Clinical comparison of tympanic membrane displacement with invasive ICP measurements.

OBJECTIVE Tympanic membrane displacement (TMD) measurements may be useful in the management of patients with hydrocephalus if they can be directly associated with measurements of ICP. We have compared TMD measurements using the Marchbanks Measurement System with invasive ICP monitoring. METHODS Twenty-nine patients who were undergoing routine invasive monitoring using a Camino fibre optic ICP measurement system as part of their clinical management were studied. Simultaneous measurements of ICP and TMD in both sitting and supine positions were successfully made in thirteen patients. RESULTS Thirty-nine pairs of readings were obtained. The invasive ICP readings varied from 1 to 36 mmHg in the supine position and from -12 to +35 mmHg sitting. Corresponding TMD values varied from 275 to +277 nL in the supine position and -133 to +466 nL sitting. Linear regression showed a significant negative relationship between the two measurements (r = -0.57, p = 0.0013). CONCLUSIONS There is a strong negative linear association between mean TMD and invasively measured ICP and this relationship is highly significant. Nevertheless, TMD is a poor surrogate for ICP in clinical terms because the predictive limits of the linear regression are too wide. However, serial intra-patient measurements may be useful to determine changes in ICP with time.