Practice Patterns and Opening Pressure Measurements Using Fluoroscopically Guided Lumbar Puncture

Discussion

Despite the absence of practice guidelines for fluoroscopically guided LP, neuroradiologists generally appear to perform these in a similar fashion. Most perform thecal sac puncture and OP measurement on a GI fluoroscopy table with the patient prone throughout the entire procedure. A 22-ga needle is typically used to puncture at L2-L3 or L3-L4. Almost two- thirds (64%) record the OP when the CSF meniscus stops rising as opposed to the traditional, and likely more accurate, teaching of OP measurement when systolic-diastolic or respiratory fluctuation begins.² This may result in falsely high or low opening pressure measurements.

Most (80%) neuroradiologists who typically perform OP measurement with the patient prone add the needle length to the manometer reading to determine the final CSF OP. The OP is based on the CSF height within the manometer, where zero is the level of the left atrium of the heart. In the LD position, zero level corresponds to the position of the spinal canal and is determined clinically with various techniques. In the prone position, however, needle length must be added to the meniscus height or the recorded pressure will be falsely low. According to the survey data, 1 in 5 neuroradiologists does not add the needle length when measuring OP in the prone position. Underestimation of CSF pressure by omitting needle length from the final OP measurement may have clinical implications. For example, IIH is a diagnosis of exclusion that depends on an accurate OP measurement to prevent potentially irreversible vision loss. Conceivably, the respondents who do not add needle length to the manometer reading after prone OP measurement could be using flexible extension tubing, which theoretically allows prone OP measurement with zero at the level of the left atrium. This survey did not specifically address the use of flexible tubing, so prevalence of this technique remains unclear.

Most interesting, normative OP data exist only for the LD position, not for prone measurements. Clinical decisions based on these pressures deserve careful consideration, especially when the OP is borderline. We are currently studying the relationship of prone-versus-LD OP.

The lower extremity position likely affects the OP measurement. While some studies have demonstrated that the lower extremity position does not affect CSF pressure, others have shown a significant difference between OP in the flexed- and extended-leg positions.^3⇓⇓–6 In this survey, extended lower extremity position was not consistently maintained during OP measurement in the LD position. The clinical significance of this difference is debated, but lower extremity position could affect clinical decision-making in borderline cases. While this is more applicable to LPs performed at the bedside and only applies to the small percentage (28%) of OPs measured in the LD position during fluoroscopically guided LP, it illustrates a deficiency in practice guidelines that may affect opening pressures.

As obesity increases in the US population, it is likely that the incidence of IIH will also rise. The diagnosis of IIH depends on an accurate OP of ≥25-cm water measured in the LD position. In the absence of practice guidelines for OP measurement, patients may receive a false-positive IIH diagnosis or go undiagnosed if the neuroradiologist fails to add the needle length to prone OP measurements.