European Journal of Cardiovascular Medicine

Hypertension is a persistent, noncommunicable multisystem disorder that impacts various organs, including the kidneys. It is characterized by a blood pressure measurement exceeding 140 mmHg systolic or 90 mmHg diastolic, or both [1]. The prevalence of hypertension ranges from 10% to 30%, highlighting the need to investigate its impact on vital organs like the kidneys. Renal complications rank third after stroke and congestive heart failure [2]. Renal length is the most commonly used and reproducible metric for evaluating kidney size. However, other renal parameters are more influenced by disease states, such as hypertension. Studies have demonstrated that renal cortical thickness decreases early in patients with chronic renal disease due to hypertension [3], and renal parenchymal thickness has been shown to correlate well with renal function [4,5].

Research by Okoye et al. [6] using ultrasound in South-East Nigeria found a strong correlation between renal parenchymal thickness and renal length in individuals aged 18 to 80 years. Similarly, Cost et al. [4] in the United States found that renal parenchymal area measured via ultrasound provided a more accurate estimation of renal size and function. In Japan, Kojima et al. [7] discovered that individuals with essential hypertension had smaller renal cortical volume and a more heterogeneous cortex on contrast-enhanced CT scans compared to normotensive individuals. In France, Mounier-Vehier et al. [8] reported significant cortical atrophy as a marker of early ischemic nephropathy in patients with hypertension secondary to unilateral renal artery stenosis, using spiral CT angiography. Changes in the renal cortex appear earlier than alterations in other commonly measured morphological parameters such as renal length.

There is a necessity to develop a safe, cost-effective method for evaluating the impact of hypertension on kidney function. Ultrasound, being nonionizing, offers these advantages and allows for continuous monitoring and follow-up of patients.

This study aimed to compare renal sonographic parameters between hypertensive and normotensive individuals and to identify indicators that may suggest an increased risk of renal damage in those with essential hypertension. These parameters include renal length, width, anteroposterior thickness, echotexture, as well as renal cortical thickness and parenchymal volume.

The study included 356 adults, comprising 178 individuals with hypertension and an equal number of normotensive controls. Hypertensive participants were under treatment for essential hypertension and were recruited from the medicine outpatient department, while normotensive controls were selected from the general outpatient department. Exclusion criteria included diabetes, pregnancy, renal masses, renal malformations, and hydronephrosis.

Blood pressure measurements were performed using a manual sphygmomanometer with a mercury device and stethoscope. Biometric parameters, including height and weight, were recorded, and body mass index (BMI) and body surface area (BSA) were calculated. Venous blood (2 ml) was drawn under aseptic conditions from the antecubital vein for serum creatinine analysis.

Renal ultrasound assessments were conducted using a latest machine equipped with a curvilinear probe operating at a frequency of 2-8 MHz. Participants were positioned supine on a couch, with their abdomen exposed from the upper abdomen to the symphysis pubis. Longitudinal, coronal, and transverse scans of the kidneys were performed in supine, supine-oblique, and prone positions. Renal dimensions, including length, width, and anteroposterior thickness, as well as cortical thickness and renal parenchymal volume/echogenicity/echotexture, were evaluated. Cortical echogenicity was assessed in the supine position and graded as follows:

Grade 0: Normal, with renal cortical echogenicity less than that of the liver on the right and spleen on the left.

Grade 1: Renal cortical echogenicity equal to that of the liver on the right and spleen on the left.

Grade 2: Renal cortical echogenicity greater than that of the liver on the right and spleen on the left, but less than the renal sinus echo.

Grade 3: Renal cortical echogenicity equal to that of the renal sinus.

Renal dimensions were measured in the longitudinal plane with both renal poles visible, and in the transverse plane at the hilum. Using electronic calipers, renal length (L) was determined as the longest distance between the renal poles on the longitudinal scan, and renal width (W) as the maximum transverse diameter on the transverse scan. Renal thickness or depth (D) was calculated as the average maximum distance between the anterior and posterior walls of the kidney midportion in longitudinal and transverse scans (D1 and D2). Kidney volume was calculated using the prolate ellipsoid formula.

Cortical thickness was measured on a longitudinal scan as the perpendicular distance from the base of a pyramid to the renal capsule, 2 cm away from the renal poles, and at the kidney midportion. Renal parenchymal volume was determined using longitudinal and transverse scans. Maximum longitudinal, transverse, and anteroposterior dimensions of the kidney and the central sinus echo were measured, and the ellipsoid formula was applied to calculate both renal and central sinus echo volumes. Renal parenchymal volume was obtained by subtracting the central sinus echo volume from the renal volume. All measurements were performed by a single observer, with each value measured three times and the average taken to minimize intraobserver errors.

Data were analyzed using the Statistical Package for the Social Sciences (SPSS version 21). Comparisons (statistical tests of significance) were made using the Chi-square test for categorical data and the t-test for continuous variables. A two-tailed P-value of ≤0.05 was considered statistically significant at a 95% confidence interval.

