A pachymetry test is a diagnostic tool used to measure the thickness of the cornea, the transparent layer at the front of the eye. The cornea functions as a protective barrier and helps focus light as it enters the eye. This quick and painless measurement provides information about the eye’s structure.
Purpose of Measuring Corneal Thickness
An accurate measurement of corneal thickness is a factor in diagnosing and managing certain eye conditions, primarily glaucoma. For glaucoma patients, professionals monitor the pressure inside the eye, known as intraocular pressure (IOP).
A thicker-than-average cornea can lead to a falsely high IOP reading, while a thinner cornea may result in a deceptively low pressure reading. Knowing the corneal thickness allows an ophthalmologist to correctly interpret the IOP measurement, which guides treatment decisions.
This calibration helps determine whether a patient truly has high eye pressure requiring intervention or if the initial reading was simply an artifact of their corneal structure.
Corneal thickness is also measured as part of the evaluation for refractive eye surgeries, such as LASIK. These procedures correct vision by reshaping the cornea. To ensure the surgery is safe, the cornea must be thick enough for the removal of tissue without compromising its structural integrity. A pachymetry test confirms if a patient has sufficient corneal tissue for the procedure.
The Pachymetry Procedure
The pachymetry test is a quick process, taking less than a minute for both eyes. When using an ultrasound-based method, the eye doctor will first administer anesthetic eye drops to ensure the patient feels no discomfort.
There are two common methods used to perform pachymetry: ultrasound and optical. In ultrasonic pachymetry, a small, sterilized, handheld probe is gently and briefly touched to the surface of the central cornea. This probe emits high-frequency sound waves that travel through the cornea, and the device calculates the thickness based on the time it takes for the waves to bounce back.
The alternative method, optical pachymetry, is a non-contact technique. For this test, the patient sits and places their chin and forehead against a specialized machine. This equipment, often using technology like optical coherence tomography (OCT), takes images of the eye and determines corneal thickness from these pictures without anything touching the eye itself.
Understanding Pachymetry Results
The results of a pachymetry test are recorded in units called microns, which are micrometers. The average central corneal thickness for an adult is in the range of 530 to 570 microns. This measurement can vary among individuals based on factors like age and genetics. The thickness is not uniform across the entire cornea; it is thinnest in the center and becomes thicker towards the edges.
This measurement is not just a number but a piece of data that informs clinical decisions. For a patient being monitored for glaucoma, a thin cornea might prompt a doctor to be more proactive in treating an eye pressure that appears to be in the normal range, as the true IOP may be higher than what was measured. A patient with a thick cornea and a high IOP reading might be monitored without immediate treatment, as their actual eye pressure may be normal.
In the context of refractive surgery, the pachymetry reading is directly related to candidacy. Surgeons have specific thresholds for how much corneal tissue must remain after a procedure like LASIK to ensure long-term stability. If a patient’s cornea is too thin before surgery, they may not be a suitable candidate for the procedure, and the doctor might recommend alternative vision correction options.