Tonometry is a method used to determine the pressure in the eye by measuring the tone or firmness of its surface. It is an essential component of the global examination in all eye check-ups together with other tests to give a complete picture of the patient's ocular health and therefore the risks for developing or controlling a given disease such as cataracts, macular degeneration, or glaucoma.
Elevated intraocular pressure (IOP) can damage the optic nerve which collects all of the visual information from the retina of the eye and transmits that information to the brain, where the signals are interpreted as vision. Gradually, the vision can be lost permanently and this is called glaucoma – the sneak thief of sight.
Few risk factors for glaucoma are:
- Elevated IOP
- Family history of glaucoma
- Diabetes mellitus
- Previous ocular trauma
- Injury or surgery
- Anatomical variations including narrow filtering angles or anatomical damage to the filtering angles
- Many infectious diseases of the eye such as Herpes simplex, toxoplasmosis, Fuchs uveitis syndrome, or shingles (Herpes zoster), etc.
During testing, the ocular surface is anesthetized with a single drop of topical anesthetic. The tonometer device lightly touches the surface of the eye and the resistance to indentation is measured. The firmer the tone of the surface of the eye, the higher pressure is reading.
Also, the pressure reading is lower than otherwise expected if the cornea is thin. Patients with a thin cornea may obtain a falsely low IOP reading when the actual pressure is high so they must be watched more closely for glaucoma.
There are several types of tonometers available for this test and the most common are:
- Goldman applanation tonometer – A tiny pressure sensor attached to a spring-loaded arm is gently placed against the tear film, and the doctor or technician reads the pressure through the microscope under the blue light.
- Tono-Pen handheld electronic contact tonometer – Portable, handheld device that runs on hearing aid batteries and calibrates digitally with the push of a button. The sterile device tip is gently placed against the tear film by the doctor or technician, and the pressure reading appears on the digital readout simultaneous to a faintly audible beep.
- Pneumotonometer contact device – Similar to the handheld to open tonometer, but it is larger so it's not readily portable and requires a continuous gas supply and separate gauge container with analog readout attached to a long tube and pressure probe.
- The air puff noncontact tonometer – Requires no anesthetic drop, because of "no touch" technology, but it often produces falsely elevated readings, particularly in patients who squeeze their muscles upon anticipation of the air puff.
- Tactile finger applanation Over the closed eyelid by a skilled eye doctor is an age-old traditional method utilized by the experienced practitioner.
- Patient self-testing devices – Allows a patient to gently press a calibrated spring-loaded piston against the closed eyelid over the upper, outer quadrant of the eyeball while gazing downward.