Horseshoe Kidneys - Causes, Symptoms, Diagnosis, and Treatment

Introduction

Horseshoe kidney (HSK) is also known as renal fusion. It was first described in 1522 by Da Carpi while performing autopsies. It occurs around the 7th week of intrauterine life. It is characterized by the abnormality in the kidneys' orientation, location, and vasculature. The exact etiology of the condition is unclear. It shows a moderate prognosis due to the development of complications such as kidney stones, kidney cancer, etc.,

What Are Horseshoe Kidneys?

Horseshoe kidneys are one of the common anatomical variations of the kidney, which is characterized by the fusion of two kidneys by a fibrous isthmus (a fibrous tissue connecting two renal masses) at their lower pole.

How Are the Horseshoe Kidneys Formed?

The embryogenesis of the kidney begins during the fourth week of intrauterine life. Around the seventh week, the formed kidneys ascend from the pelvis to their normal position in the lumbar region. Instead, the kidneys fuse at the base forming a U-shaped or horseshoe-shaped kidney.

What Are the Theories Proposed for the Development of Horseshoe Kidneys?

Two theories have been proposed for the development of HSK,

• Classic Theory - It states that the fusion occurs at the metanephric stage, where the kidneys are still in the pelvis, and the lower poles are nearby. They join, fusing in the midline with the fibrous isthmus.

• Another Theory - This theory states that the fused horseshoe kidneys ascend at the level of lower lumbar vertebrae, and the inferior mesenteric artery prevents the further ascent of the kidney to its normal position. Hence, the kidneys get trapped in the mid-abdomen and get malrotated.

What Are the Types of Horseshoe Kidneys?

Based on the physical appearance of fusion, it can be of two types, such as:

• U-Shaped Horseshoe Kidneys - It is formed by the fusion of symmetrical kidneys in the medial position.

• L-Shaped Horseshoe Kidneys - Formed due to the lateral fusion between the horizontal and vertical kidneys.

What Is the Epidemiology of Horseshoe Kidneys?

Studies report that the incidence of horse kidneys in the general population is one in 500. The incidence is higher in patients with chromosomal defects.No evidence of racial discrimination.

What Are the Syndromes Associated With Horseshoe Kidneys?

The syndromes that are associated with this condition include:

• Down Syndrome - A chromosomal abnormality in which the affected persons have extra chromosomes.

• Edward Syndrome - Also known as Trisomy 18, is characterized by abnormalities in many body parts.

• Turner Syndrome - This condition affects females when one of the X chromosomes is missing.

What Is the Etiology of Horseshoe Kidneys?

The proposed etiologies of horseshoe kidney include,

• It may be due to the abnormal migration of nephrogenic cells across the primitive streak.

• Teratogenic drugs such as Thalidomide and alcohol consumption may cause alteration in the fetal environment leading to fusion of the kidneys.

• It may be due to structural alterations such as rotation of the caudal spine and narrowing of the arterial forks during migration.

• No clear evidence about the genetic link in horseshoe kidneys.

What Is the Pathophysiology of Horseshoe Kidneys?

The horseshoe kidneys differ from the normal kidney in three ways;

• Location:

Usually, the kidneys are located in the retroperitoneum ( the area behind the peritoneum) at the level of the lower lumbar vertebrae. However, in the case of HSK, it can be located anywhere along the path of the renal ascent from the pelvis to the mid-abdomen.

• Orientation:

The renal ascent of the normal kidney is coupled with a medial rotation for its proper orientation and function. However, HSK exhibits malrotation due to an isthmus joining the kidneys. As a result of this alteration, the ureters need to pass down the anterior surface of the kidneys or pass the isthmus leading to further complications.

• Vasculature:

Studies report that the HSK shows a significant variation in the origin and the number of vasculatures supplying it. In addition, 23% of patients with horseshoe kidneys demonstrate anomalies in the renal veins.

What Are the Symptoms of Horseshoe Kidneys?

In some cases, it does not produce any symptoms. But in most cases, it causes a problem due to the change in location, orientation, and vasculature compared to normal kidneys. It includes,

Urinary Tract Infections (UTI) - As the ureters are blocked due to the morphological variations in the horseshoe kidney, the free flow of urine will be restricted, leading to the proliferation of bacteria in the urinary tract causing infections. The symptoms of UTI include:

• Fever.

• Chills.

• Pain or burning while urinating.

• Urge to pee.

Kidney Stones - As a result of improper urine drainage, the calcium and oxalate salts in the urine accumulate, forming renal stones. The symptoms include:

• Pain in the back of the abdomen.

• Chills.

• Fever.

• Nausea.

• Vomiting.

• Hematuria (blood in the urine).

Hydronephrosis - It is caused due to the prolonged accumulation or build-up of urine in the kidneys, causing the kidneys to swell and, in turn, hindering kidney function. The symptoms include:

• Less urine output.

• Frequent urinary tract infections.

• Presence of a mass or lump in the abdomen.

• Other complications.

Other less common complications include;

• Wilms Tumor - It is a type of cancer affecting kidneys in children.

• Polycystic Kidney Disease - It is characterized by the formation of multiple cysts in the kidneys.

• Hydrocephalus - It is the fluid collection in the ventricles within the brain.

How Are Horseshoe Kidneys Diagnosed?

Most of the time, it is an incidental finding during routine scans or scans for other purposes. The investigation methods used in diagnosis are;

1. Ultrasound - Ultrasound examination would reveal the presence of horseshoe kidneys and detect stones and blockages.

2. Computed Tomography (CT) /Magnetic Resonance Imaging (MRI) - It helps in diagnosing the condition and also provides a piece of detailed information about the surrounding vasculature. It includes information about renal stones, cancers, etc.

3. Radionuclide Scan - A radioactive dye is injected into the kidneys, and scans are taken to look for the defects.

4. Voiding Cystourethrogram - To detect urethral abnormalities.

5. Blood Test - To determine the state of kidney function.

6. Urine Test - To check for urinary infections.

How Is Horseshoe Kidney Managed?

No specific treatment has been described for this condition; management aims to treat the associated complications.

• Removal of Kidney Stones - The renal calculi in HSK patients should be removed to prevent further complications. In addition, surgical procedures such as shock wave lithotripsy and percutaneous surgery remove renal stones.

• Treating Urinary Tract Infections - UTIs can be managed with appropriate antibiotics.

• Treating Kidney Cancers - Cancer can be removed by either a surgical or laparoscopic approach.

What Is the Prognosis of Horseshoe Kidneys?

Although it is a benign condition, the prognosis is reported to be moderate as these patients are more prone to renal stones and cancers of the kidney. In addition, morbidity is reported to be high due to complications of MSK.

Conclusion

A horseshoe kidney is an anatomic malformation that fuses the kidneys to form a U-shaped or horseshoe-shaped kidney. In most cases, it's diagnosed incidentally. However, when it's symptomatic, it's due to symptoms associated with the complications of HSK. The goal is to treat the complications and preserve kidney function.