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Show Effect of Patient Positioning on Cerebrospinal Fluid Opening Pressure Anne S. Abel, MD, Jeffrey R. Brace, MD, Alexander M. McKinney, MD, Deborah I. Friedman, MD, MPH, Scott D. Smith, MD, MPH, Per L. Westesson, MD, DDS, PhD, David Nascene, MD, Frederick Ott, MD, Michael S. Lee, MD Background: Prone is the preferred patient position for fluoroscopic-guided lumbar puncture (LP). Normative data for cerebrospinal fluid (CSF) opening pressure (OP) exist for lateral decubitus (LD) positioning only and have not been defined for the prone position. This study compares CSF OP values in the prone and LD positions and examines the effect of body mass index (BMI) on OP. Methods: Patients undergoing diagnostic or therapeutic fluoroscopic-guided LP were recruited prospectively at 2 tertiary care centers from 2009 to 2012. Following prone fluoroscopic-guided LP, patients were rolled to the LD position for repeat CSF OP measurement. In addition to comparing the mean OP in each position, the relationships between OP, body position, and BMI were also explored. Results: Fifty-two patients were enrolled. A mean OP difference of 1.2 cm H2O was observed (prone: 26.5 cm H2O; LD: 27.7 cm H2O; P = 0.07). No correlation between CSF OP and BMI was seen in either position. Conclusions: No statistically or clinically significant differ-ence between prone and LD OP was identified. BMI does not appear to affect CSF OP measurement in either position. Journal of Neuro-Ophthalmology 2014;34:218-222 doi: 10.1097/WNO.0000000000000074 © 2014 by North American Neuro-Ophthalmology Society Neuroradiologists increasingly perform diagnostic and therapeutic lumbar puncture (LP) under fluoroscopic guidance, especially for patients whose body habitus may obscure bony landmarks. Although the diagnostic criteria for idiopathic intracranial hypertension (IIH) specify the mea-surement of cerebrospinal fluid (CSF) opening pressure (OP) in the lateral decubitus (LD) position (1), most fluo-roscopic- guided LPs measure OP with the patient in the prone position (2). Most neuroradiologists appropriately add the needle length to the manometer when measuring prone OP, but some do not. While normal CSF OP in the LD position is well defined in both adults and children, (3-5) normative data for prone CSF OP do not exist. Conceivably, increased intra-abdominal or intrathoracic pressure (6) or change in respiratory rate dur-ing prone body positioning could raise CSF pressure. In 2002, Jacobson and Coppens (7) reported a prospective pilot series of 7 patients with IIH, in which they did not observe a significant difference between OP measured in the prone and LD posi-tions. Schwartz et al (8) reported a series of 55 patients in which a statistically significant difference between prone and LD positions was observed. Several studies describe conflicting relationships between CSF OP, lower extremity position, and body mass index (BMI) (8-15). Our study prospectively com-pared CSF OP values in the prone and LD positions and examined the effect of BMI on CSF OP. METHODS Study Design After approval from the Institutional Review Boards (IRB) of the University of Minnesota and the University of Rochester, patients undergoing fluoroscopic-guided LP with OP measurement for clinical care were recruited prospec-tively. Patients aged 18-89 years were included. Those who Departments of Ophthalmology (ASA, MSL), Neuroradiology (JRB, AMM, DN, FO), Neurosurgery (MSL), and Neurology (MSL), University of Minnesota, Minneapolis, Minnesota; Departments of Neurology and Neurotherapeutics (DIF) and Ophthalmology (DIF), University of Texas Southwestern, Dallas, Texas; Department of Ophthalmology (SDS), Cleveland Clinic Abu Dhabi, Abu Dhabi, United Arab Emirates; and Department of Imaging Sciences (PLW), University of Rochester, Rochester, New York. Supported by an unrestricted grant from Research to Prevent Blindness (New York, NY) and the Lions Club of Minnesota (M.S.L.). The authors report no conflicts of interest. Address correspondence to Michael S. Lee, MD, 420 Delaware Street SE, MMC 493, Minneapolis, MN 55455; E-mail: mikelee@umn.edu 218 Abel et al: J Neuro-Ophthalmol 2014; 34: 218-222 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. were unable to give informed consent or requiring intrave-nous sedation were excluded. Six neuroradiologists at the University of Minnesota, and 2 registered and certified phy-sician assistants from the Department of Imaging Sciences at the University of Rochester performed the LPs. Study Protocol Informed consent was obtained for study participation with fluoroscopic-guided LP including OP measurement in both the prone and LD positions. Patients undergoing fluoroscopic-guided LP at the University of Minnesota and the University of Rochester were