Discussion

Hypertension is the most common chronic disease and is one of the major causes of heart failure, stroke, and chronic renal failure. The earlier the onset of hypertension, the longer the exposure time, resulting in a greater risk for cardiovascular events. Although there is not sufficient information about the pathogenesis of young-onset hypertension in the literature, it has been suggested that the main underlying haemodynamic abnormality may be increased peripheral vascular resistance, which causes vascular remodelling in the arteries. Increased arterial stiffness is an important parameter to determine the cardiovascular risk. ^{Reference Laurent, Boutouyrie and Asmar20–Reference Ben-Shlomo, Spears and Boustred22}

In addition to increased aortic stiffness being determined by pulse wave velocity measurement, it has been shown in the literature that there is a relationship between aortic stiffness measured by pulse wave velocity and age, ethnicity, and future CVDs. ^{Reference Leung, Dumont, Sandor, Potts and Potts23–Reference Laurent, Cockcroft and Van Bortel26} In a meta-analysis, a 1 m/second increase in pulse wave velocity was reported to be associated with an 11% increase in cardiovascular death. ^{Reference Vlachopoulos, Aznaouridis and Stefanadis27} Pulse wave velocity is an indirect measure of arterial stiffness, and many studies have measured pulse wave velocity using different methods, such as Doppler ultrasonography and tonometry. ^{Reference Mattace-Raso, van der Cammen and Hofman28,Reference Mitchell, Hwang and Vasan29} The most accurate evaluation of aortic pulse wave velocity can be performed by the measurement of intra-arterial pressure. However, this is an invasive modality, and therefore not suitable for widespread clinical use. ^{Reference Lehmann30,Reference Mitchell, Guo and Benjamin31}

Tonometry and ultrasound are more commonly used in pulse wave velocity measurement. However, both modalities only provide an estimate of aortic pulse wave velocity due to inadequate acoustic windows and insufficient spatial resolution along the length of the aorta. ^{Reference Leung, Dumont, Sandor, Potts and Potts32,Reference Alecu, Labat and Kearney-Schwartz33} Pulse wave velocity measurement using MRI is widely used as a non-invasive and reliable technique. The most common method of measuring aortic pulse wave velocity is to calculate the aortic length and transit time, i.e., the time it takes for the systolic wave to travel from one reference point within the aorta to another. ^{Reference Maroules, Khera and Ayers34–Reference Kaess, Rong and Larson36}

MRI allows for an accurate evaluation of the blood flow rate with sufficient temporal and spatial resolution to investigate the travelling of the aortic systolic flow wave. The true path length of the pulse wave along the aorta can be directly assessed by MRI even in cases of aortic tortuosity, and the regional elastic properties of the aorta can be examined depending on the number of aortic segments studied. ^{Reference Lehmann30,Reference Bolster, Atalar, Hardy and McVeigh37–Reference Grotenhuis, Westenberg and Steendijk39}

The presence and severity of hypertension is known to accelerate the increase in aortic pulse wave velocity. ^{Reference Tomiyama, Arai and Koji40} In a 6-year study measuring aortic stiffness as carotid-femoral pulse wave velocity, the presence of hypertension was found to be associated with progression of aortic stiffness compared to normotensive subjects. ^{Reference Benetos, Adamopoulos and Bureau41} Similarly, in our previous tonometry study conducted with young-hypertension cases, we found pulse wave velocity to be significantly higher in patients with hypertension. ^{Reference Gökaslan, Özer Gökaslan, Demirel and Çelik42}

Ohyama et al, who followed up hypertension cases of multi-ethnic origin aged 45 years or over for 10 years, measured their aortic stiffness by MRI and reported that blood pressure control was effective in stopping the progression of aortic stiffness. ^{Reference Teixido-Tura and Ambale-Venkatesh43} Van Elderen et al evaluated type I diabetics with normal renal functions in terms of their pulse wave velocity values measured on MRI and found that this patient group had increased pulse wave velocity, independent of renal dysfunction. ^{Reference Van Elderen, Westenberg and Brandts44} Similarly, other researchers showed that the pulse wave velocity increase measured by MRI in patients with type I diabetes was associated with cardiac dysfunction and cerebral small vessel disease. ^{Reference Van Elderen, Brandts and Westenberg45}

In another study, the pulse wave velocity measurements were undertaken by MRI in end-stage renal disease cases with decreased arterial compliance and were found to be significantly higher compared to the control group. ^{Reference Zimmerli, Mark and Steedman46}

In the current study, the pulse wave velocity values calculated by MRI were found to be increased in hypertensive patients compared to the control group, which is in agreement with the literature. To the best of our knowledge, in the literature, aortic stiffness has not been previously investigated with MRI in patients with young-onset hypertension. Our findings suggest that increased arterial stiffness may also contribute to the aetiology of hypertension that begins at a young age.

The sample size was relatively small, and larger studies are needed to confirm the clinical role of aortic stiffness in cardiovascular risk classification and treatment optimisation. There is also a need for further studies comparing aortic stiffness measured by MRI with other methods.

In conclusion, aortic stiffness may play a role in the aetiology of young-onset hypertension and serve as a non-invasive and reliable screening tool when measured by MRI.