Pachymetry is the medical term for the measurement of the corneal thickness.
Hence, pachymetry involves the measurement of the corneal thickness. It is a quick test that does not require ambiguous procedures and does not cause any pain or discomfort.
Pachymetry measurement will enable your doctor to understand properly your intraocular pressure readings in order to develop a treatment plan that is accurate for your specific condition.
Central corneal thickness is seen as an indicator of glaucoma risk by the ocular hypertension treatment study (OHTS), and this has made pachymetry an important aspect of glaucoma testing.
Corneal experts use accurate results from pachymetry measurements to manage the health conditions of the cornea like corneal ectasias like keratoconus and keratoglobus as well as other causes of corneal edema.
Pachymetry has also formed a very important part of glaucoma care and management of ocular hypertension.
Where is the cornea located in the eye?
The cornea is the clear part of the eye that allows light to enter the eye for a clear vision of images and objects. The cornea lies directly in front of the iris and the pupil. The cornea measures about 12mm in horizontal diameter and about 11mm in vertical diameter.
The cornea consists of 5 layers namely;
- The Bowman’s layer
- The stroma
- The Descemet’s membrane
- The Endothelium of the cornea
- The corneal epithelium
What is the function of the cornea?
As discussed above, the major function of the cornea is that it allows light to enter the eyes so that images of objects can be formed on the retina. Other functions of the cornea are;
- It provides the major focusing power of the eye, a percentage of about 65% while the remaining is provided by the crystalline lens of the eyes.
- Most eyesight abnormalities like long-sightedness, short-sightedness and astigmatism are caused by less optimal curvature of the cornea.
Abnormalities of the cornea
Just like every other part of the eye, the cornea has abnormalities that can be caused by common aging changes in the eye or when the cornea loses its focusing ability.
The common abnormalities that can occur in the cornea are;
- Arcus senilis
- Corneal abrasion
- Corneal ulcer
- Corneal dystrophy
- Fungal keratitis
- Corneal ectasia
- Acanthamoeba keratitis
What is the expected corneal thickness?
The average expected corneal thickness is 540 microns i.e basically half of a millimeter.
Recent research has shown that corneal thickness varies slightly in a population. Corneal thickness can affect the reading of the degree of pressure present in the eye
Checking the corneal thickness is very important for the following reasons.
- People whose cornea is thin with a thickness of about 500 microns or less are easily susceptible to the risk of glaucoma although the mechanism through which this happens is not yet clear.
- It helps to determine when a person has corneal edema
- It also helps in monitoring patients for refractive LASIK surgery
- Corneal thickness also helps in the calibration of applanation tonometer readings which checks the eye pressure by pressing on the cornea.
- Knowledge of corneal thickness is also important in determining the right patients for laser vision correction.
What Is A pachymeter?
Pachymeter is a device that is used in the measurement of corneal thickness. It is used before corneal molding evaluation as well as in LASIK surgery.
Pachymeter is also used for screening patients who are being evaluated for glaucoma. The modern design of the pachymeter uses ultrasound technology to measure the thickness of the cornea.
Procedures For Performing Pachymetry
There are mainly two methods of performing the pachymetry test and these two methods employ somewhat similar approaches to measuring the thickness of the cornea of the eye.
While each of the techniques has specific characteristics of their own, they have been described as reliable in taking the measurement of corneal thickness.
It is important to know that systematic differences abound in each of the techniques hence may result in presenting different values as results.
So, a substitute for measurement between the different modalities will be required.
The two main procedures for pachymetry are;
1. Ultrasound pachymetry:
It uses ultrasound medical systematic principles to measure the thickness of the cornea. The devices used in ultrasound pachymetry are cost-effective and very portable so performing the test is made easier and simpler.
The most important downside of using the ultrasound pachymetry to measure corneal thickness is that the part of the ultrasound instrument that is meant to touch the cornea must be positioned properly as any slight displacement will cause the reading to become inaccurate.
Some ultrasound pachymeters are more designed for the testing of glaucoma.
2. Optical pachymetry:
Optical pachymeters are mounted on a biomicroscope mostly used by ophthalmologists called slit lamps.
The optical pachymeter does not come in contact with the cornea, unlike the ultrasound pachymeters that need to come in contact with cornea before the measurement is taken.
The most popular pachymeter widely in use today is the optical coherence tomography which does not touch the cornea for measurement to be taken and it uses an optical principle called interferometry.
The most important setback witnessed by users of optical pachymetry is that most insurance companies will normally pay for ultrasound pachymetry so when you go for optical pachymetry your doctor may ask you to pay for the services.
In optical pachymetry, the thickness of the cornea estimated using a medical equation that is based on assumptions of the refractive index radius of curvature of the cornea.
Another major setback of the optical technique is assumption based estimation of the measurements which most times leads to slightly inaccurate results.
Other types of procedures for pachymetry include;
Laser Dopler interferometry
This is a non-contact technique of the measurement of corneal thickness that makes use of laser Dopler interferometry made up of a dual-beam infrared device. Although the technique is reliable yet there is little research data available regarding the usage of this technique.
The Pentacam Scheimpflug Camera
it is a non-contact technique of corneal thickness measurement that makes use of a rotating Scheimpflug camera to quickly capture images of the anterior segment of the eye.
This technique is used to measure corneal thickness from one limbus to another.
This technique involves contact with the cornea and focuses a confocal microscope through the thickness of the cornea. It measures the details of the cellular sublayer structure and that is a major advantage of this technique. It is also used to identify corneal microbial pathology.
How to Prepare for Your Pachymetry Test
Once you have an appointment booked with an ophthalmologist the next thing that comes in mind is the preparations needed for a successful test.
For the corneal thickness measurement test, no special preparations are made.
On the day of your appointment prepare and go to the hospital, where the doctor will before the test apply anesthetic eye drops to the using a probe that looks more like a pen.
After the eye drops, the doctor will employ one of the procedures listed above to measure the thickness of your cornea and that is all
Who can undergo Pachymetry?
Corneal thickness measurement can be performed on patients of all ages including children who are able and willing to cooperate with the doctor to obtain a reliable result.
Pachymetry is also performed on all patients who are at risk of glaucoma as well as those diagnosed with corneal diseases.
It is also performed on patients who undergo corneal transplant or refractive surgery in the cornea.
How painful is a pachymetry test?
The pachymetry test is not a painful examination and does not constitute any danger to the overall health of the eyes.
Anesthetic applied before the test helps to prevent pain and discomfort while the doctor touches the surface of the cornea with a pen-like probe.
An optical pachymeter is also a non-invasive procedure as there is no contact with the ocular surface.
How to perform the exam on a patient
Once the patient arrives at the hospital, there are ways through which the ophthalmologist performs this test in order to ensure an accurate result.
The methods include;
- The patient is made comfortable on a chair.
- Aesthetics is then applied to the eyes to help ease off pain and discomfort before the test.
- During the measurement the patient’s eyes capture a source of light repeatedly for some seconds, during this time pachymeter performs a series of tests on the cornea.
- After that a computer goes ahead to conduct the morphological and morphometric analysis as well as collect the values detected by the pachymeter at different points on the corneal surface.
- The minimum value of the thickness of the cornea is then ascertained and this marks the end of the test.
- The result is given to the patient and medical advice or prescriptions administered.
In conclusion, pachymetry is a very important aspect of eye health and all individuals are advised to know at least their average corneal thickness as this will enable you to know if you are at risk of glaucoma or not.
We hope you learned a lot from this write as we have taken time to explore all you need to know about pachymetry and corneal thickness.
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