THE PARAOPTOMETRIC: MEASUREMENT OF THE ANTERIOR SEGMENT OF THE EYE

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Photos courtesy of Oculus.

Nearsightedness, farsightedness, and astigmatism are caused by imperfections in the shape of cornea which in turn results in blurred or distorted vision. The cornea is a key factor in the overall eye health. With the increase in the amount of ocular surgical procedures, it has become increasingly more important during pre-operative examinations to test the eyes to ensure the best surgical decision for the patient.

A SOLUTION
Corneal topography used to be the only way that doctors were able to measure corneal curvature and its irregularities. The limitation to this type of measurement is that the topographer is only able to measure the anterior surface of the cornea. Abnormalities present on the posterior surface of the cornea were not discernible and often responsible for poor outcomes with laser vision correction.

Oculus’ Pentacam® tomographer has been a part of revolution in eye technology that can help include or exclude patients seeking laser vision correction. The Pentacam maps the anterior segment of the eye by using a rotating Scheimpflug camera, which supplies pictures in three dimensions. The images taken can then be analyzed at different depths and from different viewpoints.

SCHEIMPFLUG PRINCIPLE
The images obtained by the Pentacam are captured using the Scheimpflug Principle. This is a principle that describes the orientation of the plane of focus of an optical system, when the lens plane is not parallel to the image plane. When a subject is not parallel to the sensor plane, the lens plane has to be tilted so the subject plane, lens plane, and sensor plane all meet at the same point. The cornea center is precisely measured by the rotating camera.

The Pentacam consists of a Scheimpflug camera and slit lamp illumination, which combined, rotates around the eye. The slit illumination illuminates a thin layer in the eye. Since the cells are totally transparent, the light is scattered—creating a sectional image that is captured by the camera. The result is sharp images from the anterior surface of the cornea to the posterior surface of the lens. The Pentacam measures 138,000 true elevation points in less than two seconds and analyzes the complete anterior segment at different depths and from different viewpoints. At the same time, any eye movements are automatically captured and corrected by a second pupil camera.

Photos courtesy of Oculus.


The Pentacam software analyzes and evaluates all the data acquired and then extrapolates this data into various colored maps of curvature, elevation, anterior chamber angle, chamber volume, and lens densitometry.

LASER VISION CORRECTION
The information collected lets the clinician view the eye in three dimensions at 25 different angles. This is important in determining whether a patient is a candidate for laser vision correction. The precise elevation maps obtained by the Pentacam can aid in the decision of whether laser vision correction will be effective and safe to perform. It can also screen for keratoconus, a degenerative disorder of the eye. In addition, the Pentacam can detect an ectatic posterior corneal surface even if the anterior surface appears normal as well as pellucid marginal degeneration (peripheral corneal thinning usually inferiorly).

CATARACT SURGERY
Useful in the measurement of cataract lens change, the Pentacam can measure the densitometry of the crystalline lens. It can also aid in calculating the precise location for implantation of phakic intraocular lenses (IOLs). Calculation of the anterior and posterior curvature of the cornea, the crystalline lens, anterior chamber depth, and lens thickness decrease error in calculation of the effective lens position. The high-quality Scheimpflug images allow post-operative monitoring of the IOL, which is useful if there is post-operative lens opacification.

CORNEAL PACHYMETRY
Measurement of the corneal topography and pachymetry has become an indispensable part of the laser vision correction pre-operative evaluation. Corneal pachymetry is essential to ensure sufficient corneal thickness in order to prevent abnormal bulging (ectasia) of the cornea. The Pentacam takes limbus to limbus measurements of the corneal thickness. It aids in determining the thinnest point of the cornea in relation to the corneal center. The corneal thickness is displayed as a color map with warmest colors indicating areas of elevation, and cooler colors showing areas of depression. It can pinpoint the exact location of the bulging of the cornea in patients with keratoconus, a useful capability in surgical corneal transplant planning.

SUMMARY
As an increasing percentage of patients are seeking ocular surgery (such as refractive, cataract removal, or corneal transplant), it has become more important to perform highly detailed diagnostic pre-surgical evaluations. With patients’ higher post-surgical expectations, more advanced screening processes are required to minimize avoidable post-surgical complications. By performing topographic evaluations with machines like Oculus’ Pentacam, imaging of the anterior segment ocular structures can be measured. This device allows users to measure corneal structure and power and to develop screening parameters for IOL implantation and refractive surgery. Creating a 3D map of the eye allows personalized patient treatment and provides greater assurance of safety and effectiveness in laser vision correction.


Janet Hunter is president of Eye Source LLC and specialized in ophthalmic technician training.


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