Neuraxial anesthesia and analgesia techniques can be challenging in the obstetric patient. These techniques heavily rely on anatomic landmarks but the expected anatomic and physiologic changes of pregnancy impair the provider in discerning these accurately. There is exaggerated lordosis and edema, softer interspinous ligaments, narrowing of the epidural space, and a smaller intrathecal space; all of which increase the likelihood of failed block or an accidental dural puncture. Furthermore, patients may not be able to flex the lumbar spine and labor pain challenges their ability to maintain positioning during the procedure. To review the theory and steps of labor analgesia click here.
Enter lumbar ultrasound for guidance of neuraxial blocks. Ultrasound improves precision and efficacy. Contrary to the other chapters were bone was limiting our ability to see deeper structures, here our goal is to identify bony landmarks as they are surrogates for location and depth.
Lumbar Sono Anatomy
We must first take a look at two of the most common scanning planes. These are the transverse (in blue) and the paramedian sagittal oblique (PSO) plane (in red). The PSO plane is a paramedian sagittal plane (in green) except the probe is tilted towards midline as opposed to a line that is parallel the midline, as seen on the diagram.
Ideally a curved array the low frequency probe (3-5Mhz) should be selected and an initial depth of 7-8 cm chosen. We then recognize the bony structures that are imaged on these planes.
Recall that bony surfaces appear as hyperechoic or white linear structures with dense acoustic shadowing (black) beneath that completely obscures any deeper structures The diagrams and their corresponding ultrasound images are shown below for illustration purposes.
Imaging planes used in lumbar ultrasound. See text for details.
Paramedian Sagital Oblique (PSO) Plane
The most useful feature of this technique is being able to identify the interlaminar space.
The ultrasound beam cuts the vertebra in such a way the spinous process is not visualized and the bony structures that we see displayed are the lamina.
The sloping hyperechoic laminae of the lumbar vertebrae form what has been described as the “sawtooth” pattern. The gaps between lamina represent the paramedian interlaminar spaces, through which other structures may be visualized. The posterior complex (letter C on the image) which is a combination of the ligamentum flavum, epidural space, and posterior dura often appear as a single linear hyperechoic structure.
The depth from skin to the posterior complex may be measured to provide an indication of the expected needle depth for neuroaxial anesthesisa. If the probe is slid in a caudad direction a single homogenous horizontal hyperechoic line of the sacrum comes into view. We can thus identify the interlaminar spaces if we can identify the sacrum and move upwards from it.
Paramedian Sagital Oblique ultrasound view of lumbar space and corresponding bony anatomy. In red, erector spinae muscle. A, erector spinae muscle, B, lamina, C, Posterior complex and D, Anterior complex (anterior dura, posterior longitudinal ligament and posterior aspect of the vertebral body or disk).
Transverse Plane- Interlaminar Transverse Space
Now that you have identified the level where you can safely perform your neuraxial technique we can use the interlaminar space seen on the trasverse view to look at distances more closely. It is easier to manipulate the ultrasound probe with this view than the PSO.
The probe is centered around the space between two spinous processes and angled upwards so that the beam can go in between these bony structures. The hyperechoic lamina and articular processes can be seen deep and to the sides of the interspinous space. Immediately posterior to the interspinous ligament we can observe an hyperechoic line corresponding to the posterior complex (labeled E in the image) and even deeper, the anterior complex (labeled F). The hypoechoic space (between letters E and F) represents the intrathecal space. The contents we have defined represent the vertebral canal. The posterior complex may not be seen at times.
The depth from skin to the posterior complex can then be measured with the built in caliper.
Interlaminal space view of lumbar space and corresponding bony anatomy. A, erector spinae muscle; B, interspinous ligament; C, transverse process; D, articular process or lamina; E, posterior complex;F, Anterior complex.
Steps to succeed with lumbar ultrasound
At this point the question in your mind is how to approach the space with a needle in your hand since it would seem you can't have the probe on these two axis simultaneously. And here there are two approaches.
1. Triangulation. One is with the use of triangulation to prescan and then proceed with epidural placement. First, define a safe lumbar level with the PSO plane (A on the image below), mark the space. Second, define the interspace level with the transverse plane and then mark that space (B on the image below). The entry point of the needle is the location these two lines intersect.