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.
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.
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.