A horseshoe kidney is the most common congenital renal fusion anomaly, occurring in approximately 1 in 400 to 500 births. In this condition, the lower poles of the two kidneys are connected by a band of tissue called an isthmus, which crosses the midline in front of the aorta and usually below the origin of the inferior mesenteric artery. On ultrasound, a horseshoe kidney may be suspected when the kidneys appear lower in the abdomen than normal, with their lower poles oriented toward the midline. The connecting isthmus may be visible as a bridge of tissue anterior to the aorta. Horseshoe kidneys are often asymptomatic but are associated with an increased risk of kidney stones, infections, and obstruction due to altered urine drainage.
Sonography Term