Chapter 188: Intraocular Pressure Measurement (Tonometry)

INTRODUCTION

This chapter is designed to assist the Emergency Physician in the decision of when it is necessary to measure intraocular pressure (IOP) and will review several reliable methods of tonometry. There are multiple traumatic, pathologic, and postsurgical causes of changes in 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 and is 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 hand-held 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. The literature describes the measurement of anterior chamber depth with bedside ultrasound to measure IOP. This may be useful in patients with significant facial trauma and are unable to open their eyes. The Emergency Physician should be comfortable with one or more of these techniques. The early detection of abnormal IOP can prevent irreversible vision loss.

ANATOMY AND PATHOPHYSIOLOGY

Aqueous humor is produced by the ciliary body in the posterior chamber of the eye directly behind the iris (Figure 188-1). Most of the aqueous humor flows forward through the pupil and 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. Aqueous humor production is equivalent to outflow in healthy eyes. 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 Normal pressure ranges from 10 to 22 mmHg.

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. Significant increases in the IOP 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 ...