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
Lets first take a look at what we could possibly see with the use of a 3D model to have an understanding of the planes and orientation. The clips from the model below show the lumbar vertebrae and sacrum. The left portion of the model has all muscle layers while the right has only the bony layers which is what is of paramount importance to us. The probe is slightly angled towards the left on the sagittal plane to get the PSO plane. Notice that the second clip starts scanning from the sacrum. The model has been modified and used with from Z-anatomy.
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
Let's again first take a look at a 3D model so that we understand orientation of this window. The probe has been positioned in transverse plane and moved up and down the model. The first shows a slight tilt of the probe to allow for visualization of the soft tissues that would be obscured by the spinous process. We used Z-anatomy to collect these clips.