Blood Pressure and Glaucoma
Chat Highlights
July 28, 2004

Norma Devine, Editor

On Wednesday, July 28, 2004, Dr. Elliot Werner, a glaucoma specialist at Wills, and the glaucoma chat group discussed "Blood Pressure and Glaucoma."

Moderator:  Welcome back, Dr. Werner.   The topic tonight is glaucoma and blood pressure.  How is blood pressure defined? 

Dr. Elliot Werner:  Blood pressure (BP) is the fluid pressure inside the arteries that is generated by the energy of the pumping of the heart.

P:  How much does BP normally vary?  

Dr. Elliot Werner:  That depends on age.  Older people always have more labile BP.  During exercise or stress, BP can normally increase 20 or 25%, depending upon physical fitness.  During sleep, the BP of many people drops by as much as a third.  

P:  Data about BP and IOP (intraocular pressure) could be easily gathered by measuring glaucoma patients' blood pressure as well as intraocular pressure (IOP) at each office visit.  Why isn't that done?  

Dr. Elliot Werner:  Good question.  It should be done.  It is not done probably because eye docs don't think of it, don't always have the equipment at hand, and find it inconvenient and inefficient.  It slows them down.

P:  Couldn't technicians in eye doctors' offices  be trained to take BP?  

Dr. Elliot Werner:  Yes, they could be and should be. 

P:  Should high blood pressure be a concern for glaucoma patients? 

Dr. Elliot Werner:  The research on the link between blood pressure and glaucoma is confusing and sometimes contradictory.  There is a relationship between blood pressure and IOP in that IOP is higher, on average, in people with higher BP.  On the other hand, the perfusion pressure at the optic nerve (the amount of blood and oxygen delivered) is also higher in people with high BP.  The effects and relationships are not well understood. [Editor's note:  see "Understanding the Role of Blood Flow in Glaucoma," http://www.willsglaucoma.org/blood.htm.]

P:  Can having low blood pressure be a factor for glaucoma patients?

Dr. Elliot Werner:  That relationship is much more well established.  Chronic low BP does seem to be a risk factor for developing progressive glaucoma.

Moderator:  How is perfusion pressure, the amount of blood and oxygen delivered to the optic nerve, measured?  

Dr. Elliot Werner:  It can be measured directly by some complex experimental techniques.  It can be estimated by subtracting the IOP from the BP.

Moderator:  Is that a new concept? 

Dr. Elliot Werner:  Not really.  The concept has been known for a long time in the eye, as well as other organs.  More recently, (perhaps for ten or so years) the relationship has been studied more extensively in glaucoma.  

Moderator:  If my BP were 90/50 mm Hg (which it has been) and my IO were 21 mm Hg, what would my perfusion pressure be?

Dr. Elliot Werner:  There is a formula for calculating that.  I don't remember the details, but you subtract the diastolic pressure from the systolic pressure, take two thirds of that, add the result to the diastolic number to get the average BP, take 80% of that average, and subtract the IOP from that number.  I think that's right.

Moderator:  What is "white-coat syndrome?"

Dr. Elliot Werner:  "White coat" refers to the coat many doctors wear when they work.  White-coat syndrome is the tendency for some patients to have higher BP in the doctor's office because of the anxiety provoked by the visit to the doctor.  

P:  Can white-coat hypertension have an effect on IOP readings?

Dr. Elliot Werner:  I don't know.  To my knowledge, no one has ever studied the relationship, but acute elevations of BP can be associated with higher levels of IOP.   Home monitoring of IOP, unlike home monitoring of BP, has not shown a white-coat phenomenon.  

P:  How does smoking marijuana affect blood pressure? 

Dr. Elliot Werner:  Marijuana lowers IOP.  That has been well known for many years.  I am not aware of the effects of marijuana on BP, but I'm sure it has been studied and is known.  I would have to look it up. 

Moderator:  I thought smoking marijuana lowered BP, which decreased the blood flow.

Dr. Elliot Werner:  It is possible that marijuana lowers BP.  If so, it would probably lower the perfusion pressure of the optic nerve.  I don't know for sure.  A more important consideration is the effect of BP medication on glaucoma.  Lowering the BP with medication, especially if it is lowered too much, too fast, can significantly affect the perfusion pressure of the optic nerve.  There are definitely cases when glaucoma became worse after treatment for high BP started.

P:  Does fluctuation of blood pressure during the day and night play a role in damage to the optic nerve?  

Dr. Elliot Werner:  Most people have lower BP when they sleep.   Some people, called nocturnal dippers, have profound decreases in their BP when they sleep.  A variety of conditions have been potentially associated with decreased blood pressure during sleep,  including strokes, heart attack and glaucoma, especially normal-tension glaucoma (NTG).

Moderator:  Is there any way to raise BP at night to help NTG patients? 

Dr. Elliot Werner:  It's very difficult to raise the BP when it is physiologically low.  If the patient is on BP meds, the meds can be given in the morning.  In such patients, taking beta blockers at night should be avoided.  Some people prescribe high salt diets, but there is no evidence they work.  Drugs that raise BP are too dangerous for long-term use.

P:  What is normal-tension glaucoma?

Dr. Elliot Werner:  NTG is glaucoma in the absence of elevated eye pressure.

