6.3 Scenario Practice for Tonometry
Key Takeaways
- Patient behaviors such as breath-holding, lid squeezing, or a tight collar can falsely elevate IOP.
- Central corneal thickness (CCT) biases applanation: thick corneas overestimate, thin corneas underestimate.
- Time of day matters because IOP peaks in the early morning; record the clock time with the reading.
- Read scenarios for the artifact first, then decide whether to re-measure or switch instruments.
6.3 Scenario Practice for Tonometry
Scenario questions describe a measurement that looks wrong and ask what is happening or what to do next. Almost every one hides one of three artifacts: a patient behavior, a corneal property, or a timing factor. Train yourself to spot the artifact before choosing an answer.
Behavioral artifacts that falsely raise IOP
These are the classic exam traps because the pressure reads high even though the true IOP is normal:
- Breath-holding or Valsalva (straining, coughing) raises episcleral venous pressure and the reading.
- Squeezing the eyelids against the prism transmits orbicularis force to the globe.
- A tight collar or necktie raises venous pressure; loosen it before measuring.
- Pressing on the globe while holding the lids open instead of bracing on the orbital rim.
The correct response in these scenarios is to relieve the cause, let the patient relax, and re-measure rather than recording the inflated value.
Corneal thickness bias
Central corneal thickness (CCT), measured by pachymetry, systematically biases applanation tonometry. Average CCT is about 540-560 microns.
| Cornea | Effect on GAT reading | Why |
|---|---|---|
| Thick (e.g., >580 um) | Overestimates true IOP | More tissue resists flattening |
| Average (~540-560 um) | Reading is accurate | The 3.06 mm calibration assumes this |
| Thin (e.g., <520 um, post-LASIK) | Underestimates true IOP | Less tissue resists flattening |
A classic stem: a post-LASIK patient reads 15 mmHg but has thin corneas. The point is that the true pressure may be higher than the dial shows, so a normal-looking number can mask glaucoma risk.
Timing and diurnal variation
IOP follows a daily rhythm and is typically highest in the early morning, varying up to ~3-5 mmHg in normal eyes and more in glaucoma. A morning reading of 22 mmHg and an afternoon reading of 17 mmHg may both belong to the same patient. This is why you always document the time of day beside each value, and why physicians sometimes order diurnal curve testing with measurements at set hours.
Worked scenario
A 45-year-old reads 27 mmHg by GAT. You notice the patient is gripping the chair, holding their breath, and wearing a buttoned tight collar. The defensible action is to loosen the collar, coach normal breathing, ask the patient to relax the lids, and re-measure. If the repeat reads 16 mmHg, the first value was a behavioral artifact, not true ocular hypertension. Only after artifacts are excluded does a high reading prompt physician notification.
Reading method for any tonometry scenario
When a stem describes a measurement that seems wrong, work through a fixed five-step read: (1) name the device and confirm it suits the patient, (2) scan for a behavioral cause such as squeezing, straining, or a tight collar, (3) check for a corneal property such as thickness, scar, or edema that biases the value, (4) note the time of day against the diurnal curve, and (5) decide whether to re-measure, switch instruments, or notify the physician. This sequence keeps you from jumping to the dramatic answer (an emergency) when the simple answer (an artifact) explains the reading.
Special-population scenarios
Several patient types generate recurring scenario questions. Children rarely tolerate the slit lamp, so a Tono-Pen or air-puff is chosen, and a screaming or breath-holding child can produce a falsely high value. Nystagmus patients have constantly moving eyes; the Tono-Pen, which samples quickly and averages, is more practical than chasing the mires on a Goldmann. Post-corneal-transplant or post-refractive-surgery eyes have altered thickness and curvature, so applanation accuracy is degraded and the reading must be interpreted alongside pachymetry.
Patients on systemic steroids are 'steroid responders' at risk of rising IOP, so a previously normal reading that has climbed is clinically meaningful rather than an artifact.
A second worked scenario
A glaucoma patient returns for follow-up. The morning Goldmann reads 23 mmHg; the chart shows last month's afternoon reading was 18 mmHg. A novice flags a worsening of 5 mmHg. The experienced technician recognizes that IOP peaks in the early morning and that comparing a morning value to a prior afternoon value is not a fair comparison. The correct action is to record the time with the value and let the physician interpret the diurnal pattern, possibly ordering a diurnal curve.
The lesson the exam reinforces is that timing context can fully explain a difference that looks like deterioration, which is why every reading travels with its clock time.
When a high reading is a genuine emergency
Not every elevated value is an artifact, and the exam also tests recognizing the true crisis. Acute angle-closure glaucoma presents with a sudden painful red eye, a mid-dilated fixed pupil, blurred vision with halos around lights, nausea or vomiting, and an IOP often 40-60 mmHg or higher. This is a time-critical emergency because such pressures can damage the optic nerve within hours; the immediate goal is to lower the pressure with medications such as topical beta-blockers, alpha-agonists, pilocarpine, and systemic acetazolamide or mannitol, with definitive treatment by laser peripheral iridotomy.
The scenario skill is discrimination: a 28 mmHg reading in a comfortable patient who was holding their breath is an artifact to re-measure, whereas a 55 mmHg reading in a patient with a rock-hard painful eye and a hazy cornea is a reading to confirm quickly and escalate to the physician without delay. Knowing both ends of that spectrum keeps you from over-reacting to artifacts and under-reacting to crises.
A patient's Goldmann IOP reads 28 mmHg. The technician notices the patient is wearing a tight necktie and holding their breath during the measurement. What is the most likely explanation?
A patient who had LASIK has thin central corneas (about 500 microns) and a Goldmann reading of 15 mmHg. How does the thin cornea affect interpretation?