enrolled between January 1, 2009 and May 31, 2012. Light sedation with oral diazepam was administered in the selected cases based on physician discretion and patient preference. The patients were placed in the prone position, and then prepared and draped in a sterile fashion. Fluoroscopy was used to identify the lumbar spinal puncture level before administering local anesthesia with 1% lidocaine subcutaneously. Thecal sac puncture was performed with a 5-inch Whitacre or Quincke spinal needle. Puncture level and needle gauge were recorded. A manometer was attached to the spinal needle with extension tubing and a stopcock. The tubing was extended horizontally between the spinal needle and the manometer. Zero was defined at the level of the spinal canal for all OP measurements. The patients were encouraged to relax and breathe normally while the CSF meniscus equilibrated. Elapsed time from thecal sac puncture to CSF meniscus equilibration was not recorded, but took less than 1 minute in all cases. On average, equilibration took approximately 15 seconds. The highest pressure reading during respiratory excursion was recorded. Needle length was added to the manometer reading to account for the distance between the manometer and the zero level of the spinal canal while in the prone position. The stopcock was then closed to prevent CSF loss, and the patients were repositioned in the LD position with legs extended and the patient relaxed. The extension tubing was then reoriented to maintain a horizontal connection between the spinal needle hub and the manom-eter. If there was any obliquity of the spinal needle with respect to the spinal column, the manometer was positioned at the level of the spinous processes nearest the needle to best approximate the level of the spinal canal. The stopcock was reopened, and the CSF meniscus was again allowed to equilibrate. Again, this took approximately 15 seconds de-pending on the pressure. The highest OP measurement during respiratory excursion was then recorded. Each patient's age, gender, self-identified ethnicity, BMI, LP indication, spinal level of thecal sac puncture, needle gauge, and OP in the prone and LD positions were recorded. Post-procedure symptoms and the ultimate diagnosis were recorded when available. All complications were reported to the IRB. Statistical Methods The paired t test was used to compare OP measured in the prone and LD positions. Unpaired t tests were used to assess both the effect of needle gauge and the use of sedation on OP. One-way analysis of variance was used to assess the effect of the level of spinal puncture on OP. The relation-ship between OP and BMI was investigated with linear regression modeling. Bland-Altman analysis was used to evaluate the agreement between OP measurements in the 2 positions. Data were analyzed using commercially avail-able software (GraphPad Prism 6; GraphPad Software, Inc, San Diego, CA). A P-value ,0.05 was considered statisti-cally significant. RESULTS Fifty-two patients were enrolled. One patient was excluded because OP was not ordered by the referring physician. The demographic and clinical data of our patient cohort are summarized in Table 1. The mean OP in the LD position was 26.5 cm H2O (range, 11.0-48.0 cm H2O) (Table 1). The mean OP in the prone position was 27.7 cm H2O, (range, 12.0-44.7 cm H2O). Prone OP was higher than LD OP in 31 (59%) patients and lower in 16 (31%) patients. Equivalent OP was measured in 4 (10%) patients. OP was considered equivalent if measurements were within 0.5 cm H2O. The mean difference between prone and LD OP of 1.2 cm H2O greater in the prone position was not statis-tically significant (95% CI, 20.1 to 2.5; P = 0.07). Using the Bland-Altman method (Fig. 1), a bias of 1.2 cm H2O higher OP in the prone position was found (95% limits of agreement = 28.1 to 10.6). BMI was available for 49 of the 51 patients. A correlation between CSF OP and BMI was not seen in either position (P = 0.91 in the prone position and P = 0.70 in the LD position). Moreover, BMI did not affect the difference in OP measured between the 2 positions (P = 0.60). TABLE 1. Characteristics of patients undergoing lumbar puncture No. Patients 51 Men 3 (6%) Women 48 (94%) No sedation 47 (96%) Sedated with oral diazepam 4 (8%) Age, years 18-72; mean 35 BMI, kg/m2 18.7-64.0; mean 35.5 LP indication Suspected papilledema 32 (63%) IIH 8 (16%) Headache 7 (14%) Vision loss/optic neuritis 1 (2%) Ataxia 1 (2%) Hemiplegia 1 (2%) Malignancy 1 (2%) BMI, body mass index; IIH, idiopathic intracranial hypertension; LP, lumbar puncture. Abel et al: J Neuro-Ophthalmol 2014; 34: 218-222 219 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Of the 39 patients undergoing LP for suspected papilledema or IIH workup, 6 (15%) crossed the 25 cm H2O threshold to elevated CSF OP in the prone