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The introduction of quantitative and qualitative videokeratoscopy
provided Eye care professionals with a wealth of new information at their fingertips. The explosion in corneal topographic
technology has not only given us information pertaining to the curvature of the entire cornea but also to generate the wavefront
information and provide us with the higher order aberration. The combination of the two allowed a better understanding of
the outcome of keratorefractive procedures, the effects of degenerations, diseases, contact lenses and trauma upon corneal
topography.
A clinician can now more readily treat a cornea knowing the shape factor, the eccentricity or Q value
before and after any keratorefractive procedure. Corneal abnormalities such as pterygium, herpes simplex lesions, and effect
of chronic dry eye can be more precisely localized. Postoperatively, it can be determined if a refractive procedure needs
enhancement or penetrating keratoplasty suture requires adjustment. Diagnosis of keratoconus, contact lens induced corneal
warpage, and peripheral marginal degeneration are easily rendered. Corneal topography can also be used in providing the excimer
laser with the asphericity of the cornea, i.e shape factor, eccentricity and Q values can optimize the outcome of the refractive
surgery by reducing the resulting spherical aberration.
Better Diagnostic Assessment and Management
Altered
corneal topography is well recognized to be the major reason for routinely poor, uncorrected acuity following corneal surgical
procedure. Scarring can produce clinically and visually significant topographical changes. A penetrating keratoplasty (or
PKP, the removal of a diseased cornea) followed by inserting and suturing in a replacement donor cornea often leaves the patient
with significant residual astigmatism and decreased acuity. The advent of Corneal Topography allows the surgeon to visualize
the areas of corneal steepening that have produced the often irregular astigmatism. Tight sutures, the cause of the steepening,
can then be loosened and repositioned to relax the folds on the cornea and allow it to resume its normal shape.
With penetrating
keratoplasty, prior to the procedure, it is possible to map the area of irregularity that is going to be excised. Post-transplant
corneal measurement is extremely helpful in determining tight sutures and the irregularity caused by the suturing technique.
It becomes very helpful in determining the amount of astigmatism to reduce and which suture to alter.
Phacorefractive
Surgery and IOL Calculation
Phacorefractive surgery is the systematic approach for the achievement of best corrected
visual acuity through addressing all aspects of cataract surgery: the corneal contour, the resultant wound, the IOL used,
and any astigmatic correction needed. Accurate topographic analysis is of value both preoperatively and postoperatively in
the management of cataract patients. Intraocular lens calculation is predictably determined using the average keratometry
value obtained by the corneal topographer at the 3 mm zone. Computerized videokeratography provides the surgeon with feedback
regarding the topographical alterations and residual astigmatism caused by the surgical approach. In managing patients with
postsurgical astigmatism, corneal topography can reveal whether a patient needs a relaxing incision, wound revision, or contact
lenses or spectacle.
The measurements of the wavefront of the cornea and the determination of the higher order aberrations,
allows practitioner to order a custom IOL (intra occular lens) designed specifically for his patient and correct higher order
aberrations, thus improving the outcome of the surgery and visual acuity.
Surgical Keratorefractive Procedures
Surgical keratorefractive procedures, including Lasik, PRK, Lasek & Epi-Lasek are intentional alterations in the corneal
curvature to correct the refractive power of the eye. Their success is dependent upon sensitive, accurate knowledge of the
patient’s corneal topography. The use of computerized videokeratography plays a major role in the preoperative work-ups for
these procedures. The preoperative topographies screen patients for form fruste keratoconus before refractive surgery. Before
surgery, the topography maps can reveal the type of astigmatisms, regular or irregular, symmetric or asymmetric orthogonal
or non orthogonal. Postoperatively, corneal topography helps us to understand the effects of these procedures and / or postoperative
phenomena and complications such as regression, visual fluctuation, decentration/tilt and central islands.
More Qualitative
Information
The development of accurate and sensitive corneal topography greatly increases our understanding of corneal
structure and biomechanics. With the continuous expansion and acceptance of keratorefractive surgery, we recognize the increasing
importance of determining corneal curvature, elevation, high order aberrations and dry eye. Optivision’s sophisticated detection
system gathers more qualitative information and is unrivaled by any other topographer in its category. Unlike other systems,
the CornealMap EH-300 is extremely sensitive to surface and tear film irregularities as well as microirregular astigmatism.
Although the anomalies may be subtle, they are often visually debilitating and frustrating. Clinicians need to be able to
recognize and diagnose these conditions to provide optimal patient care.
