Neuraxial Block

Introduction

In many patients, epidural and spinal blocks are routine procedures guided by loss of resistance and confirmation of free flow of cerebrospinal fluid, respectively. However, in patients with obesity or advanced age, these neuraxial blocks can be more challenging and may benefit from imaging guidance. Ultrasound can estimate the location and level of spinous interspaces. There also is evidence that ultrasound guidance improves the learning curve and reduces epidural failure rates of residents in training. Ultrasound imaging has been reported as useful for guiding neuraxial anesthetics in patients with prior surgical instrumentation or scoliosis. However, there remain current limitations to the use of ultrasound technology to guide neuraxial blocks.

Neuraxial imaging with ultrasound can be difficult because of the depth of the structures of interest and the surrounding bone. The narrow acoustic window makes online approaches (imaging during needle placement) inherently challenging. Simultaneous ultrasound imaging and needle placement for neuraxial procedures is difficult in adult patients. Online approaches to neuraxial procedures are more commonly used in pediatric patients. Most practitioners use the offline technique (skin markings prior to needle insertion) when using ultrasound to guide neuraxial blocks in adults. With the offline technique, the needle follows the same angle that is used for optimal visualization of the neuraxial structures.

Selection of the correct interspace is important to the success of the subarachnoid block. The interspace selected for injection of spinal anesthetic drugs affects the resultant distribution. The failure rate at lower lumbar interspaces can be as high as 7%. These failures probably relate to the site of the injection with respect to the peak of the lumbosacral curve. One of the potential benefits of ultrasound is to help establish the correct interspace for neuraxial block, especially in challenging patients with scoliosis or obesity (Class I recommendation).

The accuracy of ultrasonography in correctly identifying lumbar interspace levels is in the 71% to 76% range for patients undergoing magnetic resonance imaging to evaluate the lumbar spine. The ability to estimate the interspace level is especially complex in patients with transitional vertebrae. These anomalies include lumbarization of the sacral spine (an unfused first sacral vertebra) and sacralization of the lumbar spine (fusion of L5 with the sacrum). The number of ribs also can vary, making estimation of level relative to the thoracic vertebrae challenging. Although ultrasound has limited ability to assess the interspace level, assessment by palpation is more inaccurate.

Longitudinal paramedian imaging planes provide the best visualization of neuraxial structures. With these views, the width of the acoustic window (the intervertebral space) is largest relative to the shadowing of the corresponding vertebral bone. Several authors have described the epidural space and adjacent bone as having a sawtooth configuration in this parasagittal view. The “saw sign” of longitudinal paramedian views inclines toward the skin surface in the caudal direction.

Midline transverse imaging planes are often used for offline markings for midline approaches for lumbar epidurals and spinals. This transverse view has been described as having a “flying bat” or “cat ears” appearance. In this view the articular processes (the rounded mammillary processes) of the facet joints form the ears of the bat and indicate the widest part of the interlaminar space. Although transverse imaging planes most closely resemble midline approaches, this view is limited by overhanging bone of the spinous processes and shadowing by the interspinous ligaments. Transverse imaging planes have been shown to be effective at lumbar interspaces for marking the needle insertion point and estimating needle depth to loss of resistance. However, these views are not helpful at midthoracic levels because of the narrower acoustic windows across the midline produced by the steep inclination of the spinous processes. In addition, the thoracic region lacks prominent articular processes that can serve as sonographic landmarks. It can be difficult to obtain symmetric midline transverse views of the neuraxis (in particular, the rounded articular processes) in patients with scoliosis due to rotation of the spine.

Ultrasound is an accurate imaging modality for depiction of the dura mater. The dura appears highly echogenic on ultrasound scans, defined by a single- or double-layer hyperechoic signal. It can be difficult to resolve the echo signals of the ligamentum flavum and posterior dura, and therefore this signal is sometimes referred to as the posterior complex ( Table 63.1 ). The anterior complex consists of the anterior dura, posterior longitudinal ligament, and vertebral body. This produces a wider hyperechoic band that is deeper and parallel to the first band (the equals sign). Because the subarachnoid space contains few endogenous scatterers of ultrasound, it appears echo free on ultrasound scans ( Table 63.2 ).

Suggested Technique for Offline Lumbar Epidural Catheter Placement

Use a low-frequency curved array (center frequency near 4 MHz) to obtain the best midline transverse view of the lumbar epidural space. The best transverse view is one in which the articular processes are prominent and symmetrically displayed on the screen. The probe slides in a cephalocaudal fashion to obtain an interlaminar view (so that no spinous process is seen within the field of imaging). The probe is then tilted to enhance imaging of the articular processes, with some rotation to make the articular processes symmetric on the display. Sliding in a medial-lateral fashion may be necessary to center the articular processes on the display. If the transducer is significantly rotated, then some repositioning of the patient may be necessary. Similarly, flexion of the lumbar spine can improve visibility and access to lumbar interspaces. The depth to the epidural space is then measured (this is most accurately done using the readout from a cursor scrolled over the anterior aspect of the posterior echo complex). The tilt (inclination) of the transducer is also noted (which is usually around 15 degrees cephalad in the lumbar region).

Mark the interspace with crosshairs on the skin using indelible ink. The intersection of the crosshairs can be marked by firmly indenting the skin with a needle hub. Mark the patient in the same position as for the interventional procedure. Changes in patient positioning reduce the accuracy of the markings.

Check or verify the level by moving the probe down to the sacrum and counting back up to the marked interspace using a longitudinal paramedian view. If this interspace is deemed too high or low, the adjacent spaces can be scanned and marked.

Remove all the gel with dry gauze and then proceed with lumbar epidural catheter placement by testing for loss of resistance in the usual sterile fashion. The noted depth can be off by as much as 0.5 to 1 cm ( Table 63.3 ). If unsuccessful on the first attempt, try redirecting the needle in a cephalocaudal fashion. This is suggested because the greatest uncertainty with the offline technique is the tilt (inclination) of the transducer.