Aqueous humor outflow facility by tonography does not change with body position

Area of Science:

• Ophthalmology
• Physiology

Background:

• Intraocular pressure (IOP) is known to vary with body position.
• Changes in episcleral venous pressure (EVP) account for most, but not all, of this IOP variation.
• Other aqueous humor dynamic parameters may also influence positional IOP changes.

Purpose of the Study:

• To investigate the variation of aqueous humor outflow facility with changes in body position.
• To determine if body posture affects aqueous humor dynamics beyond IOP.
• To assess the role of outflow facility in positional IOP variations.

Main Methods:

• Healthy volunteers (24-45 years old) underwent constant weight tonography using a modified electronic Schiotz tonometer.
• Measurements were taken in both seated (70 degrees from horizontal) and supine positions, with at least 30 minutes between.
• Tonography data were analyzed using second-order polynomials to determine IOP and outflow facility.

Main Results:

• Forty-two eyes from 21 subjects were analyzed.
• Intraocular pressure (IOP) was significantly higher in the supine position (19.9 +/- 1.6 mm Hg) compared to the seated position (17.8 +/- 1.7 mm Hg) (P < 0.001).
• Mean aqueous humor outflow facility showed no significant difference between the seated (0.30 +/- 0.31 microL/mL/mm Hg) and supine (0.28 +/- 0.09 microL/mL/mm Hg) positions (P = 0.37).

Conclusions:

• Aqueous humor outflow facility, as measured by electronic Schiotz tonography, does not significantly change between supine and sitting body positions.
• The observed positional variations in IOP are likely not mediated by changes in aqueous humor outflow facility.
• Further research may be needed to identify other contributing factors to positional IOP changes.