Caffeine is the most widely consumed drinking nutrient in the world. Caffeine effects various organs and the vascular system. It decreases ocular blood flow due to vasoconstriction.

Ingestion of caffeine in glaucoma patients and healthy subjects may decrease peripapillary and macular blood flow in the back of the eye.

The primary objective of this study is to assess the acute changes in peripapillary and macular blood flow before and after an intake of oral caffeine (200 milligram tablet) in glaucoma patients and healthy subjects by using optical coherence tomography angiography (OCTA) scans. This novel imaging tool creates three-dimensional angiograms to assesses signal changes caused by flowing blood cells in a non-invasive angiogram scan. Blood flow is shown as vessel density measured in percentage.

Inclusion Criteria:

• age 18 to 90 years
• diagnosis of primary open angle glaucoma (Mild, Moderate and Advanced)
• healthy subjects with no eye disease

Exclusion Criteria:

• Diseases, ophthalmic or systemic, that are likely to affect OCTA results
• greater than moderate cataract
• nystagmus
• inability to look at target
• macular degeneration other than mild drusen or pigmentary changes
• diabetic retinopathy
• neovascular glaucoma or non-glaucoma optic neuropathies
• current macular edema, prior laser to retina, inflammatory retinopathy or choroidopathy
• keratoconus, corneal ectasia, central corneal scarring
• rheumatologic diseases or Raynaud's phenomena
• pregnant and lactating women
• mental illness or alcohol addiction
• pre-existing bladder symptoms, cardiac disease or sleep disorder
• refractive spherical diopter greater than 5 or cylinder greater than 3
• possible tolerance to caffeine (drinking more than 1 cup coffee per day).