Embryology

A bud from the lower end of the mesonephric (Wolffian) duct grows backwards behind the peritoneum to the sacral region. The stalk of the bud forms the ureter and its dilated upper extremity the renal pelvis. From 6 weeks to 8 months, the primitive pelvis bifurcates repeatedly to form first the calyces and, after several subsequent divisions, the collecting ducts.

The renal parenchyma is derived from the metanephros which is the last in a series of embryonic renal masses: the primitive pronephros is supplanted by the mesonephros which is in turn succeeded by the metanephros. The continuity of glomerular apparatus and nephric tubules formed within the metanephros is established through connection with the collecting ducts. Between weeks 5 and 8 of embryonic development, each kidney ascends the posterior abdominal wall to reach its normal position in the loin. At the same time, it rotates so that its hilum faces medially instead of forwards as it did previously.

In humans the lobulation of the foetal kidney is usually lost as the lobules become bonded together by the growth of new cortex and the renal capsule. In some mammals, e.g. oxen and bears, foetal lobulation is retained.

Surgical anatomy

The parenchyma of each kidney usually drains into seven calyces, three in the upper and two each in the middle and lower calyces (Fig. 64.1). Each of the three segments represents an anatomical and physiologically distinct unit with its own blood supply.

Congenital abnormalities of the kidney (Table 64.1)

Absence of one kidney

Congenital aplasia of one kidney is usually discovered incidentally on intravenous urography, computerised tomography (CT) scan or renal Ultrasonography. If the mesonephric duct has failed to bud, the ureter will be absent and on cystoscopy no ureteric orifice is visible on that side. Alternatively, the ureter and renal pelvis are present but the kidney is absent or almost so. In either case the solitary contralateral kidney is likely to be hypertrophied. An absent or grossly atrophic kidney is found in about 1:1400 individuals.

Renal ectopia

In approximately 1:1000 people the kidney does not ascend. Ectopic kidneys are usually found near the pelvic brim and, for reasons which are not understood, they are usually left sided. The contralateral kidney is generally present and in its normal position. Renal ectopia may present diagnostic problems when acute disease develops in the kidney and there is always a danger that an unwary surgeon may be tempted to remove it as an unexplained pelvic mass.

Horseshoe kidney

If the most medial subdivisions of the mesonephric bud meet and fuse in the midline, the normal ascent of the kidneys is impeded by structures arising in the midline. The result is a pair of ectopic kidneys that are usually fused at their lower poles with the junction in front of the fourth lumbar vertebra. Horseshoe kidney is found in 1:1000 necropsies and is more common in men. Fusion of the upper poles is uncommon.

Clinical features

Horseshoe kidneys are liable to become diseased, possibly because the ureters are angulated as they pass over the fused isthmus (Fig. 64.2). This may lead to urinary stasis with consequent infection and nephrolithiasis. More commonly, there is a true pelviureteric junction obstruction as an associated abnormality, with the same consequences. Presentation as a fixed mass below the umbilicus is uncommon and horseshoe kidney is usually a radiological diagnosis. The most frequent appearance on the urogram shows the lower pole calyces on both sides being directed towards the midline. More rarely, all or most of the calyces are reversed (Fig. 64.3). The ureters characteristically have vase-like curves. Although horseshoe kidney is not a contraindication to pregnancy, urinary complications are more frequent.

Division of the isthmus between the kidneys is usually only indicated in the course of surgery for abdominal aortic aneurysm. Although the tissue is likely to be less vascular than normal renal parenchyma, care must be taken to avoid the vascular supply of the horseshoe which is typically eccentric, springing unpredictably from adjacent major vessels.

Unilateral fusion

Unilateral fusion (syn. crossed dystopia) is rare but the urogram appearance is striking. Both kidneys are in one loin and are usually fused. The ureter of the lower kidney crosses the midline to enter the bladder on the contralateral side. Both renal pelves can lie one above each other medial to the renal parenchyma (unilateral long kidney) or the pelvis of the crossed kidney faces laterally [unilateral S-shaped kidney (Fig. 64.4)].

Congenital cystic kidneys (syn. polycystic kidneys)

Polycystic kidneys are hereditary and can be transmitted by either parent as an autosomal dominant trait. This is important in genetic counselling because the risk of an offspring inheriting the condition can be as high as one in two depending upon the penetrance of the gene. The disease is not usually detectable on standard imaging until the second and third decades of life and does not usually manifest itself clinically before the age of 30 years.

Pathology

The kidneys become enormously enlarged, the cysts giving the appearance of a collection of bubbles below the renal capsule. On histological section the renal parenchyma is rid­dled with cysts of varying size, containing clear fluid, thick brown material or coagulated blood. In 18 per cent of cases there is a congenital cystic liver disease. The pancreas and lungs are occasionally affected as well. The aetiology of all renal cysts is uncertain although theories abound.

Clinical features in the adult

The condition is slightly more common in women than men. There are six clinical features:

 

  irregular upper quadrant abdominal mass;

  loin pain;

  haematuria;

  infection;

  hypertension;

  uraemia.

