Niels-Peter Buchholz ( Departments of Surgery (Section of Urology), The Aga Khan University. Karachi. )
Farhat Abbas ( Departments of Surgery (Section of Urology), The Aga Khan University. Karachi. )
Syed Raziuddin Biyabani ( Departments of Surgery (Section of Urology), The Aga Khan University. Karachi. )
Masood Afzal ( Departments of Radiology, The Aga Khan University. Karachi )
Qaiser Javed ( Departments of Surgery (Section of Urology), The Aga Khan University. Karachi. )
lmtiaz Rizvi ( Departments of Radiology, The Aga Khan University. Karachi )
Jamsheer Talati ( Departments of Surgery (Section of Urology), The Aga Khan University. Karachi. )
Objective: In order to establish some preliminary data of our population, we determined the ultrasonographic kidney dimensions in individuals withoud known renal disease. We assessed whether age, sex, side, body mass index (BMI) and presence or absence of diabetes mellitus and hypertension affect the renal size.
Methods: Ultrasonographic kidney measurements were performed on 194 adult patients without known kidney lesions. Measurements included length, width, cortical thickness and estimation of renal size which was obtained by multiplying the first three variables. The effect of age, gender, side, height, weight, BNll, hypertension and diabetes mellitus was statistically analyzed.
Results: The mean kidney length was 10.4 ±0.8 cm, mean with 4.5 + 0.6 cm and mean cortical thickness 1.6 ± 0.2 cm. The estimated mean renal size was 76 ±22 cm3. Kidney length did not significantly differ between zight and left, however, kidney width, cortical thickness and size did (p<0.05). Right kidneys were smaller than the left ones. In univariate analysis, the mean renal size correlated with age, sex, side, BMI and absence or presence of hypertension and diabetes mellitus. In a multivariate analysis, however, the only significant factors affecting renal size were sex and BMI.
Conclusion: We conclude that renal size is related to age, side, sex and the individual’s height and weight. the normal values for the Pakistani population Population-based studies are needed to establish (JPMA 50:12, 2000).
The kidney size of a patient is a valuable diagnostic parameter in urological and nephrologic practice. While the leading anatomy text describes the adult kidney as 12 cm long, 6 cm wide and 3 cm deep1, further review of the literature shows that renal size varies with age, gender, body mass index, pregnancy and co-morbid conditions2,3. Renal size may be an indicator for the loss of kidney mass and therefore, kidney function2,3. It is valuable in monitoring unilateral kidney disease through comparison with the other, compensatorily increased side4 and for the discrimination between upper and lower urinary tract infections9. Renal infections/inflammations, nephrologic disorders, diabetes mellitus and hypertension are the most important co-morbid conditions affecting renal size4-9,5,16
Since the renal size is affected by various factors, it is necessary to first establish the normal values. The information available in the West may not be extrapolated to our population since the renal size may differ between ethnic groups and according to body size11,17-19. While population-based studies are needed to establish the normal values for Pakistani individuals, in our study we determined the ultrasonographic renal size in a group of individuals with no known renal disease and assessed the effect of age, gender. side and BMI. In addition, we assessed the effects of hypertension and diabetes mellitus in a subgroup of our patients on renal size and compared our findings with the literature.
Subjects and Methods
For this prospective study, 201 consecutive patients between 13 to 80 years of age. who underwent an abdominal diagostic ultrasound at the Department of Radiology of the Aga Khan University Hospital, Karachi between August 1997 and March 1998, had an additional screening and measurements of the kidneys. Pregnant patients, patients with known kidney or urinary stone diseases and patients with an ultrasound for the diagnosis of lumbar or urinary symptoms were excluded. Seven additional patients were excluded due to incomplete data to leave 194 study patients.
The kidney dimensions measured include length (distance pole to pole), width (transversal axis) and cortical thickness, in millimeters. We estimated the kidney size, defined as length x width x cortical thickness, which correlates closely to the renal volume. All ultrasounds were performed by one experienced radiologist to exclude inter-observer variation. Additional data recorded either at presentation or at subsequent telephonic contact include age, gender. height, weight, BMI (weight [kg]/height [meter2]) and history of estblished hypertension and diabetes mellitus. Data were entered in a Microsoft ExcelR database and statistically analyses using SPSS Version-8. Comparative analyses were done by means of a student\\\'s "t" test. A p-value <0.05 was regarded as statistically significant.
