Horseshoe Kidneys

Content of This Page

1- Definition & Types

2- Causes (Aetiology)

3- Pathophysiology

4- Clinical Features & Examination

5- Investigations

6- Management

7- Complications

8- Core Concepts

Definition & Types

A horseshoe kidney is a congenital anomaly where the lower poles of both kidneys are fused, forming a U- or horseshoe-shaped organ. It is the most common renal fusion anomaly, with a prevalence of about 1 in 500 births.

Fusion Anatomy:

  • Usually involves the lower poles

  • The connecting bridge is called the isthmus, which may consist of:

    • Functional renal tissue (in ~80%)

    • Fibrous tissue (in others)

Embryology & Aetiology

  • Normally, kidneys develop in the pelvis and ascend to the upper abdomen during fetal life.

  • In horseshoe kidney, fusion occurs before ascent, around the 4th to 6th week of gestation.

  • The inferior mesenteric artery (IMA) acts as a barrier to full ascent, causing the fused kidney to remain low in the abdomen (usually L3-L5 level).

Pathophysiology

Due to their abnormal position and orientation, horseshoe kidneys have:

  • Anteriorly facing renal pelvis (instead of medial)

  • Abnormal ureteral course—prone to:

    • Ureteropelvic junction (PUJ) obstruction

    • Kinking or compression by isthmus or vessels

  • Stasis of urine → predisposition to:

    • Infection

    • Stone formation

    • Obstruction

The abnormal anatomy also affects blood supply:

  • Multiple aberrant renal arteries

  • May arise from aorta, common iliac, or inferior mesenteric artery

Clinical Relevance & Symptoms

Most people are asymptomatic, but some may present with:

  • Abdominal pain or flank discomfort

  • Palpable midline mass (rare)

  • Urinary tract infections (UTIs) due to impaired drainage

  • Haematuria (microscopic or gross)

  • Urolithiasis (kidney stones)

  • Hydronephrosis if there’s PUJ obstruction

Associated Conditions

  • Ureteropelvic junction (PUJ) obstruction

  • Vesico-ureteric reflux (VUR)

  • Renal ectopia or malrotation

  • Wilms tumour (increased risk in children)

  • Turner syndrome

  • Trisomy 18 (Edwards syndrome)

These associated anomalies increase the importance of long-term monitoring.

Investigations

Imaging:

  • Renal Ultrasound: first-line; shows low-lying kidneys joined at the lower poles.

  • CT / MRI Urogram: confirms anatomical details, rotation, and vascular supply.

  • DMSA scan: assesses renal function and scarring.

  • MAG3 renogram: evaluates drainage and obstruction.

  • MCUG: used if vesico-ureteric reflux is suspected.

Management

If Asymptomatic:

  • No intervention needed

  • Education and regular follow-up (BP, renal function, urinalysis)

If Symptomatic:

  • Infections: treat UTIs promptly

  • Stones: standard management (hydration, lithotripsy if indicated)

  • Obstruction: surgical correction (e.g. pyeloplasty)

  • Surgery is technically challenging due to:

    • Aberrant blood supply

    • Anteriorly placed renal pelvis

    • Difficulty in mobilising fused tissue

Complications

  • Hydronephrosis due to PUJ obstruction

  • Recurrent UTIs

  • Stone formation

  • Reflux nephropathy

  • Wilms tumour (rare, paediatric patients)

  • Trauma risk due to low position in abdomen

Core Concepts

  • Horseshoe kidney is usually benign and asymptomatic.

  • Found incidentally on imaging.

  • Always screen for reflux, obstruction, and renal function.

  • Requires lifelong surveillance of renal health if anomalies are present.

  • Surgical planning should consider vascular anomalies.

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