Eyeworld

EW GLAUCOMA 78 February 2016 by Tony Realini, MD, MPH What is the second relevant pres- sure? That depends on who you ask. Ocular perfusion pressure Epidemiologic studies have consis- tently found weak links between sys- temic blood pressure and glaucoma. The unifying concept linking blood pressure, IOP, and glaucoma may be the ocular perfusion pressure (OPP), which is most easily thought of as blood pressure minus eye pressure. OPP is the pressure at which blood enters the eye to perfuse ocular tissues. In a number of major studies, low OPP—particularly diastolic OPP (diastolic blood pressure minus IOP)—has been associated with a higher risk of both glaucoma de- velopment and progression. What causes OPP to be low? Low OPP can result from low blood pressure, high IOP, or a combination of both. In fact, at night when asleep in the supine position, IOP is at its circadian peak while blood pressure is at its circadian trough—a perfect storm of low OPP and reduced per- fusion of ocular tissues, according to Felipe Medeiros, MD, San Diego. "What is the significance of nocturnal IOP peak pressure?" Dr. Medeiros asked. One explanation: Reduced perfusion to the optic nerve could result in ischemia and produce the cupping seen in glaucoma and may provide a mechanism for the vascu- lar theory of glaucoma. One key point of clinical rele- vance is the potential for low blood pressure at night due to overtreat- ment of systemic hypertension. "In one major study, low DPP was associated with glaucoma in patients on antihypertension meds but not in untreated patients," said Alon Harris, PhD, Indianapolis. One possible explanation is that treated patients may be having significant nocturnal blood pressure dips. Cerebrospinal fluid pressure The optic nerve spans the junction between the intraocular and the intracranial space. Importantly, the optic nerve is the site of glaucoma damage. "The lamina cribrosa is the main structural element of the optic nerve head," Dr. Harris said. "In essence, the lamina forms a pressure with elevated IOP often never devel- op glaucoma. Emerging data suggest that glau- coma may be best considered not a disease of a single pressure—IOP— but rather as a 2-pressure disease. IOP has grown more distant, as we have observed that elevated IOP is neither necessary nor sufficient to explain all—or even most—cases of glaucoma. Indeed, glaucoma can occur at normal IOP levels, and eyes F or more than 150 years, glaucoma has been thought of as a disease of elevated intraocular pressure (IOP). In more recent years, the relationship between glaucoma and Glaucoma: A 2-pressure disease?

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