Of the total 194 patients, 98 were males and 96 females. The mean age was 44.7 ± 14 years (range 13-80); 46.1 ±, 15.4 years (13-80) for males and 43.3 ± 13.2 years (15-80) for females.
Renal size and effect of gender and side: The mean kidney length was 10.4 ±. 0.8 cm (7.9 - 12.3), the mean kidney width 4.5±0.6cm (3.3 - 6.3) and the mean cortical thickness 1.6 ± 0.2 cm (I . 1 - 2.0). Kidney size (length x width x cortical thickness) was 76.16 ± 21.7 cm3 (36.8 - 138).
There was no significant difference in kidney length between right and left side (P<0.06). Howevei; differences in width, cortical thickness and size were all significant (P<0.05), with the right kidney being significantly smaller than the left (Table 1).
As a group, female kidneys were significantly smaller than the male kidneys (Table 2).
Renal size and effect of age: In our study, there were 9 individuals in their 2nd decade of life, 23 in the 3rd, 48 in the 4th, 52 in the 5th, 32 in the 6th, 22 in the 7th and 8 in the 8th decade of life. All renal dimensions increased with age till the 3rd decade, remained more or less stable through the middle age, then declined beyond the 6th decade (Table 3; Figure 1).
Renal size and effect of body mass index:
Information on body mass index (BM1) (weight [meter]2) was available in 118 study patients who were then divided into 3 groups, i.e., BMI 10-20, 21-30 and 31-40. The mean renal size correlated well with BMI and correspondingly increased with BMI. This observation, however, was not made for the right kidney in the 31-40 BMI group (Figure 2).
Renal size in hypertensives versus non-hypertensives and diabetics versus non-diabetics: Information about the absence or presence of established hypertension or diabetes mellitus was available in 118 individuals.
Table 4 shows that the mean renal size in both sides was smaller in hypertensive individuals when compared with non-hypertensives, but the difference was statistically insignificant. Similarly, diabetics had larger kidneys than non-diabetics, but the difference was insignificant. Since BMI could be an important confounding factor in this analysis, we compared the groups again in BMI matched individuals (Table 5)
and except for the left kidney dimensions in hypertensive versus non-hypertensive individuals, the hypertensive kidneys were smaller and diabetic kidneys were larger.
Multivariate Analysis: We finally performed a linear regression analysis to assess the effect of the variables studied on renal size. Only BMI and sex of the individual independently influenced the kidney size on both sides. Both, hypertension and diabetes mellitus, which on univariate analysis had shown significant influence on renal size, were non-significant in the multivariate analysis (Table 6).
The normal size of a kidney is variable and is affected by age, gender, BMI, as well as the side. The size provides a rough indication of the renal function. The minimal size of a fully functional kidney is 9 cm in length2. Decrease of size and function are seen with chronic renal failure4, renal arterial occlusion2 and late stage renal venous thrombosis5. However, although kidney size seems to be related to a number of vascular diseases, there is no correlation with blood pressure20. Physiologically, renal length decreases 0.5 cm per decade after middle age11-3.
On the other hand, there is an increase in kidney size in early stage renal thrombosis5, early stage diabetes mellitus7,8 and renal inflammation6. A physiological increase of glomerular filtration rate and kidney size can be observed in pregnancy10. Kidney size also increases with increased protein intake in mice21.
In order to estimate aberrations of kidney size, normal values must be established first. Not many studies have been done on this issue. There are, as far as we know, no reliable reference tables because the measurements vary between men and women, between people of different ethnic backgrounds and even between kidneys of the same individual. Also, it has to be borne in mind that kidney size measurements with ultrasound (US), as well as with CT and MRI result in a 24% underestimation of the renal value22.
