The intervention of interest will be the ultrasound-assisted lumbar puncture (UALP). To do this, the treating physician will perform a bedside ultrasound of the spine to identify and mark the level of the conus medullaris and preferred puncture site prior to LP

The control group will have a standard landmark-based lumbar puncture

The ultrasound probe type will be selected by the treating pediatric emergency physician who has been trained according to standards outlined below. They will first identify the conus medullaris and make a horizontal marking with a sterile pen on the patient's back. Using a transverse view, they will then identify the midline of the patient's spine (using adjacent spinous processes) and will make 2 vertical skin markings on either side of the probe. Next, they will orient the probe in a longitudinal view to identify the desired vertebral interspace and will make 2 horizontal skin markings on either side of the probe at this level. Finally the 4 lines will be joined together at an intersection point, which will be the predetermined site for puncture attempt.

First-time lumbar puncture success rate is defined by the presence of at least 0.5 mL of cerebrospinal fluid with red blood cell count < 1,000/mm3.

defined by the presence of at least 0.5 mL of cerebrospinal fluid with red blood cell count < 1,000/mm3 in any number of attempt