Horseshoe kidney overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:
Overview
Horseshoe kidney is the congenital disorder of kidneys in which the isthmus of both kidneys fuse together during embryonic development leading to appearance of "Horseshoe". It is the most common fusion defect of kidneys.
Historical Perspective
Classification
Based to morphology, horseshoe kidney is classified into 3 main types.
- 'U' Shaped, when the lower poles of kidneys fuse together.
- Inverted 'U' shaped when upper poles of kidneys fuse together.
- 'L' Shaped when one vertically placed kidney fuse with another horizontally placed kidney leading to 'L' shape.
Pathophysiology
Most people with horseshoe kidneys are totally asymptomatic and their kidneys function normally throughout their lives. About quarter of cases are found incidentally, usually during a CT scan due to some other problem. Nonetheless, there are lots of other patients who are predisposed to certain medical issues. These issues include:
- Ureteropelvic junction obstruction : It is the most common problem in horseshoe kidney. It is caused by crossing of ureter over the fused isthmus leading to its obstruction.
- Kidney Stones : Stones usually occur when there is obstruction leading to decreased urine output and stasis of urine which create environment for stone formation.
- Recurrent infections : Again due to stasis of urine, because stasis of urine create medium for bacterial growth
- Hydronephrosis : Basically it is caused by back-flow of urine due to obstruction of ureteropelvic junction.
- Increased incidence of certain cancers such as renal cell carcinoma, wilms tumor and carcinoid tumors.
Causes
During early fetal development, the kidneys are developed in the pelvis and then later they move upwards in abdomen. Sometimes due to unknown reason the lower ends of both kidneys fuse together forming a 'U' shaped single kidney. Although the exact cause is unknown, but there are two main theories. One theory suggests that during the fifth week of fetal development both kidneys are so close together leading to mechanical fusion of lower end of both kidneys. This kind of fusion creates the fibrous isthmus because it is made of connective tissue. Other theory suggests that posterior nephrogenic cells, which are the cells responsible for formation of kidney, migrate to wrong place leading to joining of lower end of both kidneys. This time it is called parenchymal isthmus because it is made of kidney cells.
The horseshoe kidney remains lower in abdomen than normal kidneys due to blockade caused by inferior mesenteric artery during its movement from pelvis to abdomen in 7-8 weeks of development.
Differentiating Horseshoe kidney from Other Diseases
There are other forms of kidney fusion defects. Usually they can be differentiated based on appearances on CT scan.
Epidemiology and Demographics
- It is fairly common condition with 1 case in every 500 live births.
- Males cases are twice more common than female cases.
- Although it has been seen in members of same families, but no any particular gene has been found to cause this disease.
Risk Factors and Associations
There are no particular gene discovered yet which are directly related to horseshoe kidney but it has been seen associated with other genetic disorders such as Turner syndrome and Edwards syndrome, further suggesting that has links to some genes.
Screening
There is no particular screening test for horseshoe kidney itself, although CT scanning or renal ultrasonography can be used to screen for the presence of stones, masses, or hydronephrosis.
Natural History, Complications, and Prognosis
- Hydronephrosis
- Renal stones
- Urinary tract infections
- Renal Cancer