Ultrasonography-Guided Lumbar Puncture

Editor's Note: NeoReviews is excited to present the first in a series within Video Corner with a focus on neonatal point-of-care ultrasonography (POCUS). In this series, we delve into various practical applications of this clinical tool in the field of neonatology. Join us as we explore some insightful perspectives on applying POCUS to your day-to-day practice from experts in this arena. Stay tuned for this exciting new series.Lumbar puncture (LP) is a procedure commonly performed in neonates and infants, most often to evaluate for infection. This procedure can be at times challenging, with significant consequences of unsuccessful attempts, including missed diagnoses of meningitis, prolonged hospitalizations, unnecessary use of antibiotics, significant patient discomfort, and rarely even neurologic injury. Point-of-care ultrasonography has emerged as a powerful tool to optimize success in a variety of procedures in pediatrics, including LP.Indications, contraindications, and risks of this procedure are identical to those established for traditional LP. However, ultrasonography guidance can be helpful to identify patients in whom LP might be challenging or dangerous. Some patients are found to have miniscule amounts of cerebrospinal fluid by ultrasonography. Other patients are found to have spinal cord anomalies, such as a low-lying conus or a tethered cord. Many of these patients can still have an LP performed, although instead of using a static approach, dynamic guidance may be preferred to ensure procedural safety (see the steps later herein).The materials required for this procedure are mostly the same as those used for traditional LP. An ultrasonography machine is additionally necessary, and the probe used for this procedure is usually a linear probe. There are a multitude of different linear probe models depending on the manufacturer, but both large footprint and small footprint linear probes can be used depending on the patient's size. These probes are high frequency (typically ≥10 mHz, with some probes reaching frequencies as high as 22 mHz), which results in higher-resolution images; this is quite useful for delineating the fine structures in the spinal column. If the user plans to designate landmarks for static guidance after sterilizing the patient, or if electing to use dynamic ultrasonography guidance, a sterile probe sleeve and sterile ultrasonography gel are required. A sterile surgical marking pen is also necessary for static guidance to designate landmarks of the spinal anatomy on the skin.Ultrasonography-guided LP can be performed using either static or dynamic guidance. When performing this procedure under static guidance (Video 1), the spinal anatomy is first visualized by ultrasonography, and landmarks are identified to determine the optimal intervertebral spaces to access the spinal canal to obtain cerebrospinal fluid for sampling (Fig 1). Infants are the ideal patients for this ultrasonography-guided procedure given that their bones are incompletely ossified, which allows for the ultrasonography beams to penetrate through the spinous processes, preserving the definition of the structures within. Spinal ultrasonography is more challenging in older children with ossified bones, where bony structures create posterior acoustic shadowing over the spine, obscuring its view and allowing only for the identification of the intervertebral spaces.With the static guided procedure, the first plane that images are obtained in is the longitudinal/sagittal plane (Fig 1). This view is important to determine how much cerebrospinal fluid is present and to confirm normal anatomy. Quantification of cerebrospinal fluid volume is not technically feasible, but if a layer of hypoechoic fluid is visualized under ultrasonography (and the patient is not deemed to be "dry," with no fluid visible), an LP is reasonable to attempt. In addition, depth to the dura mater can be determined, which becomes useful when performing the LP to ensure that the needle is not advanced too deeply. As demonstrated in Video 1, a midline marking can be made when a crisp and centered view of the conus medullaris (Fig 2A) and spinal canal is obtained. The probe can then be rotated 90° into the second plane, which is the transverse/axial plane. Once in this plane, the probe is slid up and down the back centered over the spine until the hypoechoic spinal cord is again visualized. Once identified, the probe can be slid inferiorly just until the point where the cord disappears, which can be marked off as the termination of the conus medullaris (Fig 2B). This marking is extremely important because it ensures procedural safety by confirming that the spinal cord is out of the path of the needle. From here, the probe can be slid further inferiorly until the spinal canal narrows, which can be marked as the lower limit of where LP should be effectively and more successfully attempted. Figure 3 denotes the landmarks to be placed on the infant's skin when the infant is lying in the supine position.The more advanced ultrasonography-guided LP approach uses dynamic guidance. In this method, the same structures are identified, but the needle is then inserted into an intervertebral space and advanced into the spinal canal under direct visualization with ultrasonography guidance. This can be technically challenging and typically requires some experience with other dynamic ultrasonography procedures, such as vascular access. To perform this procedure, the needle is inserted just into the skin at the site of an appropriate intervertebral space (typically identified with ultrasonography just before insertion of the needle). The probe is then placed just next to the needle in a longitudinal/sagittal orientation along the spine, cephalad to the needle in relation to the patient's anatomic position. Once the needle tip is identified, the probe is held still, and the needle is advanced until it is seen puncturing the dura mater and entering the spinal canal. It is important that the patient remains still to properly visualize the needle as it is being advanced, which can sometimes require the use of nonpharmacologic analgesia methods or pharmacologic sedation.Point-of-care ultrasonography can be an intimidating venture to dive into, especially with a patient population consisting of neonates and small infants. Ultrasonography-guided LP is a great example of one of the first procedures that neonatal physicians, nurse practitioners, and physician assistants interested in ultrasonography can successfully accomplish. The views are simple and easy to obtain, and novices can perform these scans without difficulty. Interpretation of these images is also simple; most of the images look similar to familiar anatomic illustrations found in textbooks. Competency in this procedure can be obtained with relative ease because the scanning and patient marking can be completed without performing an LP; only a few supervised spine ultrasonography images with confirmation of correct anatomic identification are necessary. Static ultrasonography guidance for LPs has become an adjunct to traditional LP, increasing safety and success rates (with the cited study demonstrating an increase in the first-attempt success rate from 31% without ultrasonography to 58% with static guidance). (1) Additional studies have shown that the use of ultrasonography decreases rates of traumatic taps (with both cited studies demonstrating lower red blood cell counts in the fluid obtained when ultrasonography was used for LP guidance). (2)(3) With some practice, dynamic guidance can be used to successfully obtain cerebrospinal fluid from practically any infant, making it extremely useful especially when managing complex patients. Although point-of-care ultrasonography to guide LP is not the current standard of care for this procedure, its use by trained and experienced neonatal clinicians can be valuable in the NICU population.