Radiological methods:
Ultrasound in evaluation of CFHBI:
Ultrasound (US) is a relatively inexpensive and widely available tool to assess the liver. The initial role of US was established in a single-center study involving a cohort of 106 children with CF followed for approximately 10 years. In this study, a heterogeneous echogenic pattern on liver US was utilized to evaluate children at risk for aCFLD (defined as a nodular liver in US) with or without portal hypertension58. The authors demonstrated that children with CF with a heterogeneous echogenic appearance had a 5.2-fold increased incidence of a nodular appearance and a 6.1-fold increased incidence of portal hypertension compared with participants with a normal echogenic pattern 58. These results were confirmed by the Prospective Study of Ultrasound to Predict Hepatic Cirrhosis in CF (PUSH study) which showed the heterogenous pattern had 9.5 times increased incidence of the nodular pattern when compared to participants with normal liver appearance over a six-year period 59.
In addition to showing an association between a heterogenous US pattern and the development of a nodular pattern on US, the PUSH study also demonstrated significant differences between US groups (normal, heterogenous, homogenous and nodular) for spleen size, and non-invasive hepatic markers depicting the ability of a research ultrasound in the evaluation of CFHBI and aCFLD 44. FIB-4, GPR, and APRI were significantly increased, and platelets decreased in participants with a heterogeneous pattern when compared with PwCF with a normal liver appearance 60. In another study with longitudinal follow up, the same group of PUSH study investigators noted that in approximately 6.3 years of average follow-up, six participants with nodular liver appearance developed esophageal varices, two had bleeding related to varices, and two had liver transplants 61. The control population had no liver-related adverse events highlighting the feasibility of research US in discriminating the progression of aCFLD from the individuals with normal liver appearance.
While the data regarding US in CF is encouraging, there remain significant clinical concerns. US lacks the ability to detect early stages of aCFLD reliably and a normal US does not preclude significant liver fibrosis 12. Another major limitation is a high percentage of interpretation variabilities (both intra- and interobserver) between sonologists specifically in clinical settings12, an important point to note given the PUSH study was conducted by trained and experienced radiologist within a research protocol. US is likely best utilized clinically to compliment serum biomarkers. This is highlighted by the proposed algorithm by Sellars et al integrating conventional liver indices to guide when to perform a liver ultrasound (US). They utilized GGT as an initial screening test and further utilized APRI and GPR to determine when an abdominal ultrasound is needed 27. This is supported by the final PUSH multivariate analysis showing that the combination of initial US pattern, age and GPR produced the best AUC in predicting the development of a Nodular US 60.
Elastography in evaluating CFHBI:
With progressive fibrosis, the stiffness of the liver increases with decreasing elasticity which can be utilized for screening using several elastography techniques 38. A high cutoff for LSM increases the specificity of diagnosis (ruling in) advanced liver fibrosis and conversely a low cut-off provides a relatively higher sensitivity for the evaluation of early stages of (minimal or no) fibrosis. Common radiological methods include vibration-controlled transient elastography (TE), ultrasound-guided elastography such as point shear wave elastography (SWE) or 2D-shear wave elastography, and magnetic resonance elastography (MRE) have been studied.
TE (FibroScan®) has been utilized in the early evaluation and monitoring of liver fibrosis in both children and adults with CF39-42 Lewindon et al demonstrated that liver stiffness measured by TE was significantly higher in children with CFHBI (10.7 kPa, SD 2.4) when compared with children without CFHBI (4.6, SD 0.1) and healthy control population (4.1 kPa, SD 0.1)41. Using 5.5 kPa as a screening cut-off for CFHBI, the liver stiffness measurement (LSM) identified children with CFHBI with an AUROC of 0.82, 70% sensitivity and 82% specificity (P <0.0001). A higher cutoff of 8.7 kPa improved the specificity of the diagnosis and differentiated children with aCFLD (diagnosis of aCFLD based on ultrasound findings or by liver biopsy, stages F3-F4) vs patients with early fibrosis (stages F1-F2), with an AUROC, 0.87; 75% sensitivity, 100% specificity 41. Also, the combined utility of APRI and LSM identified children with CFHBI with higher diagnostic accuracy (AUROC of 0.89, 87% sensitivity, and 74% specificity). Also, the combination of APRI and LSM improved the differentiation of patients with severe fibrosis vs mild fibrosis 41. Lam and colleagues did a meta-analysis by pooling results from six studies (605 participants) and demonstrated an ideal cut-off for LSM by TE and APRI as ≥5.95 kPa and ≥0.329 respectively 62 The sensitivity, specificity, and AUROC were 55%, 87%, 0.76 and 52%, 93%, and 0.84 for LSM and APRI respectively. When both parameters were combined, the sensitivity, specificity, PPV, and NPV were 43%, 99%, 92%, and 87% with a diagnostic odds ratio of 75.
