We have previously reported [1] on the association between tympanic membrane displacement (TMD) and intracranial pressure (ICP). This relationship is based upon a patent cochlear aqueduct and different methods can be used to help with this assessment. We have compared three different methods based upon values obtained when the patient is supine and sitting.
All three methods showed a significant association between TMD and ICP with relatively tight confidence intervals but predictive limits are wide. Using the comparison method of assessment, at 100nL the variability of the ICP distribution is such that it could only be predicted to be between approximately -20 and +40mmHg with 95% certainty.Although there is clearly a strong relationship between TMD and ICP such accuracy limits the use of the technique. Even with different methods of assessing cochlear aqueduct patency the variability of the response makes it difficult to make clinical decisions.
[1] Chambers IR, Shimbles S, Dodd C, Banister K, Mendelow AD (2004): Clinical comparison of tympanic membrane displacement with invasive ICP measurements, Twelfth International Symposium on Intracranial Pressure and Brain Monitoring. Hong Kong, 2004, p54.
Shimbles S, Banister K, Dodd C, Mendelow D, Chambers I. Cochlear aqueduct patency in tympanic membrane displacement measurement for intracranial pressure assessment. In: Lumdstrom R, Anderson B, Grip H, editors. 13th Nordic Baltic Conference on Biomedical Engineering and Medical Physics; 2005; Umea: Swedish Society for Medical Engineering and Medical Physics; 2005. p. 268-269.
Dr. Iain R. Chambers, Cleveland Unit, Middlesbrough. Tel: +44 (0)1642-854-778.
Regional Medical Physics Department, Newcastle General Hospital, Newcastle upon Tyne, NE4 6BE, UK.
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