Intraocular pressure is the difference between the pressure in the eye and the pressure in the surrounding environment. In general, atmospheric pressure does not affect glaucoma, as it does not change this trans-corneal pressure: individuals living at sea level are not at higher risk of glaucoma than those living at high altitudes. However, the effects of a localized region of low pressure on the eye are less certain.
In a recent study, Ethier et al. investigated the effects of negative pressure goggles on IOP. The goggles generate a localized region of negative pressure around the eyes, and were modified for this study to allow pneumatonometry measurements through a latex membrane. The authors developed a lumped-parameter mathematical model to predict the effects of negative pressure on the eye and compared it with clinical measurements obtained from subjects wearing the modified goggles. The model predicted a reduction in episcleral venous pressure and an associated decrease in IOP. The model also predicted an increase in globe volume, associated with an initial increase in blood volume which gradually decreased as aqueous humor accumulated. The predicted results corresponded with clinical measurements of IOP performed on human subjects.
However, the results reported in this study raise significant concern about the safety of negative pressure goggles in glaucoma patients. The clinical results and the mathematical model both show that the decrease in IOP is less than the decrease in pressure applied by the goggles. With an applied negative pressure of -12.15 mmHg, only a -5.6 mmHg decrease in IOP was achieved clinically, and -6.4 mmHg predicted in the model. The authors interpret the results as showing an IOP decrease, but they defined IOP as the difference between the pressure in the eye and the surrounding room pressure (i.e., where the pneumatonometer was placed) ‐ this does not reflect the pressure difference experienced by the eye. Using a more conventional definition of trans-corneal pressure, the IOP was actually increased by the goggles, by 6.55 mmHg clinically (5.6 mmHg decrease in the eye, but a 12.15 mmHg decrease just outside of the eye under the goggles). This increase in conventionally defined IOP is reflected in the model, which predicted an increase in ocular volume and would be associated with distension of ocular tissues including the scleral canal. This scleral distention may predispose patients to lamina cribrosa distention1 and glaucoma development or progression.
It is clear that negative pressure goggles increase trans-corneal pressure, causing ocular tissue distension, and must be considered contra-indicated in glaucoma patients
The authors hypothesize that there may be value in defining IOP with respect to atmospheric pressure instead of trans-corneal pressure. In particular, they speculate that the retrolaminar pressure may not be affected by the goggles, and hence the trans-laminar pressure difference may be reduced. While there has been interesting work suggesting a role of trans-laminar pressure difference in glaucoma pathogenesis,2 it seems improbable that retrolaminar pressure would be unaffected by the goggles since this region resides in the orbit and not the cranial vault. Nevertheless, the effect of negative pressure goggles on this parameter is currently unknown and should be the subject of future research. In the interim, it is clear that negative pressure goggles increase trans-corneal pressure, causing ocular tissue distension, and must be considered contra-indicated in glaucoma patients.