Aqueous flow in open-angle glaucoma.

OBJECTIVES: To measure aqueous dynamic variables in patients with open-angle glaucoma when their ocular hypotensive treatment was withdrawn and to determine if the circadian rhythm of the aqueous humor flow was present in open-angle glaucoma. METHODS: We studied 20 patients with open-angle glaucoma, and a group of 20 healthy subjects served as controls. Aqueous humor flow was measured by clearance of topically applied fluorescein with a fluorophotometer, tonography was used to measure outflow facility, and applanation tonometry was used to measure the intraocular pressure. RESULTS: We did not find any statistically significant difference when comparing aqueous flow during the daytime in subjects with open-angle glaucoma with that in controls. However, we did measure a higher aqueous flow at night in the group with open-angle glaucoma, compared with the normal group. The circadian rhythm of aqueous humor flow was present in the subjects with glaucoma. CONCLUSIONS: Aqueous flow is not suppressed in glaucomatous eyes that are not undergoing treatment. Rather, the flow is slightly higher at night during sleep compared with the flow in normal sleeping subjects, but not high enough to have a clinically significant effect on intraocular pressure.

The effect of oral dexamethasone on the circadian rhythm of aqueous humor flow in humans.

PURPOSE: To determine if the circadian rhythm of plasma corticosteroid activity is necessary for the circadian rhythm of aqueous humor flow to occur in humans. METHODS: Twenty normal volunteers were recruited for this randomized, double-masked, placebo-controlled study. Oral dexamethasone was given at a dose of 0.5 mg every 6 hours; this is equivalent to more than two times the normal 24-hour endogenous adrenal corticosteroid production. This dosage schedule will maintain a relatively constant level of corticosteroid action throughout a 24-hour period. Topical fluorescein and a scanning fluorophotometer were used to measure the rate of aqueous humor flow. Subjects were studied on two separate days. On one study day, the subject took 0.5 mg of dexamethasone every 6 hours; on the other study day, the same subject took a placebo every 6 hours. RESULTS: During the morning, aqueous flow was 4.06 +/- 0.70 microliters/min (mean +/- SD) in subjects taking dexamethasone and 3.82 +/- 0.85 in subjects taking a placebo. This 6% higher dexamethasone flow was not significant (P = 0.10). During the afternoon, aqueous flow was 3.83 +/- 0.78 in subjects taking dexamethasone and 3.52 +/- 0.77 in subjects taking a placebo. This 9% higher dexamethasone flow was statistically significant (P = 0.02). The nighttime aqueous flow was 1.38 +/- 0.45 in subjects taking dexamethasone and 1.43 +/- 0.34 in subjects taking the placebo. There was not a significant difference between placebo and dexamethasone during the night (P = 0.40). On each day, intraocular pressure was measured at 8:00 AM, 4:00 PM, and 6:00 AM. When comparing dexamethasone to placebo, no significant difference was observed in any of the intraocular pressures. CONCLUSIONS: The study is interpreted as showing that the reduction of aqueous humor flow during sleep can occur in the absence of a comparable fall in plasma corticosteroid pathway.

The effect of topical timolol on epinephrine-stimulated aqueous humor flow in sleeping humans.

PURPOSE: It has been shown that intravenously administered epinephrine can increase the rate of aqueous humor flow in sleeping humans. This experiment was conducted to determine if this stimulatory effect can be blocked by the beta-adrenergic antagonist timolol. METHODS: Twenty normal human subjects were studied for one sleep cycle at night. Epinephrine was infused intravenously at a rate of 1 microgram/min. One eye received a single drop of 0.25% timolol, and the fellow eye received a placebo. Aqueous humor flow was measured by the rate of disappearance of fluorescein from the eye. RESULTS: The rate of aqueous humor flow was 14% lower in the timolol-treated eye than in the placebo-treated eye during 6 hours of sleep. This difference was interpreted as being due to blockage of part of epinephrine's effect on aqueous humor flow by topical timolol. CONCLUSION: It was concluded that the effect of systemically administered epinephrine on aqueous humor flow is at least partly mediated locally on the eye and that some portion of timolol's well-known effect on daytime aqueous humor flow could be due to inhibition of the ocular effects of epinephrine.

Aqueous humor dynamics in low-tension glaucoma.

We measured the diurnal aqueous humor flow in patients with low-tension glaucoma by clearance of topically applied fluorescein. Ten patients with low-tension glaucoma participated in the study, and ten age-matched healthy volunteers served as control subjects. The diurnal aqueous humor flow was 2.58 +/- 0.78 microliters/min (mean +/- standard deviation) in the control group and 2.48 +/- 0.61 microliters/min in the low-tension glaucoma group. The corresponding nocturnal data were 0.98 +/- 0.22 microliters/min and 1.24 +/- 0.45 microliters/min. The differences were not statistically significant. Tonographic facility of outflow and intraocular pressure were also measured. There were no statistically significant differences between the two groups in either facility or pressure. No evidence was found in this small group of patients to support the hypothesis that the normal tension in this disease results from a combination of low facility and low flow.

A feasibility study of 23Na magnetic resonance imaging of human and rabbit vitreal disorders.

PURPOSE: To assess the clinical feasibility of sodium magnetic resonance imaging for the visualization and characterization of intraocular tissues. METHODS: 23Na magnetic resonance images were obtained of enucleated human eyes and of rabbit eyes in vivo. The magnetic resonance imaging technique used in this study provided slices of < 2 mm thickness and in-plane resolution of < 2 x 2 mm. From each of these slices local values of spin-spin relaxation time (T2*) were calculated from pixel intensities in each of the eight echoes. RESULTS: The images clearly display normal anatomic details of the lens and vitreous humor, and important pathologic details such as intravitreal and subretinal hemorrhages, ocular melanoma, and retinal detachments. Intraocular tissue identifications based on relative spin-spin relaxation time values and pixel intensities correlate with those made by standard diagnostic techniques. CONCLUSIONS: 23Na magnetic resonance imaging may be used for the visualization and characterization of intraocular tissues. Differentiation among vitreous humor, lens, aqueous humor, subretinal fluid, or hemorrhage and tumor may be based on image intensity and/or spin-spin relaxation times.