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
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
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
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).