Figure 2-18 Ultrasound biomicroscopic visualization of the entire anterior segment, including structures behind the iris pigment epithelium, thereby permitting precise determination of the sulcus-to-sulcus measurements prior to phakic refractive
implant. (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.)
Anterior Segment Optical Coherence Tomography
Optical coherence tomography (OCT) is a noninvasive technology that produces 2-dimensional, high-resolution, and high-definition cross-sectional images of ocular tissue. These images are similar to ultrasonographic images, but they are based on the emission and reflection of light (lowcoherence interferometry). The extremely fine resolution of the images (5 to 10 μm) allows exquisite delineation of the layers of the cornea, anterior chamber, and iris. There are 2 types of anterior segment OCT: time-domain (TD-OCT) and Fourier-domain (FD-OCT), also called spectral-domain (SD-OCT). Recently, a new instrument was released that combines Placido disk corneal topography with TD-OCT.
OCT angle scans can measure the depth, width, and angle of the anterior chamber (Fig 2-19). The corneal pachymetry feature of these devices is useful in the preoperative evaluation of patients with Fuchs corneal dystrophy and in the postoperative follow-up of endothelial keratoplasty cases in which the shape and thickness of the donor lenticule can be quantified. In LASIK patients, this function can be used to measure the thickness of the corneal flap and the residual stromal bed to determine the safety of an enhancement (re-treatment). Recent software included with OCT devices provides information on corneal curvature and epithelial thickness, which is helpful for screening refractive
surgery patients. Software is also available to calculate the true corneal power, which can be used in IOL power calculation after LASIK or photorefractive keratectomy (Fig 2-20).
Goins KM, Wagoner MD. Imaging the anterior segment. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2009, module 11.
Jancevski M, Foster CS. Anterior segment optical coherence tomography. Semin Ophthalmol. 2010;25(5-6):317–323.
Figure 2-19 Anterior segment optical coherence tomography (OCT) image of a phakic eye. The central anterior chamber depth is 2.73 mm, and there is moderate narrowing of the anterior chamber angle. (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.)
Figure 2-20 Anterior segment OCT display showing true corneal power, which is useful for calculation of intraocular lens
power. (Courtesy of Robert W. Weisenthal, MD.)
Confocal Microscopy
The scanning confocal microscope can be used to study cell layers of the cornea, even in patients with edema and scarring. Compared with ultrasonography or OCT, confocal microscopy provides greater spatial resolution and higher magnification, particularly in the z-axis. This allows for in vivo optical sections of the cornea with a resolution at cellular and subcellular levels. Confocal microscopy has been used to help diagnose infectious crystalline keratopathy, fungal keratitis, and amebic keratitis. It has also been used in the follow-up of refractive surgery patients to analyze haze formation and the complications of LASIK flaps, such as epithelial ingrowth.
Four types of confocal microscopes have been described for clinical use: (1) the tandem-scanning (TSCM), (2) the scanning-slit (SSCM), (3) the laser scanning (LSCM), and (4) a single-sided disk design that is not commercially available. The first 3 are approved by the US Food and Drug Administration. They differ in several ways, but, in general, the TSCM provides a shallower depth of field and better anterior-posterior localization and reconstruction. The SSCM is more user friendly and, as a result, is the most commonly used technique. The LSCM provides the highest resolution, to approximately 1–2 μm (Fig 2-21).
Goins KM, Wagoner MD. Imaging the anterior segment. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2009, module 11.
Petroll WM, Cavanagh HD, Jester JV. Confocal microscopy. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed.
Vol 1. Philadelphia: Elsevier/Mosby; 2011:205–220.
Figure 2-21 Confocal microscopic image at the level of deep stroma shows fungal hyphae. Carets denote branching hyphae
(bh). (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.)
External and Slit-Lamp Photography
External eye photography is usually performed with a single-lens reflex camera. Magnification up to 1:1 (life-size) can be obtained with a bellows, extension ring, or close-focusing lens. Digital or 35mm cameras may also be attached with an adapter to a slit lamp and will produce excellent-quality images, particularly if used with external illumination.
Slit-lamp photography and videophotography allow a permanent record of most anterior segment conditions.
Mártonyi CL. Slit lamp examination and photography. In: Krachmer JH, Mannis MJ, Holland EJ, eds. Cornea. 3rd ed. Vol 1. Philadelphia: Elsevier/Mosby; 2011:89–118.
Specular Microscopy
Specular microscopy (contact and noncontact techniques) can be an important diagnostic tool,