What is the Pachymetry Test for Eyes?

Best corrected visual acuity. Ryan Etter

Definition: Pachymetry is a simple, painless test that quickly measures the thickness of the cornea. 

Why measure corneal thickness?

Corneal thickness as measured by pachymetry is important in the eye care field for several reasons:

  • Pachymetry can tell doctors if the cornea is swollen. Medical conditions such as Fuch's Dystrophy can increase fluid in the cornea and cause an increase in overall thickness. Even wearing contact lenses can sometimes cause significant corneal swelling. This may be difficult to see under the microscope. However, pachymetry will show a definite increase in thickness.
  • Corneal thickness is extremely important in refractive surgical procedures such as LASIK. Knowledge of corneal thickness is important to determine if a person is a candidate for laser vision correction. Because part of the procedure includes removing tissue which will leave the corneal thinner, it is important to know exactly how much will remain. Some people may have a cornea that is just much thinner than normal. It does not cause problems or disease, but it could spell tragic vision loss if a refractive procedure is performed on someone whose cornea is extremely thin.
  • Pachymetry has also become important in glaucoma care. Glaucoma is a disease in which eye pressure (intraocular pressure)can be elevated. Elevated eye pressure can cause nerve fiber loss in the retina which may result in blindness or decreased vision. Most methods involve a method of measuring eye pressure in which the instrument touches the cornea. Researchers discovered that corneal thickness can vary slightly in the population. Corneal thickness can influence the actual reading of the amount of pressure in the eye. Furthermore, identification of central corneal thickness as an independent indicator of glaucoma risk by the Ocular Hypertensive Treatment Study (OHTS) has made corneal pachymetry an important part of glaucoma testing.

    How is pachymetry performed?

    Pachymetry can be performed by two methods, by ultrasound techniques or by optical techniques. 

    • Ultrasound Pachymetry - Ultrasound pachymetry as the name implies, uses ultrasound principles to measure the thickness of the cornea. This method uses devices that are cost effective and portable. The biggest drawback to measuring corneal thickness by ultrasound is that the probe used to touch the cornea has to be position perfectly. Any slight displacement and the reading may not be accurate. Some ultrasound pachymeters are designed more for glaucoma testing and include built-in risk factor calculators.
    • Optical Pachymetry - Optical pachymeters vary on design. Some optical pachymeters are designed to be mounted onto a biomicroscope that eye doctors use called the slit lamp. Other devices can measure pachymetry using specular microscopy. This device does not come into direct contact with the cornea. One type of optical pachymetry that has gained in popularity is OCT or optical coherence tomography pachymetry. OCT pachymetry also does not touch the cornea to achieve the measurements. OCT uses a principle called optical interferometry. Another optical based method using something called Scheimpglug imaging is very good at obtaining multiple peripheral measurements of corneal thickness in addition to central corneal thickness and obtain these measurements very quickly. The one drawback of optical pachymetry is that insurance companies traditionally pay for ultrasound pachymetry and not always for optical pachymetry so it is possible that your doctor may ask you for payment.

    Continue Reading