Skin ultrasound in the evaluation of rheumatic diseases: Systemic sclerosis and psoriatic disease

Abstract

The clinical relevance of skin ultrasound evaluation in systemic sclerosis (SSc) and psoriasis (PsO) is significant, as it provides a non-invasive, objective, and reproducible method for assessing skin involvement, which is crucial for diagnosis, monitoring disease progression, and evaluating treatment efficacy. High-frequency ultrasound (HFUS) and shear wave elastography (SWE) are particularly useful in quantifying skin thickness and stiffness, parameters that correlate with disease activity and severity. This approach offers a more sensitive alternative to traditional methods such as the modified Rodnan skin score (mRSS), which can be subjective and variable. This article details the specific clinical applications and benefits of skin ultrasound (US) in SSc and PsO. Although the reviewed studies primarily focus on SSc, the principles of using US for skin evaluation can be extended to PsO. In PsO, US can help assess the extent of skin involvement, monitor treatment response, and potentially differentiate between active and inactive lesions, although specific studies on PsO were not detailed in the analyzed contexts. In contrast to advancements in SSc, the application of skin US in PsO requires further exploration to establish standardized protocols and validate its clinical utility. The potential of US to provide detailed insights into PsO-related skin pathology remains promising, warranting further research to fully understand its benefits. This document presents different US techniques, imaging windows, and technical aspects of performing skin US in SSc and PsO with skin involvement, covering the advantages and limitations of each method in both diseases.

Resumen

La relevancia clínica de la evaluación por ultrasonido de la piel en la esclerosis sistémica (ES) y la psoriasis (PsO) es significativa, ya que proporciona un método no invasivo, objetivo y reproducible para evaluar la afectación cutánea, lo cual resulta crucial para el diagnóstico, el seguimiento de la progresión de la enfermedad y la evaluación de la eficacia del tratamiento. El ultrasonido de alta frecuencia (HFUS) y la elastografía por onda de corte (SWE) son particularmente útiles para cuantificar el grosor y la rigidez de la piel, parámetros que se correlacionan con la actividad y la gravedad de la enfermedad. Este enfoque ofrece una alternativa más sensible a los métodos tradicionales, como la puntuación cutánea modificada de Rodnan (mRSS), que puede ser subjetiva y variable. El presente artículo detalla las aplicaciones clínicas específicas y los beneficios del ultrasonido cutáneo en la ES y la PsO. Si bien los estudios analizados se centran principalmente en la ES, los principios de uso del ultrasonido para la evaluación de la piel también pueden aplicarse a la PsO. En la PsO, el ultrasonido puede ayudar a evaluar la extensión de la afectación cutánea, monitorear la respuesta al tratamiento y, potencialmente, diferenciar entre lesiones activas e inactivas, aunque no se detallaron estudios específicos sobre la PsO en los contextos analizados. En contraste con los avances en la ES, la aplicación del ultrasonido cutáneo en la PsO requiere una mayor exploración para establecer protocolos estandarizados y validar su utilidad clínica. El potencial del ultrasonido para proporcionar información detallada sobre la patología cutánea en la PsO sigue siendo prometedor, lo que justifica investigaciones adicionales para comprender plenamente sus beneficios. En el presente documento se presentan las diferentes técnicas, ventanas ecográficas y aspectos técnicos de la realización de ultrasonido de piel para ES y PsO con compromiso cutáneo, con el objetivo de abordar sus ventajas y limitaciones en cada una de las dos enfermedades.

Skin ultrasound in systemic sclerosis

Introduction

Systemic sclerosis (SSc), also known as scleroderma, is a multisystem autoimmune disease, and skin involvement is the cardinal clinical finding. It manifests as extensive, localized, or diffuse cutaneous fibrosis, with severity determining disease status and predicting possible visceral involvement. In clinical practice and clinical trials, skin sclerosis is assessed using the modified Rodnan skin score (mRSS), a semiquantitative test currently considered the gold standard for evaluating disease activity and treatment efficacy. While it is easy to perform and feasible, it has limitations, including low inter- and intra-observer agreement and poor sensitivity to change. Therefore, it is essential to implement an objective and highly sensitive tool to assess and characterize cutaneous involvement for diagnostic, monitoring, and prognostic purposes [1].