position only. Two additional patients (1 undergoing workup for headache and 1 for optic neuritis) also crossed the 25 cm H2O threshold while prone. Four patients (8%) were sedated with oral diazepam before LP. A significantly larger OP differential (prone-LD) was seen in sedated patients (95% CI, 0.55-10.1; P = 0.04). Thirty-seven (73%) LPs were performed with a 22-gauge needle. Fourteen (27%) were performed with a 20-gauge needle. There was no significant difference between the OP differential (prone-LD) with either needle gauge (P = 0.30). Twenty-eight (55%) LPs were performed in the L2/L3 inter-vertebral space, 16 (31%) in the L3/L4 intervertebral space, and 7 (14%) in the L4/L5 intervertebral space. The lumbar spinal level puncture had no significant effect on the OP difference measured in the 2 positions (P = 0.69). Following LP, 8 (16%) patients complained of new or persistent headache. Four (8%) patients required epidural blood patch. One (2%) patient complained of persistent fever, and 1 (2%) complained of localized pain and transient lower extremity paresthesias, which resolved spontaneously. There were no complications related to study participation. DISCUSSION In cases of suspected IIH, spontaneous intracranial hypoten-sion, shunt malfunction, and normal pressure hydrocephalus, the accuracy of OP measurement is critical. Specifically for IIH, the diagnostic criteria require a CSF OP . 25 cm H2O measured in the LD position with legs extended and the patient relaxed (1). While nearly 90% of neuroradiologists perform fluoroscopic-guided thecal sac puncture in the prone position, less than one-third of neuroradiologists rotate the patient to measure OP in the LD position (2). Consequently, prone OP measurements direct clinical decision making, despite the absence of established OP norms in the prone position. The small observed difference between CSF OP mea-sured in the prone and LD positions of 1.2 cm H2O was not statistically significant. In our opinion, we believe that difference also is clinically insignificant and likely falls within the margin of error for the measure. Cautious com-parison is warranted between our study and that of Schwartz et al (8), which found a 2.7 cm H2O difference between the prone and LD positions. The patient popula-tions between the studies were very different. Most patients (56%) in Schwartz's study had myelograms, whereas all our patients had diagnostic or therapeutic LPs. Moreover, the patients in Schwartz's study had lower OPs, with 27% of patients' OP , 10 cm H2O (average: 12.6 cm H2O LD, 15.3 cm H2O prone). The average OP in our study was considerably higher (average: 26.5 cm H2O, 27.7 prone). Regardless, we disagree with Schwartz's conclusion that 2.7 cm H2O is clinically significant. A study population with even higher average OP (.30 cm H2O) would be useful. There may be a greater difference observed among patients with a normal or low OP. We found a statistically significant higher OP differential between the prone and LD position in patients sedated with oral diazepam. This finding indicates that sedation may induce an artifact when measuring OP in the prone position. Given the small number (8%) of sedated patients, further studies are needed to determine whether the effect of sedation is clinically significant. The relationship between CSF OP and BMI is contro-versial. We did not find a correlation between BMI and CSF OP in either position. Although 1 retrospective study (15) reported a linear relationship between BMI and CSF pressure among patients with OP , 25 cm H2O, 5 other prospective studies found this relationship to be clinically insignificant (8,12-14,16). Avery et al (5) found a small, positive correlation between CSF OP and BMI in children. Others have suggested that increased intra-abdominal or intrathoracic pressure can increase CSF OP (6,16). This is of particular concern in IIH, given its prevalence among obese women. According to published diagnostic criteria for IIH, 25 cm H2O is the threshold needed to diagnose IIH (1). Our 8 patients, who crossed the 25 cm H2O threshold only while prone, deserve closer examination. The LP indication for these "threshold-crossing" patients was suspected papil-ledema (n = 4), IIH workup (n = 2), headache (n = 1), and vision loss/optic neuritis (n = 1) (Table 1). Six of these 8 "threshold-crossers" had borderline-elevated OP (21- 24 cm H2O) in the LD position, whereas, only 2 had LD OP within the normal range. The average BMI for these 8 threshold-crossers was 36.9 kg/m2, which was not statistically significantly different from the rest of the pa-tients (P = 0.16), further supporting our conclusion that BMI does not affect CSF OP. Sugerman et al (16) postulated that excess intra-abdominal fat increases intra-abdominal pressure, thereby raising pleural FIG. 1. Bland-Altman plot of prone vs lateral decubitus (LD) opening pressure (OP). OP difference (prone-LD) plotted on the y axis, and mean OP plotted on the x axis. The bias (dashed line) of 1.2 means that on average, a patient's OP was 1.2 cm H2O higher in the prone position vs the LD position (95% limits of agreement [dotted lines] = 28.1 to 10.6). 