Glaucoma is a serious eye disease, which may cause vision loss and ultimately result in blindness, if not treated in time. The common way to determine whether Glaucoma is round the corner in your case is to get your eyes tested for Intraocular Pressure (IOP).
There are many ways to determine IOP, but applanation tonometry is considered to be the gold standard. Once the IOP or tension exceeds 20 mgHg in the eyes of a patient, doctors immediately prescribe appropriate treatment to lower the tension.
The normal value of IOP is essentially based on the data collected meticulously by a large number of investigators. In most of the studies, the mean value of IOP has been found to be 15.5 mmHg with standard deviation of 2.5 mmHg. The distribution of IOP values is not normal in the statistical sense, and the eye pressure in healthy people has been found to vary between 10 and 21 mmHg. The values of IOP taken over a large number of people are skewed towards higher values. It means more people have pressures higher than 15.5 than those having lower than it. Doctors, trying to hunt for glaucoma get concerned if the IOP of a patient exceeds 21 mgHg.
From January 1985 to November 1988, an epidemiological eye study was conducted in East Baltimore, Maryland. More than 5000 persons aged 40 or more were subjected to comprehensive testing for glaucoma. They were screened by all the available diagnostic tools available at that time. The investigators found that in case of a glaucomatous eye, its IOP being more than 22 mmHg was 8.6 times higher than the likelihood of its having IOP less than 22 mg. That showed a strong positive correlation between glaucoma and the IOP being more than 22. Hence, the figure of 22 mgHg got stuck in the collective memory of the physicians as the cutoff point. To further simply, the figure of 22 was rounded off to 20. Things became simpler: if your eye shows IOP more than 20 mgHg you are having either glaucoma or at least a high risk for this disease.
That was the scenario till 2002 when a large American study called Ocular Hypertension Study (OHTS) was published. It found a statistically significant correlation between the central corneal thickness (CCT) and IOP. The study showed that CCT is a reliable predictor of development of glaucoma. If your cornea is thin, that is, less than 555 microns, you have three times greater risk of developing glaucoma than those fortunate people whose cornea is more than 588 microns. The implication was that thin corneas should be viewed with greater respect by the physician as they have a more nuisance value as far as glaucoma is concerned. Of course, it does not mean that the thin cornea is a causative factor in the development of glaucoma. The OHTS implied, inter alia, that the role of thin corneas is to blow the whistle in conjunction with high IOP.
OHTS has conclusively shown that a thicker cornea tends to inflate the value of IOP, and vice versa. For example, your IOP may be 24 mgHg and yet it may not be alarming if your CCT is 587 microns. Conversely, your IOP may be 15 mmHg but it may be alarming if your CCT is 470 microns. The practical problem in the prevention and management of glaucoma is that an ophthalmologist, who is trained in the eighties or earlier, might not be familiar with the newly found relationship between the CCT and IOP. He still goes by the rule of the thumb that more than 20 mgHg pressure consistently shown in the eye by applanation tonometry calls for preventive or curative measures for glaucoma.
Of course, it may not be always fair to blame the ophthalmologist for it. In the villages and small towns of India, there is hardly an ophthalmologist who is equipped with the instrument for measuring corneal thickness. Sometimes he does not have even a slit lamp or an applanation tonometer. In those conditions it is very difficult for him to correlate corneal thickness with IOP and reach a satisfactory value of IOP for diagnostic purpose. I have seen that even in the USA, the 20 mgHg rule of the thumb is often applied. I specifically know a dear friend of mine whose glaucoma was discovered when it was too late. During his routine eye checkups, his IOP had been consistently within the normal range for many years. However, they did not care to check his corneal thickness. Had they checked up his corneas for thickness they would have found them relatively thin.
Before 2002, ophthalmologists knew that some people do have glaucoma even when the tension in their eye is normal. They called it normotensive glaucoma. However, they did not understand the why of it. Even now, they are not sure about the whole story. But After the Ocular Hypertension Study in 2002 they now at least know about the part played by the corneal thickness in the diagnosis of glaucoma. Pachymeter, which measures corneal thickness, is a powerful weapon in the arsenal of eye-care specialists who are waging a relentless war against glaucoma.
There are many ways to determine IOP, but applanation tonometry is considered to be the gold standard. Once the IOP or tension exceeds 20 mgHg in the eyes of a patient, doctors immediately prescribe appropriate treatment to lower the tension.
