Unilateral blaschkoid lichen planus
A 42-years-old male presented with itchy rash which initially appeared over left side of the abdomen and later spread to involve left upper and lower limbs within 4 weeks. On examination, there were violaceous papules and plaques with scaling arranged in inverted S-shape manner over left side of abdomen. There were linear lesions on left upper limb from axillae to the palm and also on left lower limb from thigh to dorsum of foot. Mucosal surfaces, nails, and scalp were uninvolved. There was no significant past history and there was no drug intake or any local application prior to appearance of lesions. Laboratory investigations including complete blood count, renal and liver function tests were within normal limits. Tests for hepatitis C virus and human immunodeficiency virus were negative. Histopathological examination showed hyperkeratosis, hypergranulosis, and band-like lymphocytic infiltrate at dermo-epidermal junction confirming diagnosis of lichen planus.

Blaschkoid and unilateral LP may be misdiagnosed as naevoid psoriasis, inflammatory linear verrucous epidermal nevus (ILVEN), lichen striatus, and contact allergic dermatitis. Lesser pruritus and involvement of other classic sites of psoriasis like scalp and nails and biopsy can differentiate naevoid psoriasis from lichen planus. ILVEN mostly occurs below 5 years of age and generally over legs with intense pruritus. Lichen striatus predominantly occurs in children below 15 years. It is seen as asymptomatic linear papules arranged in form of band with slight scaling and hypopigmentation over proximal part of limbs with spontaneous resolution in 3 to 6 months.

Source: http://www.idoj.in/temp/IndianDermatolOnlineJ105606-2544584_070405.pdf