Study explores mechanism of increased nocturnal IOP

September 1, 2007

A study of the mechanisms involved in nocturnal changes in IOP indicates that a decrease in outflow facility is insufficient to compensate for a decrease in aqueous outflow. Episcleral venous pressure could be a factor in the circadian variations, however.

Key Points

Fort Lauderdale, FL-Recent research has indicated that IOP follows a circadian rhythm that includes a nocturnal increase, which occurs despite a nighttime decrease in aqueous humor flow. A new study offers some clues to understanding the complete mechanism of nocturnal IOP increases.

Researchers found that tonographically measured outflow facility does not decrease enough at night to compensate for the decrease in aqueous outflow but that episcleral venous pressure could be the cause of that change. It is also possible that multiple mechanisms are involved, said Arthur J. Sit, MD, who presented results of his research at the annual meeting of the Association for Research in Vision and Ophthalmology. He is assistant professor of ophthalmology, Mayo Clinic College of Medicine, Rochester, MN.

To investigate whether changes in outflow facility during the night could account for higher IOP, Dr. Sit conducted a study of the circadian variation of aqueous dynamics in adults. He recruited men and women, aged 18 to 45 years, from among the employees and students at the Mayo Clinic and the local community in Rochester.

The mid-diurnal protocol called for fluorophotometry and IOP measurement-sitting and supine-after 5 minutes, at 2 p.m., and again at 4 p.m. Additional tests at 4 p.m. included tonography followed by fluorescein drops. The subjects were awakened at 2 a.m. for the mid-nocturnal testing protocol, which included fluorophotometry and IOP measured sitting and supine. The same tests plus tonography were administered at 4 a.m.

All 68 eyes of 34 subjects were used in the analysis; 40 eyes were of patients aged 18 to 29 years (mean age, 23), and 28 eyes were of patients aged 30 to 45 years (mean age, 37). The clinical or baseline IOP was 13.2 mm Hg for all ages-12.8 mm Hg for the younger subgroup, and 13.6 mm Hg for the older subgroup.

Results presented

Results presented at ARVO meeting showed that sitting IOP decreased slightly from day to night. For all ages, mean daytime IOP was 13.8 mm Hg; for nighttime it was 12.9 mm Hg (p = 0.02). For patients aged 18 to 29, IOP dropped from 13.8 mm Hg to 13.1 mm Hg (p = 0.18), and for patients aged 30 to 45, pressure decreased from 13.9 mm Hg to 12.7 mm Hg (p = 0.05).

Supine IOP also decreased from the diurnal to the nocturnal interval. For all ages, the mean decline was from 18.9 mm Hg to 17.8 mm Hg (p = 0.001). For the younger subgroup, pressure dropped from 18.7 mm Hg to 17.6 mm Hg (p = 0.03); for older subjects, it declined from 19.2 mm Hg to 18.0 mm Hg (p = 0.006). When the sitting and supine IOPs were considered together, a marked increase in nocturnal IOP was evident when measured in the physiologic positions of sitting while awake and supine while asleep.

A highly statistically significant decrease in aqueous flow rate was observed, which declined by about 50% from day to night, Dr. Sit said. For all ages, it decreased from 2.26 to 1.12 μl/min (p < 0.001). For the 18-to-29-year subgroup, it declined from 2.21 to 1.08 μl/min, and for those subjects aged 30 to 45, it declined from 2.33 to 1.18 μl/min (both, p < 0.001).

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