USMLE Guide: Psoriasis

Introduction

Psoriasis is a chronic, immune-mediated skin disorder characterized by red, scaly plaques that can cause significant physical and psychological distress to patients. It affects approximately 2-3% of the global population and has a multifactorial etiology involving genetic predisposition, environmental triggers, and immune dysregulation. This guide will provide an overview of the pathophysiology, clinical features, diagnosis, and management of psoriasis, focusing on key concepts frequently tested in the USMLE exams.

Pathophysiology

Psoriasis is primarily driven by an abnormal immune response involving T cells, dendritic cells, and cytokines. The exact cause is not fully understood, but it is believed to result from a combination of genetic susceptibility and environmental triggers. Key points to remember include:

• Genetics: Psoriasis has a strong genetic component, with certain human leukocyte antigen (HLA) alleles, such as HLA-Cw6, being strongly associated with the disease.
• Immune Dysregulation: In psoriasis, there is an overactivation of T cells, particularly Th1 and Th17 cells, leading to an increased production of pro-inflammatory cytokines like tumor necrosis factor-alpha (TNF-α), interleukin-17 (IL-17), and interleukin-23 (IL-23).
• Koebner Phenomenon: Psoriatic skin lesions can develop at sites of trauma or injury, a phenomenon known as the Koebner phenomenon.

Clinical Features

Psoriasis can manifest in various forms, but the most common presentation is chronic plaque psoriasis. Key clinical features to recognize include:

• Plaques: Well-demarcated, erythematous, and scaly plaques typically found on extensor surfaces (elbows, knees) and the scalp.
• Silvery Scale: The plaques are covered with silvery-white scales, which can be easily scraped off.
• Auspitz Sign: Pinpoint bleeding points after scraping off the scale (Auspitz sign) is a classic finding in psoriasis.
• Nail Involvement: Nail changes, such as pitting, onycholysis, and oil spots, are common in psoriasis.
• Psoriatic Arthritis: Up to 30% of individuals with psoriasis may develop psoriatic arthritis, characterized by joint pain, swelling, and stiffness.

Diagnosis

The diagnosis of psoriasis is primarily clinical, based on the characteristic appearance of skin lesions and associated findings. However, additional investigations may be required to confirm the diagnosis and assess disease severity. Important diagnostic considerations include:

• Skin Biopsy: A skin biopsy may be performed in atypical cases or to rule out other differential diagnoses. Histopathology typically reveals epidermal hyperplasia, parakeratosis, and inflammatory infiltrates.
• Psoriasis Area and Severity Index (PASI): PASI is a scoring system used to assess the severity and extent of psoriasis involvement. It incorporates objective measures such as erythema, scaling, and plaque thickness.
• Rule Out Differential Diagnoses: Psoriasis should be differentiated from other conditions with similar clinical features, such as seborrheic dermatitis, tinea corporis, and lichen planus.

Management

Management of psoriasis involves a stepwise approach, starting with topical treatments and escalating to systemic therapies for severe or recalcitrant cases. Key management strategies to be familiar with include:

• Topical Treatments: First-line therapy includes topical corticosteroids, vitamin D analogs (calcipotriene), and tar preparations. Emollients and keratolytics can also be used to reduce scaling.
• Phototherapy: Narrowband ultraviolet B (NB-UVB) phototherapy is an effective treatment option for widespread psoriasis. Psoralen plus ultraviolet A (PUVA) therapy, combining photosensitizing agents with UVA exposure, may be considered in severe cases.
• Systemic Therapy: Systemic agents, such as methotrexate, acitretin, and cyclosporine, are reserved for moderate-to-severe psoriasis or when topical treatments fail. Biologic agents targeting specific cytokines (e.g., TNF-α inhibitors, IL-17 inhibitors) have revolutionized the treatment of psoriasis.
• Patient Education: Psoriasis is a chronic condition, and patient education is crucial. Counseling on triggers (stress, infections), lifestyle modifications, and adherence to treatment are essential components of psoriasis management.

Conclusion

Psoriasis is a chronic immune-mediated skin disorder with a significant impact on patients' quality of life. Understanding the pathophysiology, clinical features, diagnosis, and management of psoriasis is essential for medical professionals preparing for the USMLE exams. By familiarizing oneself with the