Figure 2-15 Topography of a normal cornea with regular astigmatism. The white circle indicates the pupil. Simulated

keratometry is 41.3, 46.2@102. (Courtesy of John E. Sutphin, MD.)

Corneal Tomography

Placido disk–based topography describes only the surface corneal curvature (power), whereas corneal tomography provides details such as the anterior and posterior corneal curvature, corneal thickness, and anterior chamber depth, as well as information on the iris and lens. The Orbscan IIz (Bausch + Lomb, Rochester, NY) combines an advanced Placido disk system with slit-scanning technology and derives its posterior elevation map mathematically. This map may overestimate the posterior corneal curvature, however, especially in patients who have undergone LASIK procedures. The Scheimpflug system creates an optical section of the cornea and lens, producing a 3-dimensional image of the anterior segment. The Pentacam (Oculus, Lynwood, WA) uses a rotating Scheimpflug camera, whereas the Galilei (Ziemer USA, Inc, Wood River, IL) combines a dual Scheimpflug camera system with Placido disk technology.

Scheimpflug camera–based systems present considerable information, including anterior curvature, corneal thickness, anterior chamber depth, anterior and posterior elevation, and pupil indices; they also provide keratoconus detection and classification. As a result, the pachymetry and

topography of the entire anterior and posterior surface of the cornea can be displayed (Fig 2-16). There is also a densitometry function that measures the amount of corneal or lens opacification, information that is useful in observing patients with Fuchs corneal dystrophy or those who have undergone endothelial keratoplasty. In addition, these systems can provide a measurement of the true corneal power for use in IOL power calculation.

Figure 2-16 Scheimpflug image of a 65-year-old patient with Fuchs endothelial dystrophy and cataract. The general display clearly depicts epithelial and endothelial opacity of the cornea with a densitometry measurement of 49.7 (normal, 22–30) and the lenticular opacity with a densitometry reading of 37.0. In addition, keratometry, axis of astigmatism, corneal thickness,

and anterior chamber depth are provided. (Reproduced with permission from Goins KM, Wagoner MD. Imaging the anterior segment. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2009, module 11.)

Indications

Corneal evaluation is important in the preoperative evaluation of cataract and refractive surgery patients. For most normal corneas, keratometry measurements and corneal topographic maps are accurate and can be used in contact lens fitting or IOL power calculation. They are also useful in detecting irregular astigmatism in which the reflected images cannot be superimposed or are not regular ovals. In these patients, the surface regularity index will be reduced. Patients with corneal warpage (irregular astigmatism and/or peripheral steepening, distorted keratoscopic mires) due to contact lens wear should be instructed to discontinue the lenses until the corneal map and refraction stabilize.

Corneal topography is helpful to screen for forme fruste or subclinical keratoconus, particularly

in prospective refractive surgery patients. Corneal tomography may provide more useful information in these patients, as it may reveal subtle changes in the posterior corneal curvature that may precede the development of anterior steepening. Pellucid marginal degeneration is characterized by peripheral steepening or a “crab claw” configuration on corneal topography. Measurements of the posterior corneal curvature and pachymetry provide important confirmation of thinning and steepening, ensuring an accurate diagnosis (Fig 2-17).

Figure 2-17 Topography of a patient with pellucid marginal degeneration. The “crab claw” appearance is fully developed, with central flattening and inferior steepening; forme fruste keratoconus may have a similar but less definite appearance. (Courtesy

of John E. Sutphin, MD.)

Corneal evaluation can also be used to show the effects of keratorefractive procedures. Preoperative and postoperative maps may be compared algebraically to determine whether the desired effect was achieved. Corneal mapping may help explain unexpected results, including undercorrections, aberrations, induced astigmatism, or glare and halos, by detecting decentered or inadequate surgery; in addition, it may help confirm the expected physiologic effects of refractive surgery. For example, in LASIK for myopia, the ablation profile leads to flattening of the central cornea and a relative peripheral steepening.

Corneal mapping is useful in managing congenital and postoperative astigmatism, particularly following penetrating keratoplasty. Complex peripheral patterns may result in a refractive axis of astigmatism that is not aligned with a topographic axis. Before removing sutures or performing surgery, the surgeon must identify the steep axis based on the corneal curvature and not the refraction (incisional surgery is done on the steep axis, compression sutures on the flat axis). See also Chapter 15 for discussion of the management of astigmatism after corneal transplantation.

In patients with previous radial keratotomy, photorefractive keratectomy, or LASIK, neither the keratometer nor the corneal topographer measures the true central corneal power, and other methods

are required for IOL power calculation. In these patients, corneal tomography can be very helpful. See also BCSC Section 3, Clinical Optics, and BCSC Section 13, Refractive Surgery.

Belin MW, Asota IM, Ambrosio R Jr, Khachikian SS. What’s in a name: keratoconus, pellucid marginal degeneration, and related thinning disorders. Am J Ophthalmol. 2011;152(2):157–162.

Courville CB, Klyce SD. Corneal topography. In: Foster CS, Azar DT, Dohlman CH, eds. Smolin and Thoft’s The Cornea: Scientific Foundations and Clinical Practice. 4th ed. Philadelphia: Lippincott Williams & Wilkins; 2004:175–185.

Martinez CE, Klyce SD. Keratometry and topography. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:161–176.

Additional Imaging Techniques of the Cornea and Anterior

Segment

Ultrasound Biomicroscopy

Anterior segment echography, or ultrasound biomicroscopy (UBM)—specifically, high-frequency ultrasonography—uses a water-bath immersion technique to image the anterior segment. With this technique, the depth of tissue penetration is approximately 5 mm and the resolution is 35 to 70 μm. UBM allows structures to be viewed through opaque media. Figure 2-18 is an example of ultrahighfrequency biomicroscopy of the normal limbus. This technology allows impressive visualization of the iris, ciliary body, and ciliary processes, thereby enabling accurate white-to-white, sulcus-to- sulcus, and angle width measurements, which are essential for selection of the appropriate lens size prior to placement of a phakic refractive implant. UBM can also be used in the diagnosis and followup of iris cysts and iris/ciliary body melanomas. In addition, UBM is particularly useful in cases of trauma, as it allows visualization of the iris and lens even in the patient with an “eight-ball” hyphema. Also, using UBM, one can search for possible angle recession or cyclodialysis in the patient with severe blunt trauma to the eye.