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Comment & Response
November 2018

Measuring Optic Nerve Sheath Diameter as a Proxy for Intracranial Pressure

Author Affiliations
  • 1New York University School of Medicine, New York
JAMA Ophthalmol. 2018;136(11):1309. doi:10.1001/jamaophthalmol.2018.3426

To the Editor Having a rapid, safe, noninvasive, inexpensive, and reliable method to determine intracranial pressure (ICP) would be an essential tool for clinicians across many disciplines in medicine. The recent work by Wang et al1 is a noteworthy step toward that goal. However, it would be helpful if the authors would address 2 concerns. First, regardless of the validity of their results, it is unclear that they are actually measuring the optic nerve sheath diameter. As noted by Copetti and Cattarossi,2 the anatomy of the eye is such that it would be unlikely for the optic nerve and sheath to be aligned in such a way as to produce the sonographic images studied. While this point does not alter the implications of the findings by Wang et al,1 it is essential that we actually understand what in fact is being measured. Additionally, the use of correlation may be an inadequate method to represent and analyze the data. To be clinically useful, the change in the size of the sonographic finding must reliably indicate a change in ICP. Correlation simply states that the 2 findings move in the same direction. As can be seen from the extreme right side of their Figure 1, a patient whose ICP falls by nearly 200 mm of water may have a change in sonographic diameter from as little as 0.5 mm to as much as 2.5 mm, and these extremes overlap with nearly every other patient studied. Representation of the data using a Bland-Altman plot would have indicated this limitation.3

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