Evaluating Infant Positioning for Lumbar Puncture Using Sonographic Measurements

Abstract

ACADEMIC EMERGENCY MEDICINE 2011; 18:215–218 © 2011 by the Society for Academic Emergency Medicine

Abstract

Objectives:  Hypoxia has been observed when infants undergo lumbar puncture in a tight flexed lateral recumbent position. This study used sonographic measurements of lumbar interspinous spaces to investigate the anatomic necessity and advantage derived from this tight flexed positioning in infants.

Methods:  This was a brief, prospective, observational study of a convenience sample of patients. Twenty-one healthy infants under 1 month of age were scanned in two positions: prone in a spine-neutral position and lateral recumbent with their knees bent into their chest and their neck flexed. In each position, a 5- to 10-MHz linear array transducer was used to scan midline along the lumbar spinous processes in the sagittal plane. The distances between the spinous processes were measured near the ligamentum flavum using the ultrasound machine’s calipers. Pulse oximetry was monitored on all infants during flexed positioning.

Results:  In the spine-neutral position, all studied interspinous spaces were much wider than a 22-gauge spinal needle (diameter 0.072 cm). The mean (±SD) interspinous spaces for L3–4, L4–5, and L5–S1 in a spine-neutral position were 0.42 (±0.07), 0.37 (±0.06), and 0.36 (±0.11) cm, respectively. Flexing the infants increased the mean lumbar interspinous spaces at L3–4, L4–5, and L5–S1 by 31, 51, and 44%, respectively.

Conclusions:  This study verified that tight, lateral flexed positioning substantially enhances the space between the lumbar spinous processes and that a spine-neutral position also allows for a large enough anatomic interspinous space to perform lumbar puncture. However, further clinical research is required to establish the feasibility of lumbar puncture in a spine-neutral position.

Lumbar puncture is a routine procedure in the emergency department (ED), used primarily to exclude meningitis in infants. When undergoing the procedure, infants are traditionally placed in a tight lateral recumbent position with the knees drawn up to the chest and the neck flexed. There are studies on adults and children showing that the spaces between the lumbar spinous processes increase with hip flexion.^1–3 However, there is no literature examining the anatomic advantage derived from flexed positioning in infants. Lumbar puncture may be a stressful procedure in infants, and some of its deleterious effects may be attributable to flexed positioning. Hypoxia and clinical deterioration (including bradycardia and/or cyanosis) are documented, although rare, complications of lumbar puncture in sick infants and neonates.^4,5 Some authors report an association between drops in transcutaneously measured oxygen saturation and recumbent positioning, with just knees flexed to chest^6,7 or in combination with full neck flexion.⁸ These observations have lead some authors to suggest avoiding significant neck and trunk flexion altogether in infants.^6,8

In this study, we used ultrasound to evaluate the anatomic necessity of flexed positioning in infants by measuring the lumbar interspinous spaces of infants held in a spine-neutral position. We then examined whether tight lateral recumbent positioning substantially enlarged these spaces.

Methods

Study Design

This was a prospective, observational study on the anatomic interspinous spaces of infants less than 1 month of age. The MetroHealth System’s institutional review board (IRB) approved the study. The requirement for written consent was waived by the IRB. Verbal consent was obtained for each participant from the presenting parent(s).

Study Setting and Population

The study was conducted in the MetroHealth ambulatory outpatient pediatric clinic. Participants included a convenience sample of healthy infants under 1 month of age who were visiting the clinic for well-child exams. Exclusion criteria included known spinal/vertebral disease, respiratory complaints, vital sign instability, or parental decision not to participate.

Study Protocol

Each infant was scanned in two positions: 1) spine-neutral—prone on a raised rounded piece of memory foam with the legs straddling the foam, the back straight, and the head turned to the side; and 2) spine-flexed—lateral recumbent with hips and knees flexed so thighs were touching the chest and neck flexed (Figure 1). The memory foam was used because it aided in immobilizing the infant. Pulse oximetry was placed during the flexed position for safety monitoring. The flexed position was to be discontinued if oxygen saturation fell below 90%. In each position, a Sonosite Titan (Sonosite, Bothell, WA) 5- to 10-MHz linear array transducer was placed midline along the lumbar spinous processes in the sagittal plane and the lower back was scanned. Ultrasound is well established as a diagnostic tool for the clinical evaluation of the infant spine^9,10 and for measuring components of the spine.^2,3,11,12 The spine was scanned in real time to obtain the maximum apparent distance between the lumbar spinous processes. The vertebral body level was determined by using the lumbosacral junction. Using the method described by Beek et al.,¹² the lumbosacral junction was identified at the first clearly angulated vertebral body in the lower spine. This is where the straight line of the lumbar vertebra transitions to the gentle kyphosis of the sacral vertebra. The vertebral body immediately cephalad to the first angulated vertebral body was identified as L5. If this junction was unclear, the spine was gently flexed and extended and the sacral vertebra were confirmed by their movement as a block. Once identified, the L3–4, L4–5, and L5–S1 interspinous spaces were measured near the ligamentum flavum using the ultrasound machine’s electronic calipers (Figure 1).

Figure 1.  Infant position and acquisition of sonographic measurements.

Data Analysis

Enrollment numbers were based on a pilot study of 10 infants less than 1 month of age who were scanned prone in a spine-neutral position to approximate the hitherto unquantified interspinous space in this age group at levels L3–4, L4–5, and L5–S1. These means with standard deviations (SDs) were used to determine the number of patients who would need to be enrolled in the main study to see a >15% difference in the interspinous space with flexed positioning, if it existed, at a 90% power level assuming an alpha of 0.05. Depending on the interspace, enrollment numbers were estimated at between 6 and 21. All analyses for this study were performed using STATA SE 9.0 (StataCorp, College Station, TX). The Shapiro-Wilk test was applied to ensure that the continuous outcomes measured were normally distributed. The data are presented as means (±SD) and differences (with 95% confidence intervals [CIs]). The differences between positions were examined using a paired t-test. p-values are two-tailed, with p < 0.05 considered statistically significant.

