The premise of this pilot study was to understand in more depth body composition in psoriatic disease, particularly its phenotypic and metabolic associations. Several aspects of body composition, specifically, the amount and distribution of body fat and lean mass, are now understood to be independent health predictors in adults and may form an important part of the ongoing clinical assessment of patients with psoriatic disease.
As expected, patients represented the whole spectrum of psoriatic disease; those with chronic plaque psoriasis and concomitant psoriatic arthritis being by far the largest group. In our cohort, women revealed overall lower body mass and volumes yet higher body fat when compared to men, whereas men revealed comparatively higher visceral fat; such characteristic sex differences in body composition have been well established (28, 29). This female pattern of fat distribution is known to be associated with a more favourable cardiovascular risk at a similar BMI; however, ectopic fat deposition within the abdomen, pericardium and neck is more strongly implicated in women’s adverse cardiovascular risk than that of men. Sex dimorphism in the heritability suggests that female fat distribution may be more genetically affected than males, and biological pathways are differentially involved in the determination of body fat distribution (30). The molecular mechanism for this sex dimorphism may also be beyond the modulation of sex hormones (31).
Regarding the psoriatic group, they demonstrated adverse body composition profiles across the board, including higher body mass, whole-body volume, subcutaneous and visceral fat. This relationship could not be explained by lifestyle factors such as physical activity levels, diet or smoking; ironically, patients with PsD were seen to be maintaining as much vigorous exercise as their healthy counterparts. More patients in the PsA subgroup had active disease, as measured by MDA, and demonstrated higher visceral fat, although this effect was not revealed in the purely cutaneous PsO patients, exemplifying the fact that there are often more diverse contributory factors and nuances to disease activity in a PsA population. If we consider the observed dysmetabolism is a consequence of inflammation, it is however, not clear if the underlying psoriasis itself or the visceral adipose tissue is the key player. Moreover, the association between psoriatic disease activity and MRI-derived visceral fat distribution was noted to be starker in men than women. This finding could have important consequences when assessing individuals’ composite metabolic risk and its potential impact on efficacy of systemic therapies.
The data indicated correlations between patients’ unfavourable body composition profiles, disease activity and cardiometabolic measures of the archetypal metabolic syndrome, specifically, cholesterol:HDL cholesterol ratio and triglycerides. Epidemiological studies have tended to focus on weight or BMI to define obesity rather than altered body composition. Interestingly, there is conflicting data on the association between psoriasis severity, such as PASI, and body composition parameters, indicating that a causal link is by no means definitive. Previous studies have alluded to a dose-response relationship between psoriasis severity and metabolic syndrome (32), supported by translational studies showing T-helper cell cytokine upregulation in the blood and skin of psoriasis patients, leading to effects on lipid metabolism and insulin resistance (33).
Quantification and accurate localisation of various adipose tissue depots is of high research interest in chronic disease particularly those of an inflammatory nature. The last decade has seen an impetus in the development and validation of new modalities for the assessment of body composition. The ratio between abdominal VAT/SAT has been identified as an independent predictor of death and coronary events, irrespective of cardiovascular risk factors and the presence of coronary artery disease (34). Similarly, quantification of VAT volume and VAT/SAT volume ratio by MRI has been found to be a reproducible biomarker associated with cardiometabolic risk factors in subjects with impaired glucose metabolism (35). Whole-body fat quantities derived from a continuously moving table Dixon sequence MRI have shown high reproducibility of results ratifying its potential for future research studies (36). Moreover, the accuracy of this method and the high reproducibility of results indicate its potential for clinical applications.
FatSegNet is a novel, fully automated deep learning pipeline that utilises a competitive dense fully convolutional network (CDFNet) architecture to localise VAT and SAT on abdominal Dixon MR images. It can accurately segment visceral and subcutaneous adipose tissue inside a consistent anatomically defined abdominal region and has been shown to outperform manual rating of VAT (0.850 vs. 0.788) and SAT (0.975 vs. 0.982). In accordance with previous studies on small datasets (37, 38), our data showed a sex and age-specific difference of VAT accumulation, wherein men and older patients were more likely to have higher VAT compared to women and younger patients. This method of fat segmentation is efficient, well-tolerated and reliable (26). Furthermore, FatSegNet has been shown to go one better than other architectures employed in body composition mapping, and in our case, proved to be far more informative than the technique of air displacement plethysmography for demonstrating important phenotypic and metabolic differences between psoriatic patients and controls.
Some studies have employed manual techniques for the assessment of visceral fat in chronic disease, such as the Visceral Adiposity Index (VAI), a gender-specific empirical mathematical model based on simple anthropometric (BMI and waist circumference) and functional parameters (TG and HDL), and indicative of fat distribution and function (39). There is, however, a distinct lack of prospective evidence showing VAI to have a prognostic role in CV risk, especially in the context of inflammatory disease, and given the relative simplicity of MRI-based assessment, we suggest that the VAT and VAT/SAT could become an easy tool for the evaluation of adipose tissue dysfunction and its associated cardiometabolic risk in various patient populations, for example, those at risk for a metabolic syndrome.
Studies of spondyloarthritis, RA and psoriasis have reported a reduced efficacy, drug survival and adherence to tumour necrosis factor inhibitors (TNFis) in obese patients (40-45). There are also data linking the human TNF receptor fusion protein, Etanercept, with weight gain (46). In PsD, the impact of obesity on TNFis remains unclear since available studies are small, present diverging results and lack long-term follow-up data. Treatment with anti-IL-12/23 inhibitors has been associated with more favourable body composition profiles than TNFis, findings which parallel previous observations of increases in BMI seen with this class of drug (47, 48). IL-17, one of the key proinflammatory cytokines in psoriasis, mechanistically links inflammation with insulin resistance and adipocyte dysfunction (49). IL-17A producing cells are thought to be pathogenic in driving inflammation in obesity and progression of obesity-related inflammatory diseases, suggesting that causality between psoriasis and adipogenesis is likely to be bidirectional (50). From this perspective, there are likely to be therapeutic implications of targeting proinflammatory factors such as IL-17 or IL-12/23 in metabolic dysfunction associated with psoriatic disease. A recent prospective, open-label study (Immune Metabolic Associations in Psoriatic Arthritis) evaluated the effect of the phosphodiesterase-4 (PDE4) inhibitor apremilast on body weight and composition, and observed weight loss, principally abdominal subcutaneous fat, and improvement in psoriatic disease activity independent of weight change (51). Considering this, we postulate that further individualised treatment strategies based on multimodal insight into adverse metabolic profiles and biomarkers, such as high visceral fat, may improve outcomes and overall care of psoriatic patients. An automated model of fat segmentation – being less expensive and time-consuming than manual segmentation - could facilitate future research of similar patients using large population-based cohorts.
Strengths
To our knowledge, this is the first time that a deep learning application for MRI-derived body composition, especially that of VAT and its metabolic significance, has been studied in psoriatic disease and compared to matched controls. We have reported on a novel, automated method for image acquisition and validated its functionality in a clinical cohort with chronic inflammatory disease.
Limitations
We are aware that this is a pilot study and as such will likely need to be repeated on a wider scale. The cross-sectional nature of the study confers challenges with determining causal relationships. The relatively small sample size of patients and controls and diverse age range will also have hampered the data analysis and ability to draw certain conclusions. We believe that further research in this field will enhance the validity of our results whilst keeping a realistic view of the expected numbers of patients that can be recruited to a similar study.