Description
Horseshoe kidney is a congenital anomaly characterised by the fusion of the poles of each kidney across the midline, almost always at the lower poles (90% of cases). This results in the kidneys appearing as a ‘horseshoe’ shape, hence the name. The condition encompasses three primary anatomical anomalies: ectopia (displacement from the normal location), malrotation (abnormal rotation), and aberrant vascular supply.
Pathogenesis
Horseshoe kidney arises from abnormal migration of the metanephric kidneys during embryonic development. Instead of ascending to their usual anatomical position, the lower poles of the kidneys fuse, resulting in a single, horseshoe-shaped structure typically located lower in the abdomen than normal kidneys.
Epidemiology, Risk Factors & Associations
- The prevalence of horseshoe kidney is approximately 1 in 500 (0.2%) to 1 in 1,000 (0.1%) individuals.
- The condition affects males twice as often as females.
- Horseshoe kidney is associated with several syndromic conditions, including:
- Turner Syndrome (45,X): Found in up to 15-30% of cases. A chromosomal disorder which affects females and can cause a variety of health problems, including short stature, infertility, and heart defects.
- Trisomy 18 (Edwards Syndrome): Found in 10-15% of cases. A chromosomal condition characterised by a variety of severe medical problems including heart defects, growth deficiency, and intellectual disability.
- VACTERL Association (Vertebral defects, Anal atresia, Cardiac defects, Tracheoesophageal fistula, Renal anomalies, Limb abnormalities): Renal anomalies are found in 50-80% of individuals with VACTERL Association, and of those, a subset will have horseshoe kidney.
- Trisomy 13 (Patau Syndrome): Exact prevalence is unknown.
Clinical Features
- May be asymptomatic, with the condition detected incidentally.
- Potential complications include hydronephrosis, renal stones, infection, and an increased risk of renal malignancies.
- The anomalous kidney can sometimes be palpated as a suprapubic mass.
- Symptoms related to associated syndromic conditions may also be present.
- Potential complications include susceptibility to traumatic injury due to the kidney’s lower and anterior position, ureteropelvic junction obstruction, vesicoureteral reflux, urolithiasis, infections
- Increased risk of various malignancies, such as renal carcinoid (neuroendocrine tumor), Wilms tumour in children, and a slightly increased risk of urothelial cancer.
Pathological Features
Histopathology
- Kidney tissue in horseshoe kidneys is usually normal in appearance, though the presence of associated conditions or complications may alter this.
Radiological Features
General Features
- Horseshoe kidneys appear as two renal masses located lower in the abdomen than usual, connected by an isthmus of renal tissue across the midline.
- Kidneys may be fused symmetrically in the midline (90% of cases) or asymmetrically (10% of cases), with left-to-right asymmetry more common.
- The isthmus is usually positioned anterior to the aorta and limited superiorly by the inferior mesenteric artery (IMA)
- Renal pelvis and calyces may appear unusual due to malrotation, often facing anteriorly rather than medially.
- Anomalous renal arteries may originate from the superior mesenteric artery (SMA), IMA, iliac arteries, or median sacral artery.
- Complications such as hydronephrosis or renal stones may also be visible.
Differential Diagnosis
- Renal ectopia: The kidney may be located in an abnormal position but there is no fusion of the renal poles.
- Crossed fused renal ectopia: One kidney crosses the midline to fuse with the contralateral kidney, but the fused kidneys are located on one side of the body rather than spanning the midline.
Management
- Most horseshoe kidneys do not require specific treatment.
- Complications such as renal stones or infections should be managed as they would be in patients with normal kidney anatomy.
- Regular surveillance for malignancy may be recommended due to the slightly increased risk.
- Surgical intervention is reserved for complications that cannot be managed medically.