The baseline characteristics of the hypertensive and normotensive groups are presented in Table 1. The hypertensive group had a higher mean weight compared to the normotensive group. Similarly, the body mass index (BMI) was significantly higher in the hypertensive group compared to the normotensive group. Additionally, serum creatinine levels were elevated in the hypertensive group compared to the normotensive group, with a significant difference (P < 0.05).

Table 1: Baseline parameters in the study groups

Renal ultrasonography (USG) findings are detailed in Table 2. There was no significant difference in parenchymal volumes between the hypertensive and normotensive groups for both the right and left kidneys. However, the hypertensive group had reduced cortical thickness in both right and left kidneys compared to the normotensive group. Kidney lengths did not significantly differ between groups for the right or the left kidney.

Table 2: Renal USG findings in hypertensive and normotensive subjects

The distribution of cortical echogenicity grades for both kidneys is summarized in Tables 3 and 4. For the right kidney, a significantly higher proportion of hypertensive subjects had higher echogenicity grades compared to normotensive subjects (P < 0.01). Similarly, for the left kidney, a significant difference in echogenicity was observed (P < 0.01).

Table 3: Cortical echogenicity comparison for Right kidney

Table 4: Cortical echogenicity comparison for Left kidney

In summary, hypertensive individuals exhibited significant differences in weight, BMI, blood pressure, serum creatinine, cortical thickness, and cortical echogenicity when compared to normotensive individuals, indicating potential renal implications associated with hypertension.

The present study revealed that the mean left renal parenchymal volume was greater than the right. The difference in right and left renal parenchymal volumes was statistically significant in the normotensive group but not in the hypertensive group. Dixit et al. [9] utilized ultrasound and the ellipsoid formula to measure renal parenchymal volume in healthy children in India, finding a significant correlation between renal parenchymal volume and somatometric parameters such as age, height, weight, and body surface area (BSA). The differing findings between the studies can be attributed to the fact that the Indian study population consisted of children. Gao et al. [10] measured renal parenchymal volume in normotensive adults using nonenhanced multidetector CT in China, reporting higher values than those in this study, possibly due to the different imaging modalities and the radiographic and contrast-induced renal magnification associated with CT. Ultrasound has been documented to underestimate renal volume because the kidney is not a true ellipsoid [9, 11]. CT was not used in this study due to cost and ionizing radiation concerns. Given that renal parenchymal volume varies to meet individual metabolic demands and is closely related to renal function, as documented by Johnson et al. [12], the lower values observed in the hypertensive group may be associated with reduced renal function, potentially due to long-standing, poorly controlled hypertension.

The study also demonstrated that mean cortical thickness was higher in normotensives compared to hypertensives. The differences in values between hypertensive and normotensive individuals were statistically significant. These values were higher than those found in the study by Beland et al. [3] in Rhode Island, likely because their study population consisted of individuals with chronic renal failure (CRF). Similarly, Siddappa et al. [13] in India measured cortical thickness in adults with CRF and reported higher values than those found by Beland et al. [3], noting that cortical thickness decreased with increased cortical echogenicity. Mounier-Vehier et al. [8] in France also examined renal cortical thickness among other parameters in hypertensives with unilateral renal artery stenosis, finding significant cortical atrophy in kidneys with stenosed arteries. Buchholz et al. [14] in Karachi, Pakistan, observed higher left renal cortical thickness compared to the right. Kojima et al. [7] in Japan documented decreased cortical tissue and increased heterogeneity in patients with essential hypertension compared to age-matched normotensives using CT, attributing this to the early involvement of the renal cortex in hypertension, which appears to affect the cortex more than the medulla.

Cortical echogenicity in hypertensives was higher than in controls in this study, with a statistically significant difference for both kidneys. Siddappa et al. [13] reported increased cortical echogenicity on ultrasound in chronic kidney disease patients. Although increased cortical echogenicity is not a specific sign, it is recognized as an indicator of renal parenchymal disease in many studies. A study by Araujo et al. [15] found increased cortical echogenicity on ultrasound in various disease conditions, noting that echogenicity increased as the disease progressed. Furthermore, Moghazi et al. [16] in Atlanta, USA, found that cortical echogenicity was the sonographic parameter that best correlated with renal histopathological findings such as glomerular sclerosis, tubular atrophy, interstitial fibrosis, and inflammation. Similar results were reported by Nwafor et al [17].

Serum creatinine levels were significantly higher among the hypertensive group in this study, despite most hypertensive patients being on therapy with stable blood pressure control, potentially indicating underlying hypertension-induced renal damage.

Among hypertensive individuals, parameters such as renal length, renal parenchymal volume, and cortical thickness were generally lower, with the reduction in cortical thickness being statistically significant. Additionally, cortical echogenicity was significantly elevated in hypertensive individuals compared to their normotensive counterparts. The findings of this study indicate that essential hypertension significantly impacts cortical thickness and echogenicity. Given the association of these parameters with renal function, they may serve as predictive markers for renal involvement in hypertensive patients. Ultrasonography of the kidneys proves to be a valuable tool, especially in resource-limited settings where advanced renal imaging modalities that involve ionizing radiation are often cost-prohibitive and inaccessible.

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