Lichen planus may occur as a bullous eruption confined to the mouth. Erosive or ulcerative lesions usually develop in this type of involvement. When the characteristic white reticulate or papular lesions are absent, diagnosis is difficult. Careful study of the histologic features will point to the diagnosis of lichen planus in these confusing cases.
Shklar G. Erosive and Bullous Oral Lesions of Lichen PlanusHistologic Studies. Arch Dermatol. 1968;97(4):411-416. doi:10.1001/archderm.1968.01610100051007