Pachymetry (Corneal Thickness Measurement)

Pachymetry is the measurement of corneal thickness (from the Greek "pachys" meaning thick). This quick, painless test is essential for accurate glaucoma assessment and determining candidacy for refractive surgery like LASIK. Corneal thickness affects eye pressure measurements and provides important information about corneal health.

Why Corneal Thickness Matters

Glaucoma Assessment

The Problem:

• Intraocular pressure (IOP) is measured through the cornea
• Standard tonometry assumes average corneal thickness
• Thicker corneas give falsely HIGH IOP readings
• Thinner corneas give falsely LOW IOP readings

The Solution:

• Measuring corneal thickness allows IOP interpretation in context
• Thin corneas are an independent risk factor for glaucoma progression
• Helps determine true glaucoma risk

Refractive Surgery Planning

For LASIK/PRK:

• Laser removes corneal tissue to reshape it
• Must leave enough residual thickness (typically >250 microns)
• Thinner corneas may not be safe candidates
• Determines amount of correction safely possible

Corneal Disease

Indicates Corneal Health:

• Thin cornea may indicate keratoconus
• Thick/swollen cornea may indicate corneal edema
• Helps monitor conditions affecting corneal health

How Pachymetry Is Performed

Ultrasound Pachymetry (Contact)

The traditional and most common method:

Procedure:

1. Numbing drops are placed in the eye
2. A small ultrasound probe gently touches the center of the cornea
3. Sound waves measure the corneal thickness
4. Multiple readings are taken and averaged
5. Takes only a few seconds per eye

What to Expect:

• No pain (cornea is numbed)
• Brief touch sensation
• Very quick (less than a minute)

Optical Pachymetry (Non-Contact)

Methods Include:

• Optical coherence tomography (OCT)
• Scheimpflug imaging (Pentacam)
• Specular microscopy

Advantages:

• No contact with eye
• Can map thickness across entire cornea
• Often done as part of other imaging

Normal Values

Average Central Corneal Thickness

Category	Thickness (microns)
Below average	< 520 µm
Average	520-560 µm
Above average	> 560 µm

Important Notes

• There's a normal range—not everyone is "average"
• Varies by ethnicity (African Americans tend to have thinner corneas)
• Generally stable throughout adulthood
• May change with certain conditions or surgeries

Effect on IOP Readings

The Correction Factor

While there's no universally accepted formula, general guidelines:

Corneal Thickness	Effect on IOP
545 µm (average)	Measured IOP accurate
Each 25 µm thinner	True IOP ~1 mmHg higher
Each 25 µm thicker	True IOP ~1 mmHg lower

Example:

• Measured IOP: 22 mmHg
• Corneal thickness: 495 µm (50 µm thinner than average)
• True IOP may be approximately 24 mmHg

Clinical Applications

Glaucoma Suspect Evaluation

When IOP is borderline elevated:

• Thick corneas may explain "high" IOP readings
• May mean less aggressive treatment needed
• Thin corneas suggest higher actual risk

Ocular Hypertension

Patients with elevated IOP but no glaucoma damage:

• Pachymetry helps stratify risk
• Thin corneas = higher risk of developing glaucoma
• Influences treatment decisions

Refractive Surgery Screening

LASIK Criteria:

• Generally need >500 µm for standard LASIK
• Must leave >250 µm after ablation
• Higher prescriptions need more tissue
• Thin corneas may need surface ablation (PRK) instead

Keratoconus Screening

Keratoconus causes progressive corneal thinning:

• Pachymetry part of screening protocol
• Thinnest point location is important
• Serial measurements can monitor progression

Corneal Edema Monitoring

In conditions causing corneal swelling:

• Fuchs' dystrophy
• Post-surgical edema
• Contact lens-related edema
• Pachymetry tracks swelling

What Affects Corneal Thickness

Factors That Thin the Cornea

• Keratoconus
• Refractive surgery history (LASIK, PRK)
• Some medications
• Contact lens wear (temporarily)
• Dehydration

Factors That Thicken the Cornea

• Corneal edema (various causes)
• Morning measurement (cornea swells overnight)
• High altitude
• Fuchs' dystrophy (early stages—edema)

When Pachymetry Is Ordered

Common Indications

• Glaucoma evaluation
• Ocular hypertension assessment
• Refractive surgery consultation
• Keratoconus screening
• Corneal dystrophy evaluation
• Monitoring corneal conditions

Frequency

• Usually done once (measurement doesn't change much)
• May be repeated if:
  • Monitoring keratoconus
  • Following corneal edema
  • Unexplained change in IOP readings

Frequently Asked Questions

Does pachymetry hurt?

No. Numbing drops are used for the contact (ultrasound) method, so you feel only light pressure. Optical methods don't touch the eye at all. The test is quick and painless.

Why do I only need this test once?

Corneal thickness is largely stable throughout adulthood. Unless you have a condition affecting corneal thickness (like keratoconus or edema), the measurement won't change significantly.

My cornea is thin—does that mean I have glaucoma?

Not necessarily. Thin corneas are a risk factor for glaucoma and can make IOP readings appear lower than they actually are. Your doctor considers pachymetry along with other factors (optic nerve appearance, visual field, etc.) to assess your glaucoma risk.

Can I still have LASIK with thin corneas?

It depends. Thin corneas don't automatically disqualify you, but they limit how much correction is safe. Your surgeon will calculate whether adequate tissue remains after the procedure. Surface procedures (PRK) may be an alternative.

My IOP was high but my corneas are thick—should I be worried?

Thick corneas cause IOP to measure higher than it actually is. Your doctor will interpret your IOP in light of your corneal thickness. You may have lower true IOP than the measurement suggests, which is reassuring.

Is corneal thickness related to overall eye health?

Corneal thickness is one piece of information about your eyes. Abnormally thin corneas may indicate conditions like keratoconus. Abnormally thick corneas may indicate edema. Normal thickness doesn't guarantee eye health, and abnormal thickness doesn't mean disease—it's interpreted in context.

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