What Is Horseshoe Kidney?

As the name suggests, a horseshoe kidney is an abnormality where the two kidneys get fused together to form a horseshoe. However, it's not just the shape and structure of the kidneys that is abnormal. Their location is abnormal as well.

Rather than being present in the upper abdomen, underneath the rib cage and next to your spine, a horseshoe kidney is typically present much lower down in the pelvis. This is not the only genetic abnormality of the kidneys' location or structure. Another common example is something called the "ectopic kidney." Before we can understand why a horseshoe kidney forms and its implications, it is necessary to understand the normal formation of the human kidney during our development in the uterus.

Normal Human Kidney Development

When we are in an embryonic stage and developing into a full-fledged human being, our kidneys go through three stages of development before a fully functional and mature kidney is formed:

1. Pronephros
2. Mesonephros
3. Metanephros 

Imagine a primordial soup of cells and primitive structures which will conglomerate together into a fully functional developed kidney. The metanephros stage is achieved by about 6 weeks of gestation. It is composed of the so-called "metanephric mesenchyme" and a "ureteral bud." These structures will eventually form the kidney and the ureter.

Why do we need to understand this formative process? Well, once we appreciate that the human kidney undergoes certain structural and positional changes until it culminates in its final form, it becomes easier to understand an abnormality like the horseshoe kidney. It is therefore interesting to note that the above-described metanephros stage (which precedes a developed kidney) is actually positioned in our pelvis, and not where a mature kidney lies (upper abdomen)!

As we mature from an embryo into a baby, the growth of our bodies results in a change in the relative position of this developing kidney such that it moves from the pelvis and gradually rises to its final position (underneath the rib cage and next to the spinal column). Not only do the kidneys ascend, so to speak, they actually rotate internally so that the so-called "renal pelvis" now faces the spinal column. This process is called rotation, while the ascent of a kidney to its final location is called migration. This process is completed by the time an embryo is 8 weeks old.

Now that we have an overview of the formation of the human kidneys, we can begin to realize that any disruptions with the rotation or migration processes will mean that not only could our kidneys be located in a wrong location, they also could perhaps end up being fused into one mass, rather than distinct right and left kidneys.

Horseshoe Kidney Formation

A horseshoe kidney is what we call a "fusion anomaly." As the word suggests, a fusion anomaly will occur when one kidney gets attached to the other. This will occur because of any disruption with the normal migration process of both kidneys. A bit rarer is a phenomenon where abnormal migration affects only one kidney rather than the other, which leads to both kidneys being present on one side of the spinal column. This is referred to as a "crossed fused ectopic kidney."

In the usual horseshoe kidney, the lower pole of the kidneys will fuse together and therefore give rise to a typical horseshoe shape. The tubes that drain urine from our kidneys (called the ureters) are still present and drain each side separately. The fused portion of the kidney is referred to as the "isthmus.

This isthmus may or may not lie symmetrically over the spine. If it lies more to one side than the other, we call it an "asymmetric horseshoe kidney." Functional kidney tissue may or may not constitute the isthmus, and therefore it is not unusual to just see the two kidneys attached by a nonfunctioning fibrous tissue band.

Prevalence

On average, studies have reported the presence of horseshoe kidney in anywhere from 0.4 to 1.6 patients, for every 10,000 live births. However, this is the reported incidence only. The actual incidence could be higher since the presence of a horseshoe kidney is often unknown to the affected patient.  

Symptoms

The majority of people born with horseshoe kidney won't have symptoms. In fact, horseshoe kidneys are often incidentally picked up on imaging studies that are done for other reasons. However, when symptoms are present, they're usually related to abnormalities in the urine flow created due to the abnormal location and orientation of the kidneys. Some of the symptoms are:

• Burning during urination, increased urinary frequency, urinary urgency—all created due to an increased tendency to develop urinary tract infections. This tendency is seen because of sub-optimal urinary drainage. This leads to pockets of static urine, which is an excellent medium for bacteria to grow and thrive.
• Flank or pelvic pain due to obstruction to the flow of urine
• An increased risk of kidney stones. These, in turn, will cause flank or pelvic pain as described above, but could also cause the appearance of blood in the urine. Stones by themselves can lead to urinary tract infections as well.

• Presence of the urine reflux from the bladder to the ureters, which can lead to an increased risk of urinary tract infections as well as scarring in the kidneys. This is referred to as VUR (vesicoureteral reflux).
• Hydronephrosis- this refers to an enlarged, obstructed urinary drainage system in the kidney. This obstruction can be created by kidney or ureteral stones, as well as by compression of the ureters by external structures.
• Other genital abnormalities- since horseshoe kidney could be part of a wider genetic abnormality spectrum, other malformations of the urogenital tract can also be noted. These include undescended testes in boys or abnormal uterus structure in girls.

Complications

Most complications stem from the above-mentioned symptoms and signs of a horseshoe kidney, often associated with obstruction in the urinary tract.

Interestingly, patients with horseshoe kidney seem to have an increased risk of a particular kind of kidney tumor called the "Wilms tumor." The reasons for this risk are not fully understood. This was first established by the well-known National Wilms Tumor Study which ran for almost 30 years and identified 41 patients with Wilms tumor who also happened to have a horseshoe kidney.

Perhaps a more pressing concern on a day-to-day basis is the fact that a horseshoe kidney is more susceptible to injury from blunt abdominal trauma. For instance, in a seat belt injury sustained during a motor vehicle accident, a seat belt could squish the abdominal contents, including the horseshoe kidney against the spine. Normal human kidneys which sit higher up and are not connected together are typically not at as much risk. 

Diagnosis

As mentioned above, horseshoe kidney will typically be detected on incidental abdominal imaging. Further investigations are usually needed if the above-described symptoms, signs, or complications have been noted. For instance, if you have been afflicted with repeated urinary tract infections in the setting of a horseshoe kidney, your kidney doctor will typically recommend something called a voiding cystourethrogram (VCUG) to determine whether any urine reflux is present. Other tests that might be ordered include:

• Tests of the kidney function: these typically include blood tests like BUN and creatinine levels and GFR estimation. Urine tests for protein or blood are also helpful.
• Renal flow scan to confirm obstruction 
• CT Urogram

Treatment

If no major complications or concerning symptoms are present, and the kidney function is normal, no further treatment is required. The patient, however, should still be warned about the susceptibility of their kidney to blunt abdominal trauma. If there are complications noted due to obstruction to the flow of urine, the patient should be evaluated by a specialist (nephrologist and urologist) to determine further course of action and to see if surgical correction could relieve the obstruction. In most patients, the long-term prognosis is good.

A Word from Verywell

Remember that a horseshoe kidney is a relatively rare abnormality of the kidneys' position and structure. While most patients would not have symptoms and their horseshoe kidney will be discovered incidentally on imaging, be mindful that symptoms can be reported in a minority of patients and are usually related to obstruction to the flow of urine, kidney stones, or urinary tract infections.

If symptoms are present, treatment, including surgical treatment to relieve the obstruction, might become necessary, but most patients can be safely monitored and no further evaluation or treatment is necessary. Although you do need to be mindful of the increased risk of physical injury to a horseshoe kidney (especially from blunt abdominal trauma), remember that the long-term prognosis is favorable!