There is growing evidence supporting the role of high-frequency ultrasound (HFUS) (>18 MHz) in measuring skin thickness, demonstrating good to excellent intra- and inter-observer agreement, as well as correlation with mRSS and cutaneous histopathology [2]. Elastography has also been used to assess skin stiffness, but further validation is needed. These two tools are objective, reproducible, and acceptable measures across clinical trials, observational studies, and clinical practice. However, standardization of image acquisition and ultrasound (US) reporting is necessary [3]. The routine use of US in clinical practice is not yet widespread and requires further validation. Additionally, establishing normal values for the general population is needed, accounting for the anatomical site of evaluation and demographic variables [2].

In a pilot study with 29 SSc patients and 29 healthy controls, shear wave elastography (SWE) was performed to estimate skin thickness (epidermis and dermis) and stiffness on the dorsal and volar aspects of both forearms. Results showed significantly greater skin thickness and stiffness in SSc patients compared to healthy controls, with a predominance on the dorsal aspect in the SSc group. In the control group, skin thickness was greater on the dorsal side, and no significant differences in stiffness were observed between anatomical sides. Skin stiffness correlated with mRSS, and no differences in age or sex were observed [3].

In contrast, another cross-sectional study by Santiago et al. included 140 healthy participants (80 women), aged 20–72 years, categorized by decades. Skin thickness was measured using grayscale US (18 MHz), and skin stiffness was evaluated using elastography (VTIQ, 9 MHz) at the 17 mRSS sites. Images were reviewed by four evaluators with varying levels of US experience. Significant differences in univariate and multivariate analyses were found in skin thickness and stiffness by age and sex. Higher values were observed in men compared to women at all mRSS sites except the chest. Age-related differences were also found in certain sites for both skin thickness (face, arm, forearm, hand, thigh, and leg) and skin stiffness (abdomen, arm, forearm, hand, thigh, and foot). Additionally, this study demonstrated good to excellent intra- and inter-observer correlation in grayscale and elastography US [1]. These conclusions are valuable for establishing normal values and add to the growing evidence supporting the reproducibility and validity of US.

Standardization protocols are the cornerstone for advancing the use of US in SSc by closing the gap in current findings, which are hindered by the lack of homogeneous acquisition and analysis techniques that prevent generalization. Clinical evidence includes specific recommendations by expert groups recognizing this need. Key points include assessing skin thickness and echogenicity using high-frequency B-mode US (>18 MHz), positioning the probe perpendicular to the skin surface with minimal pressure over a generous layer of gel, and using elastography to measure skin stiffness – procedures that should be performed by trained professionals [2]. The recommended anatomical sites for evaluation are those used in the mRSS, based on current evidence, and to achieve comparable results. Future research aims to simplify the anatomical areas examined to facilitate their use in clinical settings without compromising validity [2].

Reporting of US skin findings in clinical studies must include basic recommendations for image analysis. It is essential to specify whether the image meets quality criteria for acceptance and interpretation, use B-mode to evaluate the epidermis, dermis, and subcutaneous tissue, identify the gel film to confirm minimal pressure on the skin surface, report the anatomical location based on the mRSS sites (as described by Moore et al. [4]), and specify the number of images obtained per site examined (Fig. 1) [2].

Fig. 1.

Left: visualization scheme of cutaneous structures on US in skin affected by SSc. In a transverse section with the transducer in longitudinal orientation, from superficial to deep layers, the stratum corneum, the epidermis, and the basal cell layer are observed. Deeper, the dermis is visible, showing an absence of vascular structures and a marked thickening compared to normal skin. This dermal thickening, as illustrated in the diagram, is caused by prominent collagen fiber deposition. Note the scarce inflammatory infiltrate and the predominance of collagen fibers and extracellular matrix. Right: clinical skin US image showing changes due to SSc, with skin thickening primarily dependent on increased dermal thickness. The US image on the right, labeled D1, shows the measured epidermal thickness in a patient with the disease, which is 0.56 mm. The cutaneous thickening in the pathophysiological process of the disease is primarily due to increased dermal thickness.

Theoretical foundations

Skin US is an important tool for evaluating patients with SSc, as it provides objective, quantifiable measurements of skin involvement – a hallmark of the disease. HFUS and SWE are particularly valuable in this context.

