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Psoriasis Scalp Severity Index

Physician-Reported Clinical Assessment

Calculate the PSSI for scalp psoriasis from erythema, induration and desquamation grades multiplied by the scalp area involved. Includes change from baseline, PSSI 50 to PSSI 100 responses and the PSSI 12 trial threshold.

About

About the PSSI

Clinical Background

The Psoriasis Scalp Severity Index (PSSI) is a scalp-specific adaptation of the Psoriasis Area and Severity Index. It was introduced by Thaci and colleagues in 2001 as the efficacy measure of a large open study of calcipotriol solution in 3,396 patients with scalp psoriasis, in which the mean score fell from 18.4 to 5.6 after eight weeks, and it has since been the standard scalp endpoint in the biologic and small-molecule trial programmes for plaque psoriasis with scalp involvement. Because scalp psoriasis is a special site that responds differently from trunk and limb disease and that is often under-represented in the whole-body PASI, the scalp is assessed on its own.

Three signs are graded on the scalp as a whole: erythema, induration (thickness) and desquamation (scaling), each from 0 (none) to 4 (very severe). The extent of the scalp involved is scored from 0 to 6 using the PASI area brackets: 0 for no involvement, 1 for under 10%, 2 for 10% to 29%, 3 for 30% to 49%, 4 for 50% to 69%, 5 for 70% to 89% and 6 for 90% to 100% of the scalp. The PSSI is the sum of the three sign grades multiplied by the area score, giving an integer from 0 to 72. No regional weighting is applied, and lesions on the face and neck are not included in the assessment.

Response is expressed as the percentage reduction from the baseline PSSI, using the same convention as the PASI. PSSI 75 was the primary endpoint of the etanercept scalp study (Bagel et al. 2012), PSSI 90 was the primary endpoint of the secukinumab scalp study (Bagel et al. 2017), and PSSI 100, complete clearance of the scalp, is reported in the ixekizumab (Reich et al. 2017) and brodalumab (Elewski et al. 2022) programmes. Trial entry for moderate to severe scalp psoriasis has been defined as a PSSI of 12 or more together with at least 20% to 30% of the scalp surface involved in the secukinumab and deucravacitinib studies, whereas the etanercept study required a PSSI of 15 or more.

No validated mild, moderate and severe bands exist for the PSSI, so the score is interpreted through its change over time and the trial thresholds above rather than through fixed categories. The index shares the limitations of the PASI: the sign grades are subjective, the area brackets are coarse at low levels of involvement, and the multiplicative structure makes the score insensitive when either the signs or the area are minimal. A scalp-specific Physician Global Assessment is usually recorded alongside the PSSI in trials, and hair density, pruritus and quality of life are not captured by the index.

References

  1. Thaci D, Daiber W, Boehncke WH, Kaufmann R. Calcipotriol solution for the treatment of scalp psoriasis: evaluation of efficacy, safety and acceptance in 3,396 patients. Dermatology. 2001;203(2):153-156. doi:10.1159/000051731
  2. Bagel J, Lynde C, Tyring S, Kricorian G, Shi Y, Klekotka P. Moderate to severe plaque psoriasis with scalp involvement: a randomized, double-blind, placebo-controlled study of etanercept. J Am Acad Dermatol. 2012;67(1):86-92. doi:10.1016/j.jaad.2011.07.034
  3. Thaci D, Unnebrink K, Sundaram M, Sood S, Yamaguchi Y. Adalimumab for the treatment of moderate to severe psoriasis: subanalysis of effects on scalp and nails in the BELIEVE study. J Eur Acad Dermatol Venereol. 2015;29(2):353-360. doi:10.1111/jdv.12553
  4. Reich K, Leonardi C, Lebwohl M, Kerdel F, Okubo Y, Romiti R, et al.. Sustained response with ixekizumab treatment of moderate-to-severe psoriasis with scalp involvement: results from three phase 3 trials (UNCOVER-1, UNCOVER-2, UNCOVER-3). J Dermatolog Treat. 2017;28(4):282-287. doi:10.1080/09546634.2016.1249820
  5. Bagel J, Duffin KC, Moore A, Ferris LK, Siu K, Steadman J, et al.. The effect of secukinumab on moderate-to-severe scalp psoriasis: results of a 24-week, randomized, double-blind, placebo-controlled phase 3b study. J Am Acad Dermatol. 2017;77(4):667-674. doi:10.1016/j.jaad.2017.05.033
  6. Elewski B, Rich P, Lain E, Soung J, Lewitt GM, Jacobson A. Efficacy of brodalumab in the treatment of scalp and nail psoriasis: results from three phase 3 trials. J Dermatolog Treat. 2022;33(1):261-265. doi:10.1080/09546634.2020.1749546
  7. Tyring S, Bagel J, Lynde C, Klekotka P, Shi Y, Kricorian G. Patient-reported outcomes in moderate-to-severe plaque psoriasis with scalp involvement: results from a randomized, double-blind, placebo-controlled study of etanercept. J Eur Acad Dermatol Venereol. 2013;27(1):125-128. doi:10.1111/j.1468-3083.2011.04394.x

Development

The clinical calculators on this site are free to use. They were developed in partnership by the Centre for Medical and Surgical Dermatology and Dermi(opens in a new tab), a Toronto company that makes clinical imaging software for dermatology practices, which continues to maintain them.

Frequently Asked Questions about the PSSI

The Psoriasis Scalp Severity Index (PSSI) is a physician-scored measure of scalp psoriasis severity adapted from the PASI. It was introduced by Thaci and colleagues in 2001 and is the standard scalp endpoint in clinical trials of biologics and small molecules for plaque psoriasis with scalp involvement. Scores range from 0 to 72.
Erythema, induration and desquamation of the scalp are each graded from 0 (none) to 4 (very severe) and the three grades are added. The extent of scalp involvement is scored from 0 (none) to 6 (90% to 100%) using the PASI area brackets. The PSSI is the sum of the sign grades multiplied by the area score. For example, grades of 3, 2 and 3 with 50% to 69% of the scalp involved give (3 + 2 + 3) x 4, a PSSI of 32.
They are response thresholds defined as reductions of at least 75%, 90% and 100% from the baseline PSSI. PSSI 75 was the primary endpoint of the etanercept scalp trial, PSSI 90 was the primary endpoint of the secukinumab scalp trial, and PSSI 100 means complete clearance of the scalp. A patient improving from 32 to 3 has a 90.6% reduction and reaches PSSI 75 and PSSI 90 but not PSSI 100.
No validated severity bands exist for the PSSI. Clinical trials have defined moderate to severe scalp psoriasis for enrolment as a PSSI of 12 or more with at least 20% to 30% of the scalp involved (secukinumab and deucravacitinib studies), while the etanercept scalp study required a PSSI of 15 or more. The score is interpreted mainly through its change from baseline.
The PASI scores erythema, induration and desquamation in four weighted body regions (head, upper limbs, trunk and lower limbs) and ranges from 0 to 72. The PSSI applies the same sign grades and area brackets to the scalp alone, with no regional weighting, and also ranges from 0 to 72. The PASI head region includes the face and neck, whereas the PSSI excludes them.

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