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What Is a Pachymeter?

A corneal pachymeter measures the thickness of the cornea, the clear dome on the front of the eye.
Diseases that affect the cornea may result in expansion of the corneal thickness.
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  • Written By: Toni Henthorn
  • Edited By: W. Everett
  • Last Modified Date: 26 September 2014
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A corneal pachymeter is a scientific instrument used for the measurement of the thickness of the cornea, the transparent window-like tissue on the front of the eye. This process, which is called pachymetry, utilizes laser, light waves, or ultrasound to determine the distance in micrometers between the front and back surfaces of the cornea. Once the device measures the corneal thickness, the pachymeter displays either the number of micrometers or a corneal waveform (CWF) to the investigator. Ophthalmologists use pachymetry measurements to detect, assess, and track a variety of ocular conditions. Furthermore, refractive surgeons routinely use a pachymeter in the preoperative evaluation of Laser-Assisted In Situ Keratomileusis (LASIK) patients and the implementation of Limbal Relaxing Incisions (LRI).

Measurement of the corneal thickness with a pachymeter provides critical information in the management of glaucoma, a degenerative optic nerve disease associated with high eye pressures. Studies show that traditional methods of eye pressure detection will underestimate the eye pressure in patients with abnormally thin corneas. Patients with thin corneas and glaucoma may be at increased risk for glaucoma damage, due to the underestimation of eye pressure that occurs. On the other hand, assessment of patients with thicker-than-normal corneas may yield high eye pressure readings, even though the actual eye pressures may be normal. Several methods exist to adjust the measured eye pressure to account for inaccuracies due to variations in corneal thickness.

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Diseases that affect the cornea may result in distension and expansion of the corneal thickness. For example, Fuch’s corneal dystrophy is a progressive disease that destroys the endothelial cells that line the inner cornea and generally remove fluid from the cornea. Without normal function of the endothelial cells, the cornea eventually becomes opaque, with pachymetry readings exceeding 600 micrometers. An ophthalmologist will use a pachymeter to follow the corneal thickness and evaluate the effectiveness of his treatments.

LASIK is a procedure that alters the shape of the cornea to change the way that the eye focuses light. Ultraviolet laser light planes the cornea, flattening areas that are too steep. When a patient undergoes LASIK, the laser thins the cornea a fixed amount for every diopter of power that it corrects. Ophthalmologists use a pachymeter preoperatively to determine whether a patient has enough corneal material to allow a full treatment without making the cornea too thin. Corneal ectasia, or bulging, can occur postoperatively if the cornea becomes too thin, producing an irregular corneal surface and distortion in the vision.

In cases of astigmatism, ophthalmologists may cut the cornea in the steep zones to allow the profile to flatten in that area. These incisions are called limbal relaxing incisions. The surgeon will measure the peripheral corneal thickness using a pachymeter before setting the depth of an adjustable diamond blade, which he uses to create the incisions. Ideally, the incision depth should be about 90 percent of the total thickness in the area of the incision. Pachymetry helps the surgeon avoid a perforation through the entire cornea.

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