Results

Twenty-four infants had a parent or guardian consent to the study. Three infants initially consented were unable to be placed in the study positions and were not enrolled. Twenty-one infants were enrolled; their ages were normally distributed between 6 and 27 days, with a mean (±SD) age of 14.6 (±5.6) days. The L5–S1 interspace was only measured in 17 of these infants due to technical difficulties in obtaining clear measurements at this level with the edges of the spinous processes being poorly identified. One operator performed the study. No adverse hypoxic events occurred during flexed positioning, with all infants maintaining a pulse oximetry saturation of greater than 90% during the approximately 1- to 2-minute scan period. Study results are presented in Table 1. The outcome measures appeared normally distributed when the Shapiro-Wilk test was applied to the data. The interspinous spaces of infants in the spine-neutral position were much wider than a 22-gauge needle (diameter 0.072 cm). All studied interspaces were at least five times this needle width. Results show the interspinous spaces increased with flexed positioning (p < 0.0001 for all studied spaces).

Table 1.    Interspinous Space Measurements in Spine-neutral and Spine-flexed Positions

Space (No. of observations)	Spine-neutral		Spine-flexed		Difference (% increase)	95% CI
	Mean (cm)	SD	Mean (cm)	SD
L3–4 (21)	0.42	0.07	0.55	0.08	0.13 (31%)	0.17–0.08
L4–5 (21)	0.37	0.06	0.56	0.11	0.19 (51%)	0.23–0.15
L5–S1 (17)	0.36	0.11	0.52	0.10	0.16 (44%)	0.22–0.10

Discussion

When undergoing a lumbar puncture, infants are traditionally held in a tightly flexed lateral recumbent position. Although no hypoxic events (saturation < 90%) occurred in our healthy infants during flexed positioning, other studies suggest this may not always be the case in premature and sick infants.^4–8 Silverboard et al.⁴ performed lumbar punctures on 48 consecutive preterm infants less than 35 weeks’ gestation in the neonatal intensive care unit. The majority of lumbar punctures occurred at 48 to 96 hours of age. Three infants experienced acute bradycardia and/or cyanosis. Out of the 29 patients who had their arterial blood gas monitored pre- and postprocedure, 10 had a drop in PO₂ of at least 10 mmHg.⁴ Subsequent studies, two enrolling sick neonates^6,7 and one enrolling healthy preterm infants,⁸ have also shown decreases in PO₂ with knee to chest positioning, even with neutral neck positioning. This trend is further seen in older infants undergoing lumbar puncture for sepsis evaluation in a conventional flexed lateral decubitus position. Eleven of 12 infants (median age 6 weeks) had oxygen saturations below 85% in one study.⁵ These observations raise concern that hypoxia may occur in populations other than healthy infants, and there may be adverse effects to tight lateral recumbent positioning in some instances. This is troubling, given that lumbar punctures are rarely conducted on healthy infants, but rather on sick infants and neonates who may already show signs of respiratory distress.

We are unaware of any prior study in young infants examining the anatomic necessity or advantage of traditional lateral flexed positioning. Our data show that the studied interspinous spaces of infants less than 1 month of age held in a spine-neutral position were much wider than a 22-gauge needle. This may allow for the performance of a successful lumbar puncture in a more extended position, although clinical research would need to establish the feasibility of such positioning. The spinous processes are easy to palpate in young infants given their reduced body fat, so arched positioning is probably not as important in feeling out the interspinous spaces in neonates. Our findings appear to demonstrate support for the anatomic feasibility of the extended upright position or the modified lateral position, without the knees into the chest and the neck neutral, as already advocated by some authors.^3,6,8

To the best of our knowledge, there are no prior studies verifying the assumed benefits of traditional flexed positioning in widening the lumber interspinous spaces in infants. Three reports were found that quantify a benefit to flexed hip positioning in adults^1,2 and children.³ Our findings show that the studied interspinous spaces are substantially widened when infants are placed in a tightly flexed lateral recumbent position relative to spine-neutral. Intuitively, this should lead to greater procedural success, but how much greater needs to be clinically studied. This may imply that in some situations it may be worth assuming the potential risks described with lateral flexed positioning if needle passage has proven difficult in a more neutral position, although more clinical study is needed.

Limitations

The study was a convenience sample of healthy infants. The observer in the study was not blinded to patient position or the study hypothesis. Only two extreme forms of positioning were compared (spine neutral and extreme neck and hip flexion). This limitation was accepted because we were unable to hold infants (beyond a couple of weeks of age) still enough in a seated or more extended lateral recumbent position to allow for consistent ultrasound measurements to be made given continual movement. This same technical difficulty has been reported by other investigators in this age group.³

Conclusions

In this study, we verified that flexed positioning substantially enhances the space between the lumbar spinous processes in infants, but that a spine-neutral position may also allow for a large enough anatomic interspinous space to perform lumbar puncture. However, future studies are required to evaluate the actual clinical feasibility of performing lumbar puncture in a spine-neutral position.

The authors thank the staff at the outpatient pediatric clinic at Metrohealth for their assistance with patient enrollment and Charles L. Emerman, MD, for his assistance with manuscript development. The authors also thank the Barcombs for allowing the photographing of their child.