[Clinical measurement of intraocular gas]

Increased use of gases in retinal/vitreous surgery requires a good clinical estimate of intraocular gases. This is needed, if a supplement of gas is intended, its disappearance to be predicted (e.g. to 10% to permit travelling by plane) or the amount to be calculated to cover a break. Clinical estimation of gas in the pupil is limited by its size. Accuracy of pupil measurements will be tested and compared with experimental results obtained by estimating the level of gas meniscus at the retina.

Ten cadaver eyes (5 phakic, 4 aphakic, 1 pseudophakic) were fixed in a plastic mold, vitrectomized and injected with multiple randomized volumes of air. Two observers measured the level of meniscus in disc diameters above or below the superior margin of disc and then anteriorly in the pupil in millimeters from the superior limbus. Measurements were converted to milliliters through tables constructed on mathematical models of the phakic and aphakic eye.

Observations at the retina and in the pupil were equally accurate for intermediate gas volumes, for which the meniscus fell within the pupil (P = 0.372). Measurements at the retina proved less vulnerable to error induced by misalignment of the observer's eye. Above the pupil observations at the retina were still reliable for small bubbles up to 4 dd above the disc (error < 5% in the aphakic, < 10% in the phakic eye). Below the pupil observations at the retina yielded a result increasing less than the injected volume because of refraction at the anterior segment gas interface (error > 10% of the ocular volume).

Observations of the level of gas meniscus at retina with indirect ophthalmoscopy is an accurate method for estimating intraocular gas volumes. It extends the range of measurement significantly beyond the pupil. Tables based on a mathematical model convert disc diameters to milliliters.