Leaving on a Jet Plane

Is any intraocular gas bubble safe for air travel?

MARK HAMMER, MD • IVAN SUNER, MD

In general, an intraocular gas bubble will begin to expand when the atmospheric pressure surrounding the patient decreases. Since the sclera is relatively inelastic, the intraocular pressure will rise until the fluid volume of the vitreous cavity is decreased enough to compensate for the requisite gas bubble expansion. There is uncertainty about whether the fluid outflow can occur fast enough to prevent a damaging rise in intraocular pressure, resulting in central retinal artery occlusion. For this reason, the standard recommendation of retinal surgeons to their patients is not to fly until the bubble is gone.

Might there be exceptions to this rule? In the extreme, a patient would experience no expansion of the intraocular gas bubble if the cabin pressure could be kept at atmospheric pressure. Certainly this could be achieved if the patient were placed in a special pressurization tank that strictly maintained atmospheric pressure around the patient inside the airplane.

The SJ30, a lightweight business jet, is at least one plane that is advertised to accomplish this goal. The SJ30 has the unique feature of maintaining a sea-level cabin pressure up to 41,000 feet. However, this could create a deferred problem if the arrival altitude is significantly higher than the departure altitude, eg, flying from Miami to Denver. This is a sep arate topic, but upon exiting the plane, an immediate decompression from a sea-level cabin pressure to an altitude of 5,000 feet could result in a dangerous elevation of over 100 mm Hg in intraocular pressure for a patient with an intraocular gas bubble.

The usual cabin conditions in scheduled commercial aircraft are quite different. The flight plan determines the altitude of the plane. The commercial scheduled aircraft cabin pressurization system modifies the atmospheric pressure to result in a cabin pressure that the patient's eye experiences. The pressurization system can keep the cabin pressure at or near sea-level atmospheric pressure up to a threshold altitude varying from 7,000 feet to 20,000 feet. Thus, if the plane stays at or below this threshold altitude, the patient will experience no bubble expansion or pressure increase. This may be experienced on very short flights, eg, between the Hawaiian Islands. Kokame and Ing¹ reported that a patient with a 65% gas bubble experienced no rise in eye pressure during a flight between Hawaiian islands, which reached a maximum altitude of 3,000 feet. Before making any real plan based on this strategy, the assumptions should be verified by the pilot, as some smaller aircraft have no cabin pressurization system.

For longer commercial flights, considerations of security, weather, fuel economy and safety relative to other air traffic make it impractical and unlikely that the flight plan could be kept below the threshold altitude. Nevertheless, this is a potential strategy for a “ high-value” political, military or business patient with a demanding schedule.

Figure 1 shows the typical transcontinental or transoceanic flight plan. The flight ascends to 37,000 feet in about 25 minutes and then cruises at 30,000 to 40,000 feet two hours before landing. Figure 2 shows the cabin pressure in terms equivalent to altitude during the flight.

Figure 1. Altitude during the typical transcontinental or transoceanic flight.

Figure 2. Cabin pressure in terms of altitude during a threehour flight.

The cabin pressure can be converted to absolute pressure in terms of millimeters of mercury (mm Hg), which are the units we usually use in describing intraocular pressure, as seen in Figure 3.

Figure 3. Cabin pressure in absolute pressure (mm Hg) during a three-hour flight.

If the eye has no bubble, the initial intraocular pressure remains unchanged throughout the trip. If the eye contains a bubble that cannot expand, the IOP is the initial IOP plus the difference of starting cabin pressure and the cabin pressure at altitude.

Let us assume that the initial IOP=18 mm Hg; initial cabin pressure at sea level=760 mm Hg; and at airplane altitude of 30,000 feet absolute cabin pressure=609 mm Hg. Then IOP=18+760-609=169 mm Hg. However, the gas bubble can expand mainly by trabecular, and perhaps trans-scleral, filtration in normal eyes and very slightly by scleral stretching.

