ReviewIn vivo confocal microscopy for the oral cavity: Current state of the field and future potential
Introduction
Confocal microscopy (CM) is a noninvasive method for imaging superficial soft tissues, to approximate depths of 200–300 μm [1], [2]. A confocal microscope consists of a point source of light which is focused to a small three-dimensional illuminated spot or a “voxel” within tissue. Light from this illuminated voxel is then collected onto a detector through a small aperture or a pinhole, and forms a “pixel”. Light from out-of-focus regions is spatially filtered and rejected. In a confocal configuration, we can illuminate and detect only a single voxel at a time, but in reality we need to look at large areas or volumes of tissue. So, the voxel is scanned in two dimensions to create a thin plane of illumination. As the voxel is scanned, we collect light from each location in the illumination plane, which is detected through the pinhole to produce pixels, which then produce an image. In this manner, a thin section of tissue may be imaged with high resolution and high contrast. This process is known as “optical sectioning”. We can optically – i.e. noninvasively – section tissue, instead of having to physically cut the tissue into thin slices, as done for conventional light microscopy. The optical sections or images are oriented en face, meaning parallel to the tissue surface. By translating the microscope lens along the optical axis toward or away from the tissue, one can image deeper or shallower, and capture a stack of optical sections – i.e. a stack of images – in depth. At any chosen depth, a two-dimensional matrix of images may be collected and stitched together to create a mosaic. A mosaic shows a larger area of tissue, equivalent to displaying at lower magnification, as is necessary for pathology.
The imaging is with high lateral and optical sectioning [1], [3], enabling subcellular structures to be visualized, and good correlation with histology [3]. The current main in vivo clinical applications for CM have been in the areas of ophthalmology and cutaneous dermatological conditions. These latter cutaneous uses include cosmetic studies, dermato-oncology (diagnosis of pigmented and non-pigmented lesions, mapping of the area of concern, targeted biopsy of lesions and follow-up), allergic reactions and inflammatory dermatosis evaluations [2].
CM is also a promising tool for diagnosing and helping to manage oral mucosal pathologies. A number of near-real time (ex vivo) studies have shown excellent correlation of CM findings, aided by imaging enhancing agents (such as acetic acid and acriflavine hydrochloride), with histopathology in oral mucosal diseases [4], [5], [6], [7]. The in vivo use of CM for oral mucosal pathology is the logical next step towards advancing patient care. This, however, requires careful evaluation before it can be routinely used in clinical practice.
The purpose of this review is to analyse the current scope and evidence for in vivo applications of CM in diagnosing and managing patients with oral mucosal pathology. The secondary purposes are to highlight some current developments with CM devices relevant for oral applications, and to identify future areas where CM holds potential for management, but has yet to be evaluated in the literature based on our search results.
Section snippets
Methods
We searched Ovid Medline® from inception through to May 2015 using 7 focused literature searches for relevant articles (Fig. 1). Search terms were each mapped to a subject heading (MeSH), and combined by the Boolean operator ‘AND’.
A duplicate search was conducted with 7 Google® searches. A further 3 searches were done using the terms “inflammatory oral confocal”, “mucosal confocal”, and “confocal squamous cell oral” in Google® alone, as Ovid Medline® did not have exact matching MeSH headings
Results
Fig. 1 shows the results of the systematic literature search. Table 1 summarizes the 25 included papers [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32]. The included studies were predominantly case reports or case series, with some having consistently applied histopathology as a reference standard (evidence levels 3 and 4, Oxford Centre for Evidence-Based Medicine [33]). There were two
Discussion
The results from our literature review showed that the current scope and evidence for in vivo CM diagnosis and management of oral mucosal pathology is limited. The focus so far, understandably, has been first to appreciate the appearance of normal oral epithelium. This is important, as the oral mucosa differs in several ways from cutaneous epithelium. For example, the differing regions of keratinization in the mouth, the taste glands, and the minor salivary glands are all important to
Conflict of interest statement
Dr. Milind Rajadhyaksha is a former employee of and owns equity in Caliber ID (formerly, Lucid Inc.), the company that manufactures and sells the VivaScope confocal microscope. The VivaScope is the commercial version of an original laboratory prototype that was developed by Dr. Rajadhyaksha when he was at Massachusetts General Hospital, Harvard Medical School.
All other authors declare no conflicts of interest.
Acknowledgements
Dr Nigel G. Maher was awarded an Australian Postgraduate Award from the University of Sydney for his Master of Philosophy degree.
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