Eye Pressure and Glaucoma

Dr. Kisling Uncategorized Leave a Comment

Eye Pressure is one of the tests we use to determine if you have glaucoma. Fluid is always being produced in the eye and always draining out into the circulatory system. When the drainage system starts to fail, the fluid pressure goes up. Rarely there may also be over production of fluid, but normally the plumbing system of the eye starts to clog up or collapse the meshwork channels composing the drainage structure.

To test the eye pressure we use an instrument called a tonometer. Tonometers have a long history, having been first invented in 1861. It was not until the introduction of the Shiotz tonometer in 1905 that eye pressures started to be recognized as a cause of glaucoma. Until recent years, eye pressure was thought to be the direct cause of glaucoma. Today we view it as one of a number of risk factors. The Shiotz tonometer resembles a medieval torture device, with a small plunger placed on your eye while you recline and weights are added. It indents the cornea from the resistance of the pressure inside the eye. The Goldmann tonometer was invented by Hans Goldmann and replaced the Shiotz tonometer in the 1950’s. It is an applanation tonometer meaning it flattens a specific size of the corneal tissue on the front of the eye and measures the resistance needed to do this. Its requires mild numbing drops and is done sitting up while looking at a small blue light. The Goldmann tonometer has been considered the Gold Standard since its introduction, although some question the truth of this statement. The Goldmann tonometer is quick and painless but on rare occasions can leave a small scratch on the eyes surface and cause discomfort for 24 hours. There is also the possibility of cross contamination if the tip is not disinfected,  There was a large screening done a number of years ago by several ophthalmologists that created a large outbreak of conjunctivitis due to improper disinfection of the tonometer.

The next advancement in tonometry came from an optometrist, Bernard Grolman, O.D.,who worked for the Research and Development Group at the American Optical Corporation. At the time, optometry was a drug free profession and had no good methods to measure eye pressures. Dr Grolman started work on a non contact tonometer in the late 1950’s and worked 10 years to introduce the first model. The non contact aspect meant nothing ever touched the eye like the Shiotz Goldmann Tonometers did. Dr Grolman first started with the idea that every fluid had a resonant frequency that could be utilized, but changed his model as he developed the non contact tonometer (or NCT as it is commonly referred to). The NCT works by blowing a small puff of air at the front of the eye. When the air flattens the corneal tissue it acts like a mirror and reflects a light beam back to the instrument. The time for this to happen is related to how high the eye pressure is and a digital read out gives the pressure reading. The introduction of non contact tonometry allowed mass screenings for glaucoma for the first time in history and was a truly revolutionary technology. It has saved countless people from blindness and is familiar sight at health fairs. Since there is no contact with the eye there is no chance for spreading an infection from prior patients. It does not require an eye doctor to use the NCT, a technician can measure your pressures. There does seem to be more variability in readings as catching partial blinks probably
artificially raises the findings. The same thing can occur with Goldmann tonometers but is more easily observed. Newer models of non contact tonometers have been reported to correlate well with Goldmann tonometers.

The advent of LASIK surgery brought a new awareness of the shortcomings of all methods of measuring eye pressure. In the process of removing tissue from the cornea, it was noticed that eye pressures were lower. Due to some occasional serious complications after LASIK measuring the thickness of the cornea prior to surgery became a standard technique.  After further investigation, the two were connected and  it was concluded that the eye pressures were not actually lower but the thinner tissue offered less resistance and changed the readings. While this problem was known for years the extent of the effect had not been fully appreciated. Nomograms, or charts to help correct pressure for different tissue thickness amounts were developed. Currently many optometrists believe that it is not the thickness but the floppiness or how the tissue rebounds after pressure that alters the readings.

Tonometers are just starting to enter a new generation to compensate for these difference in the structural character of the cornea. The Pascal Dynamic Contour Tonometer utilizes technology to dynamically match the sensor tip to the curvature of the cornea and eliminate some of the effects do to thickness and structural variations in the cornea. It also takes numerous readings in a very brief period of time and averages them. There is a second  measurement reported to be related to the blood flow to the retina. The ocular pulse amplitude (OPA) compares the pulse of blood flow to the retina in the back of the eye with the heart rate. Alterations are supposed to correlate with poor blood flow which can result in glaucoma even when the pressure readings are normal.

Eye Drops To Treat Glaucoma Fort Collins

Eye Drops To Treat Glaucoma Fort Collins

The Reichert Ocular Response Analyzer take a different approach, and like the non contact tonometer utilizes a puff of air. It takes a second reading and measures how long it takes the cornea to rebound and recover its shape. This gives a gauge of how floppy the structure is. Corneas that are structurally more floppy (a property referred to as hysteresis) are more likely to develop progressive glaucoma at normal pressure levels. All of this technology is at the early stages and will probably take some time before we truly know how useful it will be.

All tonometers report the pressure in mm of Mercury, related to how much pressure a column of mercury 1mm high exerts at its base. Mercury became the standard for pressure measurements due to a protégé of Galileo in 1643. He developed the first barometer. At the time, the existence of a vacuum was in question. Water rises to a maximum height of about 34 feet with a vacuum, Mercury is much more dense and only rises to a maximum of 32 inches, making it easier to work with. Even though mercury is not utilized in measuring your eye pressure the equivalent pressure is reported as mm of mercury or mmHg, a reminder of the 17th century science origins of pressure readings. Eye pressures overall average about 15.5 mmHg but the gradually rise with age. Normal is considered between 5 to 6 at the low end and 20 at the high end. Some pressure is required to give the eyeball structure like an inflated ball. Normally very low pressures are due to trauma or fluid leaks as complications of eye surgery.

As explained above, pressure is not a perfect measurement for defining glaucoma. You can have high pressures and not have glaucoma. There is a small segment of glaucoma patients that have normal pressures but still have glaucoma. Pressures readings may spike for a short period at one point of the day only, so your eye doctor may want to take tonometry readings at different times of the day. Unfortunately, we can’t do this easily in the middle of the night or on a 24 hour basis like a cardiac monitor measures your heart. Today eye pressure is viewed as a risk factor for glaucoma instead of a direct cause. Glaucoma is beginning to be viewed as a disease of circulation and inflammatory factors, with eye pressure having a fairly high correlation.

The good news is that glaucoma can be treated, especially if caught early. We still have a long way to go to perfect eye pressure readings but we are improving by leaps and bounds. Eye pressure checks on a regular basis along with a thorough eye exam and history of factors that raise your risk can prevent most blindness due to glaucoma. Don’t delay your preventative care that can make such a difference for your future health. Most vision insurance plans like Vision Service Plan and Medicare cover glaucoma testing with your eye exams.

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