Tonometry

CHAPTER 68 Tonometry



Deepa A. Vasudevan, Grant C. Fowler


Tonometry is used to detect increased intraocular pressure, which is common in patients at risk for glaucoma. Glaucoma is actually caused by a group of conditions, all of which can lead to optic nerve damage and a loss of visual function. More than 2 million persons in the United States are estimated to have some degree of blindness caused by glaucoma. It is the third most common cause of blindness, and its incidence increases with age. Despite the fact that most blindness caused by glaucoma is preventable, or at least able to be delayed, there are estimates that less than half of patients with glaucoma have been diagnosed. Tonometry remains one of the easiest methods of screening for glaucoma; however, patients with glaucoma can have normal intraocular pressures and, vice versa, not all patients with increased intraocular pressures have glaucoma. Researchers are currently pursuing other risk factors associated with glaucomatous changes in the eye to develop additional practical screening techniques. When this succeeds, glaucoma will be more readily diagnosed, even in those with normal intraocular pressure. Until then, tonometry, in combination with funduscopic examination and visual field testing, is the most sensitive and specific method for detection of glaucomatous changes in the eye. Patients at high risk for glaucoma should be screened with all three.


There are three basic types of tonometry. Impression tonometry measures the depth of the impression produced on the ocular wall by a given force, and the Schiøtz tonometer uses this method. Noncontact/air-puff tonometry, originally considered the least accurate and therefore designed for screening (especially for children), has turned out to be fairly accurate even when compared with applanation tonometry, the “gold” standard. Applanation (Goldmann) tonometry measures the force necessary to flatten an area of the cornea. Because applanation tonometry is more accurate than Schiøtz tonometry, most optometrists and ophthalmologists use this technique; however, it requires the ability to use a slit lamp (see Chapter 67, Slit-Lamp Examination). Consequently, the Schiøtz tonometer is still the standard for measuring intraocular pressure in the offices of primary care clinicians, in urgent care centers, and in emergency departments. The Schiøtz tonometer is also less expensive (about $300). Many urgent care centers and emergency departments also now have available a hand-held portable device (e.g., Tono-Pen; Reichert, Inc., Depew, NY) that uses applanation technology, although it is somewhat more expensive (about $3000) than the Schiøtz tonometer. Regardless of the method of tonometry, the clinician may recommend other tests or a referral if the initial test result is abnormal. This chapter discusses the techniques used in impression (Schiøtz and Tono-Pen), and applanation (Goldmann) tonometry.



Indications



Screening Those with Risk Factors



• Age older than 45 years (>35 years in African Americans).

• Family history of glaucoma (fourfold to ninefold increased risk).

• African Americans (fourfold to sixfold higher risk than whites).

• Hispanics age older than 60 years.

• Asian Americans (increased risk of angle-closure glaucoma, but this causes <10% of glaucoma).

• Diabetes mellitus.

• Hypertension.

• Decreased visual acuity (myopia or hyperopia).

• Central corneal thickness less than 0.5 mm.

• Prior injury: blunt or penetrating trauma, eye surgery, eye inflammation (e.g., uveitis) can cause glaucoma acutely or years later.

• Prior or current medications: use of corticosteroids (including high-dose inhaled corticosteroids), mydriatics, phenothiazines, and sympathomimetics can precipitate glaucoma.


Diagnosis



• Ocular pain (especially if associated with signs or symptoms of angle-closure glaucoma such as acute-onset red eye, cloudy or smoky cornea, fixed mid-position pupil, blurry vision, halos around lights, nausea and vomiting, frontal headache)

NOTE: Patients with angle-closure glaucoma may complain more about nausea and vomiting than eye symptoms.


• Red eye (especially if associated with signs or symptoms of angle-closure glaucoma)

• History of visual field loss


Determining Baseline Intraocular Pressure



• Uveitis: patients with iritis or uveitis can develop both open- and closed-angle glaucoma. Corticosteroid treatment of uveitis can also cause glaucoma.

• After blunt ocular trauma: patients with hyphema often have an acute rise in intraocular pressure.

• Orbital fracture: an intraocular pressure greater than 22 mm Hg or a difference greater than 3 mm Hg between eyes is a reasonable marker for ocular injury in the patient with an orbital fracture.


Contraindications



• Eye infection (Although Tono-Pen can be used because it uses disposable tip covers, noncontact tonometry may be preferred.)

• Corneal abrasion (Although Tono-Pen can be used in another area of the cornea, tonometry is commonly deferred until a subsequent visit.)

• Recent eye trauma (Tono-Pen/Schiøtz tonometry should not be performed in patients with suspected penetrating ocular injury; pressure on the globe can result in extrusion of intraocular contents. Slit-lamp examination should be performed to exclude perforation.)

• Patients who cannot keep their eyes still or open (Schiøtz and applanation tonometry cannot be used in patients with severe eyelid swelling or those unable to open their eyes; however, the Tono-Pen can sometimes be used.)


Equipment



• Topical ophthalmic anesthetic drops, such as proparacaine hydrochloride 0.5% or tetracaine 7%

• Gloves, mask, goggles (whatever is needed to follow universal blood and body fluid precautions)

• For Schiøtz: Schiøtz tonometer kit (each kit contains the tonometer, three plunger weights, a concave test block, conversion tables, pipe cleaners, and instructions for care)

• For Tono-Pen: Tono-Pen unit, condom-style disposable tip covers, optical-grade canned air for cleaning

• For applanation: Slit lamp with Goldmann applanation device attached, sterile fluorescein strips


Preprocedure Patient Preparation


Explain the reason for measuring the intraocular pressure with the tonometer (e.g., presence of risk factors, symptoms, or positive physical findings). Briefly explain the procedure as well as the fact that this test only detects increased intraocular pressure. The patient should know that anesthetic ophthalmic drops may sting a little as they are instilled. The patient should also know that having normal intraocular pressures does not completely exclude glaucoma. If glaucoma is suspected, even with normal intraocular pressures, tonometry should be combined with the other tests mentioned previously.



Technique



Schiøtz or Tono-Pen



1 Check and record the patient’s visual acuity. Examine the eyes.Immediately after the funduscopic examination, place two drops of topical ophthalmic anesthetic in each eye. (The anesthetic will have time to take effect while you prepare the tonometer.) The clinician should follow universal blood and body fluid precautions.

2 Assemble the Schiøtz tonometer with the 5.5-g weight in place, and test for accuracy on the convex metal test block (Fig. 68-1). The Schiøtz tonometer is precalibrated by the manufacturer and must be returned for repair if it does not read “0” when resting on the test block. The Tono-Pen (Fig. 68-2) should also be calibrated before each use. Turn the transducer tip straight down and push the operation button twice within 1.5 seconds. It will beep and the liquid crystal display (LCD) will read “CAL.” Although it may take up to 15 seconds, the Tono-Pen will then beep and display “UP.” Immediately invert the tip straight upward, and if it is functioning properly, it should beep again and display “Good” in the LCD, which means it is calibrated. It is now ready for use, so the tip should be covered with a disposable cover.
image

Figure 68-1 Schiøtz tonometer.

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May 14, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Tonometry

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