Several diseases share similar clinical features with psoriasis. The commonest ones that can easily mimic psoriatic plaque over the lower legs are chronic eczema, nodular prurigo and lichen planus. Patients with chronic eczema tend to have personal or family history of atopy, asthma, allergic rhinitis, allergic conjunctivitis or potential allergen or irritant [3]. Dermoscopy of chronic eczema may present with red dots, yellow serocrust, focal dull white scales and vessels in clusters [4]. Histological findings in chronic eczema are nonspecific. These include spongiosis, intraepidermal vesicles, lymphocytic exocytosis [5].
For nodular prurigo, the itch is usually intense for days on end. The rashes are warty and crusted. Dermoscopy may reveal “white starburst pattern”. Histological examination shows increase in nerve fibre ending with a negative direct immunofluorescence staining [6].
For lichen planus, 50% of patients would have mucosa involvement and 10% will have nails involvement [7]. Wickham striae can be seen clinically or dermoscopically [8]. Histological findings include immunoglobulin deposition over the epidermal basal region from direct immunofluorescent stain.
Pruritus affects 64–97% of psoriasis patients [9]. The likely mechanism is neurogenic inflammation; a combination of abnormally expressed neuropeptides, increased innervation and peripheral opioid system dysfunction [10]. Treatment relies upon resolution of the psoriatic skin lesions, either with topical medications in most cases or systemic drugs in extensive or extra-cutaneous involvement.
Our case highlights the importance of distinguishing psoriasis from other skin disorders especially lichen planus despite obvious symptoms (severe pruritus) and clinical appearance (red-violaceous, flat-topped papules and plaques with minimal scales on ankles and knees). A dermoscopic examination of the skin lesion and nail changes may give some clue, which in our case, pointed towards psoriasis (dermoscopic Auspitz sign, light red background, white scales with a regularly distributed dotted blood vesse; nail ridges). However, other pruritic lesions may present with Auspitz sign [11].
Nail ridges are not specific for psoriasis. Psoriatic nail dystrophy can present as pitting, leukonychia, onycholysis, subungual hyperkeratosis, lines or ridges, nail plate crumbling and splinter hemorrhage [12]. Lichen planus nails involvement may present as nail plate thinning, grooves or ridges, nail darkening, nail thickening, onycholysis, pterygium and anonychia [13]. Other causes of vertical nail ridges include eczema, elderly, and nutritional deficiency [14].
Hand osteoarthritis (HOA) is common in female patients aged 40 years or older with positive family history. Joint pain usually occurs upon usage and morning/inactivity stiffness is usually mild [15]. Heberden nodes develop years after recurrent joint pain and swelling. PsA in its early course may have similar radiological appearance as HOA as seen in this case.
The occurrence of distal IPJ arthropathy; the absence of joint pain preceding the DIPJ deformity and Heberden nodes formation; the presence of resting joints pain, early morning joints stiffness, skin rash and nail changes added further credence to our suspicion of PsA. Ultimately, we confirmed the diagnosis with skin biopsy findings and clinical improvement with oral methotrexate and topical steroid. The rapidity of the hand joints deformity in our case indicates a very aggressive disease.
Psoriasis and lichen planus share some common features such as a red and scaly look. However, both have different pathology and distinct management. There are few reports of psoriasis and lichen planus coexistence [16]. There are also cases of lichen planus mimicking psoriasis [17]. However, case reports and pictures on psoriasis mimicking lichen planus are scarce.
To the best of our knowledge, this clinical mimicry has never been documented, especially among Asians. Our case also demonstrates the challenges of relying on history and gross physical examination alone for final identification of lichen planus versus psoriasis. It is important to remind clinicians of this association to avoid misdiagnosis and delay in appropriate treatment.
Non-invasive and accessible dermoscopic examination has proved to be a valuable tool in diagnosing such conditions [18]. Therefore, clinicians who frequently encounter skin diseases need to be acquainted with this important diagnostic tool. PsA may have clinical presentations that mimic other diseases or coexist with them. When response to treatment deviates from its expected course, clinicians should consider coexisting diseases and misdiagnosis.
