Tonometers

Tonometers measure the internal pressure of the eye and tonometry is one of the principal tests for glaucoma, but until relatively recently their use in the eye examination was far from routine. 

A patient's intraocular pressure (IOP) should normally be 15. Any reading in the region of 21 or 22 signifies an increased likelihood that the patient will go on to develop glaucoma.

The link between ocular pressure and what was later identified as glaucoma seems to have been recognised as far back as the 10th century AD by Al-Tabari an Arabian surgeon. By 1622 Bannister was discussing the use of the fingers by the practitioner to feel for the pressure. This is called palpation or (confusingly, given the other modern meaning of the word) 'digital' tonometry.

Observation of the optic disc, via ophthalmoscopy, and the prospect of a surgical ‘cure’ by iridectomy encouraged those who sought a means more effective than palpation to test for glaucoma. Space does not permit an outline of all the various diagnostic techniques developed in the late nineteenth and twentieth centuries but we might say that from the 1880s Jannik Bjerrum was foremost in documenting the defects in the visual field that are characteristic of glaucoma and can be detected with the perimeter. Other techniques include gonioscopy (1918) by virtue of which glaucoma came first to be defined in open and closed angle varieties, dynamometry and the 1950s technique of tonography. The principal technique however has proved to be tonometry. 

In 1862 Von Graefe, a professor in Berlin, was the first to design an indentation tonometer for testing the pressure of a seated patient’s eyeball and reading it off a scale. The instrument recorded the depth of indentation caused on the eye by a known weight.

In 1865 Donders  designed the first tonometer intended for use against the sclera, the white part of the eye and Priestly Smith would independently come up with something similar in 1884.

Ernst Pflüger was a Bern ophthalmologist whose clinical work in the 1880s concentrated on glaucoma. The incidence of glaucoma specialists is a notable development. Pflüger’s thesis of 1871 had already dealt with tonometry, which can thus be seen to have established itself as a primary technique. That said, even large university clinics often lacked a tonometer and the tension of the eye was still most often measured by pressing a finger onto it, with all the consequent risks of infection.

Impression Tonometers 

Professor Hjalmar Schiøtz, the first Director of the Eye Department at the Rikshospitalet, Oslo, from 1897 devised his impression tonometer, originally for use against the sclera of the eye, in 1905. He then developed this into a corneal plunger. the higher the pressure of the eye, the lower amount of indentation would result.

For the next half century the Schiøtz was generally accepted as a reliable means of measuring IOP and became the first tonometer to achieve mass sales. Various fakes also hit the market. 

It required a calibration scale, as designed by Friedenwald, taking into account that placing a tonometer onto the eye automatically raises the IOP.

The Gradle-Schiøtz tonometer of 1911 held the weights in place much better and was much more easily adjusted.

Schiøtz manufactured the instruments personally and from the 1920s offered a calibration service in Norway for some years before licensing various companies, including Weiss & Co and Theodore Hamblin Ltd, to produce it. At the time of its introduction it had the advantage that the pressure of the fingers in steadying the instrument was virtually nil. It was slightly modified in 1924 when an un-weighted version, the X-tonometer, was introduced. An example in the BOA Museum, formerly used by the London Refraction Hospital has supporting documentation from 1956 to show that it was tested by the W.H.Ross Foundation and the Faculty of Ophthalmologists Tonometer Testing Station in Edinburgh and found to comply with the International Standard for Tonometry accepted by the International Council of Ophthalmologists in June 1953. 

The Schiøtz was still offered as a ‘traditional’ instrument (with a choice of straight or oblique scale) in Keeler catalogues of the 1980s but it is now rarely used in the developed world. Before the procedure could begin the footplate had to be sterilised with absolute alcohol or by heating. The practitioner then had to wait for evaporation or cooling to occur. The patient had to be placed in a supine position, without any pillow, and undergo corneal anaesthesia (using Xylocaine). Application of an antibiotic ointment was necessary after the procedure was complete. Impression tonometers can only record a relative measurement and various unavoidable errors might occur due to contraction of the extra-ocular muscles or an eye having a particularly rigid outer coating. Accommodation (which causes IOP to drop) was also a problem, as the patient would somewhat naturally attempt to focus on the instrument heading straight for his eye.

Mention should also be made of the French tonometer of Paul Bailliart (1923) and the Berens-Tollman Indicator of 1950 although this latter instrument was intended for glaucoma screening rather than diagnosis.

Applanation Tonometers 

The alternative applanation technique, in which the amount of corneal flattening produced by a known weight is measured, was pioneered by Von Graefe's pupil, Weber (1867) and Alexei Maklakov of the Moscow Eye Hospital (1885), the latter specialist helped by the availability since 1884 of localised anaesthesia. Maklakov tonometers are still used in some parts of the world today.

The Goldmann Tonometer, introduced by another Bern professor in 1954, pressed a plexiglass plate onto the cornea, controlled by a coiled spring and lever system. Because of the very small area touched, ocular rigidity did not affect the quality of the readings. The Goldmann method of applanation differed from Maklakow’s in that it measured the weight required to produce a given amount of corneal flattening. It carried far less risk than the Schiøtz impression method, meaning that it was now more acceptable to take the five readings in a single day necessary to determine the diurnal curve chart of a new glaucoma patient. The instrument came to be available in two types. Type R remained mounted to a slit lamp and was compatible with most brands of that instrument, including those made by Keeler, Haag-Streit or Zeiss. Type T could be brought out for use when desired and fitted a guide plate on the slit lamp. It could be used in two possible positions. A permanent fixture became more attractive to optical practitioners. The AO Applanation Tonometer, marketed heavily in the early 1970s, mounted permanently to an American Optical Company or Haag-Streit slit lamp and could be incorporated into the practitioner’s normal biomicroscopic examination.

