COA Tonometry and Glaucoma 2 — Questions and Answers

Question 1: Goldmann applanation tonometry (GAT) measures intraocular pressure by determining the force required to flatten a corneal area of what specific diameter?

2.0 mm
3.06 mm (Correct answer)
4.5 mm
6.0 mm

Correct answer: 3.06 mm

The Goldmann tonometer prism flattens a circular corneal area of 3.06 mm diameter. At this diameter, the forces of surface tension and corneal rigidity cancel each other out, providing an accurate IOP reading.

The Goldmann principle is based on the Imbert-Fick law: for a dry, thin, spherical surface, the pressure equals the force divided by the area. The 3.06 mm applanation diameter is chosen because at this size, the capillary attraction of the tear meniscus and the corneal rigidity forces cancel out, simplifying the calculation. The dial reading in grams × 10 gives IOP in mmHg. Accurate GAT requires fluorescein staining, cobalt blue slit lamp illumination, proper prism alignment, and correct dial zeroing.

Question 2: A patient's IOP is measured by GAT as 14 mmHg OD and 15 mmHg OS. The COA notes the corneal pachymetry is 490 microns OU. How should the COA interpret this?

IOP is normal; thin corneas do not affect GAT readings
IOP may be falsely low; thin corneas tend to underestimate true IOP with GAT (Correct answer)
IOP may be falsely high; thin corneas overestimate true IOP with GAT
Pachymetry has no relationship to IOP measurement accuracy

Correct answer: IOP may be falsely low; thin corneas tend to underestimate true IOP with GAT

GAT assumes a standard central corneal thickness (CCT) of ~550 microns. Corneas thinner than 550 microns are more easily applanated, requiring less force, causing GAT to underestimate true IOP. A CCT of 490 microns is significantly thin.

The Goldmann applanation tonometer was calibrated for a standard CCT of approximately 520–550 microns. Thin corneas (<500 microns) offer less resistance to applanation, causing the dial to read lower than actual IOP — a falsely low reading. Thick corneas (>600 microns) give falsely high readings. This is clinically significant: a thin-cornea patient with 'normal' GAT pressure of 14 mmHg may actually have elevated IOP. Pachymetry is recommended before interpreting GAT results, especially in glaucoma suspects. Correction tables or formulas exist, though no single correction algorithm is universally accepted.

Question 3: Which type of glaucoma is characterized by a narrow or closed iridocorneal angle that obstructs aqueous outflow?

Primary open-angle glaucoma (POAG)
Normal tension glaucoma (NTG)
Angle-closure glaucoma (ACG) (Correct answer)
Congenital glaucoma

Correct answer: Angle-closure glaucoma (ACG)

Angle-closure glaucoma results from apposition or adhesion of the peripheral iris against the trabecular meshwork, blocking aqueous humor outflow through the iridocorneal angle and causing IOP elevation.

The iridocorneal (drainage) angle houses the trabecular meshwork, through which aqueous humor exits the eye. In angle-closure glaucoma, the peripheral iris physically blocks the trabecular meshwork — either intermittently (subacute) or completely (acute). Risk factors include small eyes (hyperopia), shallow anterior chambers, thick lenses, and plateau iris. Acute angle closure is an emergency causing rapid IOP spikes, pain, halos, nausea, and corneal edema. POAG features an open angle with reduced outflow despite no structural obstruction. Normal tension glaucoma has optic nerve damage with IOP in the statistically normal range.

Question 4: What is the recommended first step before performing Goldmann applanation tonometry?

Dilate the pupil with tropicamide
Instill a topical anesthetic and fluorescein (Correct answer)
Check that the slit lamp is set to white light
Ask the patient to remove contact lenses only if soft lenses are worn

Correct answer: Instill a topical anesthetic and fluorescein

GAT requires topical anesthesia for patient comfort and fluorescein sodium to create the characteristic tear film meniscus rings that enable proper mire alignment. Both are instilled before the procedure.

The correct GAT preparation sequence is: (1) instill one drop of topical anesthetic (e.g., proparacaine or tetracaine) for patient comfort; (2) instill sodium fluorescein or use a fluorescein-impregnated strip moistened with saline. The cobalt blue filter on the slit lamp illuminates the fluorescein mires (two semicircular green arcs). Contact lenses — both soft and rigid — must be removed before GAT to avoid fluorescein absorption and lens damage. Pupil dilation is not needed for tonometry. White light is replaced with the cobalt blue filter.

Question 5: The optic nerve cup-to-disc ratio (CDR) is evaluated during ophthalmic assessment. Which CDR value is MOST suggestive of glaucomatous optic nerve change?

0.2 OU
0.3 OD, 0.3 OS
0.8 OD with inferior rim thinning (Correct answer)
0.5 OU with symmetrical appearance

Correct answer: 0.8 OD with inferior rim thinning

A CDR of 0.8 is large and, combined with inferior rim thinning (loss of the ISNT rule pattern), is highly suspicious for glaucomatous optic neuropathy. Both the ratio and the rim quality must be assessed.

The cup-to-disc ratio describes the proportion of the optic disc occupied by the central cup. Most normal eyes have CDR ≤ 0.5. A CDR ≥ 0.6–0.7 is suspicious; ≥ 0.8 with asymmetry or rim thinning is highly concerning. The ISNT rule states that the rim is normally thickest Inferiorly > Superiorly > Nasally > Temporally. Violation of the ISNT rule (especially inferior notching) is a key glaucoma sign. Symmetric small cups (0.2–0.3 OU) are generally normal. A single CDR value without context (rim quality, visual field, IOP trend) is insufficient for diagnosis.

Question 6: Non-contact tonometry (NCT or 'air puff') measures IOP by analyzing what?

The light reflected from the fluorescein mires
The time or force required for an air pulse to applanate the cornea (Correct answer)
The change in pupil diameter after air contact
The depth of the anterior chamber under pneumatic pressure

Correct answer: The time or force required for an air pulse to applanate the cornea

The NCT instrument delivers a rapid air puff that deforms the cornea. An infrared detection system measures either the time required to flatten a standard corneal area or the force of the puff at the moment of applanation, which correlates with IOP.

Non-contact tonometry uses a collimated air pulse directed at the corneal apex. As IOP rises, more force is required to achieve the same degree of corneal applanation. The instrument's optical detection system monitors corneal reflection intensity, detecting the moment of maximum applanation. Older NCT models measure elapsed time from air pulse onset to applanation; newer models measure air pressure at applanation. NCT does not require anesthetic or fluorescein and has no corneal contact, reducing infection risk. It is less accurate than GAT, particularly at extreme IOP values, but is widely used for screening.

COA Tonometry and Glaucoma 2

Goldmann applanation tonometry (GAT) measures intraocular pressure by determining the force required to flatten a corneal area of what specific diameter?