Title: Intraocular Pressure
Author: Gavin Gorrell, 4th Year Medical Student, University of New Mexico
What is it?
Just as the measurement of blood pressure and intracranial pressure have clinical significance, measurement of intraocular pressure (IOP) is useful in evaluating overall eye health and narrowing a differential diagnosis in presenting eye disease. The eye is filled with a clear jelly (vitreous humor) behind the lens and a continuously produced, nourishing fluid called aqueous humor that fills the space in front of the lens (the anterior chamber). Because the amount of vitreous humor is relatively static, IOP is primarily a function of aqueous humor production and drainage.
Aqueous humor is produced in the posterior chamber by the ciliary body, then it flows past the zonules supporting the lens, between the iris and lens, through the pupil into the anterior chamber, then through a trabecular meshwork into Schlemm’s canal into the episcleral veins (venous system underneath the scleral surface).
Normal IOP of 10-21mmHg is important in maintaining eye shape and perfusion of the cornea and lens (avascular structures which rely on aqueous humor for nutrients, oxygen and clearance of their metabolic waste). Low IOP can cause vision impairment but almost exclusively occurs in post-surgical eyes. Elevated IOP is important to identify as it is the only modifiable risk factor for glaucoma. “Normal” IOP is a term that has fallen out of favor due to the fact that glaucoma can occur at almost any IOP—reaffirming the recommendation that all patients 40 and over have a thorough, dilated eye exam.
When to measure?
IOP is considered one of the “eye vital signs” and should be measured in every patient with an eye complaint or in whom the clinician suspects glaucoma. The American Academy of Ophthalmology recommends all patients to have a baseline eye exam at age 40, during which exam IOP and other glaucoma risk factors are assessed.
When to postpone measurement?
Checking IOP should be avoided in cases of ocular trauma where there is a concern for ruptured globe or potential cornea perforation (thin cornea, recent corneal surgery, large corneal ulcer). In cases of hyphema or retrobular hemorrhage, the IOP is an important diagnostic clue but needs to be done with extra caution and should be deferred to an eye specialist.\
How is it measured?
Measurement of IOP is based off the Imbert-Fick principle which basically states the pressure inside a thin walled sphere can be determined by the force required to flatten part of the sphere; P =F/A, where P = pressure, F = force, A = Area. A simple analogy is how hard your thumb must press on a basketball to make a dent when its deflated vs inflated.
There are several tools that can be used to measure IOP. The gold standard is the Goldmann applanation which requires a slit lamp and a cooperative, mobile patient. When Goldmann applanation isn’t feasible, a quick and convenient way to check IOP is to use a portable hand-held device such as the Tono-Pen® or Icare tonometer.
- “Icare Tonometer – Portable, Handheld IOP Measurement.” Icare Usa. Accessed June 25, 2017. http://icare-usa.com/.
- “IOP and Tonometry – EyeWiki.” Accessed June 25, 2017. http://eyewiki.aao.org/IOP_and_Tonometry.
- “Ocular Hypertension: Background, Pathophysiology, Epidemiology,” May 25, 2017. http://emedicine.medscape.com/article/1207470-overview#a2.
- “Tono-Pen AVIA® Applanation Tonometer.” Accessed June 25, 2017. http://www.reichert.com/product_details.cfm?pcId=474&skuId=2980&skuTk=1037022486#.WU_rM-vyuM8.