To the best of our knowledge, this is the only study assessing endothelial function in the absence of diabetes, morbid obesity, smoking and poorly controlled hypertension in patients with biopsy-proven NAFLD.
NAFLD is a group of disorders that occur in people who do not drink excessive amounts of alcohol and that are characterized by abnormal fat accumulation in the liver, leading to histological features that are similar to those seen in alcoholic liver disease11.
This clinical entity is comprised of a spectrum that begins with simple fatty liver (steatosis) and may end with cirrhosis. A high prevalence of ultrasonographic findings of fatty liver and insulin resistance has been reported in patients with CVD12.
In the current study, we evaluated endothelial function in patients with conditions ranging from normal liver histology to NASH. We also eliminated factors that are clearly associated with endothelial dysfunction and CVD, including diabetes, smoking, chronic or uncontrolled hypertension, known cardiac disease and liver cirrhosis. It can be argued that the increased atherosclerosis and CV events in NAFLD patients are related to the associated metabolic syndrome, especially diabetes. It was recently shown that patients with type 2 diabetes and ultrasonographically diagnosed NAFLD had higher rates of coronary, cerebrovascular and peripheral vascular disease than age‐ and sex‐matched patients with diabetes without NAFLD6. We did not exclude patients with hyperlipidemia; however, cholesterol levels were not elevated in the majority of our study population and were similar between all three groups. It has been well established that vascular dysfunction can be induced by smoking. Smoking-induced vascular dysfunction is initiated by reduced nitric oxide (NO) bioavailability and the increased expression of adhesion molecules, leading to subsequent endothelial dysfunction. Smoking also increases platelet and macrophage adherence and promotes the development of a procoagulant and inflammatory environment13. The endothelial response to various agents and physical stimuli is impaired in patients with hypertension. The decline in endothelial function in systemic hypertension is associated with a progressive decrease in NO bioavailability and an increase in the production of endothelium-derived contraction factors14. We did not completely exclude hypertensive patients in the current study; however, we included patients with only a relatively short-term history of hypertension, who were on one or two antihypertensive agents and whose hypertension was well controlled. Endothelial dysfunction was also reported in patients with cirrhosis and advanced liver disease. Multiple studies have clearly shown systemic vasodilation in patients with cirrhosis; NO is overproduced in cirrhosis, and the measured serum levels are significantly elevated in both patients with cirrhosis and animal models of cirrhosis15,16,17. NO is an endothelial-derived relaxing factor that leads to arterial vasodilatation. Therefore, patients with advanced liver disease were also excluded.
Saudi Arabia is among the countries with the highest prevalence of metabolic syndrome, DM, and obesity18,19,20,21. This made the recruitment of patients difficult and prolonged, but it also contributed to a more accurate assessment by eliminating the negative impact of metabolic syndrome on endothelial function.
Brachial FMD is the most common tool used to assess endothelial function in the majority of clinical studies22,23,24. It is a noninvasive and reproducible method with relatively accurate results. Therefore, we used FMD to evaluate endothelial function in the current study, and this method enabled us to compare our results with those of other clinical trials. Our vascular evaluation protocol was designed to minimize limitations; thus, the test was performed by a single well-trained and experienced technician in a quiet and temperature-controlled room. In our study, we clearly demonstrated that there was a higher vasodilatory response in the patients of the control group than in the NAFLD patients. This difference was more evident when the control and steatohepatitis groups were compared. The lower endothelial flow‐mediated vasodilation is likely a reflection of the presence of subclinical atherosclerosis. Similarly, a relationship between NAFLD and atherosclerosis has been reported. Fracanzani and colleagues demonstrated that patients with NAFLD had a thicker carotid intima-media and a higher prevalence of plaques than healthy controls. Steatosis was the strongest variable to be independently associated with intima-media thickness and the development of atherosclerosis25. Villanova and colleagues demonstrated that there was evidence of endothelial dysfunction and an increased risk of CV events in NAFLD. They measured the vasodilatory response of the brachial artery in response to ischemia as well as the CV risk profile in 52 NAFLD patients and 28 age- and sex-matched controls. They demonstrated that there was evidence of endothelial dysfunction and an increased risk of CV events in NAFLD patients compared to healthy controls. However, the diagnosis of NAFLD in their study was based on chronically raised alanine aminotransferase levels and a bright liver on ultrasound. Furthermore, they excluded only patients with overt diabetes or a BMI above 35 kg/m226. Senturk and colleagues showed that patients with NASH had significantly worse endothelial dysfunction than patients with simple steatosis27. Endothelial dysfunction is considered to be a strong predictor for the development of atherosclerosis and CVD. Shechter et al. conducted a study to determine the long-term association between brachial artery FMD and adverse CV events in healthy subjects, and they prospectively assessed brachial FMD data from 618 consecutive healthy subjects with no apparent heart disease. The subjects were divided into 2 groups, namely, FMD < 11.3% and >11.3%, where 11.3% was the median FMD, and the patients were comparable regarding other CV risk factors. During a mean clinical follow-up of 4.6 years, composite CV events were significantly more common in subjects with an FMD < 11.3% than in subjects with an FMD > 11.3% (15.2% vs 1.2%, p < 0.0001, respectively). Univariate and multivariate analyses demonstrated that the median FMD was the best independent predictor of long-term CV adverse events (odds ratio 2.93, 95% confidence interval (CI) 1.28 to 6.68, p < 0.001)28.
One of the limitations of this study is the relatively small sample size in each group. The lack of serum biological markers of endothelial dysfunction in the current study was another limitation. Nevertheless, this study clearly revealed a variable response in brachial artery FMD across the spectrum of NAFLD.