pelvic kidney

**Embryology/Pathophysiology** - The metanephros (from the ureteric bud and metanephric blastema) develops in the pelvis. Between weeks 6 and 9 the kidney "ascends" to the lumbar region and rotates 90° medially, so the renal pelvis faces medially. - In a pelvic kidney, ascent fails. The kidney stays in the pelvis, usually at the sacral promontory or below the aortic bifurcation. - Because it stops low, it keeps a blood supply from nearby vessels (common iliac, internal iliac, or distal aorta). Ascending kidneys normally pick up higher vessels and lose lower ones, so ectopic kidneys often have multiple or anomalous renal arteries. - Malrotation is common. The renal pelvis often faces anteriorly, and the ureter may be short. - The adrenal gland develops separately and is usually in its normal position.

**Epidemiology** - Simple renal ectopia occurs in about 1 in 900 autopsies. Pelvic kidney specifically occurs in about 1 in 2,000-3,000. - It is more common on the left, and slightly more common in males. - It is often associated with other anomalies: - Contralateral renal agenesis or other anomalies (in about 50% of cases) - Genital anomalies, such as a bicornuate uterus or vaginal agenesis (MRKH), and hypospadias or undescended testis - Cardiovascular and skeletal anomalies, as in VACTERL, Turner syndrome, and horseshoe kidney (fused crossed ectopia)

**Clinical presentation** - Most cases are asymptomatic and found incidentally on imaging or at surgery. - Complications include: - Hydronephrosis from ureteropelvic junction obstruction or vesicoureteral reflux (the most common problems) - Recurrent UTI - Nephrolithiasis - Pelvic or abdominal pain - It can present as a palpable pelvic or abdominal mass. It may be mistaken for an ovarian or other pelvic mass, appendicitis, or a tumor, and biopsy or surgery done in error can be harmful. - In pregnancy it may cause dystocia or obstructed labor, so it is a consideration in women of childbearing age. - Hypertension can occur from aberrant vasculature. - Pelvic kidneys are more prone to trauma because they are not protected by the rib cage.

**Diagnosis** - Ultrasound shows an empty renal fossa with absent kidney in the normal location. - CT or MR urography/angiography defines the location, vasculature, and associated anomalies. - Nuclear renal scan (DMSA or MAG3) assesses function and obstruction. - A VCUG evaluates for reflux.

**Management** - No treatment is needed if the kidney is asymptomatic and functioning. - Treat complications as they arise, such as infection, stones, and obstruction (pyeloplasty). - Preserve the kidney if it is the patient's only functioning kidney. - Avoid misdiagnosis, because the kidney should not be removed as a "mass."

**High-yield board points** - It is the classic cause of an "absent kidney in the renal fossa" with a pelvic mass on exam or imaging. - Ectopic kidneys have an abnormal blood supply and short ureters. Normal ascent is blocked in horseshoe kidney by the inferior mesenteric artery (IMA), which traps the fused isthmus. This is a different mechanism from pelvic kidney. - A pelvic kidney is not the same as renal ptosis (a kidney that is mobile and drops with posture), and it differs from crossed fused ectopia. - Contrast it with: - Renal agenesis: absent kidney with no ectopic organ (Potter sequence if bilateral) - Horseshoe kidney: fusion of the lower poles, associated with Turner syndrome, trisomy 18, and an increased risk of Wilms tumor - Duplex collecting system: associated with ureteral duplication, and the Weigert-Meyer rule applies - An associated Mullerian anomaly (e.g., unicornuate or bicornuate uterus) should prompt a search for renal anomalies, and the reverse is also true.