October 2010




Water, water ... everywhere

by Matt Young EyeWorld Contributing Editor


Even water can affect ocular parameters. That's according to a new study published online in May in Experimental Eye Research titled, "Water drinking influences eye length and IOP in young healthy subjects."

Researchers studied the effects of water loading on IOP, ocular pulse amplitude (OPA), and axial length. "Significant increases in both IOP and OPA were found to occur following water loading (P<0.0001), with peaks in both parameters occurring at 10min after water loading," according to lead study author Scott A. Read, Ph.D., Contact Lens and Visual Optics Laboratory, School of Optometry, Queensland University of Technology, Brisbane, Australia. While the changes may have no significant clinical impact, "these findings highlight the fact that hydration levels can significantly influence ocular dimensions," Dr. Read noted.

Understanding the water impact

Dr. Read analyzed 21 young adult subjects. Among them were 11 myopes and 10 emmetropes. "To reduce the potential confounding influence of diurnal variations in IOP and axial length, all measurements were carried out between 1pm and 4pm," Dr. Read noted. "For all subjects, a period of fasting (with no ingestion of food or liquid) was observed for 2 [hours] prior to the experiment."

The researchers also tried to minimize any possibility that previous tasks involving accommodation would impact results. "To reduce the influence of prior accommodative visual tasks upon our results, a period of 7 min of a distance viewing task (i.e. watching TV at a distance of 6 m) was undertaken by the subjects immediately prior to the baseline measures," Dr. Read reported. "Following these baseline measures, all subjects then drank 1000 ml of water in a 5 [minute] period."

IOP, OPA, and ocular biometric measurements were studied before as well as 10, 15, 25, and 30 minutes after 1,000 ml of water was ingested. "In between measurement sessions, subjects maintained distance fixation (watching TV)," Dr. Read wrote. After doing everything they could to ensure the results would be accurate, the researchers found that water loading did cause important changes. "In our population of young healthy adult subjects, water drinking was found to lead to significant changes in IOP, OPA and axial length," Dr. Read concluded. On average, the highest subject IOP was found to occur 10 minutes after water drinking. The mean +/SEM (standard error of the mean) increase was 2.24 +/0.31 mm Hg. Mean increases of 1.67, 1.59, and 0.93 mm Hg were demonstrated at 15, 25, and 30 minutes after water loading, respectively. "Pairwise comparisons revealed the changes in IOP were significantly different to baseline at all measurements following water drinking," Dr. Read reported. Baseline mean +/SEM IOP was 15.65 +/0.44 mm Hg. "OPA exhibited similar changes, with a highly significant increase in OPA found following water drinking (P < 0.0001)," Dr. Read wrote. "The mean OPA at baseline was 2.28 +/0.19 mm Hg, with the peak in OPA observed on average 10 min after water drinking (mean increase 0.46 +/0.06 mm Hg)."

Therefore, at 10 minutes after water drinking, average IOP increased by 14.21% and OPA increased by 24.37% on average."This indicates that whilst the magnitude of increase in OPA was smaller than the change in IOP, proportionally the increase in OPA was larger than the change in IOP," Dr. Read concluded. Axial length also demonstrated a small but statistically significant change after water drinking. Mean axial length was 24.33 mm, and the largest reduction was found at 10 minutes after water loading. The mean change was 12 microns. This research could have some impact on future studies involving diurnal variations. "It has previously been shown that axial length undergoes a significant diurnal variation," Dr. Read reported. "Changes in hydration across the day could potentially contribute to a portion of these previously observed diurnal variations."

Dr. Read hypothesized that changes in axial length following water drinking could be related to changes in choroidal thickness. "Given the high blood flow and vascular permeability of the choroid, it is plausible that systemic water loading could lead to an increase in its volume," Dr. Read reported. He also suggested the IOP rise could be attributed to an increase in choroid volume. Meanwhile, John D. Sheppard, M.D., professor of ophthalmology, microbiology, and immunology, Eastern Virginia Medical School, Norfolk, Va., said he believes even subtle changes in axial lengthperhaps even resulting from water loadingcould have clinical implications. "We measure the axial length to determine the power of an IOL," Dr. Sheppard said. "Even a tenth of a millimeter difference can produce a significant change in calculated IOL power."

Editors' note: Dr. Read reported no financial interests related to this study. Dr. Sheppard has no financial interests related to his comments.

Contact information

Read: +61 7 3864 5714, sa.read@qut.edu.au
Sheppard: 757-622-2200, docshep@hotmail.com

Water, water ... everywhere Water, water ... everywhere
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