P:  If the perfusion BP is inadequate during sleep, how much damage could that do if a person normally sleeps six to seven hours? 

Dr. Elliot Werner:  Six hours versus eight hours of sleep probably doesn't make much difference, but nobody knows for sure because it hasn't been measured.

P:  Are you suggesting that beta blockers not be used by persons with normal-tension glaucoma, regardless of blood pressure, in case the beta blocker lowers blood pressure, which is detrimental to the eye?

Dr. Elliot Werner:  In my opinion, NTG patients with low BP, or who are nocturnal dippers, should avoid beta blockers, if possible.  If such patients absolutely need to use beta blockers, once a day in the morning is probably best.

P:  I heard that taking a beta blocker first thing in the morning is a good idea. The reason is that the eye generates less aqueous fluid while sleeping, and that IOP will be lower during the daytime. 

Dr. Elliot Werner:  That is true, as well as the effect on BP during sleep if the beta blocker is taken at night.

P:  Does pulse rate play any role? 

Dr. Elliot Werner:  Probably not, but I don't know if it has been studied.  BP is more important than pulse rate.

P:  IOP rises when the head is lowered relative to the rest of the body.  What is the dynamic here, and does BP play a role?  A person might consider sleeping with a slightly elevated pillow to avoid a rise in IOP, but if that could negatively affect perfusion pressure, the net effect might be deleterious rather than protective.  What do you think? 

Dr. Elliot Werner:  IOP increases when lying down or when the head is below the heart, because the pressure inside the veins of the head increases due to gravity.  IOP is directly related to the pressure in the veins of the head.  People with advance glaucoma should avoid prolonged head-down position.  Sleeping with the head elevated is a good idea.

P:  The production of aqueous humor decreases in bed during the night and early morning.  But the IOP is commonly highest at 6:00 or 7:00 a.m.  That seems paradoxical.  

Dr. Elliot Werner:  That probably has to do with the effects of increased cortisone production on aqueous outflow shortly before awakening.

Moderator:  Are glaucoma patients more prone to heart disease or strokes?

Dr. Elliot Werner:  That is another controversial question, and the literature contains contradictory evidence.  Most recent population studies have not shown that to be true, but there are some studies showing that life expectancy is somewhat lower in glaucoma patients.  The reasons for that are unclear. 

Moderator:  Are any studies being done on the effect of BP on eye pressure and optic nerve damage?

Dr. Elliot Werner:  There are many studies, but the results are not conclusive.  A relationship between low BP and glaucoma seems to exist.  High BP does not seem to have as strong a relationship.

P:  Do you think that the positive correlation between IOP and BP is causal, or that both might vary in relation to something else? For example, if you lower BP by regular exercise, your IOP might also be lowered, not as a result of the lowered BP, but by a directly beneficial effect on IOP.   In other words, many variables that affect one might affect the other, and the correlation between the two might be something besides one causing the other.

Dr. Elliot Werner:  The effect of BP on IOP is probably a mechanical one.  There is a direct connection between the blood vessels and Schlemm's canal, so that the flow and pressure of the aqueous in the eye is connected to the pressure in the blood vessels. 

P:  Can a beta blocker such as Timoptic cause a baby's blood pressure to be lowered?  She is nearly six-months old and has congenital open- angle glaucoma.  Should she be checked by her pediatrician for side effects?  I notice she sleeps more often. 

Dr. Elliot Werner:  Yes, Timoptic can lower BP in an infant and can cause drowsiness.  Certainly the pediatrician should check her BP to see if the Timoptic is having any adverse effect.

P:  Can a medication that causes fatigue in some patients lower blood pressure?  Or at least the pulse rate? 

Dr. Elliot Werner:  That depends upon the medication.  Some meds can cause fatigue, but have no effect on blood pressure or heart rate.   Any med that depresses the cardiac output can have an adverse effect on glaucoma.

P:  My doctor has never asked me about my blood pressure, and most patients probably wouldn't think of volunteering that information, unless they are on BP meds.

Dr. Elliot Werner:  A general medical history is part of a complete eye exam.  The doctor should obtain a history of any treatment or evaluation of BP that has been done.

P:  If the IOP is directly related to the pressure in the veins in the head, would an increase of circulation in the head from using herbs such as ginseng cause the IOP to rise?

Dr. Elliot Werner:  I don't know.  I don't think that has ever been measured.  One of the frustrating things about human biology is that it is very hard to guess or speculate about things.  You have to perform experiments and take measurements. 

P:  Shouldn't our general practitioners communicate with our eye doctors?  

Dr. Elliot Werner:  Yes.  Any time more than one doctor of any specialty is involved in caring for a patient, the doctors should (indeed must) communicate with each other.

P:  What about the variation in IOP during the day? Why does the doctor ask when we took our eye drops?  

Dr. Elliot Werner:  The IOP varies during the course of the day. Some glaucoma patients have a very high variation of IOP over 24 hours, which can cause progression of glaucoma.  Eye docs usually ask what time you took your drops just to be sure you are taking them.

[Editor's note:  See "Understanding the Role of Blood Flow in Glaucoma." http://www.willsglaucoma.org/blood.htm]

End of highlights for July 28, 2004.

On August 4, Dr. Wilson discussed "Target Pressure" in the Chat room. Click here for highlights of that meeting.

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