SPECIAL REPORT

Imaging Technologies to Optimize Corneal Surgery Outcomes

By John D. Sheppard, MD, MMSC, and Michael Gazzolo

The imaging demands of a cornea surgeon are high. We rely on our imaging technology to examine the anterior and posterior surfaces of the cornea, to obtain accurate corneal thickness measurements, to evaluate complications, to plan and manage refractive surgery and to improve IOL implantation in cataract surgery. To fully utilize digital imaging technology, we need quantification software, digital storage capacity, interface facility, user facility and, above all, outstanding clinical staff to procure the best possible images.

The technology is continuously improving and thanks to competition in the marketplace, our choices are expanding, making the process of equipping a practice more complex. First, you should decide what is really necessary for the practice and then determine whether or not you have the volume to support a new device. That said, I believe certain imaging devices are must-haves.

Digital Photography

There are still good reasons to obtain photographs for reference in a chart. We use digital photography for a variety of conditions, including subtle corneal ulcers, particularly when an expensive therapy, such as a ProKera lens (Bio-Tissue Inc.), will be used, or if debridement is necessary. If a patient has a nevus on the conjunctiva or vessels in the cornea, sometimes the best way to monitor for progression or regression is a simple photograph. We can always measure the diameter of a lesion, but lesions tend to be irregular, and digital photographs are useful, especially when different doctors will be examining a patient throughout the course of an illness or observation period. Digital photographs are also useful for patient education (Figure 1).

Figure 1. Digital photography clearly demonstrates dislocated I0L and capsular tear in patient with 20/40 acuity and some diplopia who was hesitant to have I0L exchange until viewing this photograph.

When evaluating a digital camera, be sure it's simple to operate so everyone in the office can master it. That way, a technician can set it up, open the file master and have all systems ready for archiving the photographs, freeing you from these time-consuming tasks.

One issue with photographic imaging devices is that they extend the length of the beam splitter. If you place most of these devices on a normal slit lamp, you must pull your neck back farther because your eyes are farther from the patient's eyes. So that should always be considered in your choice of an instrument. An all-digital imaging system can bypass this ergonomic inconvenience, I recommend a biomicroscope. A corollary to the essential beam splitter is that some light is lost with each image and as a result, the illumination in your photo may not be as bright as it would be at the normal setting. Thus, you must increase the aperture and the illumination power, allowing more light for photos as well as for your routine examination.

Corneal Topography

Corneal topography is the imaging technology we use most often. It's our workhorse. Numerous machines are available, but only a few provide outstanding selectivity in terms of both anterior and posterior surface imaging. With anterior and posterior surface imaging, we can generate a secondary pachymetry or thickness reading, which is very useful. Pachymetric precision is essential in planning LASIK, photorefractive and phototherapeutic keratectomy, corneal collagen crosslinking, corneal transplantation and lamellar keratectomy procedures. The first device to do that through topography and not ultrasonography was the Orbscan (Bausch + Lomb), followed by many others including the Pentacam (Oculus Inc.) and the Epic (Marco).

Posterior corneal surface images have great utility for diagnosis because some diseases show a significant posterior float change. For example, disproportionate posterior float steepening changes often occur in herpetic stromal keratitis (Figure 2). In these cases, there is significant steepening of the posterior float, so that the corneal ectasia is more aggressive from the back of the cornea. That hint can provide a diagnostic test and cinch your diagnosis along with corneal sensitivity and standard physical findings in response to medication.

Figure 2. This 46-year-old woman had recurrent stromal keratitis that was poorly responsive to topical trifluorothymidine. Digital topography shows classic eccentric disciform thinning by pachymetry (lower right) and steepened posterior float (upper right), highly suggestive of herpes simplex stromal keratitis. Systemic valacyclovir stabilized her previously frequent relapses.

With corneal topographers, we need clarity, multifunctionality and interface facility with electronic medical records (EMR). Many devices, such as the Epic, provide not only topography but also a wide variety of parameters that are useful in evaluating a cataract patient.

The other hot item on the topography market is the Pentacam, which is the topographer of choice for many ongoing corneal collagen crosslinking studies, including Topcon's protocol. It provides additional parameters and analyses that are useful for detecting subtle changes in corneal topography. It has good serial analysis capability, as well as next-generation optics and software, giving us more reliable and better digitally enhanced images.

Tomey topographers are also outstanding devices. They are sophisticated and integrate well with EMR. They are versatile enough to work well for a cornea practice, a referral practice or a cataract practice.

