3. Case Discussion
Linear psoriasis is a rare clinical presentation of psoriasis characterized by the linear distribution of psoriatic lesions along the lines of Blaschko. The main differential diagnosis is ILVEN. Gross morphological distinction between these two entities is difficult. Furthermore, the two entities share similar histological findings and the coexistence of ILVEN and psoriasis has also been reported [3, 4]. Histological examination of ILVEN shows areas of hypergranulosis and orthokeratosis alternating with areas of hypogranulosis and parakeratosis [5]. Under these circumstances, immunohistochemical studies are helpful in distinguishing these two cutaneous disorders. For example, involucrin is a marker whose expression is absent in the parakeratotic areas of ILVEN but present in psoriasis [6]. Moreover, the number of Ki-67 positive nuclei is higher in psoriasis compared to ILVEN, while the number of keratin-10 positive cells is higher in ILVEN [7]. In practice, these tests are rarely ordered. It is often more practical to simply initiate therapy for psoriasis if the diagnosis is suspected.