What is the normal range for intraocular pressure?

The statistically normal IOP range is 10-21 mmHg, based on a 2 standard deviation distribution from the population mean of approximately 15.5 mmHg. However, normality alone is not sufficient to exclude glaucoma -- some patients develop glaucoma with IOP within the 'normal' range (normal-tension glaucoma), and many with IOP above 21 mmHg never develop glaucoma (ocular hypertension). IOP must be interpreted with optic nerve appearance and visual field data.

How does corneal thickness affect applanation tonometry readings?

Goldmann applanation tonometry assumes a standard corneal thickness of approximately 520 micrometers. Thicker corneas (above 570 microns) resist deformation more, requiring more force to applanate -- this gives a falsely high IOP reading. Thin corneas (below 520 microns, including post-LASIK corneas) deform more easily, giving a falsely low reading. Central corneal thickness (CCT) is measured by pachymetry and is considered when interpreting IOP, particularly in glaucoma suspects.

What is the correct endpoint when reading Goldmann applanation tonometry mires?

When the tonometer prism contacts the fluorescein-stained cornea under cobalt blue illumination, two bright green semicircular mires appear -- one above and one below center. The correct endpoint is when the inner edges of the two mires just touch, like two parentheses meeting in the middle. If they overlap, the force is too high; if they are separated, the force is too low. At the correct endpoint, the drum reading gives the IOP in mmHg.

Why should three NCT readings be taken and averaged?

A single NCT reading may be influenced by blinks, patient movement, corneal surface irregularities, or instrument misalignment. Taking three readings and averaging them reduces the effect of these random errors and provides a more reliable estimate of true IOP. If the three readings vary by more than 3-4 mmHg, the measurements may be unreliable, and Goldmann applanation tonometry should be considered for a more accurate measurement.

What factors cause a falsely elevated IOP reading?

Factors that falsely elevate IOP readings include: thick central cornea (higher resistance to applanation), lid squeezing or blepharospasm (external pressure transmitted to the globe), Valsalva maneuver or breath-holding (increases episcleral venous pressure), tight collars or neck ties (compresses jugular veins, raises episcleral venous pressure), and too much fluorescein (thick mires make it appear the endpoint is reached sooner). Documenting patient behavior during measurement helps interpret borderline readings.

Basic Tonometry Screening

Intraocular Pressure Screening

Intraocular pressure (IOP) measurement is one of the most performed procedures by the CPOA. IOP is a critical parameter in glaucoma screening, diagnosis, and management. While IOP measurement alone cannot diagnose or exclude glaucoma (the optic nerve and visual fields are equally important), accurate IOP recording is foundational to monitoring glaucoma patients and identifying those at risk. The CPOA must be skilled in at least one tonometry method and understand the factors that affect IOP accuracy.

Normal IOP

Normal IOP is generally considered to be 10-21 mmHg, with a statistical mean of approximately 15.5 mmHg in the general population. However:

Some patients develop glaucoma with normal IOP (normal-tension glaucoma)
Many patients have IOP above 21 mmHg without developing glaucoma (ocular hypertension)
IOP fluctuates diurnally -- typically highest in the morning (upon waking) and lowest at night
The target IOP for a treated glaucoma patient is set individually by the physician

Non-Contact Tonometry (NCT / Air Puff)

Non-contact tonometry uses a brief, calibrated puff of air to flatten (applanate) the central cornea. The instrument measures the time or force required for applanation. No drops are required, making it suitable for screening and high-volume settings. The CPOA commonly performs NCT as a preliminary test.

NCT Technique

Seat the patient at the instrument; adjust the chin rest
Instruct the patient to keep both eyes open and look at the target light inside the instrument
Align the instrument with the patient's corneal reflex
Capture 3 readings per eye and record the average
Do not touch the eyelids to hold them open -- this alters IOP by applying pressure

💡 Clinical Tip: NCT readings can be unreliable if the patient blinks or squeezes during the air puff, or if the alignment is off-center. Take 3 readings per eye; if there is significant variability (more than 3-4 mmHg between readings), note this and consider Goldmann applanation tonometry for more accurate assessment.

Goldmann Applanation Tonometry (GAT)

GAT is the gold standard for IOP measurement. It is performed at the slit lamp using the prism and fluorescein to directly measure the force required to flatten a defined area (3.06 mm diameter) of the corneal surface.

GAT Technique

Instill topical anesthetic (proparacaine) to minimize discomfort
Instill fluorescein (from a fluorescein strip or sodium fluorescein drops)
Switch to the cobalt blue filter on the slit lamp
Advance the tonometer prism until it makes contact with the cornea
Observe the two fluorescein semicircles through the eyepiece
Adjust the force dial until the inner edges of the two semicircles just touch ("kiss" appearance)
Read the value on the drum and multiply by 10 to get mmHg (if the drum scale reads in gram-force units) -- or read directly in mmHg depending on the scale
Record the reading; repeat for the fellow eye

The Fluorescein Mires

When properly aligned, two semicircles (mires) are visible through the prism -- one above and one below. The clinician adjusts the force drum until:

The mires are equal in height (if unequal, the prism may be off-axis or corneal astigmatism affects the appearance)
The inner edges of the two mires just touch (not overlapping, not separated)

⚠️ Common Mistake: If the mires are too thin (not enough fluorescein) they may be difficult to see; if too thick (too much fluorescein), the IOP reading will be falsely low. The mires should appear as two bright green semicircles, each with a width roughly one-tenth of their radius.

Factors Affecting IOP Accuracy

Factor	Effect on IOP Reading
Thick cornea (CCT >570 microns)	Falsely HIGH reading
Thin cornea (CCT <520 microns)	Falsely LOW reading
Blepharospasm / lid squeezing	Falsely HIGH (external pressure on globe)
Breath-holding by patient	Falsely HIGH (Valsalva increases IOP)
High corneal astigmatism (>3 D)	Inaccurate unless prism rotated to 43 degrees
Fluorescein too thick / too thin	Falsely LOW or difficult to read
Patient staring; tight collar / tie	Falsely HIGH
Post-LASIK cornea (thinned)	Falsely LOW

Tono-Pen and Rebound Tonometer

Tono-Pen: a handheld electronic applanation device; useful for patients who cannot be positioned at the slit lamp (wheelchair-bound, children, hospital patients). Readings are less precise than GAT.

Rebound tonometer (iCare): uses a small probe that bounces off the cornea; requires no drops or contact lens; particularly useful in children or uncooperative patients. Also handheld.

IOP Measurement in Glaucoma Management

In glaucoma patients, IOP is measured at every visit to monitor response to treatment. The CPOA records the reading, the time of measurement (diurnal variation is significant), and which medications the patient took before the visit. A significant rise from baseline (e.g., >3-4 mmHg) or an IOP above the physician-set target triggers physician review.

Key Takeaways

Normal IOP is 10-21 mmHg; mean is ~15.5 mmHg; diurnal variation peaks in the morning
NCT (air puff) is the standard screening method; take 3 readings and average
Goldmann applanation tonometry is the gold standard; uses fluorescein + cobalt blue; mires' inner edges should just touch
Thick cornea = falsely high IOP; thin cornea = falsely low IOP; CCT must be considered in interpretation
Lid squeezing, Valsalva, and tight clothing around the neck falsely elevate IOP
Document IOP readings with the time taken; diurnal variation is clinically significant in glaucoma

Basic Tonometry Screening | CPOA Exam