106: Psoriasis and Psoriatic Arthritis

Key Points

• Disease summary: Psoriasis and psoriatic arthritis (PsA) are complex genetic disorders involving multiple genes and epigenetic factors that contribute to disease manifestation and progression.

  • Typical psoriasis lesions are well demarcated, scaly erythematous plaques that occur in various sizes and shapes. Psoriasis most often manifests on the extensor surfaces of the elbows and knees, as well as the scalp and sacral lesions.

  • PsA is an inflammatory arthritis associated with psoriasis. Approximately 30% of patients with psoriasis will at some point develop psoriatic arthritis. The psoriasis precedes the onset of arthritis in approximately 70% of patients, occurs concurrently in 15% of patients, and follows the onset of psoriasis in the remaining 15% of patients.

  • The diagnosis of PsA is dependent on clinical, laboratory, and radiologic assessments. The recently published classification criteria, ClASsification criteria for Psoriatic ARthritis (CASPAR) is now widely accepted for classifying this entity.

• Differential diagnosis:

  • The differential diagnosis of psoriasis includes atopic dermatitis, contact dermatitis, pityriasis rosea, pityriasis alba, seborrheic dermatitis, nummular eczema, and lichen planus. The differential diagnosis for PsA includes rheumatoid arthritis, ankylosing spondylitis, Reiter syndrome, inflammatory bowel disease (IBD) arthropathy, and systemic lupus erythematous.

• Monogenic forms:

  • There are no monogenic forms for psoriasis and PsA. The genes associated with psoriasis and PsA appear to be multiple with each displaying modest effect size.

• Family history:

  • An affected first-degree relative confers a relative risk between 4 and 10 for psoriasis. PsA displays stronger heritability than psoriasis, as an affected first-degree relative confers a relative risk between 30 and 47.

• Twin studies:

  • Twin studies in psoriasis demonstrate that there is a threefold increase in concordance in monozygotic versus dizygotic twins. The one twin study in PsA did not reveal significant heritability.

• Environmental factors:

  • Trauma, streptococcal infection or HIV infection may precipitate or worsen psoriasis and PsA.

• Genome-wide associations: Numerous genes are associated with the pathogenesis of both psoriasis and PsA (Tables 106-1 and 106-2; Figs. 106-1 and 106-2).

• Pharmacogenomics:

  • Polymorphisms exist that affect the pharmacokinetic and pharmacodynamic profiles of many drugs used in the treatment of psoriasis and PsA (Table 106-3). Pharmacogenetic data specific to psoriasis and PsA are lacking.