With all the exciting new technology available and glaucoma diagnosis going digital, it’s time to have a look at the oldest
diagnostic test for this condition – intra-ocular pressure (IOP).
Pressure of course comes into its own as a treatment measure. Numerous trials confirm pressure reduction as our only known treatment modality. For this we need a reliable baseline and reliable follow up pressures. Too often I see treatment which does not work just being continued, or second medications added rather than switched. All drugs, lasers and surgeries don’t work sometimes, and if you don’t get that individualised target pressure reduction of 20-40% then don’t continue the treatment.
NICE guidelines specify a Goldman measurement is best practice for referral and required for diagnosis and treatment. This mechanical machine is getting on now but remains the gold standard. Calibration checks are easy and quick but tend to get overlooked. Make sure you know how to do this and do it at least weekly if you are a regular user.
And make sure your practice has a process for recalibration if needed – it’s a workshop procedure, so you may need a spare machine while it is away. What to do for those who can’t manage a Goldman? People with neck, back or tremor issues are difficult. You might have a Perkins, which will do. My current favourite back up is the ICare, but be aware correlation with a Goldman measure isn’t great (52% within 2mmHg).
Clean tonometer tips are required. In a recent survey 60% of eye departments exclusively use disposable tips. The re-usable tips are also fine as long as you have a proper disinfection process in place, and they cost less. My advice is to do one or the other. With mixed use, sure as taxes your trainees or cleaners will put your expensive reusable tips in the bin.
Pressure fluctuation is important in glaucoma – daily fluctuation is more marked in glaucoma and some types of glaucoma (eg pigment dispersion and chronic angle closure) may exhibit marked spikes of IOP. So, your normal IOP on one reading may not be all the story. Diurnal pressure measurement is common practice in eye departments where we try to pick up these fluctuations. I’m not a huge fan as it’s very time-consuming for patients and in my experience rarely alters treatment plans. And if you do diurnal IOP measures to try to pick up spikes or mean IOP, you should really do diurnal measurements for follow up as well. With the endemic delays in NHS clinics, decisions on effective treatment can just get put on hold while this process goes on.
Corneal thickness (CCT) is best looked at as an independent risk factor, not an IOP modifier, and I never use ‘adjusted’ IOP. If a patient has glaucoma then the pressure needs to come down whatever the thickness of the cornea. This measure is most useful when discussing risk and possible preventive treatment in ocular hypertension (OHT) and there are useful online formulae for five year risk prediction. A trial I tried to set up 15 years ago was intended to look at treatment/no treatment in OHT with pressures of 28-35 and thick corneas – pushing the boundaries of the OHT treatment trial, with corneal thickness taken into account. One still to be done if anyone is interested.
I also find CCT useful when treating normal tension glaucoma (NTG). Patients with pressure in the high teens and thin corneas may have a regular type of pressure-related glaucoma rather than a vasculopathy or otherwise unknown cause. In these cases I feel better about aggressively tackling IOP. In cases with normal CCT and a pressure of say 12 I really have to be pushed into doing major surgery as I don’t know what I’m treating.
But pressure is also a cornerstone of screening and diagnosis. Despite 40% of all glaucoma in the UK being normal tension, it’s still the high pressure glaucoma that has the worst outcomes, and finding high pressure is still very important. All secondary glaucomas are pressure-related and many occur at young age. More important than just raised IOP in my view is pressure asymmetry. Glaucoma is asymmetric in all its features in 99% of cases, and that asymmetry should add up. So the eye with the higher pressure should have the worse disc, field defect and OCT – but of these the easiest and first one to find is the IOP. This basic rule also applies to NTG; pressure matters even when it’s normal.
Whatever method you use to screen or treat IOP, make sure the machine is properly used, calibrated and that you pay attention to the results – it’s really important!
Tim Manners, consultant ophthalmologist and Newmedica Ophthalmology Joint Venture Partner, Lincolnshire.