In 1938, Wilmer first described and classified renal fusion anomalies, which was refined and expanded by McDonald and McClellan in 1957.According to them renal ectopia is of two types: Crossed type and simple/uncrossed type. Crossed Renal Ectopia may be of four types:
- Bilateral Crossed Ectopia of Unfused Kidneys.
- Unilateral Crossed Ectopia of Unfused Kidneys.
- Bilateral crossed ectopia of Fused Kidneys: commonest (90%).
- Crossed Unfused renal ectopia.
Other rare fusion anomalies are sigmoid kidney, L-shaped kidney but rarely there is incidence of familial crossed ectopia. 3.3 –11.1% of the population have congenital anomalies of the urinary system, which accounts for nearly 50% of all congenital abnormalities. The overall occurrence of ectopic kidney is 1 out of 400 autopsy cases and among them 85% had fused kidney. In a study incidence of prenatal ultrasonography detected crossed renal ectopia was 0.003% in India. The commonest (incidence of 1 in every 700 autopsies and 0.25% in the general population) renal ectopia is horseshoe kidney.
Pancake kidney is the rarest type of ectopic fused kidney disease with unknown occurrence. But Miclaus et al had calculated that 1 out of 65,000–375,000 population may get affected. Pancake kidney has male preponderance (Male: Female ratio being 2.5:1).The condition may be detected at any age, although most common is 30–60 years.Crossed Ectopic Kidney (second most common fusion abnormality) has varied presentation. In this situation, both the kidneys may be situated on the ispilateral side in a fused manner (85%), in unfused manner (<10%), or in extreme rarity it may be bilateral or solitary. The left kidney more commonly (3 times commoner than the right kidney) migrates to the opposite side. Commonly upper pole of the crossed ectopic kidney fuses with the lower pole of uncrossed kidney.
Mc Donald, Mc Clellan et al had classified Crossed Fused Renal Ectopia (in decreasing order of frequency) into six categories like [Figure 4]:
- Type A: Inferior crossed fusion .Here the superior part of the ectopic crossed kidney fuses with the inferior part of the ipsilateral uncrossed kidney. Pelvis of both the kidneys may be located anteriorly.
- Type B: S-shaped or Sigmoid kidney. Here the ectopic crossed kidney is placed inferiorly with pelvis directed laterally and the ipsilateral uncrossed kidney is placed superiorly with pelvis directed medially. In this situation, pelvis of both kidneys is placed correctly as two kidneys fuses after completion of rotation on the vertical axis.
- Type C: Lump kidney. Here,unilaterally, fusion occurs over a wide surface and pelvis of both kidneys are placed anteriorly .The ureter from the ectopically positioned kidney crosses the midline and it is placed more inferiorly than the ipsilateral one.
- Type D: Tandem or L-Shaped kidney. Here the crossed ectopic kidney lies transversely and fuses partly with the inferior part of the ispilateral uncrossed kidney.
- Type E: Disc kidney (Unilateral in position). Here both the kidneys are fused along the whole medial surface.
- Type F: Superiorly fused and crossed kidney (least common type):Here the lower part of the crossed ectopically placed kidney fuses with the superior part of the ipsilateral uncrossed kidney . Pelvis of both the kidnes is anteriorly placed.
The ascent of primitive renal tissue (metanephric blastema) and ureteric bud starts in the 5thweek of IUL which completes at 9th week of gestational age. Congenital anomalies of renal fusion and ectopia occur if there is impaired lateral shift, deviation and internal rotation during the process of cephalic migration from the mid-pelvis to the abdomen due to
- Faulty ureteral bud development.
- Under influences of teratogenic factors.
- Aberrant renovascular phenotypes where abnormally positioned umbilical artery causes opposition of the metanephric blastema resulting in fusion anomally. The retroperitoneal structures may impair fused renal mass to ascent cranially upto the lumbar position.
- Cook and Stephens had proposed that abnormal flexion or growth of the developing hind-end may cause development of pancake kidney.
So, Pancake kidney malformation is a result of extensive fusion of the medial surfaces of metanephric blastema during early intra-uterine life. The renal mass is commonly situated in the pelvis or at the level where aorta bifurcates. Pancake kidney may get blood supply from branches of the abdominal aorta or from numerous branches of both the internal and external iliac arteries. Most commonly, inferior parts of the ectopic, fused kidneys are more medially rotated than superior parts. The pancake kidney is located commonly at the level of third to fifth lumbar (L3-L5) vertebra and lies in front of the great vessels. The parenchymatous orfibrous isthmus lies where the inferior mesenteric artery arises from the aorta.