Although encouraging, LSM by TE suffers from poor intra- and interindividual variation. The poor repeatability of TE was highlighted by Rowland and colleagues in healthy children (normal range of LSM was as 2.88–6.52 kPa) across various age categories 63. A difference of >1kPa between paired measurements was noted in approximately 26% of the participants 63. Therefore well agreed standard cut-off values for TE in the evaluation of CFHBI are urgent needed.
Calvopina and colleagues demonstrated that LSM by supersonic SWE showed good diagnostic accuracy in children with CFHBI 64. LSM was significantly higher in CFHBI (8.1 kPa, IQR = 6.7–11.9) versus without liver involvement (6.2 kPa, IQR = 5.6–7.0; P < 0.0001) and control population (5.3 kPa, IQR=4.9–5.8; P < 0.0001) 64. The diagnostic accuracy improved when supersonic SWE was combined with APRI.
In a single-center study, both TE using FibroScan® and point SWE (Virtual TouchTM Quantification) were compared in children with CF for their repeatability and reproducibility34. The performance of both techniques was good and investigators concluded that either one of the two methods can be utilized for evaluating CFHBI 34. However, the intraclass correlation was superior for TE than SWE for both intra- and interobserver agreements 28. Unlike TE, the LSM measurements between studies may not be comparable in difference studies involving SWE as the results may vary based on the technology utilized (point SWE vs 2D-SWE, probe and frequency (6Hz vs 9 Hz) differences, and availability of different commercial brands 65.
MRE is the most accurate elastographic technique but its availability is limited to research settings in tertiary care centers. MRE is expensive and also poses technical challenges in young children, people with severe obesity, and advanced lung disease 66. In a single-center prospective study involving 55 percipients, all three (TE, SWE, and MRE) elastography modalities were compared67. PwCF aged 6-18 years were included and based on US findings, participants were grouped into three groups (aCFLD, heterogenous increased echogenicity, and normal/homogeneous echogenicity). All tests were done on the same day. LSM on all elastography methods was significantly higher compared to the other two groups. TE and SWE were highly correlated (r=0.9) and concordant in identifying aCFLD (Cohen’s k=0.84) while MRE was moderately correlated and concordant with TE (r=0.41; k=36) and SWE (r=0.5; k=0.50)67. Even though these elastography techniques have shown good reliability in the evaluation of early diagnosis of significant liver fibrosis, their effect on longitudinal monitoring of disease progression is lacking, and further studies are awaited1.
Spleen elastography:
Increased spleen stiffness measured by elastography methods has been utilized in both children and adults with advanced liver disease and may help to predict the severity of portal hypertension68,69. In a single-center study involving children, higher swear wave velocity was noted in CF than in control population.70. But no differences in swear wave velocity were noted in CF children with or without liver involvement and further studies are needed to explore its utilization in the early stages of aCFLD 70.
Radiological methods conclusions:
Greyscale US is often clinically utilized in evaluation of aCFLD due to wide availability and relatively low cost. However, it lacks the sensitivity to detect early stages of aCFLD reliably and also has high percentage of interpretation variabilities (both intra- and interobserver) among radiologists. US-based elastographies (TE and SWE) are relatively readily available, technically easier to perform, and cost effective. LSM obtained by various elastography techniques (TE, SWE, and MRE) are modality-dependent, and cannot be compared directly to one another. Also, these radiological tests are best utilized to complement the serum biomarkers in the evaluation of CFHBI. Finally, spleen elastography may be advantageous to evaluate portal hypertension, with or without cirrhosis, but its availability is limited and additional CF-specific studies are needed.
Genetics in the evaluation of CFHBI :
While CFHBI has been associated with pancreatic insufficiency, history of meconium ileus and CFRD, no clear genotype association has been established 7. While some studies have shown F508del homozygosity was associated with aCFLD, it is unclear if that reflects the frequency of the mutation rather a true risk factor. It is generally believed that severe mutations (class 1-3) are associated with aCFLD71, but given the relatively low incidence rate among patients with two severe mutations, it has long been believed that additional genetic modifiers contribute to aCFLD.
To date, only the SERPINA1 Z allele has been associated with the development of aCFLD 72,73, but only accounts for a small portion of those with aCFLD. However, SERPINA1 has since been shown to be an independent risk factor for advanced liver disease in a number of chronic liver conditions 74, and thus unlikely to be a CF-specific modifier. In silico modeling has also supported the idea that additional genetic modifiers beyond SERPINA1 Z may contribute to the development of aCFLD 75Currently, both the PUSH study team and researchers for the CF Foundation are conducting studies to better understand the role of genetic modifiers in CFLD (Clinical Trials NCT01144507 and NCT00804583).
Genetic modifiers conclusion:
To date, only the SERPINA1 Z allele has been associated with aCFLD but only accounts for a small portion of the population, and is unlikely to be CF-specific. Additional work is underway to better understand the potential genetic modifiers associated with CFHBI and aCFLD.