How is eye pachymetry performed and why is it necessary - studying the thickness of the cornea? Pachymetry - measurement of the thickness of the cornea of ​​the eye Name of the medical institution.

Cornea - the spherical and transparent part of the outer shell of the eye. It is an organic lens with a biconvex structure, which is attached to the sclera of the eye through thin fibrous fibers (limb).

Thanks to the cornea and the peculiarities of its structure, light waves easily pass into the deeper layers of the organ of vision and fall on.

Functions of the cornea:

• protective;
• supporting;
• light conductive;
• refractive.

It's normal characteristic features are:

• high sensitivity and ability to regenerate;
• transparency and specularity;
• spherical structure;
• strength and integrity;
• absence of capillaries;
• radius of curvature – 7.7-9.6 mm;
• horizontal diameter – 11 mm;
• light refraction power – 41 diopters.

Inflammation, injury or degenerative processes in the cornea lead to a change in its original parameters and properties.

Structure

This organ resembles lens , convex on the outside and concave on the inside.

It occupies from 1/5 to 1/6 of the surface of the outer shell of the eye. Unlike its larger part, the sclera, the cornea has no blood vessels and is completely transparent. Its thickness increases along the periphery and decreases in the center.

The cornea has five layers:

• integumentary (front), it consists of epithelial cells, performs a protective, gas and moisture exchange function;
• Bowman's membrane, maintains the shape of a sphere;
• stroma (the main and thickest layer), formed mainly by collagen fibers and fibro-, keratin- and leukocytes, it provides the strength of the cornea;
• Descemet's, promotes high tolerance of the outer layer of the eye to external and internal influences;
• endothelial (posterior), inner layer, consisting of hexagonal-shaped cells, it performs a pumping function, supplying all the membranes of the cornea with nutrients from the intraocular fluid, therefore, it is with pathological changes in this layer that corneal edema quickly develops and is detected on instrumental examinations.

The nervous innervation of the cornea is carried out by vegetative (sympathetic and parasympathetic) neural plexuses, of which there are 300-400 times more on the surface of its upper layers than on human skin. Therefore, in case of injuries with damage to the surface shell of the cornea and its ciliary nerves, the victims experience very strong pain.

Due to the fact that the cornea does not have blood vessels, its nutrition is provided by the intraocular fluid and the capillaries that border it.

A sign of impaired blood supply to the cornea may be its cloudiness, this is due to the germination of capillaries from the limbus and the vascular film into it.

Diseases of the cornea of ​​the eye

1. . They develop when small chips of wood or metal, sand, chemicals get into the eye.

Damage to the layers of the cornea can be superficial or deep. The consequence of such injury may be erosion cornea of ​​the eye. Its formation is caused by damage to epithelial cells and the loss of their ability to regenerate (recovery).

Clinical manifestations of this pathology are:

• eye pain;
• sensation of a foreign object;
• photophobia;
• itching and burning;
• foci of opacification in the cornea;
• decrease in visual acuity.

2. Birth defects buildings:

• megacornea - pathologically large sizes of the cornea, reaching more than 11 mm in diameter;
• microcornea – reduction in the size of the cornea (from 5 mm in diameter);
• keratoglobus - protrusion of the cornea and change in its shape to spherical;
• - thinning of the cornea and loss of elasticity, leading to a change in its shape to a cone.

This is what keratoconus looks like

All these diseases entail a change normal indicators vision, occurrence, blindness.

3. Inflammatory diseases () infectious and non-infectious origin.

Symptoms of such corneal damage:

• pain in the eyes and them;
• bright vascular network of the conjunctiva;
• pastosity and (or) swelling of the cornea;
• blurred vision.

A complication of bacterial (caused by strepto-, staphylo-, gono-, diplo- or pneumococci, as well as Pseudomonas aeruginosa keratitis is) creeping corneal ulcer. It can develop within a few days, penetrating through the layers to the vitreous body.

Signs of an ulcer are:

• the formation of a raised infiltrate with uneven edges on the surface of the cornea;
• purulent discharge;
• detachment of the upper layers of the cornea, its cloudiness and pain;
• visual defects.

The danger of this pathology is that perforation (breakthrough) of the ulcerative infiltrate is possible, impregnation of the eye tissue with purulent contents and its death.

4. . Occurs against the background of metabolic disorders in the body. May be congenital or acquired.

Symptoms of the pathology may not appear for a long time, and the first signs may be discovered by chance during an instrumental examination (small stripes or areas of corneal opacification). As the disease progresses, patients begin to complain:

• on and blurred vision;
• loss of visual acuity.

