180 Years of Evolution of the Tonometer

The tonometer has evolved tremendously since Sir William Bowman emphasized the importance of ocular tension measurements in 1826. In an address delivered at the annual meeting of the British Medical Association, Sir William underscored the critical role that digital estimation of ocular tension played in his practice. In his address, Sir William stated that “it is now my constant practice, where defective vision is complained of, to ascertain almost at the first instant the state of tension in the eye...It is easy enough to estimate the tension of the eye, though there is a right and a wrong way of doing even so simple a thing... With medical men, the touch is already an educated sense, and a very little practice should suffice to apply it successfully to the eye.” (Bowman, W. Br Med J, 1852; 377-382).

Soon afterwards, digital tonometry became an essential clinical skill necessary to master by all ophthalmologists. When mechanical tonometry was first introduced in the late 1800s, many ophthalmologists felt so confident with their ability to estimate IOP by palpation that they viewed the new technology as inferior. Isador Schnabel, in an address to the Vienna Ophthalmological Society in 1908, was noted to state that although he did not object in principle to mechanical tonometry, he expected “…very little from this test since digital tonometry by an expert is a much more accurate test” (Schnabel I., Klin Montasbl Augenh 1908; 48:318).

Although Grafe is credited with the first attempts to create instruments that mechanically measured IOP in the early 1860s, his proposed instruments were neither designed nor built. Rather, it was Donders who designed the first instrument capable of estimating IOP – albeit not accurately – with mechanical tonometry in the mid 1860s. The principle behind Donders’s instrument was to displace intraocular fluid by contact with the sclera. The ophthalmologist first measured the curvature of the sclera at the site of contact, then used this measurement as a reference plane to measure the depth of indentation. Smith and Lazerat refined this technology in the 1880s, and the discovery of cocaine by Carl Koller in 1884 led the way to corneal impression tonometry soon thereafter. With the aid of a powerful corneal anesthetic agent, corneal tonometry became the definitive choice of IOP measurements because it offered a well – defined and uniform site of impression when compared with the sclera.

Impression tonometry’s major shortcoming was that it displaced so much fluid upon contact with the eye that the measured readings were highly variable and mostly inaccurate. What was needed was a way to displace a minimal amount of fluid to record IOP. This breakthrough came when Adolf Weber designed the first applanation tonometer in 1867, which gave a highly defined applanation point without indentation. After two decades of skepticism, the value of applanation tonometry was re-discovered when Alexei Maklakoff and others introduced new versions of applanation tonometers. In early 20th century, there were about 15 models of tonometers in use. In fact, Maklakoff’s 1892 model is the basis of applanation tonometry today. However, digital tonometry still remained the gold standard among most ophthalmologists in the early 1900s.

The first clinically useful mechanical tonometer was designed and introduced by Hjalmar Schiotz in the early 1900s. The instrument was simple, easy to use, and highly precise. It was quickly accepted and became the new gold standard beginning the 1910s. Innovations in calibration led to its increased use, and a tremendous amount of knowledge about the normal and glaucomatous eye was quickly acquired. An adjustment for ocular rigidity was introduced by Goldmann in the 1950s, which led to the development of Goldmann applanation tonometers. The Goldmann tonometers displace such little fluid that variations in ocular rigidity are mostly negligible. The electronic and non – contact tonometers used today rely heavily on the principles and instrumentation first introduced by Maklakoff, Schiotz and Goldmann.

Today, for the most part, digital tonometry has been replaced by sophisticated technologies to estimate IOP. Today’s instruments are incredibly accurate and easy to use. Yet, there is sometimes no good substitute for digital tonometry. For example, some ophthalmologists may prefer digital tonometry when estimating IOP in patients with keratoprostheses. In these situations, fingers that have mastered Sir William’s art are highly desirable. In fact, it is said that the famous Dr. Claus Dohlman, Harvard professor of Ophthalmology at the Massachusetts Eye and Ear Infirmary, remains as accurate in measuring IOP with his fingers as any ophthalmologist using the high-tech tonometers of today!