In firstname.lastname@example.org, Rachel wrote:
I've done some research on the AT 550 also, and according to a European study, it has a very significant upward bias for thick corneas relative to the Goldmann (I happen to have a CCT of around 600)...while it does very well relative to the Goldmann for thin corneas such as David's (which would be consistent with David's observations on his website).
David, since you have done quite a bit of research on tonometers, what is your view of the Tonopen? I don't need a "self"-tonometer, and don't necessarily want to check my pressures thousands of times (though I could very well imagine becoming addicted if I bought the AT:). Studies seem to indicate that the TonoPen is less precise than the Reichert 550 or previous models. What has been your experience?
Rachel, this is an excellent question. I see you have done your homework. I'll offer my opinion.
The most important features of any tonometer for personal or home use are self-consistency, easy of use and safety. I think the Reichert AT555 tonometer comes out on top.
UPDATE: What follows is my opinion based on my own experience. Please keep in mind, while you read the rest of this article, that the Reichert AT555 tonometer is not designed for home use nor is it designed for self-tonometry. For that matter, neither are the TonoPen or any of the other good quality tonometers available in the US right now. Therefore, I am discussingnon-standard uses for these tonometers. The manufacturers probably would not support my recommendations. Unfortunately, a glaucoma patient interested in self-tonometry (or any reliable form of home eye pressure monitoring) doesn't have a good choice other than to improvise. (You should also plan on getting a prescription from a doctor to purchase one of these professional-quality tonometers.)
Most ophthalmologists in the US don't have much experience with the AT555 or AT550 tonometers (or any other non-contact tonometers). However, doctors of optometry often use both the non-contact tonometers and the TonoPen. Most doctors of optometry I have spoken with tell me that the AT555 (and even the older Reichert model tonometers) are more consistent than the TonoPen and therefore they consider them more accurate & reliable. In terms of safety and ease of use, the AT555 (or AT550) is a landslide winner. On the other hand, the TonoPen is the winner in terms of portability and cost.
In regard to CCT (central corneal thickness, which is measured with a pachymeter), this is a complex question. Adjusting eye pressure for CCT is a flawed process and I won't go into the details here. However, I will mention that one's central corneal thickness is not a constant. If yours is 600 in the morning soon after you wake up, it might be 560 later in the day, for example. (However, with those numbers your CCT would still be thicker than average -- on average.)
The important issue regarding CCT and the Reichert AT555 for home tonometry is that the AT555 tonometer will give youreliable results. The study conclusions you cited don't apply so much for home tonometry. When your ophthalmologist takes your eye pressure he or she needs to compare it to population standards. When you do home tonometry, you mostly need to track changes relative to yourself. You could do a comparison of your eye pressure readings vs. your doctor's and then apply an adjustment factor to correlate your home readings with Goldman tonometry (GAT) if you wish. (However, the adjustment factor may not be accurate unless you perform a rigorous comparison.)
True self-tonometry at home (not having to rely upon an assistant) is an advantage one cannot appreciate until one experiences it. I did a lot of research prior to having an AT555 and it was a real pain to collect measurements when I had to wait on an assistant.
Good luck and please let me know what you decide. Comments (or ratings) on this message are appreciated.