A lot of times there are new and

A lot of times there are new and exciting breakthroughs in medicine that take the medical profession and their patients by storm. Some of the blockbusters make little or no impact on the general public and some even take a long time for the professionals to catch up with the news. One of the newer modalities that have taken ophthalmology by surprise is the effect of central corneal thickness on the measurement of the intraocular pressure or IOP.

Ophthalmologists and optometrists have been laboring with measurements of the IOP and glaucoma for many years with a certain comfortable relation that higher than normal IOP, over 20 mm Hg was bad and less than 20 mm Hg was good. Of course, there were the patients who had low IOP readings and were still losing visual field. How could those patients be explained? Those patients with low IOP readings, changes in the optic nerve and loss of visual field were branded as having low tension glaucoma. Eye care professionals were treating patients as numbers of IOP, millimeters of mercury, (mm Hg) and still patients were losing vision with what was considered to be normal or low readings. About 4 to 5 years ago there were some ground breaking studies reported that caused ophthalmologists and other eye care professionals to revamp their thinking concerning glaucoma. There needed to be a new definition, new methods for diagnosis and finally to new treatment for the disease called glaucoma.

Gone are the days of worshipping the numbers of the IOP, at least for eye care professionals and their glaucoma patients. The last several years has seen a steady flow of studies aimed at glaucoma and funded by the National Eye Institute. A virtual alphabet soup of studies relating to glaucoma that affects the diagnosis, the treatment and even changing the definition of glaucoma. AGIS, CIGTS, EMGT, OHTS are the initials of the recent studies in the news for glaucoma and how glaucoma is diagnosed, treated and followed. As an example, the Advanced Glaucoma Intervention Study, (AGIS), found that patients who are maintained at Intraocular pressure measurements (IOP) of less than 18 mmHg at all times had a much smaller chance of having progression of visual field loss than those who were noted to have IOP measurements at follow up visits of greater than 18 mmHg. There was some data presented from studies that indicated an IOP of about 12 mmHg or less prevented visual field loss or reduced the chance of further vision loss for patients. The Early Manifest Glaucoma Trial, (EMGT), had data that indicated if the IOP was lowered by 1 mmHg there was a corresponding reduction of about 10% in the progression of visual field loss over time. The EMGT study concluded that for each 1 mm Hg lower in the IOP was from baseline the visual field loss was lessened. The Collaborative Initial Glaucoma Treatment Study (CIGTS) seemed to show that an aggressive management of the glaucoma patient by, surgery, medication or combination of both significantly lowered the risk for that patient for progression of the glaucoma and vision loss.

Glaucoma is now defined as a multifaceted disease of the optic nerve that leads to loss of vision if untreated. Risk factors are involved and include race, age, intraocular pressure, fluctuations of the IOP and corneal thickness. You note that the IOP level or reading is not a part of and is not stated as part of the definition for glaucoma and that is because that IOP is an individual risk factor for each patient. There are some patients that an IOP of 24 mm Hg represents satisfactory control and there are other patients that a level of 14 mm Hg or less that is still too high.

Well we are still talking about numbers but the numbers now have a meaning that appears to make more logical conclusions for the diagnosis and the treatment of glaucoma. But there are still many questions that are raised by the studies. Discussions are ongoing about the determination of the ideal IOP reading for glaucoma patients. Should every glaucoma patient have an IOP of 12 mm Hg or less as suggested by the EMGT study? What does it mean if the IOP is measured at 20 mm Hg on a single visit to the Doctors office?

Data analysis of the Ocular Hypertension Treatment Study, (OHTS), concluded that one of the most significant, if not the most significant risk factor for the development of glaucoma and the loss of visual field and vision from glaucoma was not the measured IOP but the Central Corneal Thickness, (CCT), and that the CCT was a real predictor of visual field loss for patients who already had optic nerve changes or neuropathy. The study divided the patients into two groups: those patients who had CCT readings of less than 550 µm and those patients who had CCT readings of over 550 µm. At 4 years follow-up from the study initiation those patients with CCT readings of less than 550 microns had a 38% probability of visual field loss compared to only 11% of those who had CCT of greater than 550 µm.

What has occurred is that ophthalmologists must now consider the CCT and the measured IOP when diagnosing and treating patients. The IOP in patients with thin corneas is underestimated, that is a Goldman tonometer reading of 20 mm Hg in a patient with corneal thickness of 500 µm will have a higher reading in actuality in the eye. A present there is no universal conversion factor but it appears that for every 10 µm thinner than the 550 normal a value of between 7 and 10 mm Hg must be added to the Goldman reading. Given a patient with Goldman IOP readings of 20 mm Hg and a CCT of 500 µm that patient actually has a corrected IOP of about 25 to 28 mm Hg, significantly higher than the initial reading, uncorrected. Conversely, a patient with a thicker than normal 550 µm is somewhat protected from vision loss because the Goldman IOP tonometer reading would be adjusted lower in similar fashion. That is, a patient who has an

IOP reading of 24 mm Hg and a CCT of 600 µm would actually have a true corrected IOP reading of about 19 to 17 mm Hg.

A pachymeter is used to measure the Central Corneal Thickness and most ophthalmologists now employ the use of these ultrasound probes to measure the corneal thickness in their glaucoma patients and glaucoma suspects, to have a more accurate corrected reading for the true IOP readings. There are many manufactures of pachymeters today. They are applied to the topically anesthetized eye much in the same way as a tonometer contacts the corneal surface. Readings are obtained rapidly and noted in the patient chart. There are some pachymeter instruments that are now incorporating a nomogram calculator to provide the physician with a corrected IOP for the patient based on the CCT readings from the instrument and the Goldman tonometer readings that are entered by keyboard or stylus.

So, IOP numbers are still important, physicians just have to ensure that the right numbers are being used, based on the CCT and Goldman. The IOP number is still important to follow patients to assure that the medication or surgery is working effectively over time. And the IOP readings are still important to follow up on the patient compliance with medication use. And as physicians we have to realize that every mm Hg is important to preserving the vision for our patients.

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Date and time this page was last updated02/02/2011 09:10:16 PM