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Cover illustration Pressure to measure pressure: the McLean Tonometer Albrecht von Graefe’s invention of the surgical iridectomy procedure for the treatment of glaucoma in 1857 was his impetus to design an instrument to measure the eye pressure so that he could record pressures before and after iridectomy. He also constructed a campimeter for plotting the visual field. His design for a pressure measuring instrument was developed into the first tonometer in 1863. It was however unsuccessful. This was followed by several attempts and a series of tonometers by Frans Donders and others over the next 20 years. The problem they all had was a lack of accuracy due to variable friction in the moving parts of the instrument and the fact that the probe had to be applied through the eyelid, introducing further uncontrollable variables. Carl Koller’s discovery of cocaine as a local anaesthetic in 1884 allowed the next series of tonometers that made direct contact with the cornea. The emphasis was on indentation tonometers, despite the difficulty and inaccuracy associated with their use. The introduction of Hjalmar Schiotz’s tonometer in 1905 eased some of the major problems. Even at that early stage Alexei Maklakov introduced his applanation tonometer, which too became popular. The McLean tonometer, featured on the cover of this issue, was constructed to improve or eliminate certain deficiencies in the Schiotz tonometer. The instrument was made for Dr McLean by EB Meyrowitz of New York. In an article written by William McLean, Professor of Ophthalmology at New York Medical College MD in the American Journal of Ophthalmology, June 1919 he listed the following features: c To avoid the changing of weights as in the Schiotz and Gradle tonometers. c Elimination of the chart to determine the pressure in millimetres of mercury. c To place the reading scale in a position to make it easier for the observer to both apply the instrument and take a reading. Br J Ophthalmol September 2009 Vol 93 No 9
The McLean tonometer. Courtesy of Mr Richard Keeler, Curator, museum of the Royal College of Ophthalmologists, UK. Photo: Mark Thomas. To try and prevent capillary attraction between the plunger on the tonometer and its barrel from fluid in the conjunctival sac. McLean also read a paper on his tonometer at the Oxford Ophthalmological Congress in July 1919, which was published in the British Journal of Ophthalmology 1919;3:385–99. The Schiotz tonometer however remained a popular instrument in clinical practice and is still used in some parts of the world.
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Richard Keeler, Arun D Singh, Harminder S Dua Br J Ophthalmol 2009;93:1131. doi:10.1136/bjo.2009.168872
1131
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Pressure to measure pressure: the McLean Tonometer Richard Keeler, Arun D Singh and Harminder S Dua Br J Ophthalmol 2009 93: 1131
doi: 10.1136/bjo.2009.168872
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