Confocal microscope images have played a vital part in the medical decision-making process, but does that mean there should be one in every ophthalmologist’s office?
Confocal microscopy can provide real-time, high-resolution images of thin layers of living ocular tissue. Introduced to the vision care community in the early 1990s as a revolutionary tool, it was described by some at the time as “the slit lamp of the future.”1,2
While it has yet to achieve this level of usefulness, the confocal microscope continues to provide key diagnostic information. In this article, the benefits of this instrument will be explored, emphasizing differences between confocal microscopy, specular microscopy, and the slit lamp biomicroscope.
Slit Lamp vs Confocal
Compared to the slit lamp biomicroscope, the confocal microscope provides clearer images of intracorneal structures at much higher magnification. Confocal microscopy is based on sophisticated optical technology that produces an effect similar to a strobe light in a discotheque, in which one can see a very clear image of people in stopped-motion against a dark background. Everything looks extremely clear for a very brief instant in time.
Similarly, the confocal microscope can produce very sharply focused images with much of the adjacent visual “noise” eliminated. In comparison, the slit lamp provides broader views of ocular structures with continuous light and no strobe-like effects. With the slit lamp, if structures need to be examined in greater detail, higher optical magnification is limited because adjacent structures and individual cells scatter light, limiting what features can be resolved. Thus, in theory and in practice, the confocal is a superior instrument for examining smaller structures.
Where the slit lamp provides images of ocular structures in context, showing how they relate to each other, the highly magnified confocal images provide information about tissue and cellular structures with limited context. The situation is analogous to the proverbial blind men introduced to an elephant for the first time: Each feels a different portion of the animal and tries to describe it as a whole based on his own narrow experience. The confocal provides detailed information about a very small area, and is thus complementary to the slit lamp—but it will likely never be “the slit lamp of the future.”
Specular vs Confocal Microscopy
The specular microscope is similar to the confocal microscope and is ideal for imaging the central corneal endothelium. The high contrast between the endothelial tissue and the adjacent cell-free anterior chamber produces excellent, highly focused images with no light scatter. However, the specular microscope is less effective for imaging structures elsewhere in the cornea. For example, the mid-stroma and epithelium are associated with significant reflected light, which blurs the image. Nonetheless, the specular microscope is an excellent tool for examining the endothelium: it can be used to help in the diagnosis of corneal endothelial dystrophies, as well as to obtain endothelial cell counts and to characterize the size and shape of endothelial cells.
The confocal microscope can be used to take endothelial cell counts as effectively as a specular microscope, but the reverse is not always true. The advantage of the confocal microscope is greater versatility and ease of use. The practitioner is not restricted to the central area where the specular microscope works best. Images can be collected of keratic precipitates or inflammatory aggregations on the back of the cornea. Generally, more morphologic data is available with the confocal microscope in areas of the cornea that cannot be easily imaged with a specular microscope. Both instruments need similar amounts of light for imaging the corneal endothelium.
The variety of confocal microscopy applications can be increased by the use of lenses designed to penetrate deeper and more widely into the eye (Figure 3). These specialty lenses can effectively image the anterior lens capsule and limbal blood vessels in patients with limbal stem cell deficiency, for example. Vasculature can be studied, and it is possible to look at the limbus as well as the conjunctiva and lacrimal gland. Researchers are also effectively collecting video images of dynamic processes, including blood flow through ocular capillaries and tracking of individual blood cells. The sclera, however, is largely off-limits to the confocal because the randomly organized, opaque tissue results in excessive light scatter, producing poorly detailed images.
The Confocal in the Clinic
Currently, the primary clinical application of the confocal microscope is in the diagnosis of fungal and protozoan infections (Figures 1 and 2). It is ideal for the identification of these infections because the invading organisms are relatively large. The resolution of most confocal microscopes is on the order of 1 to 2 microns, while most bacteria are that size or smaller. Protozoans (eg, Acanthamoeba) are much larger (10 to 25 microns). Infectious fungi tend to be very long and thin (4 microns), and this offers resolvable images in at least one dimension.
Confocal microscopy is highly recommended for the diagnosis of atypical infections, especially those not responding to traditional antibacterial medications; those with unusual pain or discomfort; and cases with a history of exposure to contact lenses, water, or soil. The rapid identification of structures that resemble either an amoeba or a fungus can be very valuable in these cases; this is especially true because traditional laboratory techniques often fail to identify these organisms. In addition, fungal cultures can take many days to grow, potentially leaving the patient without effective treatment while waiting for culture results. The confocal offers an opportunity to characterize these infections without biopsies and start treatment much earlier.
Confocal microscopy can also be used to observe and study corneal wound healing, especially after LASIK or PRK. Although the normal cornea appears clear, it has a large number of supportive keratocytes. Following trauma, corneal cells tend to die off and collagen becomes disorganized. The confocal microscope is very effective for tracking cell loss and return, nerve damage and regeneration, and collagen fiber reorganization. The confocal microscope is also useful in monitoring ocular blood vessels, tracking abnormal vessel growth, and following the wound-healing process.
It is important to ask, though: does tracking wound healing with this level of detail affect treatment choices? At this point, the answer is a qualified no—the impact on clinical decision-making is minimal except in selected cases. The confocal microscope has the potential to help characterize the normal wound-healing response and determine whether healing is progressing as it should. One day this capacity may turn out to be important, but we are not there yet.
