This is an important question and one that I am frequently asked here at MIAMI EYE CENTER.  One must first have a fundamental understanding of the nature of glaucome so as to appreciate the answer to the question.

Many of my patients are merely interested in “how is my pressure today?”  While that information is significant, that number represents only part of the story.  It is true that one is at higher risk of glaucoma developing and worsening with a pressure greater than 21mmHg, there are many people with pressure greater than 21mmHg  who do not develop glaucoma, that is, their optic nerve does not show signs of damage.  At the same time there are folks with normal intraocular pressure (21mmHg to 12mmHg) who have damaged optic nerves. 

So what is glaucoma?  While clinical investigators wrestle with the answer to this question, it seems to be that it is a condition wherein the optic nerve degenerates in a specific pattern with a concommitent loss of peripheral vision.  If unchecked the vision loss closes in on central acuity.  In the classic sense this degeneration is associated with eye pressure which is too high for that individual.

It is the job of the ophthalmologist to lower the eye pressure by at least 20% from the pressure at the time of diagnosis.  Lowering the pressure has been correlated with slowing or stopping the progress of the degeneration.  Someone with a pressure of 25mmHg should have their pressure reduced to 20mmHg, while another with a pressure of 15mmHg at the time of diagnosis should have their press at 12mmHg.  The pressure being lowered with eyedrops, laser therapy or surgery.

Once the pressure has been reduced by 20%, the physician must monitor the condition of the optic nerve and confirm that there is no further degeneration.  This is because a reduction of 20% may not be low enough to stop the progression in any one individual (20% is only a statistical generalization).

So the question posed by the glaucoma patient should be: “how is my optic nerve doing?”  The answer should be that it is not getting worse.

When your headlights flash an animal in the road you see the bright orange reflex of light from the retina (pictured to the left).  While that was man’s first image of the interior of the eye the view is now down to the microscopic level.  The device which makes that possible is Optical Coherence Tomography or OCT. We can visualize early and subtle changes in the retina and optic nerve in many cases before they are noticed by the individual.

How is this done?  The patient is seated before the OCT machine and waves of safe invisible ultraviolet light are beamed into the eye.  The light is reflected back just as it is with the headlight.  The reflected light is captured by a sensing devise (interferometer) and passed along to a computer which interprets the variations in the reflection as various anatomic parts of the retina and optic nerve.

Anatomic Details of the Retina The complex structure of the retina, pictured here, passes the visual information along its nerves to the Optic Nerve which in turn feeds the image to the brain.  All this processing takes about 2/10ths of a second.  Needless to say, small anatomic problems in the retina can cause big problems with the vision.  So it becomes important to view the anatomic changes asssociated with disease early in their evolution.  This can also be said about diseases of the Optic Nerve such as glaucoma.OCT Image of the Retina

To the practiced ophthalmologist, the image to the right represents all the layers of the retina in sideways view.  One can see early signs of macula degeneration, swelling due to diabetes or other circulatory problems as well as impending holes.

Among ophthalmologists, it is said that you have had to have practiced for many years to understand the nature of the disease.  This is because in most cases it is a long, slow process.  However, within that framework, it is the ophthalmologist’s job to identify those individuals who have a more aggressive form of the disease and are at greater risk of vision loss.

The eyeball must have an internal pressure in order to function properly just as a basketball needs the proper pressure to bounce correctly.  The normal range of eye pressure is 12 to 21mmHg.  If you press on a healthy eye it will indent only slightly.  Please don’t do this at home.  When the pressure raises above 21mmHg, there is an increased risk of damage to the optic nerve; glaucoma.  There are factors which muddy the water.  Some people have thin corneas and some have thicker corneas, this effects the ability to accurately measure pressure and it effects the eye’s susceptablity to glaucoma.  Using a pachymeter, the cornea thickness can be measured and the normal pressure is adjusted for the individual.  Another confounding issue is that some people with elevated pressure don’t get glaucoma (ocular hypertensives) and some with normal or low pressure do get glaucoma (normal tension glaucoma).  Based on this observation, it is clear that there is more to glaucoma than just eyeball pressure.  But medical science at this moment in time only knows that if you lower eye pressure, patients tend to retain their vision.  Some time in the future we will be directing the treatment at some neural-based regulatory mechanism controlling the circulation in the optic nerve.

Once the diagnosis is made, the question becomes, “how low does this patient’s pressure need to be so as to stop the progression of the disease.”  Not surprisingly, it is different for each individual so the approach is to attempt to lower the pressure by 20% as the initial strategy.  Once the target pressure has been achieved the vision is monitored for further loss of peripheral vision (visual field testing) and the appearance of the optic nerve is checked (optic nerve tomography) for degeneration.  If patients continue to lose perpheral vision and/or optic nerve damage is progressive, the pressure is lowered further.

Pressure control is achieved with one of a number of eye drops which work by reducing the production of fluid within the eye or enhancing the outflow or both.  If one or a combination of drops is not effective then laser surgery might be the next option or possibly traditional glaucoma surgery in the operating room.

In my experience, the majority of patients with open angle glaucoma do quite well with a lifetime of eye drops.  In those people where pressure control is not attainable with conservative means (drops or laser) one must not waste time moving forward with surgical intervention.  This is particularly important in younger individuals with advanced disease.

Glaucoma is relatively easy for the layperson to understand because medical science really dose not understand the disease either and has thereby simplified matters.  Physicians view the disease in terms of eyes with too much pressure but there is a far more complex nature to the disease.  So the condition can be looked at as an eyeball with too much internal pressure to allow for the health of the optic nerve..  As a result, the nerve becomes damaged and vision is lost.

The eyeball is a closed system with fluid (aqueous humor) continually circulating in and out via a highly regulated structure (trabecular meshwork).  If the production of fluid remains constant and there is a problem with the outflow, the pressure must rise.  The optic nerve connects the eyeball and the brain and inserts itself into the back of the eye.  This nerve is very sensitive to pressure changes and if the pressure remains too high, the nerve becomes damaged.  When damage occurs to the nerve, vision is lost in a characteristic pattern; the periphery of the vision is lost long before the center of vision is effected.  If untreated, the central vision is ultimately lost and blindness ensues.

The most common type of glaucoma is open angle and a less common form is narrow angle or closed angle.  The “angle” refers to the region on the inside of the eye where the clear cornea meets the iris (colored part of the eye).  It is in this angle that the fluid passes out of the eye.  In narrow angle glaucoma the iris bulges too far forward and touches the cornea thus obstructing the flow to the angle.  When this occurs, the pressure in the eye raises very quickly causing severe pain and loss of vision.  It becomes a medical emergency.  This relatively rare situation most often occurs without warning in people who are anatomically predisposed, i.e., they have small eyeballs with crowding of the angle.  Sometimes cataract formation in the presence of a small eyeball can precipitate the angle closure.

Open angle  glaucoma is far more common and occurs as a result of a defect in the trabecular meshwork which is the structure in the angle through which the fluid must pass on its way out of the eye.  So here the angle is open but the filtration mechanism does not function properly.  The net result is that the pressure raises above normal but usually in a much more gradual rate.  Since the pressure raise is gradual there is no pain but damage to the optic nerve nevertheless develops.  Since the resulting visual loss occurs in the periphery, most people don’t realize that there is a problem.  I saw an elderly gentleman in the office who had undiagnosed advanced glaucoma.  He had attempted to drive his car between two side-by-side trucks on a four lane highway since he only saw one of them.  This is an example of how bad the disease can become.

If left untreated the vision loss extends to the center of vision.  This is one of the reasons for a routine ophthalmic examination on a periodic basis.

Treatment options will be discussed in a future blog.