6.4 Common Traps in Tonometry

6.4 Common Traps in Tonometry

Tonometry traps on the COA exam cluster around safety and measurement error. The wrong answer is usually the one that ignores infection control or accepts a reading distorted by technique.

Infection-control traps

Contact tonometers touch the cornea and tear film, so they can transmit disease between patients. Key rules:

• Do not perform applanation on an active corneal infection, abrasion, or ulcer. Direct contact can drive pathogens into the epithelium and worsen the defect. If pressure is clinically needed, use a non-contact tonometer or measure only on intact tissue with a sterile/disposable tip.
• Disinfect the prism between every patient. Common protocols include wiping then soaking in dilute sodium hypochlorite (bleach) or 3% hydrogen peroxide, or using single-use disposable tips. Alcohol must be fully dried before the next use because residue is toxic to the epithelium.
• Epidemic keratoconjunctivitis (adenovirus) is highly contagious and survives on surfaces; herpes simplex and (theoretically) prion disease are the reasons disposable tips are favored.

Fluorescein and dye-dosing traps

The fix is to instill a controlled amount, blot excess, and re-read.

Calibration and technique traps

• An uncalibrated prism introduces systematic error; GAT tips should be checked at 0, 2, and 6 (20 and 60 mmHg).
• Astigmatism over 3 diopters distorts the circular applanation; rotate the prism so the red marker aligns with the flat (minus-cylinder) axis, or average horizontal and vertical readings.
• Repeated applanation can transiently lower IOP by massaging aqueous out; take the first clean reading rather than averaging many touches.
• Pressing on the globe to hold the lids open raises the reading; brace fingers on the orbital rim instead.

Decision checklist for a suspicious reading

• Is the cornea intact and infection-free? If not, switch device or defer.
• Is the tip clean, calibrated, and (ideally) disposable?
• Is the fluorescein dose correct and the mire endpoint exact?
• Are behavior, collar, and breathing controlled?
• Is corneal thickness and time of day documented for interpretation?

The most defensible answer protects the cornea, prevents cross-infection, and produces a reading you can trust.

Why disinfection protocol questions matter

Tonometer tips are a documented route for transmitting epidemic keratoconjunctivitis (adenovirus), herpes simplex, and theoretically prion disease, because the prism contacts tear film and epithelium on consecutive patients. The exam expects you to know that wiping alone is insufficient and that an approved protocol is required: a 5-10 minute soak in dilute sodium hypochlorite (1:10 household bleach), 3% hydrogen peroxide, or 70% isopropyl alcohol, followed by thorough rinsing and drying. Disposable single-use prisms or tip covers eliminate the reprocessing question entirely and are increasingly standard.

The recurring trap is an answer that skips disinfection or that re-uses a tip with only a dry wipe between patients; that answer is always wrong even when it is faster.

Anesthetic and dye pitfalls

Applanation requires a topical anesthetic, and the standard is proparacaine 0.5% combined with fluorescein, marketed as Fluress; it numbs the cornea in about 15 seconds and supplies the dye in one drop. Two pitfalls follow. First, anesthetic must be confirmed before contact, because touching a non-anesthetized cornea causes a blink-and-squeeze reflex that both hurts the patient and inflates the reading. Second, residual disinfectant alcohol on the prism is itself an epithelial toxin and must be fully dried, so the trap answer that uses an alcohol-wet tip is unsafe.

Fluorescein dosing is the other classic error already shown in the table: control the drop, blot the tear lake, and re-read rather than accepting a value taken through a flooded or bone-dry tear film.

Putting the safety reflex first

The single most testable habit in this section is refusing contact tonometry on an eye with active infection, an abrasion, an ulcer, or a deep epithelial defect. In those cases the correct path is a non-contact device, a Tono-Pen with a fresh disposable cover on intact tissue, or deferral until the physician decides. A familiar device name in an answer choice is never enough; the choice must also be safe for this cornea, clean for the next patient, and read with correct dye and force. Treat infection control and corneal integrity as veto conditions that override convenience every time.

Calibration error in depth

A tonometer that is out of calibration produces a confident but wrong number, and the exam treats calibration as a hidden trap. The Goldmann prism is checked against a calibration bar at the 0, 2, and 6 positions, corresponding to 0, 20, and 60 mmHg; if the dial does not match at these points the device is serviced rather than used. A subtle clinical version of this trap is corneal astigmatism greater than three diopters, which distorts the circular applanation into an ellipse and skews the reading.

The correction is to rotate the prism so the red graduation marker aligns with the flat (least-curved) corneal axis, or to take readings in the horizontal and vertical meridians and average them. The takeaway is that a number can be wrong for instrument reasons as well as patient reasons, so a complete answer accounts for both. When a stem mentions a poorly maintained instrument or a steeply astigmatic cornea, the calibration or axis-alignment answer is usually correct over one that simply records the value.