Nocturnal risks for the eyes.

For people with certain eye disorders who are also hypertensive, taking high blood pressure medication shortly before bedtime may cause a sudden loss of eyesight or accelerate a more gradual process of vision loss, reports ophthalmologist Sohan Singh Hayreh of the University of Iowa in Iowa City 

For three years, Hayreh has kept track of blood pressure in patients with "stroke" of the optic nerve or glaucoma, including "normal-pressure" glaucoma. (People with normal-pressure glaucoma have normal fluid pressure in the eye.) All types of glaucoma can gradually reduce eyesight; stroke of the optic nerve can cause an abrupt loss of vision.

Scientists know that healthy people experience a drop in blood pressure during sleep, generally without ill effect. However, Hayreh discovered that on average, people with these eye disorders experienced much steeper declines in nocturnal blood pressure.

Indeed, nighttime blood pressure measurements proved lower in study participants who suffered progressive visual deterioration than in those who did not experience progressive vision loss. Patients taking hypertension medication in the evening appear to be at particular risk of declining vision, Hayreh says.

A drastic drop in nocturnal blood pressure may be the final insult that pushes such individuals toward the brink of blindness, Hayreh suggests. He speculates that very low blood pressure may deprive the optic nerve of nourishing blood. Without adequate blood flow, parts of the optic nerve die, causing fading vision and eventual blindness. In both glaucoma and stroke of the optic nerve, scientists believe that loss of vision is due to optic nerve damage, Hayreh adds.

Hayreh says many cardiologists tell their patients to take hypertension drugs shortly before they go to sleep, a practice that prevents side effects during the day But for people with glaucoma or a history of an optic nerve stroke, this may be unwise, he says. If such people already show an abnormal drop in blood pressure during sleep, taking drugs to lower the pressure even more might further reduce blood flow to the optic nerve.

Hayreh suggests that people with eye disorders consider asking their doctors to recheck any diagnosis of hypertension. Some people are erroneously labeled as hypertensive simply because their blood pressure rises out of nervousness when they visit the doctor's office, he notes.