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How is tonometry done?
Tonometry is generally performed upon an anesthetized ocular surface. Anesthesia is generally rendered with a single drop of topical anesthetic, such as proparicaine (Alcaine) or tetracaine (Pontocaine). The tonometer device lightly touches the surface of the eye, ever so slightly indenting the cornea. The resistance to indentation is measured by a precisely calibrated pressure sensing device, the tonometer. Several types of tonometers are available for this test, the most common being the applanation tonometer:
- Goldman applanation tonometer: the "gold standard" instrument attached to the slit lamp biomicroscope used in all eye doctors' offices. It requires a cobalt blue light source and a small droplet of fluorescein on the ocular surface. 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: This widely used, portable, handheld device runs on hearing aid batteries and calibrates digitally with the push of a button. It requires a disposable sterile cover for each patient. 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: The device is operated similarly to the handheld tonopen tonometer, but due to its larger size, it's not readily portable. It requires a continuous gas supply and separate gauge container with analog readout attached to a long tube and pressure probe. This is an older technology and has largely been replaced by the handheld tonopen tonometer.
- The airpuff noncontact tonometer, which generally requires no anesthetic drop, is widely used in doctor's offices, clinics, and screening facilities. It is very safe due to the "no touch" technology, but it often produces falsely elevated readings, particularly in patients who squeeze their muscles upon anticipation of the air puff. The patient simply sits then places their chin in a rest while looking straight ahead, while the operator activates the air puff mechanism while aligning each eye individually.
- Tactile finger applanation over the closed eyelid by a skilled eye doctor is an age-old traditional method utilized by the experienced practitioner.
- Intraocular sensors for experimental or intraoperative use during surgery are utilized in research and technology development companies. Hopefully, these will become universally available for long-term use and patient self-readout.
- Patient self-testing devices are in their infancy. The ProVision device marketed by Bausch & Lomb allows the patient to gently press a calibrated spring-loaded piston against the closed eyelid over the upper, outer quadrant of the eyeball while gazing downward. The true IOP is reached when the pressure applied onto the globe produces faint lights in the eye, or phosphenes, readily perceived by the patient and thereby recorded at home.
After the eye has been numbed by the technician or doctor with anesthetic eye drops, the tonometer's sensor is placed against the surface of the eye. The firmer the tone of the surface of the eye, the higher the pressure reading. The doctor can record the pressure reading, and this can be used to diagnose or monitor the treatment of glaucoma. Also, the pressure reading is lower than otherwise expected if the cornea is thin. Thus, patients with a thin cornea may obtain a falsely low IOP reading when the actual pressure is high. These patients must be watched more closely for glaucoma.