Renal enlargement. The bilateral knobbly enlargement can hardly be mistaken when discovered in the course of a routine examination. Less florid examples may be revealed at laparotomy or abdominal imaging for some other condition. Unilateral renal swelling, in which one kidney contains larger cysts than the other, may be confused clinically with a cystic renal tumour.

Pain, felt as dull loin ache, is thought to be caused by the weight of the organ dragging upon its pedicle or by stretching of the renal capsule by the cysts. Haemorrhage into a cyst may cause more severe pain, as may the passage of a calculus from the diseased kidney.

Haematuria. Rupture of a cyst into the renal pelvis may cause haematuria which is typically moderate, lasts for a few days and recurs at intervals. Profuse haematuria is uncommon.

Infection. Pyelonephritis is common in patients with congenital cystic kidney, presumably because of urinary stasis.

Hypertension is present in up to 75 per cent of patients over the age of 20 years with polycystic kidneys. Why some escape this complication is not clear; the high blood pressure could possibly result from a separate genetic factor linked to the gene for congenital cystic kidneys.

Uraemia. Patients with congenital cystic kidneys pass large volumes of urine of low specific gravity (1.010 or less) which contains a trace of albumin but no casts or cells. Chronic renal failure develops as functioning renal tissue is replaced progres­sively by cysts. The patient complains of anorexia, headache and vague abdominal discomfort. As the symptoms are nonspe­cific the diagnosis may be missed until drowsiness and vomit­ing result from the biochemical derangement. Severe anaemia is common. Signs of end stage renal failure often begin suddenly during middle life and the patient is unlikely to survive without renal replacement by dialysis or renal transplantation.

Imaging

Ultrasound and CT will show multiple cysts in both kidneys, and sometimes cysts in the liver and other organs. The presence of blood and debris in the cysts may mimic the hetero­geneity of a cystic adenocarcinoma. By contrast, simple (acquired) cysts are usually solitary and have smooth thin walls and homogeneous contents (Fig. 64.5). Doubt about the diagnosis can be resolved by cytological examination of cyst fluid obtained by fine needle aspiration.

Polycystic kidneys have a typical appearance on excretory urography: the renal shadows are enlarged in all directions; the renal pelvis is compressed and elongated; and the calyces are stretched over the cysts and are often narrow (like spiders’ legs) or bell-like (Fig. 64.6).

Treatment

Renal failure. As kidney failure develops, a low protein diet will help to postpone the need for renal replacement. Infection, anaemia and disturbances of calcium metabolism need appropriate treatment, usually by a nephrologist.

Surgical treatment to uncap the cysts (Rovsing’s operation) is rarely indicated because few surgeons now accept that this can preserve renal function by relieving pressure on the parenchyma. If cysts are to be decapped in the hope of reducing pain, the operation can be done laparoscopically.

Infantile polycystic disease

Infantile polycystic disease is an unrelated and much rarer condition. The kidneys are large and may obstruct birth. Many cases are stillborn and most of the others die from renal failure early in life. The condition is inherited as an autosomal recessive.

Unilateral multicystic disease

Unilateral multicystic disease is much more common. It presents as a nonfunctioning mass in the flank. Exploration and removal is the treatment of choice. In the differential diagnosis, Wilms’ tumour (see below), neuroblastoma and congenital hydronephrosis are all rarer conditions.

Solitary renal cyst (syn. simple cyst of the kidney) (Fig. 64.5)

The term ‘solitary’ cyst serves to distinguish the condition from congenital cystic disease of the kidneys, but more than one cyst is often present in one or both kidneys. Simple cysts are most frequently discovered incidentally on imaging of the upper abdomen: they rarely give symptoms. A palpable mass, pain from haemorrhage into the cyst and infection are relatively uncommon presentations. Occasionally a cyst in the hilum of the kidney (a parapelvic cyst) presses on the pelviureteric junction and causes obstructive symptoms.

When discovered as a filling defect on excretion urography the true nature of a solitary cyst will be apparent by its characteristic appearance on ultrasound or CT. Percutaneous cyst puncture will yield fluid for cytological examination but this is rarely necessary with sophisticated modern imaging.

Differential diagnosis

In sheep-rearing districts, hydatid cyst of the kidney is common. On the right side if hydatid disease is suspected the swelling is liable to be mistaken for hydatid cyst of the liver. Occasionally, the patient complains of passing ‘grape skins’ (ruptured daughter cysts) in the urine. Removal of the cyst must follow the principles used in excision of hydatid cyst of the liver, i.e. the scolices must be killed by injection of a scolicide-like formalin solution before the cyst is handled. If the cyst is large nephrectomy may be the safest course.

Aberrant renal vessels

Two or more renal arteries are most common on the left. The main importance of the abnormality is as a source of potential error during operations in the retroperitoneum, espe­cially those on the kidney. The renal arteries are functional end arteries so division of an aberrant lower pole artery will lead to infarction of the section of parenchyma that it supplies. Renal veins, by contrast, have extensive collaterals and an aberrant vein can be divided with impunity. Aberrant vessels probably do not cause hydronephrosis, although a hydronephrotic renal pelvis may bulge between renal vessels, making them particularly noticeable (Fig. 64.7).