Urogenital anomalies are found to be linked with 9p tetrasomy and 9p trisomy, .Pancake kidney is often associated with other anomalies, such as un-descended testis or anomalous vas deferens, vaginal agenesis, cornuate (uni/bi)uterus, Fallot’s tetralogy, spina bifida, agenesis of sacrum, caudal regression syndrome and strabismus.
Pancake kidney mostly has deformed and rotated collecting system. The shorter length of ureters cause obstruction and stasis leading to hydronephrosis, nephrolithiasis, and vesicoureteral reflux with recurrent infection of urinary tract. So, otherwise symptomless Pancake kidney, may present with vague lower abdominal pain, features of urinary tract infection like pyrexia, hematuria. Aneurysm of iliac vessel, amenorrhea and failure to conceive may also be encountered as extra renal manifestations.The Pancake kidney is supplied by one or multiple renal arteries (branches from distal aortic or iliac artery) and is drained by renal veins (tributaries of iliac vein or inferior vena cava). In case of single vascular supply gravid uterus, pelvic mass or trauma may lead to renal ischemia. Hypertension may result from stenosis of aberrant renal arteries due to atherosclerosis of the aorta and iliac arteries in the process of ageing.
In spite of scarcity of reported cases of pancake kidneys, subjects with ectopic fused kidney anomalies are more inclined towards development of various primary malignancies, including renal cell carcinoma ,Wilms tumor or rarely rhabdomyosarcoma. Person with horseshoe kidney is nearly two times more prone to develop Wilms tumour in comparison to one with normal renal anatomy.
The diagnosis is always incidental. Excretory urography was used previously which is now replaced by ultrasonography, MDCT ,CT urography and radio nucleotide scanning for better studies of the urinary system and renal vascular anatomy.Ultrasonography, is the primary modality for pre or post-natal diagnosis of kidney anomalies. CT urography with contrast enhancement (MDCT) is specially useful for studying urinary tract anatomy, which includes kidney parenchyma and collecting systems. MDCT images can also be processed using multiplanar reconstruction methods, maximum intensity projection, volume rendering process which provide a three-dimensional imaging and helps in better diagnosis.Crossed fused ectopic kidney has no specific treatment guidelines. Mere existence of pancake kidney does not herald progressive renal failure. Surgery is only indicated in case of confirmed failure or progressive derangement of kidney function due to urinary outflow obstruction/obstructive uropathy. The treatment is selective towards the associated problems like pyeloplasty for a pelvi-ureteric junction obstruction, bulking agent injection or ureteric reimplantation in case of vesicoureteral reflux. To avoid vascular injury, infarction, necrosis or postoperative renal failure , the fused renal mass should not be separated. Conservative management is indicated for symptomless subjects, after exclusion of coexisting anomalies, in the form of regular monitoring of renal function, remaining alert for the development of recurrent infections, any obstructive (viz. ureteropelvic junction) manifestations leading to calculus formation and any malignant changes. The person should be careful in avoiding trauma to low-lying pelvic kidney. In the case of pregnancy in female with pancake kidney, possible compressive effect by gravid uterus and possible trauma during child birth should also be kept in mind. During pelvic surgery, specially abdominal aortic surgery, the pancake kidney is also susceptible to iatrogenic trauma. As the blood supply to pancake kidney comes from aortic bifurcation or iliac vessels, the whole renal mass may suffer ischemia if proximal aortic cross-clamping is done.
To conclude,Pancake kidney is the rarest anomaly occurring due to extensive fusion of medial surfaces of both the kidneys which is commonly detected incidentally. There may be increased occurrence of recurrent urinary tract infections or renal calculus formation, due to the chances of ureteral obstruction by rotated and anomalous collecting systems. The Pancake kidney can be managed conservatively, if the subject remains symptom less, by regular monitoring of renal function, remaining alert for the development of any malignant changes. The person should be careful in avoiding trauma to low-lying pelvic kidney. Extensive surgeries are avoided and only selective procedures should be done so that the patient may lead a normal lifestyle.