Several studies have demonstrated the usefulness of HFUS and SWE in assessing skin thickness and stiffness in patients with SSc. HFUS and SWE can distinguish patients with SSc from healthy controls by quantifying increased skin thickness and stiffness, which correlate with disease activity and severity [5,6]. These modalities have shown strong diagnostic performance, with high areas under the ROC curves, indicating their potential for accurate disease evaluation [5].

Additionally, HFUS and SWE measurements correlate well with mRSS, a commonly used clinical tool for assessing skin involvement in SSc. This correlation highlights the clinical relevance of US measurements in reflecting disease severity [5–7]. HFUS has also demonstrated good concordance with histological findings, further validating its use as a reliable tool for skin assessment in SSc [6,7].

The reproducibility and reliability of HFUS are also noteworthy. Studies have shown high intra-observer repeatability for HFUS parameters such as skin thickness and echogenicity, supporting its use in both clinical practice and research settings [3]. Moreover, HFUS can detect subclinical skin changes that may not be evident on clinical examination, providing a more sensitive assessment of skin involvement [7,8].

Despite these advantages, challenges remain concerning the standardization of US techniques and the interpretation of results. The heterogeneity in image acquisition and analysis methods across studies underscores the need for standardized protocols to ensure consistent, comparable results [9,10].

In summary, skin US – particularly HFU and SWE – is a valuable tool for evaluating SSc. It provides objective, quantifiable measurements of skin involvement, correlates well with clinical and histological findings, and offers high reproducibility. However, further efforts are needed to standardize US techniques to enhance their clinical utility.

Technical aspects and general recommendations

Given some limitations in the current literature, including the lack of standardization in US assessment, it is important to note that parameters such as skin thickness, echogenicity, and elastography had not previously satisfied the OMERACT reporting criteria in rheumatic diseases. However, in 2022, a systematic review and Delphi consensus, led by Dr. Tania Santiago's group [2], established a set of recommendations for standardized reporting of US findings and key technical aspects of standardized skin US. The most important technical points for image acquisition and reporting, as outlined in this document, are outlined below.

General principles

These were identified through a systematic literature review and Delphi methodology by Santiago et al. The document defines five general principles and seven technical recommendations for performing and reporting skin US in SSc:

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  B-mode US and elastography are promising tools for assessing skin involvement, though their role in management decisions in SSc patients is not yet fully established.

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  Reporting of US findings in rheumatic and musculoskeletal diseases, including SSc, should follow EULAR's checklist recommendations.

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  Standardization of US techniques, particularly image acquisition and analysis, is essential for progress. The systematic review highlighted heterogeneity and gaps in several technical aspects that must be addressed to advance research.

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  Examiner training and appropriate US equipment and settings are critical. US is operator-dependent, and there is an inherent risk of observer bias, which may compromise image acquisition and analysis. Previous studies have shown that equipment and settings affect the accuracy of musculoskeletal US [11]. Proper training is necessary to reduce variability and improve evidence quality, though standardized training in skin US is still needed.

These recommendations aim to promote the validation of skin US in SSc by improving objectivity, reproducibility, and sensitivity.

Section A: Recommendations for performing skin US

Recommendation 1: Skin assessment in SSc patients should, where possible, include B-mode US for thickness and echogenicity, and elastography for stiffness measurement. Despite logistical difficulties and equipment limitations, evaluating all three domains simultaneously may help clarify the underlying pathology.

Recommendation 2: Skin US should be performed at standardized sites used in the mRSS. Studies following Moore et al. [4] proposed 17 skin sites for assessment. Some protocols evaluate reduced or full sets of sites. While this can complicate data pooling, the authors agree that examining all 17 sites is time-consuming and perhaps unnecessary. The optimal balance between validity and feasibility regarding the number of sites remains unknown.

Recommendation 3: Skin US should be performed using a high-frequency linear probe (>18 MHz), held perpendicular to the skin with minimal pressure and a generous amount of gel. All reviewed studies used probes >18 MHz (except one), allowing clear differentiation between skin layers. Lower-frequency probes cannot distinguish the epidermis-dermis interface, reducing reproducibility and precision [12,13]. To avoid anisotropy, the probe angle must be continuously adjusted. Generous gel prevents skin compression, which may distort images [11]. Elastography may require a second probe because this modality lacks high-frequency options [12].