We're now in the era of epithelium crosslinking and small-incision lamellar femtosecond refractive surgery, where we leave the corneal anterior surface intact. We are also in the era of premium IOLs, where cataract patients expect perfect vision, as have LASIK patients for many years. With these demands, we must understand and know the topography of a given eye. It is essential.

Ocular Response Analyzer

The ocular response analyzer (ORA; Reichert) is a first-in-class device in a field that is not yet fully developed. This instrument measures how an eye rebounds after momentary noncontact depressive stress. Using a pulse of air, the ORA measures the recoil of the cornea by generating an initial curve and a rebound curve. By comparing the two curves and then comparing the entire graph of the ocular response or the rebound of the shape of the cornea, a number of conditions can be diagnosed, such as ectasia, keratoconus and pre-keratoconus. Doctors are exploring various ways to utilize this technology, such as in pre- and postoperative PRK and LASIK eyes. This is exciting technology that is slowly being adopted.

Ultrasonography

Although costly and more often used in academic settings, ultrasonography has found its way onto our buying list. With retina, glaucoma and cornea specialists in our practice, this technology will be useful for all of us. The sophisticated imaging techniques of new fourth-generation machines pick up the corneal epithelium through to the endothelium. We can look at LASIK flap thickness and Descemet's detachment, wound apposition and leaks, and we can even view infectious processes at the stroma. Anterior segment ultrasonography is also useful for looking at angle abnormalities or anatomical abnormalities behind the iris, where they cannot be visualized, such as a suspected ciliary body melanoma. It is a universally heralded modality, and it is billable as an image.

There are many practical uses for ultrasonography, but we should be selective in its use because any time we view an image in a busy practice, it slows things down. We're all visually oriented, but it's only when we can't see something at all or when we want to quantify something that these imaging techniques come into play. As an example, a Descemet's detachment in an opaque cornea is something we would never see optically. Although A-scan ultrasonography can quantify LASIK flap thickness intraoperatively, only B-scan imaging (Figure 3) can delineate the flap location, describe epithelial ingrowth or analyze wound apposition.

Figure 3. High resolution B scan ultrasonography compares the multiplanar LenSx incison to a uniplanar metallic blade incision. Wound leaks, poor apposition, Descemet's detachments and epithelial ingrowth are all readily identifiable with this technology.

Optical Coherence Tomography

OCT is widely used in femtosecond cataract surgery devices, but it's only now becoming available to clinical practice. The advanced-generation devices provide striking images and often can be adapted for both anterior and posterior segment imaging. The price differential over high-resolution ultrasound is improving, but OCT office technology for the anterior segment remains costly.

Specular Microscopy

Specular microscopy has been available for decades, and it's extremely important for the cornea specialist who is evaluating endothelial posterior corneal disease, and also for the cataract surgeon, who has to decide whether or not the endothelium is viable for an elective cataract procedure (Figure 4). There are numerous specular microscopes available that can take pictures and quantify the endothelium. The most advanced technology is confocal microscopy, which uses a specialized flying slit of rapid-fire imaging to sequentially image every layer of the cornea from front to back (Figure 4). With a specular camera, you can photograph the endothelium, and that's important because it gives you a reproducible, quantitative cell count and identifies the shape of the cells as well. Regular homogeneous cells are healthier than those with multiple cell shapes and a high standard deviation in cell size and surface area, termed polymegathism. With a confocal microscope, you can also view and photography the epithelium, Bowman's layer, the stroma and Descemet's membrane, as well as the endothelium in vivo. The beauty of the technology is that it is versatile. For example, we use it to look for fungal and Acanthamoeba infections because they have a specific appearance when viewed with the confocal microscope. This technology gives us sequential optical images in sections from the front to the back of the cornea. The major difficulty in procuring good images for infectious keratitis is the severe photophobia and blepharospasm experienced by many of these patients in the context of a contact device. For endothelial dystrophy, confocal microscopy is superb.

Figure 4. Confocal microscopy of normal endothelium in patient with contralateral unilateral endothelial dysfunction due to iridocorneal endothelialization syndrome. This eye was cleared for routine cataract surgery.

Key to Translating Physical Findings

Imaging is the corneal specialist's Rosetta Stone, translating the plethora of physical examination findings in challenging cases into reliable, digital, quantifiable, reproducible data packages. The precision and variety of imaging technologies now available provide tremendous benefit to the clinician, surgeon, professional photographer and patient. ■