The normal value of IOP is essentially based on the data collected meticulously by a large number of investigators. In most of the studies, the mean value of IOP has been found to be 15.5 mmHg with standard deviation of 2.5 mmHg. The distribution of IOP values is not normal in the statistical sense, and the eye pressure in healthy people has been found to vary between 10 and 21 mmHg. The values of IOP taken over a large number of people are skewed towards higher values. It means more people have pressures higher than 15.5 than those having lower than it. Doctors, trying to hunt for glaucoma get concerned if the IOP of a patient exceeds 21 mgHg.
From January 1985 to November 1988, an epidemiological eye study was conducted in East Baltimore, Maryland. More than 5000 persons aged 40 or more were subjected to comprehensive testing for glaucoma. They were screened by all the available diagnostic tools available at that time. The investigators found that in case of a glaucomatous eye, its IOP being more than 22 mmHg was 8.6 times higher than the likelihood of its having IOP less than 22 mg. That showed a strong positive correlation between glaucoma and the IOP being more than 22. Hence, the figure of 22 mgHg got stuck in the collective memory of the physicians as the cutoff point. To further simply, the figure of 22 was rounded off to 20. Things became simpler: if your eye shows IOP more than 20 mgHg you are having either glaucoma or at least a high risk for this disease.
That was the scenario till 2002 when a large American study called Ocular Hypertension Study (OHTS) was published. It found a statistically significant correlation between the central corneal thickness (CCT) and IOP. The study showed that CCT is a reliable predictor of development of glaucoma. If your cornea is thin, that is, less than 555 microns, you have three times greater risk of developing glaucoma than those fortunate people whose cornea is more than 588 microns. The implication was that thin corneas should be viewed with greater respect by the physician as they have a more nuisance value as far as glaucoma is concerned. Of course, it does not mean that the thin cornea is a causative factor in the development of glaucoma. The OHTS implied, inter alia, that the role of thin corneas is to blow the whistle in conjunction with high IOP.
OHTS has conclusively shown that a thicker cornea tends to inflate the value of IOP, and vice versa. For example, your IOP may be 24 mgHg and yet it may not be alarming if your CCT is 587 microns. Conversely, your IOP may be 15 mmHg but it may be alarming if your CCT is 470 microns. The practical problem in the prevention and management of glaucoma is that an ophthalmologist, who is trained in the eighties or earlier, might not be familiar with the newly found relationship between the CCT and IOP. He still goes by the rule of the thumb that more than 20 mgHg pressure consistently shown in the eye by applanation tonometry calls for preventive or curative measures for glaucoma.
Of course, it may not be always fair to blame the ophthalmologist for it. In the villages and small towns of India, there is hardly an ophthalmologist who is equipped with the instrument for measuring corneal thickness. Sometimes he does not have even a slit lamp or an applanation tonometer. In those conditions it is very difficult for him to correlate corneal thickness with IOP and reach a satisfactory value of IOP for diagnostic purpose. I have seen that even in the USA, the 20 mgHg rule of the thumb is often applied. I specifically know a dear friend of mine whose glaucoma was discovered when it was too late. During his routine eye checkups, his IOP had been consistently within the normal range for many years. However, they did not care to check his corneal thickness. Had they checked up his corneas for thickness they would have found them relatively thin.
Before 2002, ophthalmologists knew that some people do have glaucoma even when the tension in their eye is normal. They called it normotensive glaucoma. However, they did not understand the why of it. Even now, they are not sure about the whole story. But After the Ocular Hypertension Study in 2002 they now at least know about the part played by the corneal thickness in the diagnosis of glaucoma. Pachymeter, which measures corneal thickness, is a powerful weapon in the arsenal of eye-care specialists who are waging a relentless war against glaucoma.
1 comment:
Suraj, very interesting and well put article.
I would also like to add that new technologies such as Transpalpebral Tonometer Diaton which measures IOP through the EYELID, with no contact with cornea (over sclera) minimizes the need for corneal thickness....or pachymetry. In other words, the IOP reading with Diaton tonometer eliminates corneal dependency.
You have mentioned rural areas in India, I would also positively comment on Tonometer Diaton in a way that it does not require sterilization and there is no need for anesthetic drops either... + since the reading is done over the lid, using transpalpebral method for screening minimizes the risk of infecting...
You can review the details @ http://www.TonometerDiaton.com
I hope you'll find it helpful...
Best regards,
DA
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