Key words : History of nephrology, Kidney defect, Ren arcuatus

Introduction: Morphological Representation of the Horseshoe Kidney

Horseshoe kidneys are the most common fusion defect of the kidneys, with a reported incidence of approximately 1:500.^1,2

The bridge connecting the right and left kidneys may present functional renal parenchyma or just fibrous tissue. Its central or lateral position in relation to the spine determines whether the horseshoe kidney is symmetrical or asymmetrical.³ Asymmetric systems are more often left dominant (70%).⁴ In 80% of cases, the isthmus comprehends functional renal parenchyma, which can present a challenge for safe division at surgery. In more than 90% of cases, fusion between the kidneys occurs at the inferior pole, although it may also occur at the superior pole, resulting in an “inverted horseshoe” (5%-10%) or a “disc kidney” at both poles.⁵ There may also be double ureters on one or both sides (6%).⁴ In rare cases, one or both ureters may migrate behind the isthmus.⁶

Individuals with horseshoe kidneys seem to be at higher risk for chronic kidney disease. A follow-up study in 146 adults (mean age 43 years) with horseshoe kidney reported that the horseshoe group compared with matched controls (ie, sex, age, serum creatinine, hypertension) had higher rates of urinary tract obstruction, kidney stones, urogenital cancer, and end-stage kidney disease.⁷

In another case series of 41 children (aged 2 months to 16 years with a median of 4 years follow-up), kidney scarring was found in 24%, proteinuria in 15%, and hypertension in 10%, with progression to chronic kidney disease in 7% of the cohort.⁸

Data on the risk of kidney injury in this population cohort are still limited, and further studies are needed to confirm these findings.

The First Description of Horseshoe Kidney in the Literature

In 1522, Berengario da Carpi described this kidney malformation for the first time in his masterpiece Isagogae breves (Introduction to Anatomy). He was Master of Anatomy and Surgery at the University of Bologna from 1502 to 1527. Berengario da Carpi made several important advances in anatomy, including the first anatomical text to be supple-mented by illustrations in Anatomia Carpi: Isagoge breves, perlucide ac uberime, in Anatomiam humani corporis.

He was the first to describe the appendix, the first to consider the arytenoids as separate cartilages, the first to recognize the greater proportional size of the thorax in men and the pelvis in women, and the first to give a clear description of the thymus. In Isagogae breves, he reported the results of an autopsy in the public autopsy room of the University of Bologna. This text had great success and quickly resulted in numerous editions and translations, including abroad. In Isagogae, Berengario gives a concise but extremely detailed description of the anatomy of the human body and includes some anatomical illustrations. He was also the first anatomist to supplement his text with illustrations based on direct observations.⁹

He wrote, “Kidneys permeable like a kidney, with two emulsifying veins, two emulsifying arteries, two ureters,” and “I myself, during a public dissection in my study at Bologna, also saw an artery [kidney] emanating from the emulsions and forming a canal on the right side beyond the kidney, which ran at an extraordinary distance below the kidney along the ureter that emerged from the above-mentioned kidney, and both ran as one canal to the bladder.”

He continued, “This emulsifying [renal] artery, however, also entered the kidney at the usual place, and in this individual the kidneys were continuous, like a kidney, and it had two veins and two emulsifying arteries and two ureters, with only one enveloping panniculus, which occupied the usual place of the kidneys and even the middle of the back, which is in the position between the spleen and the liver, a little below them.”¹⁰

Berengario da Carpi was a pioneer in describing this common renal anomaly and laid the foundation for more detailed analyses and studies.

History of Horseshoe Kidney After Berengario’s First Report

In 1564, Leonardo Botallo¹¹ further described and illustrated the features of this atypical anatomical representation: “For there are four kidneys united in a single mass, but separated from each other by canals [infundibula], passages [ureters] and vessels [arteries and veins]. There were even a greater number of canals [infundibula] emanating from the drains [calyces] than are shown here, but because of their small size I wanted to show only the most visible ones.”

He continues, “It is not necessary to set forth at length the cause of an arrangement so abnormal and foreign to the natural order, for nature has the same power and impotence over the kidneys as over other parts of the body, and sometimes makes them double, sometimes mutilated, sometimes larger, sometimes smaller, and sometimes entirely different from the natural form.”

The next horseshoe kidney described in the literature was by Leonard Doldius at an autopsy in Nuremberg in 1602.¹² More descriptions of horseshoe kidneys were given by Barthelemy Cabrol in 1604,¹³ Thomas Bartholin in 1654 who reported on a horseshoe kidney with a valuable iconographic contribution,¹⁴ Edward Tyson in 1678,¹⁵ and Stalpert van der Wiel in 1682.¹⁶

Ercole Lelli created the model “Rene normale and Rene a ferro di cavallo” (normal kidney and horseshoe kidney) in 1734, in wax on panel (Museo di Palazzo Poggi, Bologna, Italy), which is a valuable iconographic contribution.

