In the classic form of cutaneous LP, patients often complain of moderate to severe itch. This is the form that affects the overwhelming majority of children [3, 4]. The flexural aspects of the limbs are the most commonly affected sites. Individual lesions are polygonal, shiny, violaceous and flat-topped papules, and may be grouped together to form plaques (Figs. 32.1 and 32.2). Small delicate white streaks forming a network over the surface of these lesions, called Wickham’s striae, may also be seen. LP is a condition that demonstrates the Koebner phenomenon, whereby new lesions can appear on lines of trauma.

Mucosal involvement in childhood LP is much less common than the 60 to 70 % incidence in adults [7, 9, 11, 19]. Previous case series have reported rates ranging between 4 and 39 % [1, 2]. All mucosal surfaces including the respiratory tract, gastrointestinal tract and genitalia can potentially be affected. Oral LP is subdivided into the following types: reticular, atrophic, bullous and erosive [20]. The reticular form is the most common type, and usually presents with painless white keratotic lesions with Wickham’s striae on the buccal mucosae bilaterally (Fig. 32.3). Less commonly, lesions may be found on the palate, tongue and lips [21]. On the other hand, atrophic, bullous and erosive type lesions cause burning sensations and pain. The atrophic form causes atrophic changes with erythema of the oral mucosa, while the bullous form manifests as fluid-filled vesicles and/or bullae. Erosive LP can result in painful ulcerated areas that may be complicated by secondary infection and scarring.

Nail involvement occurs in up to 10 % of adult patients with LP, but is less common in the pediatric age group. Most prior studies have reported rates varying from 0 to 9 % [1, 7, 9–11, 13, 19]. However, more recent studies have found a higher incidence of nail LP in children (13.9 and 19 %) [3, 4]. Concomitant cutaneous or mucosal LP may not be present, and biopsy of the nail matrix is sometimes required to confirm the diagnosis [22]. In a major study on pediatric nail LP, Tosti et al. classified nail LP into three categories: typical nail LP, trachyonychia and idiopathic nail atrophy [23]. Typical nail LP can present in various ways: irregular longitudinal ridging and grooving of the nail plate, thinning of the nail plate, splitting or nicking of the nail margin, pterygium formation, onycholysis, subungual hyperkeratosis and red or brown discoloration. Trachyonychia is also called twenty-nail dystrophy, in which all nails are affected and show ridging and/or pitting, lack of lustre, splitting and roughening likened to sandpaper [24]. Idiopathic nail atrophy is seen only in children and refers to the rapid and diffuse destruction of nails over a few months [23].

Scalp LP is also known as follicular LP or lichen planopilaris. Keratotic plugs surrounded by violaceous erythema are seen on the scalp in this condition, and if untreated, can lead to scarring alopecia (Fig. 32.4) [1, 9].

Although LP is classically considered to be a papulosquamous eruption, there are many different variants. These include the actinic, annular, atrophic, bullous, erosive or ulcerative, eruptive, hypertrophic, linear and pigmented forms (Table 32.2). The eruptive, hypertrophic and linear types are the most common variants in children of color [3, 4]. Actinic LP and pigmented LP are more commonly seen in tropical countries such as India, possibly due to greater ultraviolet radiation exposure [6, 11, 13]. Another notable variant of LP is an entity termed lichen planus pemphigoides (LPP). This is a rare autoimmune condition in which patients have antibodies to type XVII collagen/bullous pemphigoid 180 antigen. While bullous LP is characterised by blister formation on pre-existing LP lesions, LPP is marked by the development of bullae on both lesional and normal skin [25–27]. Occasionally, LP can occur with other conditions and features of both disorders are present concurrently. Some examples include the lichen planus-lupus erythematosus and lichen planus-lichen sclerosus overlap syndromes [28, 29]. Lesions prominent in sun-exposed areas merit lupus screening.