost layer is the pia, a vascular structure that helps provide lateral support to the spinal cord.

To reach the subarachnoid space, a needle placed via the paramedian approach at the L3-L4 interspace will pass through the skin, subcutaneous tissues, the inner margin of the interspinous ligament, the ligamentum flavum, the epidural space, dura, the subdural space, and arachnoid (Fig. 109.3). Drugs administered into the subarachnoid space are placed
between the arachnoid and pia, although inadvertent subdural injection is possible. Subdural injection of local anesthetic is characterized by a spotty, incomplete block.

Ultrasound-guided lumbar subarachnoid block can be carried out by placing the patient in sitting position (Fig. 109.4). If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. To perform ultrasound-guided lumbar subarachnoid block, a three-step process is used. Although this may seem cumbersome, the three-step process allows the clinician to quickly identify critical anatomic structures while at the same time maintaining a transducer position that allows a safe and easy placement of needles into the lumbar subarachnoid space.

Step One is to obtain a paramedian sagittal transverse process view by placing the 2- to 5-MHz low-frequency curvilinear probe in the longitudinal plane 3 to 4 cm lateral to the right side of the middle of the spinous processes at the level to be blocked for the right-handed clinician and 3 to 4 cm to the later to the left side of the middle of the spinous processes at the level to be blocked for the left-handed clinician (Figs. 109.5 and 109.6). An ultrasound survey is taken, and the transducer is slowly moved medially and laterally until successive transverse processes are visualized. The transverse processes of the lumbar spine will appear as hyperechoic domes with sausagelike acoustic shadows beneath them (Fig. 109.7). This classic appearance of successive transverse processes viewed in the longitudinal plane has been named the “trident sign” after Neptune trident (Fig. 109.8).

After the transverse processes are identified in the paramedian sagittal transverse process view, the ultrasound transducer is slowly slid toward the midline until the superior and inferior articular facets are visualized (Step Two) (Figs. 109.9 and 109.10). In longitudinal paramedian ultrasound articular process view, the superior and inferior articular facets will appear as successive hyperechoic hills and valleys, with the gap in the center of each hill representing a facet joint (Fig. 109.11).