Some patients with glaucoma appear to have higher pressure in their eyes during sleep at night than during the day when it is usually measured, possibly putting them at higher risk for progression of the disease than previously thought, according to a study in the February issue of Archives of Ophthalmology.

Higher intraocular pressure, the force within the eyeball, and greater daily fluctuation in pressure may increase the risk that glaucoma will develop or worsen. Intraocular pressure is usually measured during regular physician’s visits, when patients are seated, according to background information in the article. However, previous studies have found that the intraocular pressure may be higher when a person is lying down, the authors report. This is probably because the eye is level with the heart when lying down, which increases the resistance in flow of fluid in the eye and may create additional pressure, they write.

Takeshi Hara, M.D., Jichi Medical School, Tochigi, Japan, and colleagues measured the intraocular pressure of 148 patients with untreated glaucoma at the Hara Eye Hospital in Utsunomiya, Japan. They took measurements 12 times over the course of 24 hours, including every three hours during the night. Each time, they measured the pressure when the patient was sitting as well as when the patient was lying down, so that each patient had three levels: the sitting pressure, the lying pressure and the reproduced pressure, which was calculated by combining the sitting values when the patient was awake and lying values when the patient was asleep.

The researchers then calculated the peak, average and fluctuation of each of these levels. The average peak pressure for seated patients was 16 mm Hg; for those lying down, it was 18.9 mm Hg; and the peak reproduced pressure was 17.5 mm Hg. Twenty-nine (20 percent) patients had a reproduced pressure of 21 mm Hg or greater when sleeping, compared to five patients (3 percent) when only sitting results were considered.

“More than 10 percent of the patients whose intraocular pressure was less than 21 mm Hg during clinic hours when sitting only had a reproduced intraocular pressure that was 21 mm Hg or greater,” the authors write, suggesting that measuring pressure only during the day does not paint an accurate picture of patients’ risks. “We are convinced that diurnal intraocular pressure changes should be reproduced with consideration for the posture of the patients,” they conclude.

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