Some people with Primary Open-angle Glaucoma (POAG) also suffer from Obstructive Sleep Apnoea (OSA), a common sleep disorder which is known to affect heart and blood vessels, and may contribute to glaucoma progression. Obstructive Sleep Apnoea is treated with Continuous Positive Airway Pressure (CPAP); however using this type of breathing support may raise intraocular pressure (IOP). The evidence for this is limited and the potential mechanisms involved are poorly understood.