Generality
Pachymetry is a diagnostic test that is performed in the eye area. This investigation serves, in particular, to measure the thickness of the cornea, that is the thin transparent fabric that covers the front part of the iris.
Pachymetry is useful for diagnosing and evaluating the evolution of certain eye diseases, such as keratoconus, corneal edema or glaucoma . The examination also allows the study of the anterior surface of the eye for the programming of corneal surgery or refractive correction operations .
During the examination, a probe - called a pachymeter - is gently placed in proximity or in contact with the cornea, to measure its thickness. The results thus obtained are interpreted by the doctor, who will provide the most appropriate indications for the management of the clinical case.
What's this
Pachymetry consists in measuring the thickness of the cornea, ie the transparent surface of the eye placed in front of the iris. This examination is important for establishing the diagnosis of keratoconus (together with corneal topography ) and allows the correct assessment of ocular hypertonus .
This test is also used in patients who have to undergo corneal surgery (such as cross-linking), refractive correction or transplantation .
Cornea: brief notes on anatomy and functions
- The cornea is the membrane that covers the front of the eye ; through this avascular and transparent structure it is possible to see the iris and the pupil . In practice, the cornea is the first "lens" that light meets during its natural path to the inside of the eye.
- The cornea is made up of overlapping layers, the outermost of which is the stratified paved epithelium, while the successive ones are formed by a dense weave of collagen fibrils arranged in lamellae, with a glycoprotein matrix that unites them and makes them transparent.
Why do you run
The purpose of pachymetry consists in measuring, in various ways, the thickness of the cornea in several sectors .
The execution of this examination is useful to monitor the evolution of both the diseases characterized by a progressive thinning of the cornea (such as keratoconus) and of glaucoma .
This instrumental examination is indispensable for programming refractive surgery or other corneal treatments.
When is it indicated?
Corneal pachymetry may be recommended during an eye examination .
This examination is important for the diagnosis of the following pathological conditions:
- CHERATOCONO : in association with corneal topography, the pachymetric map is an indispensable exam to establish the presence of this pathology. Within this clinical framework, pachimetra allows the evaluation of thickness at the apex of the cone, as well as monitoring the evolution of the disease (follow-up) and programming the most appropriate interventions for the patient (cross-linking, lamellar keratoplasty or perforating, intrastromal rings, corneal transplant, etc.).
- OCULAR HYPERTENSION : pachymetry is of great importance in the evaluation and monitoring of patients who present an increase in intraocular pressure, limiting the evolution of glaucomatous damage by timely medical intervention.
Corneal thickness and intraocular pressure
The thickness of the surface of the cornea is a value that must always be correlated with the measured intraocular pressure, to make the outcome of the tonometric examination reliable.
In fact, a thin cornea can give a false finding of low eye pressure (intraocular hypotone), and, conversely, a thick surface may erroneously provide a result compatible with intraocular hypertension. Therefore, the main risk factor associated with glaucoma is the risk of ignoring or underestimating (if the pressure measurements are normal or poorly altered).
Depending on the result obtained with pachymetry, the appropriate corrections must be made on the data obtained with the tonometer .
As for eye treatments, corneal pachymetry is important for the choice of the intervention or therapeutic protocol to be adopted.
- In the field of REFRACTIVE SURGERY, a thickness of the cornea greater than 500 μm makes the option of laser technique possible . When pachymetry provides a lower value, on the other hand, another procedure should be considered, such as the PRK (more superficial method). The examination is also important to exclude patients who have insufficient corneal thickness to support this type of intervention and recognize corneal ectasias in subjects who have already undergone refractive surgery.
- When the patient is suffering from keratoconus, pachymetry is one of the fundamental preliminary tests to establish indications for treatment . The measurement of the corneal thickness allows to establish, in fact, if the patient is more suitable for cross-linking (for which a minimum thickness of 400 μm is required), the intrastromal rings (minimum thickness of 400 μm at the incision) or another type of intervention, such as lamellar or perforating keratoplasty. In the most serious cases of keratoconus, however, it is appropriate to resort to corneal transplantation (mandatory if a perforation has occurred).
Associated examinations
Depending on the results obtained, to understand the nature of a problem, pachymetry is complementary to other investigations aimed at deepening the clinical picture, such as tonometry, corneal topography and confocal microscopy (allows observation of all layers of the cornea and identifies possible points of fragility).
The corneal topography, in particular, allows to evaluate the conformation of the cornea, study its surface and monitor the evolution of the disease.
Normal values
The numerical value obtained by means of pachymetry is expressed in micrometers (μm), a unit of length corresponding to one thousandth of a millimeter.
- To be clear, at the center of the cornea, the thickness is about 520-540 μm . However, values between 460 and 620 μm are also considered normal (although this finding is more rare).
People with a thinner cornea are more likely to develop certain diseases, including keratoconus, keratitis, edema and corneal ulcer. Furthermore, the reduced surface correlates to a higher probability of progression of glaucomatous damage.
People with a thick cornea, on the other hand, would be more protected from this risk.