In 1761, Giovanni Battista Morgagni discussed this condition not only as a rare anatomical curiosity found only in necropsy but also discussed its physiological aspect.¹⁷ This was in line with Morgagni’s anatomical-clinical thinking (that is, if the signs and symptoms most frequently observed in many patients in vivo regularly coincided with common changes in the affected organs, then these changes could be considered the cause of the disease and the hypothesis of mere coincidence could be ruled out).

In 1894, G. H. Edington accurately described one case of horseshoe kidney in a 5-year-old child: “The kidneys show lobation on the anterior surface but not on the posterior [...], the two organs being joined at their lower ends by a ligament which to the naked eye looks like renal tissue and measures five-eighths of an inch vertically and one-eighth of an inch antero-posteriorly.”¹⁸

The diagnosis of horseshoe kidney by careful palpation of the abdomen was first described by Israel in 1908.¹⁹ In 1911, Rovsing²⁰ meticulously described a syndrome characterized by nausea, vomiting, and abdominal pain, which can be aggravated by overdistension and lead to the diagnosis of horseshoe kidney.

History of the Imaging of the Horseshoe Kidney

With the advent of pyelography, imaging reports allowed more accurate visualization of anatomical variations of the horseshoe kidney, which was particularly useful for preoperative assessment and outcomes.

Through the intravenous pyelogram, Gutierrez introduced the horseshoe-shaped renal pyelogram “triangle.”²¹ This triangle is created by drawing a line from the most medial part of each pyelum to a reference point on the midline at the level of the iliac crests. The angle between the 2 lines is much smaller in people with 1 horseshoe kidney than in people with 2 kidneys because the pyelons of a horseshoe kidney are close together. Nowadays, horseshoe kidneys can be identified with most abdominal imaging modalities. The diagnosis of horseshoe kidney is most commonly made with ultrasonog-raphy or computed tomography.²²

Computed tomography and magnetic resonance imaging are best for visualizing the anatomy and can identify accessory vessels and surrounding struc-tures.^23,24 A computed tomography urogram allows identification of stones and urinary tract and uretero-pelvic junction obstruction. Magnetic resonance imaging examinations can be used when ionizing radiation is to be avoided or in patients who cannot tolerate the standard intravenous contrast agent. It is also sometimes possible to detect horseshoe kidneys on radiography by visualizing the perinephric fat in association with an altered renal axis.

Nuclear radionuclide scans of the kidneys can be helpful in distinguishing between true obstructions and passively dilated systems and in diagnosing uretero-pelvic junction obstructions. Micturition cystourethrograms can be used to detect vesico-ureteral reflux, which is more common in horseshoe kidney patients than in the general population.

Horseshoe Kidney Surgery Treatment and Renal Transplantation

In the nineteenth century, with the advent of renal surgery, the horseshoe kidney assumed a more important role in urological diagnosis and treatment, and its identification became more frequent.

Martinow performed the first division of the renal isthmus in 1909 to separate the fused kidney.²⁵ In 1911, Rovsing proposed further modifications to this technique,²⁰ and, in 1922, Papin added more variants to the method.²⁶

In 1940, Foley advised nephropexy in addition to symphsiotomy.²⁷ In 1962, Felton and Miller described the resection of an aortic aneurysm associated with a horseshoe kidney, demonstrating the safety in the division of isthmus to gain access.²⁸

However, the vascular anatomy of a horseshoe kidney remains usually complex. Abnormalities of the ureteral and collecting systems are also common. Because of the frequent vascular and ureteral anomalies, transplantation of a horseshoe kidney has always been a technical challenge.

Politano was the first to transplant a horseshoe kidney from a living donor in 1963. He removed half a horseshoe kidney and transplanted it into the identical twin of the deceased. The kidney worked well, although the recipient died of hepatitis 8 months after the transplant. Unfortunately, Politano failed to publish his results. Nelson published the first case of a split horseshoe kidney transplant in 1975.²⁹

In these first cases, the kidney isthmus was split along the lines of Papin’s, and a split horseshoe kidney was transplanted. The first en bloc transplant was performed in 1981 by Menezes de Goes in Sao Paulo, Brazil.³⁰

In 1996, Lucan, in Cluj, Romania, was the first to perform a living donation procedure with a horseshoe kidney. The donor of the split horseshoe kidney was the recipient’s older brother. The horseshoe kidney was split on the spot, leaving one-half to the donor. The 3 renal arteries of the donated left part were anastomosed with a synthetic patch on the back table before transplant. This information was detailed in Stroosma and associates.³¹

Finally, Stroosma and colleagues demonstrated that the results after horseshoe kidney transplant are similar to results with normal anatomy kidneys.³¹

Conclusions

The horseshoe kidney has taken its place in medicine over the years. In the Middle Ages, it was considered a curiosity or a monstrous anomaly. As medical sciences gradually developed, this anatomical variation was not only recognized as a congenital malformation, but symptoms and other lesions were discovered that could be associated with the horseshoe kidney.

Berengario da Carpi laid the foundation for better knowledge of this anatomical anomaly. Five hundred years after the first report in the literature, important advances have been made in the treatment of complications associated with horseshoe kidney and in diagnosis, confirming the need to monitor individuals with this condition who are at higher risk of developing chronic kidney disease.

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