Fluorescence optical imaging and musculoskeletal ultrasonography in juvenile idiopathic polyarticular disease before and during antirheumatic treatment - a multicenter non-interventional diagnostic evaluation

Juvenile idiopathic arthritis (JIA) is the most common chronic inflammatory rheumatic disease in childhood and can lead to severe disability [1‐3]. The validity of diagnosis, classification and prognosis as well as therapeutic decisions for each JIA patient depends on reliable detection of joint involvement. Careful clinical assessment is of great importance but may be difficult, especially in children. Subclinical inflammation could also be missed, for example in early stages of the disease or with minimal disease activity. Ultrasonography (US) in gray-scale (GSUS) and power Doppler (PDUS) modes is often available for assessment of synovial thickening, joint effusions and hyperperfusion, thus helping to detect active joints. In clinical practice this procedure is usually used on a limited number of joints due to limitations of time. Fluorescence optical imaging (FOI) is a contrast-enhanced optical procedure for visualization of changes in microcirculation. It has been used for over 30 years for various indications [4]. In experimental models of arthritis, indocyanine green (ICG)-enhanced FOI findings corresponded to histologically proven synovitis [5, 6]. This approach was also tested in humans [7, 8]. FOI is time efficient and operator independent. The image interpretation, however, is dependent on visual perceptions and imaging experience of the examiner and is limited to wrist and finger joints. Increased focal optical signal intensities visualize areas of high perfusion, altered microcirculation and/or capillary leakage, which may not show in GSUS, especially in the small finger joints [9‐11]. In adult RA patients, FOI has been shown to be a reliable technique to detect inflammation with good agreement with magnetic resonance imaging (MRI). The authors distinguished three different phases and a computer-generated composite image; their meaning still being unclear, they may give varying information such as changes in perfusion, microcirculation, capillary leakage or neovascularization. In the opinion of the authors, very early arthritis could be observed with FOI as well as subclinical inflammation [10, 11].

FOI might be a valuable tool to assess subclinical and very early arthritis, to differentiate between arthritis and pain syndrome and to assess treatment response.

This observational study intended to analyze improvement upon newly instituted treatment with a DMARD or with a biologic clinically, using commonly used tools (PedACR, JADAS, number of active, swollen and tender joints) and by imaging methods (GSUS and PDUS) as well as by FOI. We intended for the first time to compare clinical and imaging results with the new FOI method to explore the clinical utility. Of further interest is the presentation of residual inflammation which might not be detected clinically but by imaging and whether an early decrease of inflammation detected by FOI is predictive for remission later on.

The results of the first study using the FOI technique and US examination in children with JIA before and during antirheumatic treatment are reported here.

Musculoskeletal ultrasonography (in GSUS and PDUS) provides an imaging method which has a solid component in the clarification of inflammatory joint diseases. A disadvantage of the method is the relatively strong investigator dependency [18].

Morphological features of the inflammation are hypervascularization, increased blood perfusion and capillary permeability [19]. The (neo)angiogenesis is a very early event in the pathogenesis of chronic inflammation [20]. The angiogenesis of the synovial membrane leads to a dense capillary network in the inflamed joint membrane. Vasodilatation and increased capillary permeability maintain synovitis.

It has been shown in rheumatoid arthritis patients that the synovial vascularization correlates with the disease activity of an affected joint [21]. Arthrosonographic procedures, including power Doppler sonography, are limited in resolution, however, and are not capable of displaying an increased blood flow in the submillimeter region and thus on the capillary plane.

Magnetic resonance imaging (MRI) is today regarded as the gold standard for the representation of synovitis. Furthermore, bone marrow edema detectable in MRI is the strongest independent predictor of the radiological progression of rheumatoid arthritis [22, 23]. The exact significance of MRI for the diagnosis and prognosis of an early arthritis has not yet been clarified conclusively [24]. Overall, MRI is used for early diagnosis in clinical routine and above all in clinical trials. For standardization, the OMERACT criteria for the use and evaluation of MRI were developed [25]. However, there are limitations to the use of MRI in everyday practice due to the high cost of an examination and the limited availability. The use of gadolinium and some contraindications also limit their use in clinical routine. Furthermore, there were several limitations especially in younger children, such as maintaining an immobile position or the necessity for sedation.

In human medicine, FOI methods have so far been used predominantly in ophthalmology as fluorescence angiography for the imaging diagnostics of retinal and choroidal diseases. Furthermore, FOI use is described in vascular surgery, in neurosurgery after clipping of a brain aneurysm in oncology to delineate tumor tissue and in plastic and reconstructive surgery [9, 26, 27].

FOI is a relatively new imaging technique for the detection of inflammation and arthritis in the joints of the hand by visualizing altered microcirculation and angiogenesis in adults with rheumatoid arthritis [9, 28]. FOI findings agreed well with US [10, 11] and MRI [11] findings in two studies with 252 and 32 RA patients and 46 and 6 patients respectively without signs of inflammatory disease as control groups. Early arthritis as well as subclinical inflammation could be detected by FOI.

There are almost no data concerning FOI use in pediatric patients with JIA. Werner et al. [29] described FOI use and results in three children. The aim of this study was to evaluate patients with JIA of hand and finger joints using the FOI technique along with US and CE, and to show its usefulness for demonstration of improvement upon treatment. Thus a follow-up analysis before and after 12 and 24 weeks of treatment (either methotrexate or a biologic antirheumatic drug) using the FOI technique in comparison with US and CE was performed in an open-label study. Clinical findings were all as expected. A high rate of treatment success on either a newly initiated therapy with methotrexate or a biologic was documented, similar to that observed in other trials, with earlier improvement in the biologics cohort.

