by Dr. David L. Williams MA VetMB PhD CertVOpthal CertWEL FRCVS
Veterinary Ophthalmology
It’s thirty years since I started as a veterinary ophthalmologist and it’s interesting to see what advances we’ve had since then, and where things are still the same. The best way to assess tear production in the dog, to my mind at least, is still the Schirmer tear test, invented back in 1904 by Otto Schirmer the German ophthalmologist.
A year later, Hjalmar August Schiøtz from Norway presented the world with his new way of measuring intraocular pressure (iop). Here, though, we have seen significant advances. Schiotz’s tonometer measured the indentation of the cornea caused by a specific weight and allowed one to calculate the intraocular pressure using a conversion table. It does work, but is cumbersome, relatively inaccurate and needs the animal’s head to be held with the corneal surface precisely horizontal. A considerable advancement came with the applanation tonometer which measured iop by determining the pressure needed to flatten the centre of the cornea. In people, the Goldmann tonometer is used at a slit lamp but this requires a completely stationary eye and is not appropriate for our species.
When I was a new graduate we had the MacKay-Marg tonometer, a wall-mounted device with a probe one touched ono the eye to give a graphical representation of the pressure on the prove surface after which one could calculate the iop. So imagine our delight as ophthalmologists when the Tonopen came along, an electronic hand held tonometer. The Tonopen still needs some experience in how much pressure to apply as one touches the cornea and the reading depends not only on the iop but also on the rigidity of the cornea which can change with scarring or oedema. It required topical anaesthesia and a relatively compliant patient, so was difficult to use in a bouncy dog or a petulant child! It also needs a relatively large cornea, so is not appropriate for rodents or small exotic species. In 2001 a new device, the Tonovet, was produced that could be used in these laboratory animals and relied on measuring the rebound velocity of a small probe, so tiny that it did not even cause irritation to the ocular surface. And really simple to use: hold it a few millimetres from the corneal surface, press the button and there is the reading. Here was the perfect tonometer for veterinary use!
I must admit to having used the Tonovet for years and not realising it could ever be improved. How can you upgrade perfection?! But iCare, who manufacture the Tonovet, were not ones to rest on their laurels. They have produced the Tonovet Plus. One issue with the original machine was that changing its automatic calibration for different species was difficult. There was a different setting for dog or horse but everything else was ‘other’ so one was never completely sure whether tonometry on a cat or a rabbit…or even a cow, was giving a true reading. And while with experience one could easily get a reading how was one to know if the probe was held at the correct distance, or perpendicular to the ocular surface, to give an accurate reading of iop?
The Tonovet Plus
The Tonovet Plus holds you by the hand, as it were, as you hold it in your hand, to tell you if you are taking a correct reading. A red light and message signals if the probe is too close or too far or at an angle while a green light gives you the go-ahead to accept the result. It is easy to change the calibration for dog, cat, horse or rabbit and the resulting readout is much larger and clearer.
The question is whether you really need to measure the iop in an animal. You can imagine, given that I am singing the praises of the Tonovet Plus, that my answer is ‘absolutely’! Every animal that comes to you with a red eye needs its iop measured to see if it has uveitis or glaucoma. Every animal with a cataract needs its iop determined to assess if it has concomitant uveitis.
Every traumatised eye needs tonometry to assess globe health. But can you afford it? Let me tell you about a great student of mine who went for his first interview at an excellent practice. At the end of the discussion, the partners asked him if he had any questions for them:
‘Can I have a tonometer please?’
This was followed by his request with a business plan describing how many cases a week he anticipated using it on, what he planned to charge, showing that the device would be paid for easily within a year. They gave him the job…and the tonometer! You can afford a Tonovet Plus, I’m sure, and it will improve your assessment of eyes beyond your wildest dreams……well maybe that’s a bit much, but you get my drift!
For more info on Dr. Williams, visit http://davidlwilliams.org.uk/