This chapter is designed to help the Emergency Physician know when it is necessary to measure intraocular pressure (IOP) and reviews several reliable methods of tonometry to measure IOP. There are multiple traumatic, pathologic, and postsurgical causes of altered IOP. The clinical signs and symptoms of elevated IOP are similar regardless of the etiology. Digital palpation is the oldest and simplest form of tonometry and remains useful in select situations. Schiøtz indentation tonometry is discussed for historical purposes but is still an accurate method to measure IOP. The nonportable Goldmann or applanation tonometer serves as the standard for measuring accurate IOP. It requires the use of a slit lamp and can be difficult to master. The handheld Perkins and Kowa tonometers are based on the same principle as the Goldmann and require experience to use effectively. The electronic Tono-Pen is best known to most Emergency Physicians and is discussed at length. It is useful to become comfortable with one or more of these techniques because early detection of abnormal IOP can prevent irreversible vision loss.
Aqueous humor is produced by the ciliary body in the posterior chamber of the eye, directly behind the iris (Figure 156-1). Most of the aqueous humor flows forward, through the pupil, into the anterior chamber. It drains out of the eye through the trabecular meshwork located at the angle where the cornea and iris meet. This is the area referred to in open angle, narrow angle, and angle closure glaucoma. In healthy eyes, aqueous humor production is equivalent to outflow. IOP reflects the pressure of the ocular contents and by convention is expressed in millimeters of mercury or mmHg.1 The mean IOP in the general population is 16 mmHg with a standard deviation of 3 mmHg.2 Therefore, normal pressure is considered to range from 10 to 22 mmHg.
Anatomy of the anterior segment of the eye: (1) anterior segment; (2) posterior segment; (3) ciliary body; (4) trabecular meshwork; (5) cornea; and (6) iris.
Aqueous humor production and outflow can be dramatically affected by disease or injury of the eye. Even small changes in IOP over long time periods can be vision threatening. However, significant increases in pressure can cause rapid and irreversible damage to vision in just a few hours. Nontraumatic conditions that result in an elevation of IOP include primary angle-closure glaucoma and secondary angle-closure glaucoma. Traumatic conditions associated with elevated IOP include retrobulbar hemorrhage, hyphema, and traumatic iritis. Conditions associated with low IOP that threaten vision include penetrating trauma and postsurgical complications.
Patients with primary or secondary acute angle-closure glaucoma often present with ocular pain and decreased vision, usually in one eye. They may describe a headache in the brow region, with or without associated nausea and vomiting. External examination frequently reveals that the conjunctiva is erythematous, the cornea appears milky or ...