An apparatus can include a left cover, sized and shaped to fit over a left eye of a patient to define a left cavity between the left cover and an anterior surface of the left eye and a right cover, sized and shaped to fit over a right eye of the patient to define a right cavity between the right cover and an anterior surface of the right eye. The apparatus can include a left pressure source to apply a left working fluid to the left cavity, the left pressure source capable of generating a left cavity pressure including a left negative gauge pressure and a right pressure source to apply a right working fluid to the right cavity, the right pressure source capable of generating a right cavity pressure including a right negative gauge pressure, where the right pressure source can be separate from the left pressure source. The left pressure source can be configured to generate a left cavity pressure independently of the right pressure source and the right pressure source can be configured to generate a right cavity pressure independently of the left pressure source.