# Ectopic kidney

An ectopic kidney is a kidney that is not located in its usual position (ectopia) in the lumbar retroperitoneal space. It is the result of anomalous migration of the kidneys from their origin in the fetal pelvis during embryogenesis.
The diagnosis is usually made during antenatal and/or postnatal testing, when the ectopic kidney is found incidentally. Although most patients with renal ectopia are asymptomatic, some can develop symptoms due to complications such as kidney stones, urinary tract infections and hydronephrosis.

## Embryology

The kidney arises from the intermediate mesoderm and has three embryological forms: pronephros, mesonephros and metanephros. The pronephros and mesonephros are the primitive transient stages, while the metanephros is the final and functional stage that later forms the permanent kidneys.
The metanephros is formed at around fifth to sixth week of gestation and is composed of the metanephric mesenchyme (the proper kidney) and the ureteric bud (collecting system and ureter). This metanephric unit then ascends from the pelvis into the retroperitoneal renal fossa while simultaneously rotating 90° so that the developing kidney is positioned vertically with the hilum pointing medially. This whole process of renal ascent and rotation is generally completed by the eighth to ninth week of gestation.
When the mature kidney fails to ascend to its normal location in the retroperitoneal space between T12 to L3 vertebral levels, it is called an ectopic kidney. Numerous factors have been hypothesized to affect the proper ascent and rotation of the kidneys, including genetic factors, developmental defects in the metanephric tissue and the ureteric bud, teratogens and maternal illnesses and exposures during pregnancy.

## Classification

An ectopic kidney can be classified based on its location in the corresponding cavity, whether it is simple or crossed and whether there is associated fusion with the ipsilateral kidney.
The types of ectopic kidney include:
* Simple: kidney that remains on the same side of the body but in an abnormal location.
* Crossed: kidney that crosses the midline so that it's on the opposite side of the body. It can be with or without fusion to the ipsilateral kidney.
* Cephalad: kidney that ascends until its reaches the level of T10 vertebra, just below the diaphragm. This type is usually associated with omphaloceles and other abdominal wall defects.
* Thoracic: kidney that protrudes through the diaphragm into the posterior mediastinum. An intrathoracic kidney could be a "real" ectopic kidney without any diaphragm abnormalities or can be related to diaphragmatic hernias or traumatic diaphragm ruptures.
* Pelvic: kidney that fails to ascend and remains in the pelvic brim, below the level of aortic bifurcation.
* Abdominal: kidney that is anywhere between its origin in the pelvis and its normal position in the renal fossa.

## Incidence

Data on the incidence of ectopic kidneys is inconsistent since renal ectopia may be diagnosed antenatally, but it also may be asymptomatic and found only incidentally or postmortem. Average incidence is about 1 in 900 in autopsy studies and 1 in 3000 in imaging studies. The incidence also varies depending on the type of ectopy. For example, pelvic kidneys are seen in 1 per 2000 to 1 per 3000 cases and are generally more common than cephalad and intrathoracic kidneys which only occur in 1 per 22,000 cases.