Recommendation 4: No spacers or interface devices should be used in skin US for SSc. This recommendation is based on expert opinion. No studies directly evaluated spacers/interfaces [9], but high-frequency probes with ample gel provide optimal resolution and reduce artifact risks. Interfaces could increase tissue distortion.

Recommendation 5: Skin US should only be performed by well-trained examiners. This applies to clinical and research evaluations (not training sessions). US requires knowledge of basic physics and technical skills. It has become a core skill in rheumatology training in Europe [14]. While easier than musculoskeletal US, there are currently no official training programs dedicated to skin US. Despite the lack of consensus on minimum experience, proper training is essential.

Section B: Recommendations for reporting specific aspects of skin ultrasound in SSc studies

Recommendation 6: When analyzing images, the following must always be specified:

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  Quality criteria used to accept a US image

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  Skin layers evaluated (epidermis, dermis, hypodermis, subcutis, etc.)

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  Exact anatomical site of assessment

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  Number of images acquired per site

Clear differentiation between skin layers is mandatory for acceptance of B-mode images. In elastography, most software provides automatic image-quality scoring [12]. The presence of a visible gel layer confirms that minimal pressure was applied during scanning.

Studies by Moore et al. [4], Flower et al. [7], and Naredo et al. [15] emphasize the dermis as the primary site of disease and the best-assessed site, given poor inter-observer correlation in the epidermis (ICC < 0.35). Reports must explicitly indicate the layer being measured as a quality indicator.

When assessing Rodnan sites, use the precise anatomical descriptions by Moore et al. [4]. If evaluating other sites, describe their exact location and distance from anatomical landmarks. Always report the number of images per site for standardization purposes.

Recommendation 7: In relation to image analysis, always specify:

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  Number of measurements per image and their exact location

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  For elastography, the size and shape of the region of interest (ROI)

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  How individual measurements were processed to calculate site values

These details are rarely reported in prior studies. The choice of measurement sites affects US values and may impact reproducibility and generalizability. Hence, reporting image analysis methods and scoring systems is essential.

Conclusion

The importance of skin US in evaluating patients with SSc lies in its ability to provide objective, quantitative measurements of skin involvement – critical for diagnosis, monitoring, and understanding disease progression. This imaging modality allows for a deeper understanding of dermal changes, offering greater sensitivity and specificity than traditional clinical assessments [16].

Its clinical relevance is underscored by its correlation with disease severity and internal organ involvement, making it a valuable prognostic tool [17]. It is non-invasive, painless, and can be easily repeated, facilitating continuous treatment monitoring and disease progression assessment. Therefore, in today's clinical landscape, it is essential that rheumatology programs and rheumatologists be properly trained in skin US assessment.

Skin elastography

Although the mRSS has good validity, studies have shown it has low responsiveness to change and substantial inter-observer variability, ranging from 12% to 25% [18]. Therefore, there is concern that it may not detect clinically significant differences over time or with treatment [19,20].

Skin HFUS enables non-invasive measurement of skin thickness and, through elastography, quantitative and qualitative assessment of elasticity at multiple anatomical sites in a sensitive, objective, and reliable manner [4,21]. It complements conventional US techniques in the routine clinical evaluation of various traumatic and pathological conditions of the musculoskeletal system and the skin.

There are four types of elastography [22]: compression sonoelastography, transient elastography, strain elastography, and SWE.

Compared to the others, SWE is operator-independent [12,23] and has excellent intra- and inter-observer correlation coefficients of 0.98 and 0.91, respectively. Thus, it provides a new opportunity to objectively evaluate fibrosis, a crucial feature in the complex process of skin involvement in SSc. Cross-sectional studies have shown that this technique can distinguish between patients and healthy individuals by detecting higher shear-wave propagation velocities in affected as well as in apparently unaffected skin (mRSS score 0) of patients [12,24].

Furthermore, skin involvement in SSc follows a clinical progression from an edematous phase to a fibrotic phase and finally to an atrophic phase [20,25]. In healthy individuals, the collagen fiber network in the dermis decreases with aging, which is expected to affect elasticity [26]. Histological studies show that this loss begins as early as age 20 but becomes more pronounced after age 50, a trend also confirmed by SWE [27].