Corneal Topography and Standard Methods of
detecting and diagnosing Corneal Topographic Disorders
Once a diagnosis has been made, a Corneal Topographer
is extremely useful in the monitoring and management of such conditions. When pathology is in its early stages, slit lamp
corneal changes may be too subtle or not yet apparent. In this instance, when keratometry readings and retinoscopy may also
be normal, the topographical map is a key component in making a diagnosis. The greater capabilities of the CornealMap EH-300
are required to detect nonspecific steepening as well as subtle corneal and tear film abnormalities. Fourier analysis and
Zernike polynomials are very helpful in this regard. Visually debilitating anomalies, including micro-irregular astigmatism
or loss of contrast sensitivity from flap complications and central island after Lasik, are not discernible through other
methods. It is incumbent upon the clinician, therefore, to accurately detect, diagnose, and manage such evasive conditions
if he or she is to provide the best patient care and reassurance.
Corneal topography is an important component of preoperative
evaluations of refractive surgery patients. The evaluations are necessary to screen for corneal disorders, such as keratoconus.
Astigmatic keratotomy, and Lasik are currently contraindicated in patients with keratoconus. Other topographic disorders include
keratoglobus, CL warpage, distortion caused by pterygium, corneal dystrophies, degenarations, and severe dry eyes. Patients
with any of these preexisting conditions are not good candidates for refractive surgery.
Detect small Aberrations
and Details
Small aberrations and important details, which can be left out by other systems, are highlighted by the
CornealMap EH-300 system. For example, with the CornalMap EH-300 system, it is very easy to demonstrate in the RK patient
the exact locations of the RK incisions because of the high concentration and sensitivity of the data rings. Other systems
have fewer rings, spread widely apart. In order to fill in the data between the rings, other systems extrapolate the information
causing a smoothing effect on the cornea, and meaningful data is lost. ( The CornealMap EH-300 has the capability to emulate
the competitors maps by activating the smoothing filter to smooth the data).
Screening a patient prior to refractive surgery
will determine the amount and direction of the corneal astigmatism. Additionally, screening for conditions that would preclude
surgery is important. As with any refractive procedure, the accuracy of the data collected will be a determining factor in
ultimate success of the outcome. If the information is missed by the detection system, then no amount of software programming
can reinstate that which was not measured. To detect small details, set the smoothing filter to off, select the relative
scale, and choose the smallest steps (1/4 or 1/2).
Contact lens warpage
Typically, candidates for refractive
procedures wear contact lenses. Most patients exhibit some amount of corneal warpage due to the contact lenses. It is recommended
by most physicians that patients with soft contact lenses should discontinue wear for 2-3 weeks prior to any refractive procedure;
patients with RGP (rigid gas permeable), 6-10 weeks; patients wearing PMMA lenses, 12-16 weeks; to ensure that the warpage
caused by contact lenses is resolved.
To follow a preoperative refractive patient who has discontinued lens wearing,
capture a corneal topography image of the patient at the first visit. Note the time the patient removed the lenses prior to
the exam. Follow the patient at predetermined intervals (based upon type of contact lens) and capture an image at each additional
visit. The evaluation of the reduction of corneal warpage due to contact lenses can be visualized using serial displays of
exams. As the cornea regains its normal shape, the serial exam will demonstrate fewer and fewer changes in the difference
map.
A comparison of primary astigmatism axes from serial exams can alert the surgeon to shifts in astigmatism, averting
problems that would result from a premature operation. If the surgeon were to operate on a patient without the cornea being
stabilized, the outcome would not be ideal. The resulting fluctuations in a patient’s vision would be a result of the cornea
returning to its natural state. With its ability to measure and track contact lens warpage, the CornealMap EH-300 can help
surgeons determine the optimal time for surgery.
The CornealMap EH-300 system is also helpful with patients who are
photophobic and hard to manage. The Optivision CornealMap EH-300 system has a manual image-capture override that allows the
operator to capture the image precisely, without hesitation. The instantaneous manual capture of the corneal image makes these
patients easy to manage.
How does corneal topography facilitate contact lens fitting?
Easy and difficult fits
can be expedited and the number of changes required minimized with a good corneal topographer system. The Optivision CornealMap
EH-300 system has software to assist in fitting contact lenses. Its Contact Lens Program uses the topography to suggest a
custom RGP design. It allows for interactive design changes and evaluations with their corresponding simulated fluorescein
pattern.
All parameters, including base curve radius, optical zone, peripheral curves, and overall diameter, are determined
and can be altered and evaluated via fluorescein evaluations. The steep areas of pooling are shown in bright green and the
flat areas of touch are dark green or black. The initial RGP suggestion is a computer-generated, best-average alignment fit
with three peripheral curves. The initial values for these parameters are determined by the doctor’s default nomogram. The
Optivision CornealMap EH-300 system has both predefined nomograms and the ability to save physician-customized nomograms.
Software provides a selection of specific contact lenses
The lens software also provides a selection of specific
contact lens nomograms from different manufacturers. By choosing different diagnostic lenses, altering their parameters, and
evaluating the fluorescein patterns, you can quickly arrive at your final lens selection. The rapid fitting process benefits
patients and practitioner: Chair time, lens costs, shipping, and handling are all decreased; efficiency is increased.
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