From this position, slide the probe cephalad (in a paramedian plane), counting the facet joints to ensure that the initial attempt is not too caudal. Sonographers can use either the ultrasound system’s reference line or M-mode line to ensure that the center of the probe corresponds to the center of the ultrasound image. Mark the L4 and L3 facet joints (corresponding to L4-L5 and L3-L4 interspaces, respectively) on the skin surface (see Figure 3).

Rotate the transducer to the patient’s left in order to locate the true anatomic midline. Identify the bony spinous process, then slide the probe cephalad and caudad, marking the direction of the bony midline (see Figure 4). Connect the surface markings from the midline and previously identified interspace levels for an ideal location for needle entry. We recommend marking and anesthetizing two interspace levels before performing standard aseptic technique for a lumbar puncture (ensuring that the patient does not make significant movements in between skin marking and lumbar puncture needle entry). Standard lumbar puncture technique should be followed, with the clinician aware that slight needle manipulations are to be expected even though the interspace has been visualized.

Summary

Ultrasound localization for lumbar puncture can be a useful adjunct when anatomic landmarks are not palpable and/or initial attempts fail. Difficult lumbar punctures are often due to nonpalpable bony anatomy, making ultrasound localization for lumbar puncture a useful adjunct for the emergency physician.⁵ Errors are often due to inferior estimation of Tuffier’s line (a horizontal line connecting the top of the iliac crests that is thought to represent the L4-L5 interspace) as well as not locating the true anatomic midline.⁶ A simplified two-step ultrasound localization method in which the L4-L5 or L3-L4 interspace and true anatomic midline are defined can help reduce procedural error in patients with nonpalpable anatomy. The reason for ultrasound localization of bony anatomy in the difficult patient is to reduce rates of failure and reliance on consultative services.

Dr. Nagdev is director of emergency ultrasound at Alameda Health System’s Highland General Hospital in Oakland, Calif., and assistant clinical professor at the University of California, San Francisco, School of Medicine. Dr. Riguzzi and Dr. Frenkel are ultrasound fellows in the department of emergency medicine at Highland General Hospital. Dr. Mantuani is an attending physician in the department of emergency medicine and director of internal medicine point-of-care ultrasound at Highland General Hospital.

References

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