In the study by Santiago et al. [28], after five years of follow-up, a decrease in wave velocity was found in both healthy controls and SSc patients, except in the fingers of the latter. This change was not detected by the mRSS. Wave velocity was higher in patients in the edematous phase and decreased more significantly during follow-up than in those in the fibrotic phase. This change was also observed in skin with a mRSS score of 0, both in this and a previous study [12]. These findings suggest that SWE is more sensitive to change than the mRSS and may help distinguish between early stages of edema, induration, and sclerosis, as well as monitor treatment response. However, studies using elastography in this condition are still very limited.

Regarding skin thickness, US also detects changes even in areas where the mRSS is normal [8,29], and it shows excellent inter-observer correlation coefficients ranging from 0.65 to 0.94 (0.86 for maximum thickness) and intra-observer coefficients from 0.55 to 0.96 (0.92 for maximum thickness) [4].

The study by Chen et al. [6] examined the validity and reliability of skin US. It found a good correlation between skin thickness measured by histology and by US (r = 0.6926, p = 0.009). Inter- and intra-observer reproducibility were also high (r = 0.8 and 0.9, respectively). However, the correlation between histological thickness and the mRSS was low (r = 0.5867, p = 0.050), suggesting that US may better reflect histological measures than the clinical score. No correlation was found between skin thickness and SWE-measured elasticity. The authors suggest this may be because there is typically greater thickness in the edematous phase than in the fibrotic phase. Given the heterogeneous progression of the disease, US may help indicate the current pathological state and assist in evaluating the treatment response in each case. This study included a small number of patients, and its findings still need to be confirmed in larger studies. Nevertheless, it remains the only study comparing US with histology.

Skin ultrasound in psoriatic disease

Introduction

Skin US is a rapidly evolving technique for assessing psoriatic disease (PsO). Initially used in psoriatic arthritis (PsA), recent publications have established its potential role as an additional strategy for evaluating other disease domains, such as skin and nail involvement. This has led to the development of the concept of “holistic ultrasound assessment of psoriatic disease” [30]. In the study by Gutiérrez et al. [30], emphasis is placed on the potential utility of US in detecting not only structural abnormalities but also subtle changes in blood flow in the superficial soft tissues during the early stages of the disease. This article aims to summarize the scientific evidence supporting the current use of US in the evaluation of psoriatic disease, with a primary focus on skin involvement.

Clinical importance and impact of psoriatic disease

PsO is a chronic, recurrent skin disease affecting millions of people worldwide, with a general estimated prevalence of approximately 2–3% [31]. This complex skin disease has an immune-mediated, multifactorial basis involving both genetic and environmental factors. It is considered the prototype of chronic dermatoses. PsO is now recognized as a systemic inflammatory disease with immunologic mechanisms involving dermal infiltration by inflammatory cells and secondary epidermal hyperplasia, along with extracutaneous involvement, including joints and cutaneous adnexa such as the nails [32].

The diagnosis of PsO is based on clinical history and physical examination, with histological evaluation remaining the gold standard for definitive diagnosis [33]. High-frequency skin US is a non-invasive imaging method that allows both anatomical and physiological analysis of various skin features and adnexa, including nails, as well as assessment of lesion morphology and changes in underlying tissues [34]. It is increasingly used to evaluate disease extent, severity, and treatment response in PsO.

Technical aspects of assessing psoriatic involvement

To achieve optimal visualization of the skin during US examination, high-frequency or ultra-high-frequency linear transducers ranging from 15 to 40 MHz are used [35]. The use of ultra-high-frequency transducers allows for characterization of submillimetric lesions smaller than 0.1 mm. Grayscale (2D) US enables good characterization of skin and even nail changes, while Doppler US provides visualization of blood vessels, including information on vascular presence, appearance, direction, and flow velocity [34].

A detailed study of the skin and its structures requires high-frequency transducers, as higher frequency results in a shorter wavelength, thereby improving axial resolution in US images. At this depth, the US beam is narrow, enhancing lateral resolution and ensuring high-definition imaging of the evaluated tissues. During evaluation protocols for normal and psoriatic skin, lesions are initially scanned in grayscale mode to detect morphostructural changes, followed by power Doppler to detect dermal blood flow. Vascularization in psoriatic plaques is assessed with power Doppler, taking care not to apply excessive pressure to the tissues to avoid signal “blanching” from transducer compression [36].

