Introduction
Corneal pachymetry is an imaging technique used to measure the thickness of the cornea. It is an essential test for glaucoma and a component of a comprehensive eye exam. Glaucoma is an eye disease in which the intraocular pressure (IOP) or internal eye pressure rises above the average level. It is an insidious disease that develops slowly over time. Often in the early stages, it does not show symptoms. Increased intraocular pressure is the first evidence of the existence of the disease. However, in some rare cases, a patient may have glaucoma even with normal IOP.
Glaucoma can damage the optic nerve and the visual canal to the brain leading to decreased vision or even blindness. It is among the leading causes of blindness in the world. The corneal measurements set a baseline for other glaucoma tests and help to assess the health of the patient’s eyes. The examination is suitable for patients of all ages, including the elderly and children.
Also Known As
- Pachymetry
Types
There are different techniques of pachymetry, including:
Optical pachymetry:
It creates a digital map of the corneal pachymetry. Optical coherence tomography pachymetry and Scheimpflug imaging are the most commonly used methods. Scheimpflug imaging is an excellent technique for producing multiple peripheral measurements of CCT and corneal thickness quickly.
Ultrasound pachymetry:
It can provide a map that indicates corneal thickness at every point allowing the doctor to detect the location and spot with the minimum value of the corneal thickness. There are ultrasound pachymetry that are designed for glaucoma testing and have built-in risk factor indicators. The method uses machines that are cost-effective and portable.
Purpose
A pachymetry test can be conducted at the eye clinic or the doctor’s office by:
- A technician
- An optometrist
- An ophthalmologist
Pachymetry is an essential tool in diagnosing glaucoma. Details of corneal thickness help the doctor to define the reliability of the eye’s internal pressure measurement. A thin cornea can lead to an inaccurately low reading of the intraocular pressure while a thick one can show a wrong high reading. The correct readings are essential in creating a baseline guide for future assessments in patients of ocular hypertension or glaucoma. The central corneal thickness (CCT) can serve as an independent indicator of glaucoma. Studies show that CCT is an independent risk factor for glaucoma among patients with high IOP.
Pachymetry is also useful for examining some pathologies of the cornea, like keratoconus, corneal dystrophy, corneal edema, etc. It can tell doctors whether the cornea is swollen. There are medical disorders, like Fuchs’s Dystrophy, that can lead to a build-up of fluid in the cornea increasing its thickness. At times, wearing contact lenses may cause significant swelling of the cornea. It may be difficult for the doctor to observe such abnormalities under the microscope. A pachymetry can show an increase in corneal thickness.
Corneal pachymetry is also essential in surgical procedures, including:
- Corneal cross-linking
- LASIK for correcting a refractive error or vision correction
- LASIK surgery to prevent bulging of the thin cornea or ectasia
- Corneal surgeries, such as corneal transplant and Limbal relaxing incisions for astigmatism
Preparation & Expectation
Patients with glaucoma or a family history of the disease should inform the doctor before the exam. Those who wear contact lenses or glasses will have to remove them before the test. The patient may experience increased sensitivity to light after the test. The patient might be able to drive 30 minutes after the test; however, they should come with someone or arrange for transportation home. The patient should not experience any pain or discomfort during the test.
Procedure
Ultrasound pachymetry requires the use of topical anesthetic eye drops. The patient will sit on a chair or stool and the doctor will numb the patient’s eyes by applying the medication. After the drug takes effect, the doctor will rest a probe on the patient’s corneal surface to assess the corneal thickness. He/she may repeat the measurement at various points on the surface of the cornea. The test will take about five to ten minutes for both eyes.
For optical pachymetry, numbing drops are not necessary. The patient will sit facing the scanning device and place their head on a support to keep it steady. The patient will look at a small light source, and a microscopic viewer will determine the corneal thickness without any contact. The test also lasts between five to ten minutes.
Outcome
If the results show a less than average corneal thickness, the doctor may recommend an additional glaucoma test. Where the cornea gets thinner over time, it may show abnormal glaucoma test results. A patient whose corneal thickness is less than 555 microns is three times more likely to develop glaucoma than one with a depth of 588 microns. A combination of high intraocular pressure and a cornea that is less than 550 microns in depth is a red flag. It may form the basis for an anti-glaucomatous treatment to address the possible development of glaucoma soon.
The pachymetry results are also of critical significance in eye surgery. In refractive surgery with an excimer laser, the results of the test help the eye surgeon to determine whether the patient is eligible for laser vision correction. The procedure will make the cornea thinner because it involves removing some tissue. The surgeon needs to establish in precise terms how much will remain. It is a critical parameter for all refractive surgeons.
Operating on a patient with a thin cornea may have disastrous consequences such as loss of vision. The pachymetry results combined with the extent of the optical error to be corrected also help to determine the refractive technique the surgeon will follow during the operation. Where the patient’s corneal thickness is insufficient, the specialist may opt for conventional LASIK and Z-LASIK. Furthermore, the pachymetry results are essential in the corneal flap thickness or thick flap decision.
Risks & Complications
Corneal pachymetry has no risks or complications. However, the patient should not rub their eyes until the medication has completely worn off. It can cause a scratch in the cornea leading to an eye infection.