Commonly, US is used to screen and measure the kidney. In comparison with an intravenous pyelogram, US is more accurate and suffers neither from the geometric magnification of X-raying, nor from a possible increase in kidney size by osmotic diuresis through iodinated contrast material23. It has been analyzed as a reliable, repeatable (inter-observer variation) and reproducible (intra-observer variation) method24. In our study, a single senior radiologist carried out the measurements at predetermined fixed points, to minimize the inter-observer and intra-observer variation. Studies on kidney length and size have also been made by means of CT25 and MRI22. But although these methods show, not unexpectedly, a better repeatability22, we believe that US is the tool of first choice due to its ubiquitous availability, its easy handling and its cost-effectiveness.
Most studies have looked at kidney length. Ultrasonic kidney length measurement (bi-polar measurement) is the most commonly used and most practical measurement in clinical practice10 and is correlated to renal function3.
Normal renal length varies from 100 to 124 mm in different populations17-19,25 dependent on ethnic background, side and sex. While population-based studies are needed to establish the normal values for Pakistani individuals, our pilot study group showed a mean kidney length of 104 mm. This is at the lower end of the scale and together with a Malaysian study population19 is probably a reflection of the relatively small body size of most IndoAsians. Organ size is unquestionably related to the body size. Accordingly, Africans figured in the upper range 17 and Caucasians somewhere in the middle25.
In our study, the kidney length, width, cortical thickness and size were significanly larger in males than in females. This has been reported by other investigators17,19,26 and has been related to differences in body size26. We found this difference significant even in the multivariate analysis.
Throughout all studies, there is a marked but not significant difference of kidney length between the right and left side, with the left side being on an average 5% larger11,17,19. We feel it could be related to the hepatic mass which does not allow comparable vertical growth of the right kidney to that which is attained by the left kidney. In our study, except for the length, all other renal dimensions were significantly larger on the left than on the right side. We thus feel that instead of renal length the renal size as determined by us, or volume as assessed by others, may be used as the most useful parameter for evaluation and comparison.
The age of an individual has an important bearing on the kidney size. We found that the kidney size increases till the 3rd decade, remains stable through the middle age and then declines’ We have developed a prototype reference table of age and renal dimensions for both sides, as there is no such information available for our population. The validity of this information, however, needs to be established through larger, population-based studies. Such a reference table is extremely useful for routine evaluations and monitoring of urological and nephrologic diseases.
Our data shows a strong correlation between renal size and BMI (Figure 2). The renal size increased correspondingly with an increasing BMI, except for the right kidney in the group of obese individuals (BMI:3 1-40). In these patients, the body mass increase may surpass the renal growth capacity. Also, it has to be considered that gross obesity can be one of the limitations of ultrasound examination. Other investigators have also shown a strong correlation of renal volume with height, weight and BMI11,13,17,19
We assessed the effect of hypertension and diabetes mellitus on renal size because of the frequent affection of our adult population with these co-morbid conditions. While in the univariate analysis, both hypertension and diabetes mellitus were significant factors affecting renal size, their effect was not significant in multivariate analysis. It has previously been shown that high blood pressure is not proportionally related to kidney size20. In long-standing hypertension, however, the kidney size is shown to decrease due to ischemic changes with resultant fibrosis and hyalinization27. Other investigators have related an increase in kidney size with an increased glomerular filtration rate in early stage diabetes niellitus. This increase represents indeed a risk factor for diabetogenic nephropathy thus making ultrasonic kidney measurement a tool for the diagnostic and prognostic assessment of diabetic patients7,8.
Finally, of all the variables assessed in our study, the most significant factors associated with the kidney size were sex (p=0.000) and BMI (p=0.01, 0.004) in the linear regression analysis model.
In summary, normal values for kidney measurements are dependent on age, sex and body mass index. This has to be considered by the ultrasonographer. Aberrations from these values can give valuable general clues and confirmations in the diagnosis of particular diseases. A slightly small right kidney may be considered as normal and a reference table as developed by us can be used for routine evaluation. For the Pakistani population, normal kidney measurement values need to be developed by means of population-based studies.
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