220 Abel et al: J Neuro-Ophthalmol 2014; 34: 218-222 Original Contribution Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. and cardiac filling pressures, and ultimately increasing CSF OP by impeding venous return from the brain. If Sugerman's theory is correct, one would expect that the pressure from the abdominal contents would elevate CSF pressure when the patient is prone. According to Pascal's principle, pressure applied to an enclosed fluid under equilibrium conditions is transmitted equally and undiminished throughout the fluid. Therefore, if increased intra-abdominal or intrathoracic pressure were transmitted at any point along the dural sac, CSF OP would increase. However, we did not observe a cor-relation between BMI and CSF OP. Our findings argue against Sugerman's theory that central obesity causes IIH by impeding venous return from the brain. Although the average BMI of our patients was 35.5 kg/m2 and that of Sugerman's patients was 45 kg/m2, we suggest that the vector forces transmitted from central obesity to the thecal sac dur-ing OP measurement are minimal, as long as the patient lies down and relaxes (no Valsalva) with the base of the manom-eter level with the spinal canal. As obesity increases in the United States, the incidence of IIH will also likely rise, which may lead to an increase in fluoroscopic-guided LPs performed in the prone position. Based on this study's data, it seems reasonable to interpret prone OP measurements with the established normal range defined for OP in the LD position (8-25 cm H2O). An accurate prone OP must have the needle length added to the final manometer reading unless flexible tub-ing is used to bring zero to the level of the spinal canal. When OP is needed to certify a clinical diagnosis as in IIH, the trend toward higher OP in the prone position that we identified should be considered, especially when the measurement is borderline. If a clinical diagnosis is questionable because of an OP near the 25 cm H2O threshold, we think it is reasonable to roll the patient to the LD position or repeat the LP with OP measurement in the LD position. Limitations of this study include the relatively small sample size and homogenous study population. Most patients were obese women undergoing diagnostic LP for suspected papilledema or IIH workup. This selection bias likely exists because only patients who were specifically referred for LP with measurement of OP were recruited. Many physicians do not specifically order OP, so hundreds of eligible patients were potentially missed. Although this potentially limits the clinical application of our results, we have no reason to believe that the difference in OP measurements should vary with age or gender. This lack of heterogeneity, however, could potentially confound our BMI vs OP analysis. Another limitation of this study is that the clinician cannot be masked to patient positioning during OP measurement. In an attempt to minimize potential bias, the OP measurement technique was stan-dardized to the highest manometric reading during respi-ratory excursion after equilibration of the CSF meniscus in the manometer. This method is widely accepted as the most accurate OP measurement technique. Despite this strict protocol, variability could potentially exist among the 8 practitioners performing the LPs. However, we believe that this approximates the variability seen in routine clinical practice and, therefore, argues in favor of supporting the generalizability of our data. ACKNOWLEDGMENTS We are grateful to our colleagues at the University of Rochester: Stephen D'Ambrosio, MPAS, and Iris Young, PA, in the Department of Imaging Sciences for performing the LPs; Elisabeth Carter, AA, Peter MacDowell, MS, Karen Skrine, Valerie Davis, MS, RN, ANP-BC, and George O'Gara, MBA, CCRC, in the Clinical Trials Unit of the Department of Ophthalmology for their invaluable assistance coordinating the research project. At the Univer-sity of Minnesota, we would like to thank Ann Moe, RN, in the Department of Radiology for her assistance coordinat-ing the study. REFERENCES 1. Friedman DI, Jacobson DM. Diagnostic criteria for idiopathic intracranial hypertension. Neurology. 2002;59: 1492-1495. 2. Abel AS, Brace JR, McKinney AM, Harrison AR, Lee MS. Practice patterns and opening pressure measurements using fluoroscopically guided lumbar puncture. AJNR Am J Neuroradiol. 2012;33:823-825. 3. Corbett JJ, Mehta MP. 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