Late twentieth century tonometer design featured various mechanical and non-mechanical innovations. The Mackay Marg model of 1959 was the first electronic tonometer and can be said to have combined the principles of applanation and indentation.

Digilab tonometers, such as the ‘Pneumatonometer’, first designed in 1965 and a popular instrument of the 1970s, were operated by a unique lightweight pneumatic feedback system, running on a liquid gas, which automatically controlled the correct applanating force. The sensor tip was designed not to absorb lacrimal fluid or mucus. Based on the Durham and Langham Applanation Tonometer of the late 1960s this instrument appeared in various forms and measured IOP continuously. No less importantly it was portable. It could also be used effectively on a scarred cornea; scarred corneas were becoming increasingly common due to the increased number of surgical procedures being carried out. Advertisements of the time refer, only partly reassuringly, to the ‘Minimal patient trauma’ they caused. The IOP reading was obtainable within a mere five seconds of correct corneal contact being established!

In the mid 1980s as portability became an increasing advantage, allowing for testing of the bed-ridden or wheelchair-bound patient, the Perkins Hand Held Applanation Tonometer (essentially the portable version of the Goldmann) was advertised as ideal for both domiciliary and consulting room use. 

The Draeger Hand Applanation Tonometer with forehead rest, made by Möller of Hamburg in the 1970s was touted as being for one-handed operation and with a measuring system that worked independently of the position of the instrument it was supposedly ideal for patients occupying various seated or supine positions. Even an operator wearing spectacles could use it without needing to get too close. It was available with a sterilising box that disinfected the optical plastic probe with intensive UV radiation. 

The electronic Tonopen (1988) may be considered the ultimate descendant of the Mackay Marg tonometer.


Non-contact Tonometers 

Routine screening for glaucoma only really began, however, with non-contact tonometry, popularly known as the ‘Puff-of-air’ test because the applanating medium is a brief controlled air pulse. The only reaction from most patients is a blink reflex. The AO NCT was introduced by its manufacturers with a true sense of history-in-the-making. A 1972 catalogue entry for the instrument begins momentously, ‘Since the 12th century the occurrence of elevated intra-ocular pressure has been recognised as a symptom of ocular pathology’. Non-contact tonometry did not require mechanical contact with the eye, any form of anaesthesia, or any retraction of the eye-lid. Early models were available in a fetching black and green livery and included a fixation light for the patient with limited acuity in the eye being tested. 

Routine testing was now both possible and economically viable for the high street optometrist. Indeed it was stated that the NCT ‘virtually eliminates professional skill and judgment as a factor influencing accuracy or reliability’.  

A decade later this notion reached its logical conclusion with the launch of the Keeler Pulsair (1986), which had been five years in development. The company proudly declared that even ancillary staff could use it. ‘Dry eyes, opaque corneas and high IOPs are no longer a problem. With just a push of a button, Pulsair will adapt to difficult eyes without loss of accuracy’. It was portable and considered suitable for domiciliary or multi-centre practice use. Notably its genesis was the result of an approach by an electronics company, PA Technology, to a major optical supplier and was not the invention of an individual ophthalmologist. An advanced 2000 model was launched in Spring 1991. In a sign of the times the catalogue for this also mentions that the instrument minimised the risk of cross infection from HIV patients. Less apprehension from the patient (for example because it involved no gas piston noise) meant that readings were also more reliable. To assist with calming the test subject the device included a demo button - to try on the patient’s hand first. 

The Keeler Pulsair 3000 (1998) and EasyEye (2001) have drawn on the advances in microprocessors to produce instruments that are even easier to use. An operator without optometric qualifications may now, in theory, teach him or herself to use it just by reading the instruction leaflet and the instruments require no prior preparation. Meanwhile sophisticated software will monitor the performance of the air release valve and other components identify spurious readings and cancel them out. History teaches us, nevertheless, that further improvements are still likely. Reichert Ophthalmic Instruments, has recently produced a cordless tonometer, the PT100. The good news for people with glaucoma is that this web article cannot yet be finished. 


 Acknowledgments: 

This webpage has been adapted from an article From Blue to Blink, a Brief History of Glaucoma Diagnosis and Testing that originally appeared in Glaucoma Forum, 2002. The author is grateful to Dr Michael Wolffe and Mr Colin Burrows for their assistance in the preparation of that two-part article. 

See also our MusEYEum webpage about the Glaucomatous Eye

 Von Graefe
A. Von Graefe 1828-1870

 Hjalmar Schiotz
Professor Hjalmar Schiøtz

 Schiotz X Tonometer
Professor Schiøtz X-Tonometer showing the spring. Probably 1930s

 Schiotz Tonometer
Professor Hjalmar Schiøtz Improved Tonometer  c.1955

 McLean Tonometer
The McLean tonometer, first designed in 1919, was an American hand-held contact tonometer with a family resemblance to the Schiøtz. It did not employ any weights and took a reading directly in mmHg, but its readings were generally higher than other instruments (up to 40mmHg) so many considered it to be inaccurate.

 Maklakov Applanation Tonometer
Maklakov-type Applanation Tonometer, the first practical applanation instrument, introduced in 1885, worked by flattening the cornea. This later type, made in 1962, included an ink pad (in the metal case) for colouring the footplate. An imprint could then be obtained on paper after the applanation. Posner designed a plastic disposable version of the Maklakov later in the 1960s.

AO NCT
The NCT (non-contact tonometer) by American Optical seen in a trade advertisement, supplied in a fetching green colour typical of the period.

Use of Pulsair Tonometer 2005
Use of a hand-held non-contact tonometer in 2005