A mass may be found within the abdomen by accident or by design. In either event, the physician should adopt the same approach to diagnosis as would be followed elsewhere in the body (Fig. 15.1). First, the age of the child should be considered, since nearly all diseases in children occur at specific ages. In particular, malignant tumours of childhood which present commonly as incidental abdominal masses are much more common in preschool children.

The second question to be answered is: ‘From which organ does the mass arise?’ Knowledge of the organ of origin will eliminate numerous possibilities from any list of differential diagnoses. The third question is: ‘What are the features of the mass??’

This is answered by careful examination to elicit whether it is mobile or fixed, cystic or solid and smooth or nodular or whether there are signs of inflammation. These features assist in the determination of which pathological process has produced the mass. The mobility of an abdominal mass is a key feature in the diagnosis, since mobile masses must be within the peritoneal cavity itself, rather than in the retroperitoneal tissue. In addition, mobile lumps usually are benign. The last question to be answered is: ‘What secondary effects does the mass cause?’ The blood pressure may need to be measured, particularly if a Wilms’ tumour, neuroblastoma or renal disease is suspected. A varicocele may be present if the left renal vein is obstructed. Metastases may be present in distant lymph nodes or bones. Secondary effects, such as bowel obstruction, vena caval-obstruction or ascites, may be present within the abdomen itself.

When an abdominal mass is discovered, either by the parent or the attending physician, it is important to determine whether the mass is normal (Fig. 15.2). The liver and spleen usually are palpable in the neonatal period and in early infancy, when their haemopoietic function is active. They become impalpable later in childhood, when they no longer produce red cells and when the shape of the costal margin changes with growth to conceal the organs of the upper abdomen. In small or thin children, the lower poles of the kidneys are palpable in the paravertebral gutters.

When the infant is relaxed, the great vessels are palpable behind and to the left of the umbilicus. The aortic pulsation can be felt in infants and small children, since the lumbosacral promontory is relatively anterior. In both iliac fossae, and especially on the left, faeces may be palpated within the colon. On the right side, the colonic contents are pultaceous and relatively soft. In contrast, in the left descending and sigmoid colon, hard rock-like faeces may be palpable in the presence of constipation. In the middle of the lower abdomen, the bladder may be demonstrated on percussion or palpation, since in small children the bladder is not obscured by the pelvis.

Hard faeces are the commonest abdominal masses (Fig. 15.3). Usually, the faeces are palpable in the left side of the colon and may vary in consistency from pultaceous to stony hard. In chronic constipation, the sigmoid may be so enlarged that firm faeces are palpable in the right side of the abdomen. In severe cases of constipation, faeces will be palpable in the right colon as well. To determine whether a mass is present within the colon, an attempt should be made to indent the mass by compression through the abdominal wall. The pathognomonic sign of faeces is the indentation produced on compression.

The bladder is misdiagnosed commonly as a pathological abdominal mass (Fig. 15.4a). Bladder enlargement will cause protuberance of the lower abdomen. In addition, palpation will be tender and is likely to induce the urge to void. If there is doubt as to whether a mass is the bladder, the child should be re-examined after voiding or catheterization.

Two common masses, described elsewhere in this volume, are the inflammatory mass of appendicitis (Chap. 3), and the sausage-shaped mass of intussusception (Chap. 8). The appendix mass (Fig. 15.5) is caused by inflammation or perforation which is walled off by adjacent bowel and greater omentum. The mass is palpable in the right iliac fossa or pelvis and exhibits signs of inflammation, including loss of mobility. There may be associated signs of bowel obstruction. In a child over 10 years of age, a rare differential diagnosis includes inflammatory bowel disease.

In addition, a malignant lymphoma of the small bowel may mimic an appendix mass, but this is rare.

The mass of intussusception may be difficult to feel (Fig. 15.6). The child may be crying and restless, which makes the mass difficult to palpate behind the rectus muscles. It is further concealed by voluntary guarding and by increasing abdominal distension. The longer the history, the more difficult it is to palpate the mass. The intussusception mass is more medial than the usual position of the right colon because of telescoping of the mesentery (see Chap. 8 for details).

Urinary obstruction produces different types of cystic dilatation of the kidney according to the time of obstruction in relation to renal development (Fig. 15.7). In ureteric atresia, early fetal obstruction prevents normal nephron development and results in dysplastic cysts. This leads to a multicystic kidney in the neonate. The cysts are of different sizes, and the kidney feels like an irregular bunch of grapes. In late fetal or postnatal obstruction, the nephrons have had time to develop normally before obstruction occurs, and the more usual hydronephrotic kidney is produced. If the obstruction is in the distal urinary tract, a dilated ureter also may be palpable. Hydronephrosis produces a smooth cystic enlargement of the kidney which retains its shape. Gerota’s fascia confines the kidney so that it does not cross the midline, and it may be felt to move downwards on inspiration. A loin or flank mass should be ballottable (Fig. 15.8). Bimanual palpation with the lower hand pushing the kidney forwards from behind should allow the hand at the front of the abdomen to feel the mass. A mass anterior to the plane of the kidney cannot be pushed forward by a hand in the loin. Finally, a large cystic kidney in an infant with little body fat can be transilluminated provided that the room is dark and the torch is bright (Fig. 15.9).

The embryonal tumours – neuroblastoma, Wilms’ tumour and hepatoblastoma (Fig. 15.10) – are relatively common in the first five years of life. Leukaemias and lymphomas are the common neoplasms which present as an abdominal mass later in childhood. Hepatosplenomegaly, with enlargement of lymph nodes in other parts of the body, in association with fever, weight loss or night sweats, should arouse suspicion of an underlying haematological neoplasm.