How fast can fluid leave the eye so the bubble can expand and lower the IOP? This is poorly understood for normal eyes, since it is mainly described in the glaucoma literature, which deals with glaucomatous eyes. Filtration seems to be faster as IOP rises. From measurement of the time it takes for IOP to come down after 0.1 mL ranibizumab (Lucentis, Genentech) injections averaged for three different patients (Figure 4), filtration rate could be as high as 0.02 mL/min at IOPs of 60-70 mm Hg or higher. Figure 4 also demonstrates that instantaneous scleral stretching probably results in less than 0.1 mL of increased volume from scleral stretching.

Figure 4. Intraocular pressure (mm Hg) decreases after intravitreal injection of 0.1 mL ranibizumab.

Since the amount of gas bubble expansion that can occur is equal to the amount of fluid that can leave the eye plus an amount <0.1 mL from sclera stretching, the amount of fluid that can leave the eye essentially equals (filtration/minute) × number of minutes.

What is the theoretical maximum bubble size that might be tolerated? Assume that: (1) The plane ascends to 30,000 feet in 18 minutes, decreasing cabin pressure from 760 to 609 mm Hg; (2) Initial IOP=18 mm Hg; and (3) filtration rate is 0.02 mL/min. According to Boyle's Law, the bubble needs to expand by 760/609=1.25 to reach equilibrium IOP. Then the maximum outflow during the ascent phase would be 25% of the maximum permissible initial bubble size. Therefore, the maximum initial volume of the bubble=(0.02 x 18+0.1)/0.25=1.84 mL. For a vitreous volume=6 mL, the maximum theoretical initial bubble size=30%.

What is the clinical experience? Gandorfer and Kampik² reported central retinal artery occlusion when patients flew with intraocular bubble sizes of 30% to 50%. Lincoff and colleagues³ reported two patients with 1 mL (17%) intraocular bubble who had no problems on transatlantic flights. Mills et al.⁴ performed hypobaric chamber simulation of passengers on airplane flights. Ten patients with bubble size from 10% to 20% were tested at average peak cabin altitude corresponding to 598 mm Hg cabin pressure. Eight patients tolerated the simulated flight well, with IOP never rising above 30 mm Hg. However, one subject had pressures higher than 30 mm Hg, and one patient had to be withdrawn form the chamber due to very high pressures.

What are the final recommendations? Though from theoretical considerations and Lincoff et al.'s experience³ it is likely that most patients with a bubble of ≤1 mL could safely fly with standard cabin pressurization, Mills and colleagues'experiment⁴ shows there are exceptions, perhaps in as many as 20% of patients. Thus the standard recommendation is still for the patient not to fly until the bubble is gone due to unknowns for specific patients.

If the patient insists on flying with an intraocular bubble, here are some precautions: (1) It is safer if the bubble is reduced to a volume of 0.3 to 0.6 mL (5% to 10%) or less; (2) The patient could be pre-tested with injection of 0.1 mL of balanced saline solution to see how long it takes for the IOP to return to baseline. If the pressure does not decrease significantly after three minutes, the patient probably does not have a normal aqueous outflow capacity and is at higher risk of experiencing very high IOP during airplane flight; (3) Asking the pilot to keep altitude low the whole flight for longer flights is probably a strategy available only to high-value patients; (4) Letting the pilot know ahead of time that it may be necessary to decrease altitude if the patient has symptoms would probably get the patient removed from most commercial flights because of the risk and expense of complying with these needs; and (5) If a postoperative airplane flight is anticipated, silicone oil is usually substituted for gas as an intraocular tamponade. RP

REFERENCES

1. Kokame GT, Ing MR. Intraocular gas and low-altitude air flight. Retina. 1994;14:356-358.
2. Gandorfer A, Kampik A. [Expansion of intraocular gas due to reduced atmospheric pressure. Case report and review of the literature]. Ophthalmologe. 2000;97:367-370.
3. Lincoff H, Weinberger D, Reppucci V, Lincoff A. Air travel with intraocular gas. I. The mechanisms for compensation. Arch Ophthalmol. 1989;107:902-906.
4. Mills MD, Devenyi RG, Lam WC, Berger AR, Beijer CD, Lam SR. An assessment of intraocular pressure rise in patients with gas-filled eyes during simulated air flight. Ophthalmology. 2001;108:40-44.