Editorial theme | Differential diagnosis of psoriasis versus other common skin conditions

Psoriasis affects 2–3% of the general population and can affect any part of the body.¹ Due its varied clinical features, psoriasis it can be mistaken for other inflammatory, infectious, or neoplastic conditions.¹ Differential diagnosis can be difficult and may involve extensive testing to determine the correct diagnosis.¹ Here, we discuss how psoriasis may present in various parts of the body and how it can be differentiated from other similar conditions.

Scalp¹

Characteristics of scalp psoriasis are shown in Figure 1.

Figure 1. Clinical characteristics of scalp psoriasis* 

*Data from Gisondi, et al.¹

Figure 2. Differential diagnoses for scalp psoriasis*

*Data from Gisondi, et al.¹

Trunk¹

Characteristics of psoriasis of the trunk are shown in Figure 3.

Figure 3. Clinical characteristics of guttate psoriasis of the trunk* 

*Data from Gisondi, et al.¹

Figure 4. Differential diagnoses for trunk psoriasis* 

*Data from Gisondi, et al.¹

Folds¹

Characteristics of psoriasis of the skin folds are shown in Figure 5.

Figure 5. Clinical characteristics of inverse psoriasis of the skin folds* 

*Data from Gisondi, et al.¹

Figure 6. Differential diagnoses for psoriasis of the folds* 

*Data from Gisondi, et al.¹

Genitals¹

Characteristics of psoriasis of the genitals are shown in Figure 7.

Figure 7. Clinical characteristics of genital psoriasis* 

*Data from Gisondi, et al.¹

Figure 8. Differential diagnoses for genital psoriasis* 

*Data from Gisondi, et al.¹

Palms and soles¹

Characteristics of palmoplantar psoriasis are shown in Figure 5.

Figure 9. Clinical characteristics of palmoplantar psoriasis* 

*Data from Gisondi, et al.¹

Figure 10. Differential diagnoses for palmoplantar psoriasis* 

*Data from Gisondi, et al.¹

Nails¹

Characteristics of nail psoriasis are shown in Figure 11.

Figure 11. Clinical characteristics of nail psoriasis* 

*Data from Gisondi, et al.¹

Figure 12. Differential diagnoses for nail psoriasis* 

*Data from Gisondi, et al.¹ 

Erythroderma¹

Characteristics of erythroderma (due to psoriasis) are shown in Figure 13.

Figure 13. Clinical characteristics of erythroderma* 

*Data from Gisondi, et al.¹

Figure 14. Differential diagnoses for erythroderma* 

*Data from Gisondi, et al.¹

Conclusion 

Considering its wide range of clinical features and presentations, diagnosing psoriasis can be a challenge; similar to other skin diseases.¹ A correct psoriasis diagnosis is not only vital for the patient, but also for rheumatologists; psoriatic arthritis also occurs in around a third of patients with psoriasis.¹

Expert opinion

The basic message of the article can be summarized thus: "not everything that is red and that scales is psoriasis…"

The meaning

I mean that the diagnosis of psoriasis is usually straightforward for a dermatologist, but may be at times challenging because some the clinical features resemble other common skin disorders including seborrheic dermatitis, atopic dermatitis, mycosis, lichen planus, syphilis, and less common conditions, including lupus erythematosus, dermatomyositis, cutaneous T-cell lymphomas, pityriasis rubra pilaris. Diagnostic doubts may also arise when psoriasis coexists with other skin diseases, such as atopic dermatitis in the same patient.

The article provides a pragmatic guide in the differential diagnosis based on a topographical criterion, i.e., based on the localization of the lesions.

What to do if you have a diagnostic doubt?

Punch biopsy represents the gold standard for most differential diagnosis, apart from nail psoriasis. Skin histopathology shows a psoriasiform reaction pattern, defined as the presence of epidermal hyperplasia with a regular elongation of the rete ridges with bulbous enlargement of their tips. Dermal papillae contain dilated, congested, and tortuous capillaries.

The infiltrate is superficial, perivascular, lymphocytic and neutrophilic. Parakeratosis is initially focal and later confluent, containing typical collections of neutrophils (Munro’s microabscesses).

Any alternative to biopsy?

However, before skin biopsy, dermoscopy is an interesting, helpful, and moreover non-invasive procedure. In case of psoriasis, it shows red dots or globules in a regular distribution on a light red or pink background covered with white scales. The vascular features appear as bushy (glomerular) vessels.

Final comment

Differential diagnosis in dermatology is a fascinating daily challenge that is fundamental in clinical practice, because you only recognize what you know. It is a critical step before prescribing any therapy.