The use of medicinal plants for the treatment of psoriasis: A systematic review and meta-analysis

Search strategy

A systematic review was performed in accordance with the Cochrane guidelines. The protocol was registered on the International Prospective Register of Systematic Reviews (PROSPERO; Regn No. CRD42021269042). The search for relevant randomized controlled trials published between 2016 and 2020 was performed in the MEDLINE, Embase, Lilacs and in the Cochrane library databanks. References found in the potentially relevant publications were screened for additional records. The search was conducted in 2021.

Selection criteria

Studies were selected by two blinded reviewers (LW, GG). Discrepancies were settled by consensus. The PICOT (Population, Intervention, Comparison(s), Outcome and Time/study period and date started) strategy was applied to originate the guiding question for the performance of the study. This resulted in the research question: “Do patients with psoriasis exhibit improvement when using products or substances derived from medicinal plants?”

Sub-questions were asked to obtain detailed specifications such as:

• Can patients with psoriasis show improvement in itching, redness and scaling of the skin with the use of medicinal plant-based products?

• Can patients with psoriasis improve their skin disease if they use herbal products?

• Can psoriasis patients use herbal medicinal products as a combined therapeutic treatment?

• Can patients with persistent psoriatic plaques improve with the use of herbal medicinal products?

The search strategy was carried out using the Cochrane Library and PubMed platforms to select the MeSH terms. These were organised into five categories of interest guiding the review and later combined with each other for the search using Boolean operators AND, OR for a match between search terms. The resulting categories were:

• Category A - for terms specific to medicinal plants (healing plants; herbs; medicinal; medicinal herbs; medicinal plants; pharmaceutical plants; phytochemicals and flavonoids)

• Category B - for generic search for randomized clinical trials and controlled clinical trials (randomized controlled trial; controlled clinical trial; comparative study; clinical trial; randomized; placebo; drug therapy; randomly; trial; groups)

• Category C - specific for psoriasis (psoriasis; published from 2016 to 2020; arthritis, psoriatic)

• Category D - for results (inflammatory activity; inflammatory; activity; follow-up; diagnose; diagnoses; diagnosis; diagnostic; treatment; treatments; therapy; therapies; therapeutic)

• Category E - for types of psoriasis considering timeline (early; very early; immediate; immediate early; subclinical; late; late diagnoses).

A Prisma (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) flowchart was used (http://prisma-statement.org/prismastatement/flowdiagram.aspx) to describe the information in the different phases of the review, mapping the number of identified entries, included and excluded, for the studies to be eligible for consideration.

Eligibility criteria

Articles published between 2016 and 2020 were considered eligible for study. The inclusion criteria adopted were in accordance with the PROSPERO protocol:

• Population: individuals diagnosed with mild to moderate to severe psoriasis;

• Intervention: use of medicinal plants (as an aid and/or therapeutic treatment or association with other psoriasis drugs);

• Outcome: especially considering the outcomes that indicated an improvement in psoriatic lesions and/or an improvement in the quality of life of psoriasis patients, although other outcomes were also included;

• Type of studies: randomized clinical trials to assess efficacy;

• Time: period of treatment or intervention.

Articles in all languages were included. Studies with interventions or investigations performed in animals were excluded. Also excluded were studies that did not provide information on the use of medicinal plants as a therapeutic resource in patients with psoriasis and those that involved only conventional pharmacological treatments.

Study sources

Searches were conducted electronically through the Cochrane Collaboration online library and LILACS, MEDLINE via PubMed and Embase platforms. Relevant studies were detected by crossing key words.

Data extraction

Titles and abstracts were independently reviewed by two reviewers. Disagreements were settled by consensus. In cases of unsettled disagreement, the decision was to be taken by a third reviewer specialised in the subject; however, there were no unsettled disagreements and this was not necessary.

Data extraction from the articles was performed and recorded in a spreadsheet indicating the type of study, number of participants, comparator (if any), medicinal plant used, dosage, time of product use, main outcome, type of study and level of evidence. To ensure a structured review a narrative synthesis of studies meeting the inclusion criteria was conducted. The studies were screened to identify, select and evaluate their eligibility and adherence to the PRISMA 2020 recommendations. For transparency and completeness, a checklist of 27 items was applied to the review.⁸

Bias risk

All studies included were randomized and the risk of bias was jointly assessed by three reviewers, using the ROB2.0 as an instrument in which the risk in each bias domain was graded as either “high,” “unclear” or “low.”⁹

Evidence level

The certainty of evidence was assessed using the Cochrane GRADE criteria.

Study selection

The initial search retrieved a total of 2,268 studies. After applying the filters and crosswords only seven randomized clinical studies remained that constituted the final sample. Figure 1 shows the flowchart (PRISMA) of the bibliographic searches and demonstrates the results obtained initially and after application of the filters “in vivo,” “clinical study” and “period from 2016 to 2020”, indicating the scarcity of clinical trials in humans using active medicinal ingredients (both oral and topical) for the treatment of plaque psoriasis according to the criteria defined in this research.

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Figure 1:

Preferred reporting items for systematic reviews and meta-analysis flow diagram showing the methodology in selecting articles for final analyses¹⁰

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Table 1 details the characteristics of the publications included. All studies included in the review used PASI. The number of participants in each study ranged from 24 to 100 and a total of 389 patients with psoriasis completed the studies till the end. The studies ranged from 4 to 12 weeks. In 3 of the 7 studies an ointment formulation was used, while cream, lotion and oral formulations were used in 1 study each. The mode of usage was not specified in one study.

Study bias risk

Five of the 7 studies were clearly randomized [Figure 2]. In the remaining two studies, one¹⁰ did not mention the form of randomization and although the other¹¹ stated that the study was conducted in a randomized manner but did not mention the type of randomization. All studies were blinded, but the form of blinding was unclear in the study by Rerknimitr et al.¹² The risk of bias analysis was performed in 6 domains (Fig.2), with about 30% of high risk in domain 1 (randomization) and medium risk (some concerns) in about 20% in the other domains.⁹

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Figure 2:

Risk of bias summary; review of authors’ judgement about each risk of bias item for each study included

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The results were presented differently in the studies. Six studies clearly and objectively outlined the selection, measurement and description of the results, but Zorko et al.¹⁰ failed to describe how they carried out the measurement of the results, as well as the selection of results in relation to the effect measures.

Trend in favour of herbal treatments

After reviewing and analysing the included studies, a spreadsheet populated with the information collected was developed followed by a statistical analysis of the data. The Forest Plot Graphic [Figure 3] shows considerable confidence intervals between the studies, I² = 91%, 2.16 (0.97, 4.78) 95% confidence interval, (P < 0.00001) demonstrating no important inconsistency even with a high heterogeneity rate. The Funnel Plot Graphic [Figure 4] however, demonstrates that although there are less than 10 studies to carry out the meta-analysis, there was considerable homogeneity in their distribution with a trend favouring the herbal treatment.

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Figure 3:

Forest plot graphic

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Figure 4:

Funnel plot graphic

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The certainty of evidence was assessed using the GRADE framework [Figure 5] which demonstrated a high level of evidence in the conduct of clinical trials that exhibited favourable action of herbal products for psoriasis.

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Figure 5:

Grading of recommendations assessment, development and evaluation

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