Diabetic Peripheral Neuropathy
The corneal nerves are part of the peripheral nervous system, and they are affected by diabetes much like other peripheral nerves. Because the cornea is transparent and corneal nerves can be easily imaged, confocal microscopy has been suggested as a noninvasive means for assessing—even screening for—diabetic peripheral neuropathy.3,4
With this new resource, clinicians can look directly at corneal nerves to diagnose neuropathy and track disease progression. Use of the confocal microscope has many advantages—it is safer, less expensive, and less painful than traditional biopsy. Although the standard of care has not changed—diabetic peripheral neuropathy is still studied by biopsy—this could be a game-changing application for the confocal microscope.
Using the Confocal Microscope
Confocal microscopy requires a well-trained technician and/or physician to operate the instrument (Figure 4). The test itself usually takes 5 or 10 minutes once the patient is positioned.
A much higher level of skill and training is required to understand and interpret the images the confocal microscope can produce. This is a skill that must be learned, similar to learning to read retinal optical coherence tomography (OCT) or corneal topographic imaging data. As was the case in the early days of corneal topography, we still have much to learn about interpreting confocal images.
To a large degree, the future of the confocal microscope is tied to the future of its primary applications. If the incidence of corneal infection increases, increased use of confocal microscopy will follow.
In addition, unlike any other current instrument, the confocal microscope can provide images of living corneal cells. No other technology, including anterior segment OCT, can do this. For applications requiring a histologic light section of the cornea, there is only confocal microscopy. As we learn more about the appearance of corneal structures in health and disease, the confocal will become an increasingly powerful resource. Over the years, we can expect this to lead to better management of cases, impacting pharmaceutical and surgical intervention decisions.
The confocal microscope has not replaced the slit lamp in the comprehensive ophthalmologist’s office—few comprehensive ophthalmologists even own one. Indeed, the confocal microscope provides different information from the slit lamp, and in today’s world, the slit lamp provides information that is far more relevant to daily practice.
We had a confocal microscope installed in 1999. Since then, proliferation in our vicinity (“Chicagoland”) has been slow but steady, and there are now confocal microscopes situated in both academic settings and in the private offices of cornea specialists around Chicago. But the total number of confocal microscopes remains relatively small for several reasons: the instrument is expensive, reimbursement is not great, and it remains difficult to obtain and interpret the images. At present, clinical applications for the confocal microscope are limited.
In some applications—eg, diagnosis of fungal and amoebic infections—the confocal offers unique value. For other applications, there are competing technologies that are at least as good. As long as this remains the case, increased use of the confocal microscope may occur, but it is likely to remain useful primarily to subspecialists. At this point, it is very hard to foresee a future in which comprehensive ophthalmologists or retina specialists adopt confocal microscopy. I can, however, envision a time when every cornea specialist has one, but that may be years from now.
As a research instrument, the confocal microscope will likely find increased utility in the study of corneal wound healing, monitoring corneal nerves, and studying intracorneal structures. Extra-corneal imaging—eg, looking at conjunctival blood vessels and conjunctival tumors—is a potential growth area for confocal microscopy. Progress in research may well lead to clinical applications that expand the utility of confocal microscopy.
One benefit we have seen from the confocal microscope is a drop in the number of corneal biopsies. Our institute sees literally dozens of patients with suspected Acanthamoeba infection each year, but today we rarely need to biopsy these patients. Instead, we have grown accustomed to interpreting confocal images, in conjunction with traditional corneal cultures and stains, and this guides our treatment process.
As worldwide research on abnormal corneal and conjunctival structures moves forward, the confocal microscope has great potential for clinicians, but that future is still undefined and some years away. I do not foresee a rush to buy confocal microscopes in the next few years.
THE BOTTOM LINE
The confocal microscope has significant utility in the diagnosis of atypical corneal infections, as well as in the assessment of limbal and conjunctival vasculature and cellular health. Though it may not assume the central role of the slit lamp biomicroscope in the comprehensive ophthalmologist’s practice, the confocal microscope will continue to be an important tool for researchers and corneal specialists.
Elmer Y. Tu, MD, is an associate professor of clinical ophthalmology and director of the cornea and external disease section of the Department of Ophthalmology and Visual Sciences, at the University of Illinois College of Medicine, Chicago, IL. He was assisted in the preparation of this manuscript by medical writer Jerry Stein, PhD.
1. Sherrard ES, Ng YL. The other side of the corneal endothelium. Cornea. 1990;Jan;9(1):48-54.
2. Cavanagh HD, Petroll WM, Jester JV. The application of confocal microscopy to the study of living systems. Neuroscience and Biobehavioral Review. 1993;17(4):483-98.
3. Edwards K, Pritchard N, Vagenas D, et al. Utility of corneal confocal microscopy for assessing mild diabetic neuropathy: baseline findings of the LANDMark study. Clin Exp Optom. 2012 May;95(3):348-54.
4. Wu T, Ahmed A, Bril V, et al. Variables associated with corneal confocal microscopy parameters in healthy volunteers: implications for diabetic neuropathy screening. Diabet Med. 2012;Sep;29(9):e297-303.