People with glaucoma can usually fly without any problem. The cabins in planes have a controlled atmospheric pressure, which makes up for the drop in pressure as the plane reaches higher altitudes.
The eye pressure (intraocular pressure) measured by the doctor is the difference between an absolute intraocular pressure and the current atmospheric pressure. A rapid decrease in the atmospheric pressure leads to a (relative) increase in the eye pressure. This normally poses no problem for a glaucoma patient on board a plane: there is an artificial atmospheric pressure inside the aircraft's cabin that compensates for most of the natural pressure drop experienced at high altitude. The eye adjusts relatively quickly to the new situation. A moderate decrease in atmospheric pressure will therefore not induce a significant rise in eye pressure.
Another aspect to consider is the quality of air inside the cabin with its somewhat lower concentration of oxygen at high altitude and, as a consequence, a lower availability of O2. But here again, cabin ventilation provides the passengers with an almost normal level of oxygen. Nevertheless, glaucoma patients with advanced circulatory problems who fly frequently should discuss this with their ophthalmologist.