Quantitative Evans index estimation using ultrasonographic measurement of the optic nerve sheath diameter in supine and upright position.

OBJECTIVES

We aimed to quantitatively assess Evans index (EI) using ultrasonographic optic nerve sheath diameter (ONSD) measurements in supine and upright position in normal pressure hydrocephalus (NPH) patients.

METHODS

Ultrasonographically ONSD was measured in a supine and upright position before and 4-5 days after the ventriculoperitoneal shunt surgery. The changes of the ONSD between supine and upright positions were calculated as ∆ONSD = sONSD-uONSD and as the variation ONSD_V = 100% × [(sONSD - uONSD)/sONSD]. Multiple linear regression analyses were conducted to assess associations between EI and the variation of ONSD. We derived the mathematical function to predict EI. Bland-Altman analysis was applied to evaluate the accuracy and precision of the EI prediction.

RESULTS

Thirteen adult patients (mean age 61.8 ± 11.1 (SD) years; 6 (46%) female) undergone VP shunt implantation for NPH. The mean EI was 0.432 (95% CI, 0.393-0.471) preoperatively and 0.419 (95% CI, 0.373-0.466) postoperatively (p = 0.066). There is a decrease of the ONSD during positional changes from supine to upright position and pre- and postoperative EI correlated with preoperative variation ONSD_V1 (r =  - 0.610 and - 0.648, p < 0.05). The mathematical function for preoperative EI estimation was EIpreop = 0.504 - 0.022 × ONSD_V1 + 0.101 × gender (M = 0; W = 1), (Durbin-Watson value = 1.94), and for postoperative was EIpostop = 0.487 - 0.022 × ONSD_V1 + 0.117 × gender; (Durbin-Watson value 2.23).

CONCLUSIONS

Ultrasonographic ONSD measurements in supine and upright position provide a potential method to quantify EI that can be conducted at the bedside.