Cornea transplant

It is used with the ineffectiveness of conservative treatment and progressive deterioration of vision.keratoplasty , is carried out using donor material (graft).

It is classified:

• on optical, which is used to restore the transparency of the cornea;
• medicinal, which is designed to save the eye, so even cloudy donor corneas are used in it);
• refractive, it helps restore vision;
• reclamation, a technique for strengthening the corneal layer for repeated transplants.

Cornea replacement methods:

• layer-by-layer, indicated for pathologies of the upper layer, only it is replaced;
• through(partial and total), involves transplantation of all corneal layers.

Due to the absence of blood vessels in the cornea, corneal transplantation operations are considered simple in ophthalmology, with a low risk of complications. A donor implant usually takes root well and allows patients to return to their previous quality of life.

An alternative to corneal transplantation is - keratoprosthesis(use of artificial corneas). It is prescribed after unsuccessful keratoplasty (rejection, inflammation). The technique for performing prosthetics is similar to transplanting a donor cornea.

Video:

– special diagnostic procedure in ophthalmology, used less often. The purpose of this study is to study in detail the refractive properties of the cornea, which gives an idea of ​​the efficiency of the visual apparatus as a whole.

Ophthalmologists are much more likely to resort to the use of more routine methods - ophthalmoscopy and tables for determining visual acuity. Deciphering keratometry can indicate certain pathological changes in the cornea of ​​​​the eye.

Keratometry - corneal topography

Keratometry is also called corneal topography. It is a computer-controlled diagnostic technique that creates a three-dimensional map of surface curvature.

The fact is that the cornea is the main refractive structure eyeball, it is responsible for 70% of the refractive power of the visual apparatus.

Man with normal vision has a uniformly rounded cornea, but if the cornea is too flat or too round and unevenly curved, then visual acuity decreases. The greatest advantage of keratometry is its ability to detect transient pathologies that cannot be diagnosed by conventional methods.

Corneal topography provides a detailed visual description of the shape and properties of the cornea. This method provides the ophthalmologist with very fine details of the state of the eye's optical system. Deciphering keratometry helps in the diagnosis, monitoring and treatment of various eye diseases.

This data is also used to prescribe contact lenses and to plan surgeries, including laser vision correction. If laser correction is necessary, a topographic map of the cornea is used in conjunction with other methods to accurately determine the required volume of corneal tissue to be removed.

Corneal imaging technologies are advancing rapidly, mainly due to significant advances in refractive surgery. To understand the significance of new imaging techniques, it is necessary to consider the mechanism of the eye's optics.

Structure and functions of the cornea of ​​the eye

The cornea is a transparent convex lens of connective tissue structure that is part of the eyeball. It is the outermost structure of the eye.

The most main structure visual apparatus - retina. It contains a huge number of color and black-and-white receptors that capture light reflected from surrounding objects. In order for light to reach the retina in the right way, the refractive apparatus of the eye is necessary. These are the cornea, aqueous humor, and vitreous humor.

The cornea performs the main refractive function.

Optical properties of the cornea and their measurement

To describe the optical properties of the cornea, various concepts, namely:

• Curvature of the anterior and posterior surface of the cornea. It can be expressed both in radii of curvature in millimeters and in keratometric diopters.
• Shape of the anterior and posterior surface of the cornea. This characteristic can be expressed in micrometers as the height of the actual surface of the cornea relative to a reference point. This concept includes not only a description of the shape of the cornea, but also an analysis of irregularities in the surface of the cornea (for example, corneal astigmatism).
• Local changes in the surface of the cornea. They can be expressed in micrometers. The optical smoothness of the corneal surface is very important, so any microscopic irregularities can significantly reduce visual acuity.
• Corneal power. This is the refractive power of the cornea, expressed in diopters. The term refers to the optical properties of the cornea, depending on the surface shape and refractive index.
• Thickness and three-dimensional structure of the cornea. These indicators can be expressed in micrometers. Changes in the three-dimensional structure of the cornea (eg, after refractive surgery) can cause further changes in its shape due to biomechanical changes, such as altered elasticity of the residual corneal tissue.

Keratometric diopter is calculated from the radii of curvature of the cornea. A special formula is used:
K = index of refraction x 337.5 / radius of curvature.

This calculation can be called simplistic, since it ignores the fact that the refractive surface is in contact with air space. This calculation also does not take into account the slant frequency of incoming light to the periphery of the eye.

