General high blood pressure in the body is of concern to many people since it’s one of the main causes of heart failure and strokes – the medical fraternity prefers to use the term “hypertension” when discussing what high blood pressure is and how to treat it.

Blood pressure is measured and recorded using a device called a sphygmomanometer, together with a stethoscope. A pump is used to tighten a band round the arm, and assessment of the blood pressure actually involves taking two separate measurements or readings:

•    Systolic pressure – the pressure at which the heart actually pumps out the blood

•    Diastolic pressure – the pressure while the heart is resting between pumping actions

So the heart and walls of all the arteries are subject to sustained pressure varying between the two values.

Since the sphygmomanometer uses a column of mercury graduated in mm, the readings are in mmHg, and although electronic devices don’t use such a system, they are calibrated to give readings in these same units.

So, when you go to the doctor you will be given two readings, the first being the systolic and the second the diastolic – typically in the form of “120 over 80”.

The numbers 120 over 80 by the way are considered as “normal” (if of course there is such a thing as normal).

Now the problem is that you can have the numbers contradicting each other with systolic at 140 and diastolic at 70 – so how is that to be interpreted? Consequently the whole question of what constitutes high blood pressure is at times open to debate.

What about the eyes?

The eye has its own pressure, called intraocular pressure (IOP), to maintain its shape and allow all the functions within it to work properly.

The way IOP is measured is by using one of a variety of devices called tonometers. The main non-contact method used by many optometrists during eye examinations blows a puff of air at the eye surface and measures how much the eye is indented with an electronic sensor.

Other methods use a very light contact and have the advantage of being mobile and easy to use.  All methods require calibration to give pressure in mm of mercury (mmHg).

Usually eye pressure lies between 10 and 20 mm Hg – if it’s any higher than 20 and repeated in subsequent testing then a condition of ocular hypertension may be diagnosed. However, as with hypertension it’s possible that the “normal” value of 20 adopted for ocular hypertension could be open to interpretation.

What are the problems associated with ocular hypertension?

It may be that the eye is not draining away excess internal fluid from the eye, so an examination by an optometrist will be required assess this.

Also it may be that the increased IOP is a very early sign of glaucoma – again this requires assessment to make sure the structure and function of the optic nerve is not being compromised.

Will a diagnosis of hypertension give rise to the development and diagnosis of ocular hypertension?

An interesting question that is not readily answered.

One of the problems associated with endeavouring to assess any kind of relationship lies in the definitions:

•    Is a blood pressure above 120 over 80 mm Hg technically “hypertension”

•    Is an eye pressure above 20 mm Hg technically “ocular hypertension?”

Results of research – difficult to compare for reasons cited above – seem to indicate that increased systolic pressure has more effect than increased diastolic in an increase in eye pressure, but that the effects are not considered significant.

A form of glaucoma called low tension glaucoma is when the intraocular pressure is never measured over 20mmHg yet progressive damage to the optic nerve still occurs.  Interestingly this form of glaucoma is associated with low blood pressure or hypotension.

Very high persistent high blood pressure can cause a condition called hypertensive retinopathy.  In this day and age because most people that have high blood pressure (or hypertension) are seen regularly by their general practitioners hypertensive retinopathy is seen less and less.

Another common cause of high blood pressure is a condition called sub-conjunctival haemmorhage. This condition causes significant distress as it is so obvious. It does not hurt or cause and vision defect but the eye looks horrible.

It is therefore obvious that both high blood pressure and low blood pressure can affect eye health.  After all, the eye is part of the body, which is supplied it nutrients and oxygen by blood flow.  Get the body’s plumbing right and things are looking good.