Because normal skin thickness varies widely across anatomical areas, it is reasonable to compare US findings between psoriatic plaques and adjacent normal skin in the same area on the same patient. Guastalla et al. [37] suggested normal US values for the thickness of different skin layers. US can be useful in differentiating between inflammatory, vascular, or tumoral pathologies, not only for diagnosis but also for monitoring disease progression in patients under therapy.

Ultrasound of normal skin

The skin consists of two layers: the epidermis and the dermis. However, because of their proximity and frequent involvement in skin diseases, the subcutaneous tissue, or hypodermis, is often considered a third layer. Each of these three layers has a distinct US appearance, defined by its main components.

In normal skin, due to the presence of keratin, the epidermis appears as a thin, continuous hyperechoic line with uniform thickness, or as parallel hyperechoic laminar lines in glabrous skin from palmar or plantar regions. The dermis, rich in collagen fibers, appears as a homogeneous, hyperechoic band, though slightly less bright than the epidermis. The subcutaneous tissue, containing fat lobules, is characteristically hypoechoic with hyperechoic lines generated by the fibrous septa of connective tissue [38,39]. Arterial and venous vessels in the subcutaneous tissue appear as anechoic tubular structures, while the bony surface appears as a separate hyperechoic line (Fig. 2).

Fig. 2.

Normal ultrasound appearance of the skin with its differentiated layers: e: epidermis; d: dermis; h: hypodermis. (Image taken from Ref. [30]).

Ultrasound of the psoriatic plaque

In psoriatic plaques, changes are localized to the epidermis and dermis, with no involvement of the hypodermis. It is relatively easy to distinguish the US features of a psoriatic plaque from those of the surrounding normal skin. As a result of keratinocyte proliferation in the epidermis and the abnormal concentration of pro-inflammatory cells in the dermis, both layers show increased thickness on B-mode evaluation. When epidermal thickness is significantly increased, acoustic shadowing may occur, hindering or obstructing clear visualization of the underlying dermis [40,41].

HFUS of a psoriatic plaque reveals a trilaminar structure: a hyperechoic band representing the epidermis with a parakeratotic scale and suprapapillary epidermis; a subepidermal anechoic band corresponding to elongated epidermal ridges with congestion and edema of the loose connective tissue; and, at the bottom, the reticular dermis appearing as a hyperechoic band (Fig. 3) [40]. Although not specific to PsO, a hypoechoic band in the upper dermis – corresponding to inflammatory edema and vasodilation – was a commonly observed finding in some studies [42,43] and has been reported as a reliable indicator of active disease states. Other inflammatory conditions, such as acanthoma, atopic dermatitis, or contact dermatitis, may also present with this hypoechoic dermal band [42].

Fig. 3.

Left: (a) epidermal thickening in a psoriatic plaque; (b) posterior acoustic shadowing in a case of severe psoriatic plaque (images taken from Ref. [30]). Right: diagram of the skin in long-axis view with an HFUS transducer. From superficial to deep layers, the following are observed: thickened stratum corneum (parakeratosis), an enlarged epidermis with proliferative activity, and dermal infiltration with inflammatory cells without fibrosis or increased collagen fibers. Unlike SSc, there is no associated dermal fibrosis, and the inflammatory infiltrate is usually prominent.

Ultrasound as part of treatment response evaluation in psoriasis

US indicators of effective therapy described in grayscale mode include a reduction in the thickness of the dermal and epidermal layers, and, most importantly, the disappearance of the hypoechoic band in the superficial dermis [43,44]. US monitoring of activity and severity in psoriatic involvement has been the focus of several studies, particularly in patients with severe or disabling forms of the disease. Skin HFUS can be used to evaluate the response to topical treatments. Lacarrubba and collaborators, in a study of 30 patients treated with 0.05% topical clobetasol, reported a reduction in plaque thickness when treatment response was monitored by 20 MHz HFUS [45].