As a result, the keratometric diopter measurement takes into account the true refractive index of the cornea from 1.375 to 1.338. That is why diopters in in this case it is more correct to call them keratometric doptries to distinguish the two different terms.

Cornea shape

The average refractive index of the anterior and posterior surfaces of the cornea is 48.5 and -6.9 diopters, respectively. To simplify these indicators, a net corneal power indicator of 43-45 keratometric diopters is often used in clinical practice.

Typically, the cornea changes little with age. It flattens by about 0.5 diopters by age 35 and rounds by 1 diopter by age 75.

In adulthood, the cornea tends to be more convex in the vertical meridian, by about 0.5 diopters compared to the horizontal meridian, which contributes to a higher risk of astigmatism in young people.

This difference between vertical and horizontal curvature decreases with age, finally disappearing at age 75. Changes in corneal shape contribute greatly to the prevalence of astigmatism.

The normal cornea is a convex lens, meaning it has a steeper surface in the center and a smoother surface at the periphery. A reduced surface (for example, against the background of laser correction) can, on the contrary, be flatter in the center and steeper at the periphery.

The visually significant surface area of ​​the cornea is approximately equal to the area of ​​the dilated pupil. Pupil diameter decreases with age. All these indicators vary among people of different age groups. Research shows that the average pupil size in bright light for people aged 25 to 75 years is 4.5 and 3.5 millimeters, respectively.

These data have important clinical implications, since most laser techniques treat an area of ​​the cornea with a diameter of 6.5 millimeters.

Mechanical properties of the cornea

The mechanical properties of the human cornea have not been sufficiently studied. The central corneal thickness is 250 micrometers, which is considered sufficient to ensure long-term mechanical stability.

Peripheral thickness is studied less frequently, but it is certainly also of clinical importance when studying the refractive power of the eye using radial and astigmatic keratometry.

Recent advances in ophthalmology may help to study corneal mechanics in more detail.

To construct a topographic map, several light concentric rings are projected onto the cornea. The reflected image is captured by a camera connected to a computer. Computer software analyzes the data and displays the results in several formats.

Each map has a color scale that assigns each specific keratometric range a specific color. In the interpretation, not only colors are used, but also other indicators. Keratometric diopters are critical in chart interpretation.

Absolute topographic maps corneas have a given color scale with already known dioptric steps. The disadvantage is insufficient accuracy - dioptric steps change by large amounts (usually 0.5 diopters), which does not make it possible to study in detail local changes in the cornea.

Adapted maps have different color scales, constructed using special programs that identify the minimum and maximum values ​​of keratometric diopters. The dioptre range of adapted cards is usually smaller than that of an absolute card.

Only an ophthalmologist can comment on the final keratometry values. Deciphering keratometry is a labor-intensive process that requires experience.

We found that keratometry is an important diagnostic measure of the refractive power of the cornea. Unfortunately, this test is not used often, although its accuracy can rival many other methods.

You will see how keratometry is performed in the video:

According to ophthalmologists, the thickness of the cornea plays an important role in determining the pathologies of the visual organs and choosing the optimal treatment tactics. If negative symptoms occur, it is recommended to examine the outer transparent membrane. Timely diagnosed abnormalities will help stop degenerative processes in the eyes and prevent complications.

What is the cornea?

When choosing a method for examining the cornea, it is recommended to give preference to the ultrasound contact technique, as it gives more accurate results. The risk of bias is minimal, but it is important to review additional contraindications.

The outer part of the eyeball, responsible for the refraction of artificial and natural light, is similar in shape to a lens. Ophthalmologists evaluate the condition of the cornea by thickness, diameter, radius of curvature and refractive power. To identify deviations, you need to contact an ophthalmologist. Intraocular pressure indicators depend on the thickness of the cornea. Disturbances in the cornea provoke serious pathologies that lead to the following negative symptoms:

If the thickness of the cornea is incorrect, double vision appears.

• blurred vision;
• complete or partial loss of visual abilities;
• doubling of objects;
• attacks accompanied by nausea and vomiting;
• loss of the eyeball;
• frequent headaches.

Normal thickness

The physical parameters of the cornea change throughout the day, but the thickness healthy person varies within 0.06 mm. Large deviations indicate the need for additional examination to identify the type of disease. In the absence of ophthalmological pathology, the following characteristics of the outer shell of the eye should be observed, presented in the table:

How do they check?

