What is a horseshoe kidney

Horseshoe kidneys are the most common fusion defect of the kidneys, but this still amounts to only about 0.25% of the population ¹⁾. Horseshoe kidney occurs in about 1 in 500 children. It occurs during fetal development as the kidneys move into their normal position. With horseshoe kidney, as the kidneys of the fetus rise from the pelvic area, they become attached (“fuse”) together at the lower end or base. By fusing, they form into a U shape, like a horseshoe. This is thought to happen more often in males than in females (M:F 2:1) ²⁾. Horseshoe kidneys were initially described during autopsies by da Carpi performed in 1522, they are characterized by abnormalities in the position, rotation, and vascular supply of the kidney ³⁾. Horseshoe kidneys are identified as having functioning renal masses present on both sides of the vertebral column fused together with ureters that remain uncrossed from the renal hilum to the urinary bladder ⁴⁾. The isthmus connecting the two renal masses may be positioned in the midline or laterally resulting in asymmetric horseshoe kidney, 70% of which are left dominant, and consists of renal parenchyma in about 80% of cases with the remainder being composed of a fibrous band. In more than 90% of cases, fusion occurs at the lower pole, although fusion may occur at the upper pole in a minority of cases ⁵⁾.

Most people are born with 2 kidneys. But sometimes the kidneys form fused together. A horseshoe kidney consists of two normal functioning kidneys attached by a band of tissue called the isthmus. The kidneys are normally located in the retroperitoneum between the transverse processes of T12 and L3 with the left kidney slightly more superior than the right ⁶⁾. The upper poles are normally positioned slightly medially and posteriorly relative to the lower poles. Horseshoe kidneys are different in three main ways: location, orientation, and vasculature ⁷⁾. The horseshoe kidney’s ascent is often quoted to be held back by the inferior mesenteric artery at L3 however, the horseshoe kidney can also be found lower in the abdomen and pelvis. During weeks six to eight of development, the renal ascent is coupled with a 90-degree medial rotation. Due to the isthmus, however, horseshoe kidneys experience malrotation, and consequently, the ureters need to either pass over the isthmus or down the anterior surface of the kidneys which can cause urinary drainage problems and stasis ⁸⁾. Horseshoe kidneys also show a greater variation in the origin and number of renal arteries and veins ⁹⁾. These are largely dependent on where during development ascent has terminated. In one study of 90 horseshoe kidneys, 387 arteries were identified ¹⁰⁾. Despite this, the normal intra-renal vascular segmental pattern remains, and the ligation or division of any of these arteries results in ischemic segmental renal necrosis due to their poor collateral arterial supply ¹¹⁾. The incidence of renal vein anomalies in horseshoe kidneys is also high (23%) ¹²⁾.

Horseshoe kidney render the kidneys susceptible to trauma and are an independent risk factor for the development of renal calculi and transitional cell carcinoma of the renal pelvis.

Figure 1. Horseshoe kidney

Figure 2. Horseshoe kidney ultrasound

What happens under normal conditions?

The urinary tract is the body’s drainage system. It includes two kidneys, two ureters, a bladder, and a urethra.

Healthy kidneys work day and night to clean our blood. These 2 bean-shaped organs are found near the middle of the back, just below the ribs. One kidney sits on each side of the spine.

Our kidneys are our body’s main filter. They clean about 150 quarts of blood daily. Every day, they form about 1-2 quarts of urine by pulling extra water and waste from the blood. Urine normally travels from the kidneys down to the bladder and out through the urethra.

As a filter, the kidney controls many things to keep us healthy:

• Fluid balance
• Electrolyte levels (e.g., sodium, potassium, calcium, magnesium, acid)
• Waste removal in the form of urine
• The regulation of blood pressure and red blood cell counts

As a child develops in the mother’s uterus, the kidneys form first in the child’s lower belly. They slowly move up to their final position on both sides of the spine as they develop.

Can a horseshoe kidney be separated?

Yes. Symphysiotomy or division of the fused isthmus, was previously recommended when doing a pyeloplasty in patients with a horseshoe kidney, but this has changed due to the increased risk of infection, fistulas, leakages, and bleeding ¹³⁾. It has also been noted that the kidneys return to their original location after such surgery, so symphysiotomy is no longer recommended.

Horseshoe kidney causes

Despite cases of familial clustering, no clear genetic cause has been described for horseshoe kidneys, although several etiological factors may contribute to their development ¹⁴⁾. These include abnormal migration of nephrogenic cells across the primitive streak, alterations in the intrauterine environment with teratogenic drugs such as thalidomide, alcohol consumption and glycemic control causing an increase in incidence and structural factors such as flexion/rotation of the caudal spine and narrowed arterial forks during migration ¹⁵⁾. Traditionally textbooks quote fusion as occurring between weeks four and six of development, although there is some evidence for later fusion, particularly when the isthmus is fibrous rather than renal parenchyma.

