Katelyn Hanson, DO
The Article: Millington S, Silva Restrepo M, Koenig S. Better With Ultrasound, Lumbar Puncture. CHEST 2018; 154(5):1223-1229
Overview: Lumbar puncture is a common procedure performed in both critical care and emergency medicine, and is typically associated with a high rate of success and favorable risk profile. However, as procedural ultrasound becomes more widely used in both specialties, it is reasonable to consider using ultrasound guidance to reduce the rate of failure and the risk of complications, especially in patients with difficult surface anatomy.
Risks and Benefits: The authors of this review state that using ultrasound improves the rate of procedural success, reduces the number of required attempts, and decreases the incidence of specific complications that will be further described next. Ultrasound has been associated with a higher rate of first-pass success and lower risk of failure in all comers. They postulate that reducing the number of failed attempts and there for having fewer needle insertion attempts will lead to less incidence of postdural puncture headaches, back pain, spinal hematomas, epidural abscess and meningitis. They also suggest that injury to deep structures can be prevented by accurately quantifying the depth required to reach the subarachnoid space and avoiding excessive needle depth, and by accurately identifying the appropriate intervertebral spaces of L5-S1, L4-L5, or L3-L4.
Techniques:
#1 Identify Spinous Processes: It is recommended to use the curvilinear probe in the transverse plane with the probe marker position to the left of the screen. The first step is to identify the midline; it is recommended to start with traditional surface land marking by locating the superior aspects of the iliac crests to find the L4 spinous process. In cases where the iliac crests cannot be easily palpated, the authors recommend starting just above the intergluteal crest. The spinous process will appear as a peaked hyperechoic structure with acoustic shadow that will obscure deeper structures. Once the spinous process is identified in the exact middle of the screen, this location should be marked at the transducer midpoint with a skin pen to delineate the patient’s midline. The probe can then be moved cranially and caudally to identify neighboring spinous processes, and these should be marked with a skin pen as well, providing a map of L5, L4, and L3 spinous processes. Between the individual spinous processes will be the interspinous space, in which the hyperechoic spinous process will start to disappear and the hyperechoic articular processes will appear laterally, which is often referred to as the “bat sign.” The location of these articular processes can also be marked with a skin pen, and then a line between these landmarks is made, as well as a line between the superior and inferior spinous processes; the intersection between these two lines may represent the optimal site for needle insertion.
#2 Identify Intervertebral Levels: It is recommended to use the curvilinear probe in the sagittal plane with the probe marker position to the left of the screen. The first step is to identify the midline; see technique one for this part of the procedure. One the spinous processes have been located, the probe is moved caudally to identify the sacrum, which is seen as a roughly horizontal hyperechoic line. Once the sacrum is identified, the probe is moved cranially, and the first spinous process identified will be L5, with the space caudal to it the L5-S1 interspinous space. The process should be repeated for L4 and the L4-L5 interspinous space, and then center the L4-L5 interspace on the ultrasound screen and draw a line across this space with a skin marker. A line between the L4 and L5 spinous processes is also made, and the intersection between these two lines may represent the optimal site for needle insertion.
In obese patients the spinous processes can be difficult to see from a midline approach. In these patients a paramedian approach can be used, where the procedure begins at the midline and then the probe is moved laterally 2-3cm until a hyperechoic saw-like pattern is seen, which would represent the articular processes. The probe is then moved caudally as described above to identify the sacrum, and then the specific L5 and L4 articular processes can be identified. The probe is then angled toward the midline to identify the lamina and L5-S1 and L4-L5 interlaminar spaces. Again, a line can be drawn horizontally across the interlaminar space, and then a line can be drawn from the
#3 Estimate Required Depth for Needle Insertion: Using the paramedian approach described above in technique two, with the transducer in the sagittal plane, identify the posterior complex, which includes the ligamentum flavum, epidural space, and posterior dura. This complex will appear as a hyperechoic linear structure just deep to the lamina, and once identified its distance from the skin surface can be measured. The procedure can also be performed in the transverse plane from technique one, by identifying the desired spinous process and then moving the probe cranially or caudally to visualized the intervertebral space; the posterior complex can then be seen just deep to the lamina and its distance from the skin again measured; this value can be compared to the value obtained from the sagittal approach for a better estimate.
#4 Perform an Ultrasound-Assisted LP: Dynamic ultrasound guidance during the procedure is not recommended as it would require the operator to hold the transducer in their off hand during needle insertion, which would make the procedure unnecessarily complicated. After performing a thorough pre-procedural scan, set the set the probe aside and perform the procedure.
Discussion: Point of care ultrasound has become an integral part of both emergency and critical care medicine. There are many procedures into which incorporating the use of ultrasound has already become the standard of care, and although this is not currently the case for performing lumbar punctures, it stands to reason that using ultrasound to better identify landmarks of the spine could improve patient outcomes, and thus should be considered a useful skill to learn to incorporate into regular practice. For more information, images corresponding to the procedures described above, and videos, see the original article cited above.