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Lichen nitidus is a rare and benign inflammatory skin condition that was first described in 1907 by Pinkus. Prevalence is thought to be around 0.34‰ of all dermatological conditions .
It primarily affects children, adolescents and young adults, with no distinction by sex or ethnic origin. The median age of onset is 7 years in boys and 13 years in girls . However, there are cases that have been reported in individuals up to the age of 80 .
The clinical picture is one of pinhead-sized, flesh-coloured papules that are shiny (nitidus in Latin means shiny, something that shines), which can be dome-shaped or flat, range from 1 to 2 mm in diameter, and are sometimes covered with a fine scale (Figure 1). They are monomorphic and often form clusters of around ten papules, distributed into a few small plaques that are spread sparsely across the skin surface (Figure 2, Figure 3). They are usually asymptomatic, although pruritus is possible. They are usually sited on the trunk, the extensor surfaces of the limbs and the genital organs, sparing the mucosa. The Koebner phenomenon is sometimes seen. Six cases of lichen nitidus with purpura have been reported [4-9]. Some cases that have been published describe ungueal involvement consisting of longitudinal linear ridging and small cupular depressions [10-13]. On the palmar and plantar surfaces, lichen nitidus can present as keratoderma punctata or it can mimic dyshidrosis [5, 14-19].
It usually affects only a small surface area but cases of generalised lichen nitidus have been described, particularly in children. The outbreak can involve genital organs only [2, 20]. Facial, photo-induced forms (known as actinic lichen) have been reported [21-23].
A number of reports have set out an association between lichen nitidus and other pathologies: atopy [24, 25], trisomy 21 [26-31], Crohn’s disease [32-35], post-partum thyroiditis , CD4 lymphocytopenia with  or without  HIV infection, tuberculosis , following hepatitis B vaccination  or treatment with interferon alpha , although it has not been possible to prove any of these suspected associations. These reports are too rare and demonstrate too little consistency to justify investigations or follow-up in the long-term.
Some cases in families have been described, though this is uncommon [42-44].
The typical histological features (Figure 4) are a localised nodular infiltrate at the very top of the dermal papillae, pressing up against the epidermis, with elongated rete ridges of the epidermis surrounding the infiltrate from one end to another, which produces the pathognomonic “claw and ball” appearance. The infiltrate is made up of lymphocytes and histiocytes, sometimes with giant cells, which can produce a tuberculoid appearance. There is no caseous necrosis. There is atrophy of the epidermis, and in the stratum basale there are necrotic cells; the stratum granulosum is thinned or absent, and parakeratosis is found. Direct immunofluorescence is negative [45, 46]. Immunohistochemistry markers identify T cells that are predominantly CD4+ as well as CD1a+ Langerhans cells and CD86+ macrophages within the infiltrate [38, 47]. Uncommonly a perforating histological form has been reported, involving complete disappearance of the epidermis at the very top of the dermal infiltrate, although the clinical appearance remains unchanged [48-51].
There are numerous differential diagnoses consisting mainly of the usual lichen spinulosus-type skin conditions including keratosis pilaris, follicular eczema, lichen planus pilaris, atypical spinulosus-type onset of psoriasis in childhood, warts, and pityriasis rubra pilaris. Other inflammatory pathologies such as cutaneous sarcoidosis with micronodules, tubercular lichen scrofulosorum, phrynoderma in vitamin A deficiency, and papular mucinosis will be considered less commonly. Idiopathic hirsuties coronae glandis must be differentiated from an isolated genital case in young boys.
At first lichen nitidus was considered to be a variant of lichen planus. Today the clinical differences (papule size, absence of the purplish Wickham striae in lichen nitidus) and its typical histological features mean that they can now be considered to be two distinct diseases.
It usually follows a course towards complete spontaneous regression within 1 to 2 years, generally without sequelae. Residual hyperpigmentation in the papule sites has occasionally been reported . It is rare but possible for lichen nitidus to persist for over ten years [9, 25].
The aetiology of this condition is unknown. A hypothesis involving immune dysfunction can be considered in view of the presence of CD4+ T cells and Langerhans cells in the infiltrate, the reported association of lichen nitidus in a context of immune dysfunction (atopy, trisomy 21, Crohn’s disease, thyroiditis, HIV infection, tuberculosis, vaccination) and its sensitivity, at least in part, to topical or systemic immunosuppressant treatments.
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