Cutaneous involvement in PsO is associated with increased blood flow signal within dermal plaques, detectable by color Doppler, as previously reported [40]. Power Doppler signal provides a useful and valid assessment for short-term monitoring of disease activity and treatment efficacy [46]. Gutiérrez and collaborators found a significant correlation between power Doppler findings and two parameters: the extent of cutaneous involvement and the histopathological degree of vascularization before and after treatment with Etanercept. These data support the validation of power Doppler findings in assessing dermal perfusion changes in patients with plaque PsO [47].

Currently, there is increasing evidence and accumulating data supporting the routine use of skin US in evaluating treatment response in plaque PsO. In the US, in conjunction with clinical examination, oral cyclosporine has been reported as a useful tool for assessing therapeutic efficacy in 20 patients with plaque PsO. Capillaroscopic vascular response has also been monitored through videodermatoscopy to detect early signs of relapse [48].

Influence of gender and age on skin thickness and echogenicity

As mentioned earlier, the increasingly frequent use of US enables direct, real-time “in vivo” skin measurement. However, at the time of measurement, when obtaining the biological variable of skin thickness, it must be noted that this variable may fluctuate and follow a normal distribution, even in patients without pathology. This applies equally to studies on PsO, highlighting the need for more data and research to establish normative curves of skin thickness in Latin American populations, particularly given that much of the published research originates from Europe and that ethnic variations have been shown to impact skin thickness and echogenicity values.

In this regard, in 2016, Firooz and collaborators [49] published a study conducted on 30 healthy volunteers aged 24–61 years, in which they measured dermal thickness and echogenicity at five anatomical sites (Fig. 4) using high-frequency transducers at 22 and 50 MHz. The results showed that epidermal and dermal thicknesses in men were significantly greater than in women, especially in the neck and the dorsum of the foot. They also found that dermal echogenicity was higher in women at all measured sites, though this difference was statistically significant only in the neck. Additionally, epidermal and dermal thicknesses were significantly greater in the younger age group (<35 years) compared to the older group, particularly on the soles and dorsum of the feet. Overall, skin thickness decreased significantly with age, indicating an inverse relationship. The authors concluded that both gender and age have a significant impact on these parameters and that the differences may be even more pronounced in ethnic groups not yet studied, such as the Latin American population.

Fig. 4.

The five body sites for ultrasonographic skin measurement: (a) cheeks: at the zygomatic arch below the orbits; (b) neck: anterior neck over the cricoid arch; (c) palms: center of the right palm, 5 cm proximal to the metacarpophalangeal joints; (d) dorsum of the foot: dorsum of the right foot, 5 cm proximal to the metatarsophalangeal joints; (e) soles: center of the right sole, 10 cm proximal to the metatarsophalangeal joints.

The subepidermal hypoechoic band increases with age, and the echogenic dermal band thickens with age. These findings are difficult to explain; however, some studies have shown that aging reduces the biophysical properties of the skin – such as hydration and elasticity [50,51]. These changes may be correlated with skin thickness and alterations in dermal echogenicity over time.

Evaluating skin echogenicity is more complex than previously thought. Therefore, it is necessary to standardize future studies not only by anatomical site evaluated, but also by time of day, patient nutritional status, and level of physical activity prior to assessment.

Authors' contributions

Correa Giraldo: manuscript drafting, systematic literature review, preparation of figures for the main document. Díaz: manuscript drafting, systematic literature review, preparation and revision of bibliography. Castillo: manuscript drafting, systematic literature review.

Ethical considerations

This article is a narrative review of the scientific literature concerning skin US in rheumatic diseases. No human subjects were recruited, and no personal, clinical, or identifiable information was accessed or used. Therefore, ethics committee approval was not required.

The conduct of this review adhered to the ethical principles outlined in the Declaration of Helsinki of the World Medical Association, as well as to international standards for responsible research and publication established by the International Committee of Medical Journal Editors (ICMJE) and the Committee on Publication Ethics (COPE). All sources of information have been properly cited to ensure academic integrity and transparency in the use of published scientific evidence.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors declare that no artificial intelligence (AI) tools were used in the writing, editing, or preparation of this manuscript.

Funding

This research received no external funding and was supported by the authors’ own resources.

Conflicts of interest

The authors declare no conflict of interest.