The day after vitrectomy for a macular hole, the gas dissipated to less than half the amount it should be. why?

Gas Bubble: The gas bubble in your eye is absorbing perhaps too quickly. Usually face down positioning is recommended after macular hole surgery so that the gas bubble compresses your macula into the proper position. I would advise calling your retinal surgeon to make sure he/she thinks this is ok.

See a retina special: Ask your retina specialist. Different gases of different concentrations are chosen by your retinal surgeon (based on how long he or she wants it to last) and injected into the eye.

Several Reasons: The gas bubble may be smaller than expected because an insufficient amount or concentration was placed in the eye at surgery or leakage occurred from wounds after surgery. Either way there may be enough gas present to provide a successful closure with good patient positioning. Recent studies show that the hole closes in the first 4 days of face down positioning.

Bubble can vary: After surgery, the bubble size can vary, affected by a number of factors. As long as the bubble is big enough to cover the macular hole (needs to be larger if no positioning is prescribed), it should be effective. The reasons for small bubble include less thorough vitrectomy, leakage of gas from eye, etc.