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Prior Two Year IOP Trend Meaningless?

Submitted by dave on Fri, 07/07/2006 - 10:55pm

Today's measurements were all taken with the Reichert NCT II. They are all means (averages). For each IOP exam, at each of the 5 times reported in the table below, multiple measurements were taken for each eye. The table below shows 5 IOP values for each eye (for a total of 10 IOP values). These values actually consist of the means of 31 individual IOP measurements. The individual measurements are not shown (in order to make the presentation of the data clearer).

What does this data indicate about the last two years worth of IOP measurements and the significance of the downward  trend that seemed so clear?

From the time I started Xalatan, the high IOP reading was 22 and the low was 17 (regardless of the eye being measured). My ophthalmologist and I (and my personal vision coach) ascribed significance to the trend.

If today's data can be taken seriously*, it indicates that what appeared to be a trend may have simply been a coincidence. Today's IOP values encompass the range of the whole two year trend in a single day!

Based on this data, I feel that investigating the fluctuations in my IOP will be worthwhile in helping me understand my disease.

Here are today's IOP values:

Date Time Left Right
07-Jul-2006 09:00 19 19
07-Jul-2006 12:10 17 22
07-Jul-2006 17:40 17 20
07-Jul-2006 20:10 16 18
07-Jul-2006 22:55 17 21

*One reason that today's data may not be as meaningful is that all the values are obtained with the Reichert NCT II non-contact tonometer. [UPDATE: after more than 12000 IOP measurements, I am developing a hunch that the non-contact tonometers provide more sensitive information than the Goldman tonometer. NCTs pick up fluctuations due to the cardiac pulse or a forceful eye blink, for example. The Goldman tonometers simply smooth out these fluctuations. This smoothing quality of the Goldman is the reason it is considered more reliable -- the readings don't fluctate as much. However, inside the eye, the pressure does change in response to things like the cardiac pulse. Therefore, from a data mining perspective on an extremely large database, the Goldman is simply collecting less information. Over thousands and thousands of measurements, the sensitivity of the non-contact tonometers may provide useful information that the Goldman tonometers miss.]

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