Improved lumbar infusion test analysis for normal pressure hydrocephalus diagnosis

Abstract

Objectives: Constant infusion lumbar infusion test (LIT) is an important way to find which patients, of those with signs and symptoms corresponding to normal pressure hydrocephalus (NPH) who will improve from shunt operation. LIT is a stress test on the ability for cerebrospinal fluid re-absorbtion. The aim of this study is to show how the information from LIT can be improved by quantitative analysis and avoidance of methodological pitfalls.

Material and methods: The potential pitfalls, and the analysis method, are described in detail. The analysis was applied on pre-operative constant infusion LIT from 31 patients operated for NPH, with known outcome. The pre- and post-operative walking speed was used to grade pathology progression or improvement.

Results: The maximal, plateau, intra-spinal pressure at constant infusion LIT is an ambivalent indicator for NPH: while low maximal pressure indicates no cerebrospinal fluid (CSF) absorbtion pathology, too high pressure (≥47 mmHg) may mean no diagnosis, because of stenosis of the Sylvian aqueduct. When subjects with too high intra-spinal pressure were excluded, the new analysis gave a couple of diagnostic volume parameters, of which one appears to be an optimal LIT parameter for identifying NPH patients with 14% better accuracy than plateau pressure.

Conclusion: By avoiding methodological pitfalls, and optimal analysis of the results from lumbar infusion test, the number of NPH patients who do not have a successful outcome after shunt operation may be further decreased.

Keywords: lumbar infusion test; normal pressure hydrocephalus; post-operative outcome.

Figure 1

Comparison between evaluations of improvement in the lumbar infusion test subjects. Subjective feeling of improvement, yes (Y) or no (N), is related to percent walk speed increase

Figure 2

Recording of intra‐cranial pressure during a lumbar infusion test measurement in a normal pressure hydrocephalus subject. Note the exponential return to resting level when the infusion is discontinued (arrow upwards)

Figure 3

(a) Increase in the intra‐cranial cerebrospinal fluid (CSF) volume (Vin) in relation to reference voidable venous volume at plateau pressure (Vv(p)), calculated from basal intra‐cranial pressure values in Figure 2, which gives exponential approach to the Vv(p) level. The area, A, between Vv(p) and the exponential approach gives the infusion time for the CSF volume increase. (b) Logarithmic plot of the distance from the Vv(p) level in (a) with the linear part between arrows giving the exponential, K, and A = 1/K = 4.3 min, and Vin = 4.3 × 0.8 ml. The slope before the first arrow illustrates the filling of the intra‐spinal compliance

Figure 4

Initial walk speed compared to post‐operative percent walk speed increase gave a p = 0.02 separation between successful (20% walk speed increase or more) and less successful post‐operative results. Absent walking ability did not give any outcome indication. In the normal pressure hydrocephalus patients, higher initial walking speed tended to indicate lower level of post‐operative improvement

Figure 5

LIT plateau pressure in relation to post‐operative 20% increase in walk speed gave no significant separation between the successful/less successful post‐operative groups. Of the four subjects with plateau pressure above 37 mmHg, all had less than 20% post‐operative walk speed improvement, like the subject with plateau pressure 17 mmHg

Figure 6

New LIT parameters for pre‐operative NPH diagnosis. (a) The intra‐cranial increase in CSF volume, Vin, during infusion gave no significant separation between successful (20% walk speed increase or more) and less successful post‐operative results. (b) The intra‐cranial volume of voidable venous blood at plateau level, Vv(p) gave a p = 0.01 separation between successful (20% walk speed increase or more) and less successful post‐operative results. The one subject with Vv(p) below 14 ml and less successful post‐operative result had 17 mmHg LIT plateau pressure, and was on that ground no NPH subject