The incidence for horseshoe kidney is higher in those who present to urology clinics (1 in 304), and with some chromosomal disorders. These include Edward syndrome at approximately 67%, Turner syndrome at 14% to 20%, and Down syndrome at about 1% ¹⁶⁾.

Horseshoe kidney symptoms

Even though a horseshoe kidney is congenital (present at birth), one-third of children will have no symptoms and the condition often goes undetected. Up to 7 out of 10 children and adults with horseshoe kidney will have symptoms. In patients who do have symptoms, horseshoe kidney is often diagnosed as the result of an urinary tract infection (UTI), an obstruction or a kidney stone. These can include ¹⁷⁾:

• Pain in the belly
• Nausea
• Kidney stones
• Urinary Tract Infections

Horseshoe kidneys have symptoms much more often than do other types of abnormal kidneys.

Kidney cancer is rare in children, but cancer tumors are somewhat more likely to occur in horseshoe kidneys than in normal kidneys. Some symptoms of a kidney tumor are:

• Blood in the urine (hematuria)
• Mass in the belly
• Flank pain

Horseshoe kidney complications

An isolated finding of a horseshoe kidney is generally considered benign ¹⁸⁾. About a third of all patients with horseshoe kidneys remain completely asymptomatic and are often found incidentally during imaging. The intrinsic anatomical defects present within horseshoe kidneys do however predispose individuals to a number of urological sequelae due to the associated ureteric obstruction and impaired urinary drainage ¹⁹⁾. Ureteropelvic junction obstruction (UPJ) is the most common abnormality associated with horseshoe kidneys, individuals are also predisposed to hydronephrosis, infection, vesicoureteral reflux ²⁰⁾. One study showed that over half of the individuals who are symptomatic had either ureteropelvic junction obstruction or vesicoureteral reflux ²¹⁾. A recent meta-analysis suggested that 36% of patients with horseshoe kidney will develop kidney stones (nephrolithiasis) at some stage ²²⁾. Due to their ectopic position, horseshoe kidneys are also particularly susceptible to blunt abdominal trauma and can be compressed or fractured against the lumbar vertebrae ²³⁾.

Horseshoe kidneys also have an increase in frequency for some common renal cancers including transitional cell tumors (three to four times more common), Wilms tumor (twice as frequently), and an extremely large increase in very rare tumors such as carcinoid (62 to 82 times) ²⁴⁾.

Horseshoe kidney diagnosis

Often, health care providers find horseshoe kidneys while treating other conditions. A health care provider may also find them when looking for the cause of symptoms mentioned earlier. These imaging tests could help your health care provider find a horseshoe kidney:

• Ultrasound
• Intravenous pyelogram (IVP)
• Voiding Cystourethrogram (VCUG)
• Radionuclide Scan
• Magnetic Resonance Imaging (MRI)

Horseshoe kidneys can be identified using most abdominal imaging modalities. The diagnosis of a horseshoe kidney is most commonly made using either ultrasound or intravenous urography ²⁵⁾. CT and MRI are the best for demonstrating the anatomy and can detect accessory vasculature and surrounding structures ²⁶⁾. It is also possible to identify horseshoe kidneys on plain radiography through visualization of the perinephric fat in association with an altered renal axis. The lower poles are positioned more medial than normal and because the kidneys sit lower in the abdomen than expected ²⁷⁾. Nuclear medicine radionuclide renal scans can be helpful in differentiating true obstruction from passively dilated systems.

Your health care provider may also order blood tests to see how well the kidney(s) are working.

Horseshoe kidney treatment

Horseshoe kidney treatment may not be needed if there are no symptoms. There isn’t a cure for horseshoe kidney, but the symptoms can be treated if they cause problems (“supportive treatment”).

Blockage of urine flow (“obstruction”) and urine flowing backwards from the bladder (“vesicoureteral reflux”) are very common in patients with horseshoe kidney. These can both be fixed with surgery.

Pre-procedural imaging such as CT is essential during the workup for any surgery required. This is due not only to the highly variable nature of the blood supply but also the association of horseshoe kidneys with having a segment of colon posteriorly and the corresponding increases in risk of incidental bowel injury ²⁸⁾.

A horseshoe kidney is most often set lower and much closer to the front of the body than a normal kidney. It’s also more likely to be hurt when there’s trauma to the abdomen than is a normal kidney. Wearing a medical alert bracelet will let emergency care providers know to be aware of the chance of kidney damage. Children with a horseshoe kidney should avoid contact sports.

Horseshoe kidneys can become blocked just as any normal kidney can. Surgery to remove blockages or kidney stones in the ureter is usually successful.

Shockwave lithotripsy for nephrolithiasis is less effective in horseshoe kidneys due to problems localizing the energy for pelvic stones and poor stone fragment clearance due to impaired renal drainage ²⁹⁾. Larger renal stones, those greater than 2.5 cm, or those not allowing ureteroscopic approaches, can be removed via minimally invasive percutaneous surgery ³⁰⁾.

If the only complaint from the horseshoe kidney is pain, surgery will often not ease the pain.