Preparatory stage

In order for corneal pachymetry to proceed without complications and accurate results to be obtained, ophthalmologists focus on following the following rules:

• It is necessary to stop wearing optical and decorative lenses 2 days before the examination.
• When collecting anamnesis, it is necessary to inform about individual intolerance medicines. The reaction to local anesthetics and antiputrefactive agents is especially important.
• You must come to the procedure without any traces of makeup or medicinal cosmetics. Decorative eye cosmetics are completely excluded 2 days before the study.

Ultrasound pachymetry

The technique involves direct contact of the device with the mucous membranes of the eyes. The subject accepts horizontal position, and the local anesthetic drug “Inocaine” is instilled into the organ of vision. The ophthalmologist carefully moves the nozzle from the ultrasound machine along the cornea. It is important that the procedure is performed by an experienced doctor who applies minimal pressure to the outer part of the eye. Excessive pressure causes injury and poor results.

The data is automatically processed, calculated and displayed on the monitor. To prevent infection, after the examination the mucous membranes of the eyes are treated with antibiotics. The most effective medications include Tsipromed and Albucid. After instillation, it is recommended to close your eyes for 2-3 minutes. and gently massage the eyelids so that the antibacterial substance is evenly distributed throughout the mucous membranes.

The examination takes place without direct contact with the eyes using a special apparatus, thanks to which the cornea is examined under multiple magnification. An attachment made of 2 parallel pieces of glass, fixed to the slit lamp, makes it possible to measure the diameter of the cornea and its thickness. The subject sits on the side of the device and places his chin on the stand. In this case, the ophthalmologist is on the opposite side of the lamp to examine the cornea. To determine the parameters, the doctor, using a lever, rotates the upper lens and directs light to the lower one. Indicators are assessed using a measuring ruler.

In the absence of the necessary medical equipment or diagnosing contraindications for examination, you can examine the eyes with a coherent apparatus to obtain a tomogram.

To identify pathologies in the human visual organ, they are used various methods diagnosing. For example, to determine the condition of the cornea of ​​the eye, the thickness of the membrane is measured. The process of measuring corneal thickness is called pachymetry. What does this type of examination reveal? And how does pachymetry influence the correct diagnosis?

In what cases is it prescribed?

Pachymetry of the eye is an indispensable type of diagnostics of the state of the patient’s visual system. It is used to identify pathological changes if you suspect the development of such diseases:

• Edema of the cornea.
• Glaucoma.
• Dystrophic pathologies of Fuchs.
• Keratoglobus.
• Keratoconus.

Corneal pachymetry has proven itself in the process of checking the state of the visual system in postoperative period. Pachymetry is included in the mandatory list of diagnostic types before laser correction of visual function.

About contraindications

And although this type ophthalmological examination gives excellent results, however, it has a number of contraindications.

Pachymetry is not performed in the following situations:

• When the patient is under the influence of alcohol or drugs.
• If the patient has a psycho-emotional illness. If the patient is violent or has inappropriate behavior, pachymetry is not performed.
• If you plan to conduct a contact examination, then if available purulent diseases eye, this diagnosis will have to be abandoned.

Types of diagnostics

Ophthalmologists carry out studies of the thickness of the cornea using two options for pachymetry:

• Optical, in which the condition of the eye is studied without contact, using a slit lamp.
• Ultrasound - using a special device that is in close contact with the shell of the eye.

When performing optical pachymetry, an attachment is placed on a special microscope that measures the thickness of the cornea. The nozzle consists of two glass plates. The lower one is stationary, and the upper one rotates along a vertical guide.

The nozzle is fixed to the lamp perpendicularly. To take readings, the diagnostician directs a stream of light to the desired area of ​​the eye. Next, while rotating the device handle, the thickness of the cornea is assessed. Readings are taken on a scale on which each degree equals one millimeter of thickness.

More accurate indicators are determined by an ultrasonic pachymetry machine. This research option is contact. To carry it out, the patient is asked to lie down on the couch. The eye to be examined is numbed using local anesthesia. At the beginning of anesthesia, light pressure is applied to the cornea with a special device. But they do it lightly so as not to distort the results of the examination.

At this moment, the device automatically determines the thickness of the cornea and displays the received information on the monitor screen. Based on the data obtained, the ophthalmologist draws appropriate conclusions. Upon completion of the diagnosis, the patient is instilled with antibacterial drops.

