5.1　Dermoscope

Dermoscope，also known as skin surface microscope，incident light microscope，etc.，is a non-invasive microscopic imaging technique that can magnify tens of times and enables the visualization of submacroscopical structures invisible to the naked eyes，such as the lower epidermis，dermal papilla layer，and deep dermis.These features have special and clear correspondences with skin histopathological changes.Based on these correspondences，the dermoscopic diagnosis exhibits sensitivity and specificity.Thus，dermoscope is also called“dermatologist's stethoscope”.It is the most widely used imaging technique in dermatology.

As early as 1663，Kohlhaus used a microscope to observe the blood vessels in the nail bed.In 1893，Unna used immersion oil and glass slides to observe the skin lesions of patients with lupus vulgaris under a microscope，and called this method as“diaskopie”in German.In 1916，Zeiss invented a binocular skin angioscopy.In 1920，a German dermatologist Saphier used the phrase“dermatoskopie”and upgraded the dermoscope.The traditional external light source was replaced with a built-in light source，so that the capillaries of the skin lesions could be clearly observed.The prototype of modern dermoscope appeared in 1951.An American dermatologist Goldman observed and evaluated the microscopic characteristics of various skin pigmented patients（including pigmented moles and melanoma）with monocular magnifying glass.After 1970，dermoscope has been widely developed and used in clinical dermatology.In 1971，Mackie studied and summarized the significance of dermoscope for the preoperative diagnosis of pigmented dermatosis and its role in distinguishing benign pigmented nevus and melanoma.In 1981，Fritsch et al.distinguished melanoma and pigmented nevus based on the characteristics of different pigment networks upon dermoscope.In 1987，Pehamberger et al.published a paper describing the characteristics of various pigmented dermatosis upon dermoscope，established a model analysis method for the diagnosis of pigmented dermatosis，and promoted the formation of dermoscope diagnostic methods.In 1991，Kreusch and Rassner published the first atlas of dermoscopy，emphasizing the relationship between characteristics upon dermoscope and histopathology of dermatosis.Till now，dermoscope has been more and more widely used in clinical dermatology，whether it is in pigmented or non-pigmented skin lesions，vascular morphology，or even hair growth can be observed，and the diagnostic criteria of dermoscopy is constantly improved.

In general，since the optical density and refractive index of the stratum corneum are different from those of air，most of the light irradiated on the skin is directly reflected，part of which is absorbed by skin.Only a small amount of light enters the skin through scattering.Therefore，the structure under the epidermis is difficult to observe with the naked eyes.The specific operation of dermoscope is as follows（Figure 5-1）.Add an infiltrating fluid（e.g.，water，mineral oil，ethanol，and gel）on the skin to increase the light transmission of the stratum corneum and reduce the reflected light；cover the glass slide on it，flat the local skin，and the light source enters from an appropriate angle.With the assistance of optical magnification equipment，invisible structures and features such as the lower epidermis，dermal papilla layer and deep dermis can be observed.As shown in the latest research，medical alcohol as an infiltrating liquid produces fewer bubbles，thereby obtaining clearer images，with the characteristics of non-greasy，non-staining，natural volatilization without wiping，and effectively reducing bacterial contamination.In addition，in the eye or mucosal area，hydrogels（e.g.，ultrasonic coupling agents）are widely used due to their non-fluidity and non-irritating advantages.(https://www.daowen.com)

In the early dermatological clinic，the diagnosis of dermatosis mainly relied on the experience of dermatologists and histopathological examination.Although histopathology is the gold standard and currently the most reliable clinical diagnosis technique in dermatology，it is a traumatic inspection method and is difficult to inspect one by one for multiple pigmented skin lesions.Dermoscope has a unique advantage to diagnose multiple pigmented skin lesions in the early stage.It can determine the lesions that need biopsy，and diagnose the features of skin lesions in a large area to ensure the accuracy of the resection site.As a screening and diagnosis tool for many clinical diseases，dermoscope has the advantages of non-invasive，in vivo and rapid diagnosis with a good prospect.At present，there are many types of diagnostic methods for dermoscope：pattern analysis，ABCD rule，Menzies'method，7-point checklist，TADA method，etc.Among them，pattern analysis has shown great advantages in the differential diagnosis of various pigmented skin lesions，especially in the differentiation of benign and malignant melanocytic skin lesions.

Melanoma is a malignant tumor that usually occurs in skin.It has attracted more and more attention due to its increasing morbidity and mortality.Early diagnosis of melanoma is the most effective way to reduce its mortality.Before it goes into aggressive melanoma，the cure rate is 98％.Once melanoma breaks through the dermis or even metastasizes，the cure rate will be greatly reduced to 15％.Pattern analysis is regarded as the classic dermoscopic method for evaluating pigmented skin lesions.This method includes the assessment of the symmetry of the lesion，the presence of one or more colors，the global dermoscopic appearance of the lesion according to predefined patterns，and the presence of local features.The global pattern results from predominant features occupying large areas of the lesion.A global pattern usually consists of one（usually）or two（less often）predominant features.In the presence of more than two predominant features，the pattern is classified as multicomponent.Instead，local features can be recognized as single or grouped characteristics，and several of them can coexist in the same lesion.There are five basic global patterns，including reticular（resulting from pigment network），globular（resulting from multiple globules），starburst（resulting from peripheral streaks or pseudopods），homogenous（resulting from structureless pigmentation），and multicomponent（resulting from the combination of more than two of the above patterns）.The first four patterns can be seen in both nevi and melanoma，whereas the multicomponent pattern is directly suggestive of melanoma（Figure 5-2）.If a lesion exhibits one of the first four patterns，further assessment will be based on the overall symmetry，colors，and the presence of local features，so-called“melanomaspecific criteria”.In general，nevi are characterized by symmetry of structures and display one or two colors.In contrast，melanoma exhibits architectural disorder and often display more than two colors.