708 From Wellman Laboratories of Photomedicine, Boston, a the Contact Dermatitis Unit, Dermatology Department, Massachusetts General Hospital, Harvard Medical School, Boston, b and Lucid Inc, Henrietta. c Supported in part by Lucid Inc. Accepted for publication Jan 28, 1999. Reprint requests: Salvador González, MD, Wellman Laboratories of Photomedicine, BHX-630, Massachusetts General Hospital, 55 Blossom St, Boston, MA 02114. Copyright © 1999 by the American Academy of Dermatology, Inc. 0190-9622/99/$8.00 + 0 16/1/97350 Contact dermatitis (CD) is the most common form of occupational dermatosis and affects approximately 20% of the population in the United States. 1 However, its pathophysiology is only par- tially understood. CD is divided into irritant and allergic mechanisms, which are often indistin- guishable clinically. Patch testing is an effective tool in the differential diagnosis of CD. However not all positive skin test responses are true posi- tives 2 and may not reveal the true environmental antigen causing CD. Furthermore, patch tests fre- quently fail to distinguish between allergic and irritant forms of CD because of similarity of mor- phologic features, both at the clinical and histolog- ic level. 3 Conventional histology is an invasive technique that also has inherent limitations such as lack of early time data, destruction of target, and exogenous artifacts from fixation, processing, and staining. Real-time confocal microscopy (CM) of skin in vivo might overcome the limitations of patch-testing and histology. A video-rate (real-time) CM for imaging living human tissue was developed recently. 4 Cellular- level images of noninvasive (virtual) tissue sections in vivo are obtained with resolution com- parable to that of standard histology. The measured lateral resolution is 0.5 to 1 μm, and axial resolu- tion (virtual section thickness) is 3 to 5 μm.* Imaging is possible to a depth of 300 to 400 μm, which includes the entire epidermis, papillary der- mis, and superficial reticular dermis. These are the layers most affected by acute CD. Furthermore, in vivo CM is entirely painless, noninvasive, and does not affect the tissue; thus it can be performed as many times as required without altering tissue structure. Allergic contact dermatitis: Correlation of in vivo confocal imaging to routine histology Salvador González, MD, PhD, a Ernesto González, MD, b W. Matthew White, BS, a Milind Rajadhyaksha, PhD, a,c and R. Rox Anderson, MD a Boston, Massachusetts, and Henrietta, New York Background: Allergic contact dermatitis (ACD) is a common and often challenging clini- cal problem. In vivo near-infrared confocal reflectance microscopy (CM) is a new vital microscopy technique. Objective: CM was used to evaluate acute ACD. Methods: Patch testing by means of Finn Chambers technique was performed in 5 subjects to induce an acute allergic skin reaction. Noninvasive CM images from normal and eczematous skin were sequentially recorded before and after removal of the Finn Chambers. Results: The epidermis and papillary dermis were clearly seen in high resolution. Retention of nuclei in stratum corneum, epidermal edema with microvesicle formation, and transepidermal migration of inflammatory cells were observed in vivo. Isolated den- dritic cells were present in the ACD sites of 2 subjects, with morphology, size, and loca- tion consistent with Langerhans cells. Dermal vasodilation was observed as well. Conclusion: CM is a useful tool to study ACD and may be able to track Langerhans cell activation. (J Am Acad Dermatol 1999;40:708-13.) *Rajadhyaksha M, González S, Zavislan JM, Anderson RR, Webb RH. Near-infrared confocal microscopy improves imaging of human skin in vivo. J Invest Dermatol (submitted for publication).