About pachymetry standards

The average thickness of the cornea in the central part ranges from 0.49 mm to 0.56 mm. Near the limb this indicator equals from 0.7 mm to 0.9 mm. In female patients, the norm of pachymetry of the eye exceeds that of the cornea of ​​men. For women – 0.551 mm, for men – 0.542 mm. It is noteworthy that during the day the thickness of the cornea can change. But, if the average pachymetry norm is exceeded, this indicates the development of a pathological process.

The accuracy of corneal thickness depends on the machine used. So, when using an optical device, the auto pachymetry code will fluctuate between 0.490 mm -0.581 mm. When using the interferometric method - from 0.519 mm to 0.536 mm. However, these numbers in themselves do not mean anything. To determine the diagnosis, interpretation of the obtained pachymetry data is required.

Concluding

Note that among the numerous methods for diagnosing the state of the human visual system, pachymetry occupies a leading position. Since changes in retinal thickness are a significant parameter for determining pathology. Ophthalmologists often resort to this type of examination in order to obtain reliable information about the condition of the patient’s eyes.

The medical term "pachymetry" is used to refer to biometric diagnostic method aimed at measuring the thickness of the cornea of ​​the eye. It allows not only to determine the condition of the cornea, but also to identify existing deviations, as well as to evaluate the effectiveness of the treatment already being carried out.

The procedure can be carried out contact or non-contact, depending on the chosen method. The process may use a slit lamp, ultrasound equipment, or a CT scanner. It is relatively safe and does not cause discomfort, and has minimal contraindications.

You can undergo eye pachymetry at Dr. Eskina's ophthalmology clinic. We conduct comprehensive studies aimed at studying the structures of the patient’s eye and identifying any, even small, deviations from the norm. The procedure is performed by experienced ophthalmologists using modern equipment, which increases its diagnostic value.

Pachymetry in ophthalmology: a valuable diagnostic method

The first methods for calculating corneal thickness were described in the middle of the last century, and somewhat later (in 1980) the first ultrasound pachymeter appeared in ophthalmology. In order to understand the value of this diagnostic method, you need to understand a little about the anatomy of the eyeball.

The cornea is the anterior, most convex part of the eyeball and is a light-refracting medium, which in its normal state should be transparent. Being an element of the light refractive system of the eye, it transmits light rays that are collected on the retina, which allows you to get a clear “picture”.

Deterioration in visual acuity is most often associated with a violation of the refractive ability of the cornea, which appears as a result of its irregular curvature and abnormal thickening of certain areas or the entire layer. Corneal pachymetry makes it possible to identify existing deviations and determine methods for their elimination.

Indications and contraindications for pachymetry

This diagnostic method is used for pathologies that are characterized by changes in thickness and deformations of the cornea. During the appointment, if the patient has complaints, the ophthalmologist evaluates the results of simpler diagnostic studies- and, if they do not allow a diagnosis, prescribes pachymetry.

How is corneal thickness measured?

Pachymetry does not require the patient to preliminary preparation. The ophthalmologist can perform it directly on the day of the first visit, if necessary. If the patient wears contact lenses, they will need to be removed. The price of pachymetry depends on how it is performed.

Interpretation of pachymetry results

The results of the study include data on the thickness of the corneal layer in different areas. The range of normal corneal thickness is from 410 to 625 microns. The average is 515 microns. At the edges, the readings can be up to 1200 microns.

It is important to understand that the thickness of the cornea is an individual indicator, which depends on the characteristics anatomical structure eyes - however, it should not go beyond the above range. Surprisingly, but true: based on statistical data, the fair sex has a thicker cornea than men, approximately 9 microns.

Comparing the research results and normal indicators, the doctor draws conclusions about the condition of the cornea and makes a diagnosis or prescribes additional diagnostics.

Advantages of pachymetry at the Sfera clinic

Our clinic has modern diagnostic equipment, which allows us to provide our patients with accurate diagnostics. The procedure is carried out according to international standards using the Sirius diagnostic platform, manufactured by the German company Schwind Eye-Tech Solutions. It is a high-precision cornea analyzer that allows you to evaluate its refractive power, thickness and elevation. Thanks to it, our specialists are able to identify any, even minimal deviations, and accurately determine the diameter of the cornea and pupil.

The high accuracy of the results allows not only to make a correct diagnosis, but also to eliminate contraindications to the procedure. surgical interventions. Moreover, thanks to it you can accurately calculate the volume of the operation.

You can make an appointment with our ophthalmologists in Moscow by filling out a simple form on our website or by calling +7 495 139‑09-81.