Set the dial between 1 and 2 before the tip meets the eye. On this instrument the dial number is multiplied by 10 to give the pressure. So 1 to 2 on the dial stands for 10 to 20 mm Hg, which is where most eyes fall. Starting there means you only have to nudge the knob a little once the tip is on the cornea.
The Goldmann tonometer works by applanation, which means flattening. Turning the knob raises the force on the prism tip. Once the tip touches, the dye under it splits into two semicircles. You turn the knob until their inner edges just touch. If the semicircles stay apart, the reading is too low. If they overlap, the reading is too high. A dial parked far from the true pressure forces a long turn while the tip sits on the cornea. That means more contact time, more discomfort, and more chance of a corneal abrasion. Have the patient blink once to spread the dye, then hold the blink. Keep your fingers on the bony orbital rim, never on the globe. Pressing on the globe pushes the reading up and wastes the whole measurement.
Build the habit of a pre-set dial. Park it near the normal range, touch the cornea, make one small adjustment, then read and pull away. Note the dial number, multiply by 10, and chart the right eye first. If you need a second try, let the eye rest a moment and check that enough dye is still on the surface. Fast, gentle technique with an accurate endpoint is the heart of the COA content area of eye pressure measurement, or tonometry.
Incorrect Answers
- A. Zero on the dial stands for 0 mm Hg. From there you would have to crank the knob a long way with the tip already resting on the cornea.
- C. This setting stands for 40 to 50 mm Hg. Pressing that hard on the eye at first contact is uncomfortable and needless in a routine exam.
- D. The top of the scale applies maximum force to the cornea as soon as the tip lands. It risks injury and tells you nothing useful about an eye in the normal range.
Testing Pearls
- Pre-set the Goldmann dial between 1 and 2, which equals 10 to 20 mm Hg.
- Dial reading times 10 equals the pressure in mm Hg.
- Semicircles apart reads too low; semicircles overlapping reads too high.
- Hold a droopy lid against the orbital rim, never against the globe.
- Have the patient blink once to spread the dye, then keep the eye open.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 8, Comprehensive Medical Eye Examination.
