Most readers will have come across the iCare rebound tonometer by now. Indeed, it has become the standard tonometer for domiciliary care and for some specialist clinics, such as when assessing people with severe learning or behavioural difficulties, such as autism. I first tried out the instrument five years ago and, somewhat to my surprise, found it easy to use and repeatable. (1)
The technique was developed in Finland and relies on electronic conversion of the speed of a metal rod as it passes through a solenoid after being bounced from a cornea at which it has been fired with a constant force. (2) The higher the intraocular pressure, the greater the force of the rebound. Many initial studies were based on animal assessment, and indeed a version for vets, the TonoVet, has been designed.(3)
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There have been many studies comparing rebound tonometry with Goldmann applanation (4,5,6) (GAT) and most imply good correlation in the standard range (to 21mmHg), some comparative elevation at higher values, and little error when assessing away from the central corneal axis. I suggest that some disparity might be expected in the variation at higher values, as (just as with comparison trials between contact and non-contact techniques) two totally different techniques are being compared.
The reason for using GAT for pressure referrals is because this is the technique used at the centres to which we might be referring. Any implications of greater accuracy for GAT (compared with non-contact or rebound) should be dismissed based on the extent of inter-user variability and the need for subjective interpretation in use.
For any individual tonometer, a key asset has to be repeatability and the iCare, and even more so the new incarnation the ic100 (Figure 1), does produce repeatable readings without the need for a local anaesthetic and with minimal patient awareness.
The ic100
The ic100 addresses concerns about robustness and ease of use that had been levelled at its predecessor. The unit is still light and portable but the new shape and larger LED screen make it comfortable to hold and to use. The forehead rest is easier to adjust and there is a colour ring indicator around the nozzle that changes from red (2a) to green (2b) when the unit is in the correct position from the cornea. Further control is offered once readings begin by the screen displaying when the unit has drifted too close or far relative to the cornea (3a and 3b).
In the correct position, the instrument takes six readings per eye, either individually with each displayed (4) or as rapid repeat measurements on a single depression of the ‘fire’ button. I expect to see this instrument making major inroads into the non-contact tonometry market and become a regular screening tool in primary care. Any measurement outside the expected range would then need verification according to the prevailing protocol and that dictated by the centre to which referral might be made.
References
1 Harvey B, On the rebound. Optician 23.09.11.
2 Kontiola A. A new electromechanical method for measuring intraocular pressure. Doc Ophthalmol. 1996-1997; 93(3):265-76
3 Knollinger A M, La Croix N C, Barrett P M. et al. An evaluation of the TonoVet rebound tonometer for measuring intraocular pressure in dogs and horses. Invest Ophthalmol Vis Sci 2005 2005461257
4 M E Iliev, D Goldblum, K Katsoulis, C Amstutz, and B Frueh. Comparison of rebound tonometry with Goldmann applanation tonometry and correlation with central corneal thickness. Br J Ophthalmol. 2006 Jul; 90(7): 833–835.
5 S. Munkwitz, A. Elkarmouty, E. M. Hoffmann, N. Pfeiffer, and H. Thieme. Comparison of the iCare rebound tonometer and the Goldmann applanation tonometer over a wide IOP range. Graefe's Archive for Clinical and Experimental Ophthalmology, vol. 246, no. 6, pp. 875–879, 2008.
6 J. Jorge, P. Fernandes, A. Queirós, P. Ribeiro, C. Garcês, and J. M. Gonzalez-Meijome. Comparison of the IOPen and iCare rebound tonometers with the Goldmann tonometer in a normal population. Ophthalmic and Physiological Optics, vol. 30, no. 1, pp. 108–112, 2010