Is intracranial pressure monitoring in the epidural space reliable? Fact and fiction

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Object

Epidural pressures have been reported as being systematically higher than ventricular fluid pressures. These discrepancies have been attributed both to the characteristics of the sensor and to the particular anatomy of the epidural space. To determine which of these two possible causes better explains higher epidural readings, the authors compared pressure values obtained during simultaneous epidural and lumbar pressure monitoring in 53 patients and during simultaneous subdural and lumbar pressure monitoring in 22 patients. The same nonfluid coupled sensor device was used in all compartments.

Methods

All 75 patients had normal craniospinal communication. Simultaneous intracranial and lumbar readings were performed every 30 seconds. The epidural–lumbar and subdural–lumbar pressure values were compared using correlation analysis and the Bland–Altman method.

The median differences in initial epidural–lumbar and subdural–lumbar pressure values were 11 mm Hg (interquartile range 2–24 mm Hg) and 0 mm Hg (interquartile range −2 to 1 mm Hg), respectively. The correlation coefficients of the mean epidural–lumbar and subdural–lumbar intracranial pressure (ICP) values were ρ = 0.48 (p < 0.001) and ρ = 0.88 (p < 0.001), respectively. Using the Bland–Altman analysis, epidural–lumbar methods showed a mean difference of −20.93 mm Hg; epidural pressure values were systematically higher than lumbar values, and these discrepancies were greater with higher ICP values. Subdural–lumbar methods showed a mean difference of 0.35 mm Hg and both were equally valid with all mean ICP values.

Conclusions

Epidural ICP monitoring produces artifactually high values. These values are not related to the type of sensor used but to the specific characteristics of the epidural intracranial space.

Abbreviations used in this paper:CSF = cerebrospinal fluid; ICP = intracranial pressure; SD = standard deviation.

Article Information

Address reprint requests to: Maria A. Poca, M.D., Ph.D., Department of Neurosurgery, Vall d'Hebron University Hospital, Passeig Vall d'Hebron 119-129, 08035 Barcelona, Spain. email: 26382app@comb.es.

© AANS, except where prohibited by US copyright law.

Headings

Figures

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    Drawing illustrating simultaneous intracranial and lumbar pressure measurements obtained using two Ladd sensors. Each sensor is connected to a Ladd monitor, which sends the pressure signal to a two-channel analogical recorder for analysis of ICP waves. Note that the patient's legs are extended and the head is aligned to avoid cervical flexing and increases in abdominal pressure. In the inset in the upper left corner, the drawing illustrates the need for systematic correction of ICPs. Because the position of the intracranial sensor was always lower than that of the lumbar sensor (between 5 and 7 cm), 5 mm Hg was always subtracted from the cranial pressure values. In the inset in the lower right corner, the photograph shows the Ladd sensor adapted to measure lumbar pressures. The device is connected to a three-way stopcock, which connects the lumbar needle (a) with the infusion system (b) and the transducer (c).

  • View in gallery

    Histogram showing differences in ICP values simultaneously recorded by epidural and lumbar sensors (3501 readings). The median difference in the ICP values recorded by both methods was 13 mm Hg (interquartile range 5–36 mm Hg), with a minimum and maximum difference of −21 and 100 mm Hg, respectively.

  • View in gallery

    Bland–Altman plot of the agreement between simultaneous epidural and lumbar pressure recordings. The difference between the methods in the y axis (Lumbar–Epidural) is plotted against their mean, represented in the x axis (Lumbar +Epidural/2). The mean difference between the two methods was −21 mm Hg. As can be seen, the epidural pressure was consistently and significantly higher than the lumbar pressure. The higher the patient's pressure, the greater the difference. The solid line denotes the mean, and the dotted lines represent +two SDs and −two SDs (upper and lower dotted lines, respectively).

  • View in gallery

    Charts depicting simultaneous epidural and lumbar pressure recordings obtained in a patient with adult chronic hydrocephalus. Note that the absolute differences in the pressures of both compartments are lower than 5 mm Hg with identical wave amplitude and morphology. Po = opening pressure (in millimeters of mercury).

  • View in gallery

    Histogram showing differences in ICP values simultaneously recorded using subdural and lumbar sensors (1468 readings). The median difference in the mean ICP values recorded by both methods was 0 mm Hg, with an interquartile range of −2 to 2 mm Hg (maximum and minimum differences of 22 and −53 mm Hg, respectively).

  • View in gallery

    Chart showing correlation of ICP values simultaneously recorded by the subdural and lumbar sensors (Spearman rank correlation analysis). The solid line denotes the mean.

  • View in gallery

    Bland–Altman plot of the agreement between simultaneous subdural and lumbar pressure recordings. The difference between the readings in the y axis (Lumbar–Subdural) is plotted against their mean (Lumbar + Subdural/2) in the x axis. The mean difference between the two methods was 0.35 mm Hg. As can be observed in the plot, the scattering of the differences around the mean did not vary over the range of mean ICP values. Therefore both methods are equally valid throughout the entire range of ICP readings. The solid line denotes the mean, and the dotted lines represent +two SDs and −two SDs (upper and lower dotted lines, respectively).

  • View in gallery

    Charts depicting simultaneous subdural and lumbar pressure recordings. Note that traces for the two pressures are identical both in absolute pressure values and in wave amplitude and morphology.

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