The effect of whole body position on lumbar cerebrospinal fluid opening pressure

We compared cerebrospinal fluid (CSF) opening pressure measurements in the lumbar subarachnoid space between the flexed position (F-OP) and relaxed position (R-OP) in recumbent patients. We devised an equation for using F-OP to determine the existence of raised intracranial pressure (ICP). Patients (n = 83) underwent lumbar puncture while in the flexed lateral decubitus position and then were moved to the relaxed position. F-OP and R-OP were measured with a water manometer. R-OP > 180 mmH2O plus relevant clinical signs were taken as indicators of raised intracranial pressure. Mean pressures for F-OP and R-OP were 178.54 and 160.52 mmH2O respectively, p <0.001. When F-OP > 180, raised ICP could be significantly over diagnosed. The authors recommend an equation [R-OP(calculated, mmH2O) = 0.885 × F-OP(measured, mmH2O)] or using 200 mmH2O as the threshold for increased ICP with flexed posture.


Findings
The existing literature recommends that physicians measure CSF opening pressure with the patient in the recumbent position. A few citations recommend using the relaxed position (neck not flexed, leg extended and without valsalva manoeuvres) because the flexed position is believed to increase lumbar CSF pressure [1][2][3]. There are very few existing studies available on recommended positions for CSF measurement [4,5]. We have noticed in our clinical practice that many physicians usually measure F-OP rather than R-OP because of the convenience. Our objectives were to determine the effect of the body position on CSF pressure, by comparing F-OP with R-OP in the same patients and to ascertain if there are any differ-ences between the two values. In addition we also investigated the cut off value for raised intracranial pressure, and derived a valid equation for the relation between F-OP and R-OP.
Eighty three consecutive patients were enrolled prospectively and underwent lumbar puncture in the lateral decubitus position between June 1 st , 2004 and January 31 st , 2005 in the Phramongkutklao Hospital, Bangkok. The inclusion criteria were: age >14 years, an indication for lumbar puncture for diagnostic or therapeutic purposes and good cooperation from the patient. Exclusion criteria were: pregnancy, contraindication for lumbar puncture, increased intra-thoracic or intra-abdominal pressure (marked ascites or pleural effusion, on ventilator, hyperventilation, cough, sneezing, uncontrolled movement and severe anxiety or agitation), and a marked fall in CSF pressure during measurement. Demographic characteristics were recorded. The mean value of F-OP was measured by a cylindrical glass water manometer after the pressure had stabilized with minimal fluctuation for at least 2 minutes. Patients were supported while they slowly relaxed their posture as much as possible, and R-OP was measured by the same technique. A measurement of R-OP over 180 mmH 2 O, together with clinical signs (several bilateral headaches and/or papilloedema) was used as an indication for increased intracranial pressure.  Figure 1A and 1B respectively. The correlation coefficient was 0.94, p < 0.001. We used "R-OP > 180 mmH 2 O plus clinical signs" as the "gold standard" for the diagnosis of raised intracranial pressure. The number of patients with clinical signs of increased intracranial pressure who were diagnosed by F-OP > 180 mmH 2 O and "gold standard' is shown in Table 1. There was a significant difference in the number of patients who diagnosed with raised intracranial pressure between using F-OP > 180 mmH 2 O and "gold standard" (p < 0.0001). Sensitivity and specificity of F-OP > 180 mmH 2 O for increased intracranial pressure were 95.7% and 83.3%, respectively. Increased intracranial pressure was significantly over-diagnosed in 10/60 cases (16.7%) when F-OP > 180 mmH 2 O was used ( Table 1). The receiver operating characteristic curve of F-OP cut off for determining increased intracranial pressure was 200 mmH 2 O and had an area under the curve of 0.961 (95% CI of 0.925-0.997, p < 0.01). By stepwise linear regression analysis, the equation between F-OP and R-OP was found to be: R-OP = 0.885 × F-OP, p < 0.001.
When this equation was used to calculate a new R-OP from F-OP, there was a significant improvement for specificity in determining raised intracranial pressure from 83.3% to 93.3%, p < 0.001. This calculation also decreased the difference in mean opening pressures between positions from 18.02 to -2.48 mmH 2 O [95% CI for mean value (-7.46)-(2.49), ns, paired sample t-test].
Correlation and agreement between flexed and relaxed opening CSF pressure measurement in patients in the lateral recum-bent position: 1A Eighteen per cent of patients had the same value for F-OP and R-OP. Furthermore; 14.7% had an F-OP less than R-OP. Interestingly, all patients aged >60 years and BMI ≤ 20 showed the same value of F-OP and R-OP. Among subjects who had the same value of F-OP and R-OP, we found people of older age group and those with low BMI. Our hypothesis is that the older people would be more relaxed and those with a lower BMI would be less likely to have increased intra-abdominal pressure.
It is concluded that F-OP should not be routinely used in clinical practice as it can falsely diagnose raised intracranial pressure. If physicians routinely prefer to use F-OP, we suggest the cut off at 200 mmH 2 O for diagnosis of raised intracranial pressure and recommend an equation that can estimate R-OP from F-OP [R-OP (calculated, mmH2O) = 0.885 × F-OP (measured, mmH2O) ].