A new pachymeter - the 300G

We have in recent issues of Instrument Focus tried out a selection of pachymeters. Increasingly these instruments are capable of accurate measurements of corneal thickness (and in some circumstances other measurements, such as anterior chamber depth) and have been marketed to optometrists as well as ophthalmologists and hospital clinicians.
The reason for this has been the increasing realisation that applanation tonometry, whether by a contact (Perkins and Goldmann) or non-contact method will give a reading that is very much dependent upon corneal thickness. It is also well established that corneal thickness varies significantly from individual to individual, varies (usually thicknesses in normal eyes) with age, and that more patients whose thicknesses are changed by refractive surgery are attending for routine eye examinations. In general, a thicker cornea will offer greater resistance to a tonometry force and hence may offer a higher intraocular pressure reading even when the actual IOP is not any different to a second patient with a lower reading but a thinner cornea.
Though research has tended to show there is no single conversion factor that may be used to adjust a tonometry reading, most modern pachymeters allow either a selection or a single referenced conversion factor (referenced to a particular research study relating thickness to IOP).

The 300G
The latest pachymeter to come our way is the 300G from the US company and supplied by Polymeric Sciences. The instrument looks similar to other desktop units (such as the Pachette 2 featured in optician, September 24, 2004). An electronic unit with a simple LED display has an ultrasonic probe attached to it. Weighing in at just over 1.3kg the unit is still portable and lightweight; an important factor if it is to be transferrable between clinics. This is a mains-powered unit so there is no need for battery recharging between uses. The unit has a printer output if a hard copy of results is required.
For ease of use, the probe may be ordered either as a straight or an angled version. There is a further option of an S-shaped probe, suitable for certain surgical procedures, but this needs to be specifically ordered. Unlike many such instruments, the probes are easily interchangeable which might be of use to a specialist who relies on different probe types, but to an optometrist such as myself, it is perhaps more useful to know if I damage the probe. Being able to replace a damaged probe alone presents a significant cost saving, bearing in mind the unit as a whole costs 2,195 plus VAT.

Practical use
For trying out the instrument I thought it might be useful (and a challenge) to use a patient I know to be reluctant having anything held close to his eyes. Fitting trial lenses or carrying out contact applanation has, on previous occasions, been less than easy.
On switching on the instrument, it goes through a short self-test programme, calibrating it ready for use with whichever probe has been inserted. The probe is, obviously, sterilised prior to use as with any repeat contact procedure and it is essential that it is also dry prior to use as this may affect accuracy. After inserting anaesthetic, I also followed the manufacturer's guidelines to instil artificial tears which it states 'will greatly enhance the ease of obtaining measurements'. The probe is then carefully placed onto the front of the anaesthetised cornea and, once a good reading has been established, a short confirmatory 'beep' is heard from the base unit.
The manufacturer emphasises that in no circumstances should you 'scouraround the cornea looking for a good reading!' The reason for this is that the instrument automatically takes a sequence of readings and has 'noise cancelling' software to reject readings that contain too much variability. In essence, the urge is to keep moving the probe around to get the satisfactory 'beep' (a strong urge as one is getting used to the instrument). What is recommended, and is an excellent tip for using the instrument at first, is a simple rocking movement of the probe on the eye to achieve a reading if one is not forthcoming. If, however, no reading is gained at all within 30 seconds, a longer 'beep' is heard to indicate that the measurement period is ended and the 'Measure' button on the unit needs to be reactivated. There is an option available (though not on this occasion) of a foot pedal to activate the measure function.
The unit takes multiple readings and at the end of the measuring sequence displays the latest four to be accepted, an average value (in our case AVG=512), and a correction factor for IOP conversion. This is displayed as a plus or minus value to one decimal place, which is the amount the tonometry reading is to be adjusted by. A thin cornea, for example 483 microns, will show a correction factor of +3.3, indicating that any tonometry reading needs this value to be added to it to validate the measure and correct for the thin cornea. Disappointingly, there is no indication of a reference in the accompanying literature of the source of the correction factor, though it is likely to be an adjustment based on recent analyses.

Usefulness
The instrument was both easy to use and the results easy to interpret and act upon. The patient had no concerns about the procedure and the speed with which the averaged reading was obtained was impressive, even for a nervous patient. I also liked the facility to automatically reject erroneous readings without any major increase in the time for the procedure.
As part of a glaucoma monitoring service such instruments would seem to be important. As far as monitoring contact lens patients, as suggested as a use by the supplier, I can think of a few who would consider this an essential buy when compared with, for example, a topographer, particularly if the topographer also had some pachymetric facility. As part of a post-refractive surgery monitoring programme, again I can see this instrument proving useful.

The instrument we used is a prototype version and that available on the market has an improved display appearance, though is in every other way the same. A much more advanced (and expensive) version, the 1,000G is available for very small thickness measurements (flap thickness or Intacs placement measurements) but is more likely found in ophthalmology clinics.

Thanks to Polymeric Sciences for loan of the instrument. For further details email: enquiries@pachymeter.co.uk