The usual range is 10 to 21 mm Hg, so this patient's 16 mm Hg in each eye sits inside it. By custom, eye pressure is written in millimeters of mercury (mm Hg). That is the same unit used for blood pressure. You may tell her the numbers are in the normal range. The doctor still reviews the whole exam before calling the eyes healthy.
Intraocular pressure (IOP) is the pressure inside the eyeball. It rests on a balance. The ciliary body makes aqueous fluid all day long. The fluid drains out through the trabecular meshwork. If drainage is slowed or blocked, the pressure climbs. A steady reading above 21 mm Hg is higher than normal and needs another look. A steady reading below 10 mm Hg is lower than normal. Pressure that stays high can injure the retinal nerve fiber layer and the optic nerve. That damage is named glaucoma. High pressure often causes no symptoms at all, which is why the number is taken at every complete eye exam. Eye pressure is not fixed through the day either. It shifts a little in healthy people, so one reading is a snapshot and not the whole story.
Carry one range into the exam room. Ten to 21 mm Hg is normal, and anything outside it gets reported to the doctor. Also report a big difference between the two eyes. Do not tell a patient that her eyes are fine or that she has glaucoma. That call belongs to the ophthalmologist, who weighs the pressure along with the optic nerve exam and the visual field. Knowing normal values and flagging numbers outside them is basic work in the COA content area of eye pressure measurement, or tonometry.
Incorrect Answers
- B. These numbers fall below the normal floor of 10 mm Hg. Such a low reading should be reported to the doctor, not described to a patient as usual.
- C. These numbers are above the normal ceiling of 21 mm Hg. They raise concern about glaucoma instead of describing a healthy eye.
- D. Pressure this high is far above normal and calls for prompt attention from the ophthalmologist. A range like this is never quoted to a patient as the usual one.
Testing Pearls
- Normal IOP runs 10 to 21 mm Hg, and the unit is always mm Hg.
- Eye pressure and blood pressure share a unit but are not directly linked.
- A thinner cornea makes applanation readings come out falsely low.
- Record the right eye (OD) first and the left eye (OS) second.
- High pressure may cause no symptoms for years, so tonometry belongs in every complete exam.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 8, Comprehensive Medical Eye Examination.
