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Editors Selection IGR 13-4
Clinical examination methods: IOP provocative tests
Comment by Franz Grehn on:
69016 Comparison between intraocular pressure spikes with water loading and postural change, Chong CW; Wang SB; Jain NS et al., Clinical and Experimental Ophthalmology, 2016; 44: 768-775
Water loading (water drinking test, WDT) has been a provocative test for intraocular pressure (IOP) spikes since many years. Water loading and other provocative tests try to give diagnostic support whether the aqueous outflow system in the eye can compensate for variations of aqueous flow sufficiently or whether the outflow function is critical. A positive test can also be taken as a surrogate for better or worse IOP control and hence for suspected future functional loss. Provocative IOP peaks may also reflect the fluctuations of IOP outside the routine office measurements.
It is well documented by sleep laboratory research that horizontal body position, in particular the supine position test (ST), increases IOP by 2-4 mmHg in the normal eye. This is usually explained by increase of episcleral venous pressure, but other mechanisms are also considered.
The supine position test may be a safer and more comfortable alternative to the WDT
The paper by Chong et al. tested the agreement of the spike IOPs after water drinking test with those of the supine test in 21 primary open-angle glaucoma patients in a consecutive, prospective blinded trial. Patients with other types of glaucoma, previous surgery or laser, or with IOP > 28 were excluded. The water drinking test consisted of drinking 10 ml water/kg body weight within five minutes and IOP measurements at 20 minutes / 40 minutes. The supine test lasted 40 minutes with measurements of IOP at 20 minutes and 40 minutes, while the patient was transiently sitting up for measurement. The I-Care tonometer was used.
In both tests, the IOP was significantly increased at 20 minutes and 40 minutes. The Bland Altman analysis showed an nearly perfect correlation between the two tests.
The paper discusses the caveats of water loading for patients with cardiac, renal, prostatic or respiratory disease, while the supine test is less stressful for body functions and the comfort of the patient. Therefore, from the authors' perspective, the ST is easier and safer to perform.
The Discussion section of the paper gives a valuable overview on the various mechanisms behind the two different provocative tests. In the WDT, increase of aqueous inflow, osmotic pressure, peripheral and episcleral venous pressure as well as trabecular resistance are discussed for elevation of IOP. In the ST, the peripheral and episcleral venous pressure are considered the major factor for IOP increase.
In summary, the strong correlation between the IOP increase after the WDT and after ST suggests that similar mechanisms may be responsible for the elevation of IOP. The ST may be a safer and more comfortable alternative to the WDT.
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