Altered Values - Causes
The pathologies that affect the cornea by altering their thickness, shape and transparency are different and include: keratoconus, keratitis, corneal edema, ulceration and glaucoma . Among the conditions capable of compromising the corneal function are traumatic, chemical and infectious diseases. Furthermore, the cornea can be affected by non-infectious inflammations and congenital alterations of its curvature.
In keratoconus, it is possible to highlight, with pachymetry, a progressive thinning of the cornea; this pathology mainly involves the anterior layers of the corneal surface, at least in the less advanced stages. On the contrary, the thickness increases considerably, if there are alterations of the corneal endothelium (eg, bullous keratopathy).
How to recognize a corneal pathology
Symptoms that suggest corneal surface involvement include pain, particularly with exposure to light, and decreased vision . These manifestations must lead to a thorough eye examination, including pachymetry.
Keratoconus
Keratoconus is a degenerative disease that causes the progressive distortion of the corneal surface . With the progression of the pathology, the cornea - normally round - thins and begins to vary its curvature, protruding outwards. Assuming a cone-shaped appearance, the corneal refractive power is modified, therefore the correct passage of the luminous input towards the internal ocular structures does not occur and there is a loss of visual acuity . The direct consequence of corneal bulging is astigmatism (called "irregular" as it is not possible to correct with the lenses).
Therefore, the initial symptoms are related to these refractive defects. As the condition progresses, vision becomes progressively more blurred and distorted, increases sensitivity to light and eye irritation. Sometimes, keratoconus causes the appearance of edema (presence of an excess of watery content inside the cornea) and scars, which in turn determine the loss of homogeneity and transparency of the membrane.
How the exam takes place
Pachymetry is an instrumental examination that is performed, on an outpatient basis, by an ophthalmologist (specialist in eye diseases). The procedure is non-invasive and painless for the patient.
Corneal pachymetry: procedure
- To perform corneal pachymetry, the patient sits in front of the instrument and is asked by the doctor to rest his chin and forehead on special supports.
- For the acquisition of the parameters of interest, the doctor instills an anesthetic eye drops in the eye and gently applies on the surface of the cornea a small probe, similar to a pen, to detect its thickness.
- During the examination, the patient is required to continuously fix a light source for a few seconds, keeping the eye open and still. At the same time, the instrument performs a series of scans of the cornea and the computer performs morphological and morphometric analyzes of the cornea, extracting the pachymetric value detected at various points on the corneal surface and the minimum corneal thickness detected.
- The data is acquired by the operator in a few seconds. If necessary, the measurement can be repeated at various points on the corneal surface.
Pachymetry: types
Corneal pachymetry can be performed with different investigation techniques, such as:
- Ultrasonic pachymetry;
- Laser pachymetry;
- Optical pachymetry:
- With endothelial microscope;
- With confocal microscope;
- With Scheimpflug (or Scheimpflug imaging system) camera.
Ultrasonic pachymetry (contact) is the most used technique and is considered the reference test for measuring the thickness of the cornea. This type of examination is performed, following the instillation of an anesthetic eye drops, with a high frequency probe (similar to a pen), which uses ultrasound to carry out the measurement.
In the case of the optical pachymeter, instead, there is no contact with the ocular surface, with the exception of confocal microscopy, in which a gel is interposed between the instrument and the patient's eye.
The pachymetric map highlights the thickness of the cornea in each area and allows the identification of the location and value of the thinnest point.
Who does it run from?
Pachymetry is performed by an ophthalmologist.
How long does it last?
As a rule, pachymetry takes a few seconds to perform. The exam is, in fact, rapid and causes minimal discomfort in the patient.
What does the report contain?
The diagnostic conclusion is reported in the pachymetry report, with any data related to the corneal thickness attached.
Preparation
Pachymetry is performed during an eye examination. To undergo the examination, no particular initial preparation by the patient is required and pupil dilation is not necessary.
The development of ultrasound pachymetry involves the instillation of an eyewash based on novesine / oxybuprocaine hydrochloride to anesthetize the ocular surface. Therefore, it is useful to report any known allergies to this type of surface anesthetic to the doctor.
If an optical pachymeter is used, instead, it is not necessary to instill anesthetic eye drops.
Contraindications and Risks
Who can't take the exam?
Pachymetry is a painless exam, which can be performed in all age groups, as long as the patient is able to guarantee an active collaboration during corneal thickness measurement. When it does not allow to obtain a reliable result - for example, due to the involuntary closing of the eye - the procedure must be repeated.
Unlike the visual field examination and other diagnostic investigations, the results of pachymetry are stable over time, except in the presence of keratoconus.
The situation is different in the case of surgical operations and monitoring of the evolution of the pathology, where corneal pachymetry is useful for studying the surface and thickness of the cornea over time (for example, before and after treatment).
Pachymetry: are there risks associated with the exam?
Pachymetry is not a dangerous sight, since the contact or the touch between the instrument and the patient's eye is delicate and almost imperceptible.
Theoretically, the contact of the pachymetric probe with the cornea could increase the risk of infections and corneal traumas, but, in reality, this occurrence is very difficult.