As one result of our study, the number of joints with increased signal intensity in FOI was higher than the number of joints rated clinically active and even higher than the joints with signs of inflammation on US. These findings are consistent with the results of former studies using FOI in adults [10, 11]. In comparative studies of CE and GSUS, PDUS, and MRI, inflammatory changes were documented in clinically asymptomatic joints [30‐32]. It can be assumed that FOI detected clinical and also subclinical activity.

There were differences in the specific phases of the analysis. The interpretation of the different phases is still not clear. Especially, phase 2 showed increased signal intensity in more joints than any other phase, including the composite image. Comparison of FOI with CE showed that phase 2 was best at detecting clinical active joints and also showed more activity in clinical inactive joints compared with the other FOI phases. FOI phase 2 also agreed best with US signs of synovial thickening and effusions. This may be the most sensitive phase and may be best suited to detect subclinical activity.

Phase 3 rarely showed increased FOI signals. In the former studies in adults, the authors considered phase 3 to show increased capillary permeability in which ICG is more persistent than normal [10, 11].

In joints with hyperperfusion detected by PDUS, FOI phase 1 most often showed increased signal intensity, followed by phase 2. This can also be shown in the ROC curves; when comparing the different FOI phases with PDUS, FOI phase 1 reached the best AUC values. As proposed by studies in adult patients, phase 1 may mirror increased vascularity as a sign of high disease activity [10, 11].

With CE as the standard of reference, FOI and US showed moderate sensitivities and high specificities, partly due to the high number of negative results. The findings for sensitivity and specificity for FOI with US as the standard of reference were comparable. This could also be shown in the ROC curves. Compared with CE, FOI phase 2 and US B-mode joint effusions were the most accurate parameters with similar values for the AUC. FOI phase 2 also had the highest AUC values when compared with the US mode parameters of joint effusion and synovitis. Because of the lack of an assured standard of reference, the interpretation of sensitivity and specificity is a delicate issue. Of course the majority of examined joints were expected to be inactive. Specificity was influenced by higher rates of positive findings in FOI compared with the other two methods. Because of the lack of a control group and an assured standard of reference like histopathological findings, sensitivity and specificity should be valued carefully. Results for agreement were added to complete the diagnostic description, but it should be considered that high numbers are partly based on the high amount of negative results.

In the detection of inflammatory changes, FOI was more sensitive than CE. The higher rate of positive findings compared with GDUS/PDUS suggests that the method is more sensitive than the other two procedures. It could be argued that only mild increases in FOI signal intensity might not be a sign of active arthritis and discreet findings in FOI should be interpreted carefully. Only FOI signals attributable to and limited to a specific joint were rated as FOI-positive joints. Also, Werner et al. [10, 11] state in their former studies that, because healthy subjects and controls with arthralgia but without signs of inflammatory disease had virtually no pathological increased signal intensity in FOI using the Xiralite method, the detected FOI signals appear to reflect actual inflammatory activity and are not all to be interpreted as false-positive findings.

Using MRI as a standard reference method, ICG-assisted fluorescence-optical diagnostics demonstrated high sensitivity (up to 85%) and high specificity (up to 95%) for the detection of MRI synovitis and tenosynovitis, as well as high consistency rates with MRI [10]. Unfortunately we were not able to also include MRI analyses in our study.

Improvement after 12 and 24 weeks of antirheumatic treatment could be shown with all three techniques, while the decrease in the number of joints with a FOI signal was slower than the clinical changes. To a lesser extent this was seen for the US as well. This indicates that joints becoming clinically inactive with treatment may show undetected subclinical activity, even after months of treatment. This is obvious in our patient shown in Fig. 3b who had residual inflammation at week 24 demonstrated by FOI although being in clinical remission. Hence, FOI can be useful to assess treatment response.

FOI improvement at month 3 did not show reliable predictive value for clinical improvement at month 6. Although more joints had changed from active at month 0 to inactive at month 6, when they had improved in FOI semiquantitative scores at month 3 (all FOI phases) the difference from 19 to about 35% is not huge. Logistic regression analysis could not produce reliable models either. One problem might be differences in sample size, because the vast majority of joints were inactive in the first place and did not show clinical or FOI changes during the investigation. But calculating only with initially active joints did not change the probability of outcome prediction.

There were some limitations to our study. The mean age of our patient cohort was relatively high, the youngest patient being 9 years old. This was due to reduced compliance in younger patients because the methodology is limited to older children able to comply with the procedure which takes 6 minutes and to the arm length of the children.

One further limitation of the FOI method is the lack of visualization of anatomical structures such as synovial proliferation or erosion, as well as the limited ability to assess the palmar inflammation, especially in the area of the hand and MCP joints. Furthermore, any inflammation of the hand region, for example lacerations of the skin, will result in increased signal intensity. In this study all patient hands were also examined for signs of inflammation and injury other than arthritis; this was documented and visible findings were documented and considered for FOI reading. Finally, FOI examination was tolerated well, with no report of AE correlated with the examination. We did not observe any serious adverse events in our cohort and in no patients did the analysis have to be stopped or interrupted.

ICG-supported FOI with the Xiralite method is a new imaging method for rheumatological questions. This technology allows sensitive detection of inflammatory changes. It is a useful technique to supplement clinical and ultrasonographic investigation in assessment of JIA activity in the joints of the hand. It can provide useful additional information for diagnosis and assessment of treatment response in JIA. Subclinical activity probably can be detected with this method. The procedure could be particularly important for the diagnosis of early arthritis as well as for follow-up under long-term anti-inflammatory therapy or for evaluation of remission.