Spleen

Anatomy and physiology

The spleen arises by mesenchymal differentiation along the left side of the dorsal mesogastrium in the 8-mm embryo. The weight of the normal adult spleen is between 75 and 150 g. It is shaped like a segment of an orange, its convexity directed backwards and to the left. It lies between the tenth and eleventh ribs posteriorly. It abuts the diaphragm superiorly, anteriorly there is the gastric impression, posteriorly it is in contact with the kidney and the colon lies inferiorly. Within the hilum is the tail of the pancreas and the splenic vessels. It forms the apex of the lesser sac. Within the anterior leaf of the lesser sac the short gastrics pass to the greater curve of the fundus of the stomach. The splenic artery divides at the hilus into branches which run along the trabeculae of the spleen into the white pulp, where they give off branches which are almost perpendicular to the central trunk (Fig. 53.1). This produces a ‘skimming effect’ by which plasma tends to pass down the branches to the white pulp and most of the red cells pass in the trabecular artery to the red pulp. The white pulp has an immune function, whereas the red pulp filters abnormal red cells from the circulation. Phagocytosis of blood-borne particles occurs in both areas. On cutting the spleen areas of red pulp, within which can be seen pale ovoid nodules of white, are apparent.

 The white pulp consists of a central trabecular artery surrounded by nodules with germinal centres and periarterial lymphatic sheaths which provide a framework filled with lymphocytes and macrophages. At the edge of the white pulp is the marginal zone into which pass arteries from both the central artery and from the peripheral ‘penicillar’ arteries. Plasma-rich blood which has passed through the central lymphatic nodules is filtered as it passes through the sinuses within the marginal zone, and particles are phagocytosed. Immunoglobulins produced in the lymphatic nodules enter the circulation through the sinuses in the marginal zone. Beyond the marginal zone is the red pulp which consists of cords and sinuses. Cell-concentrated blood passes in the trabecular artery through the centre of the white pulp to the red pulp cords. To pass from the cords to the sinuses, the red cell must elongate and become thinner. This filters abnormally shaped or rigid cells out of the circulation.

Ninety per cent of blood passing through the spleen moves through an open circulation in which blood flows from arteries to cords and thence to sinuses. Thus, splenic pulp pressure reflects pressure throughout the portal venous system. The remaining 10 per cent bypasses the cords and sinuses by direct arteriovenous connections. The overall flow rate through the spleen is about 300 ml/minute.

 

Functions of the spleen

In the past the spleen was considered dispensable because it was found not to be essential to life. The surgeon therefore sacrificed the spleen with impunity. Increasing knowledge of the untoward effects of splenectomy has led to conservatism in the management of the spleen. Indeed, when splenectomy is performed as part of another operation, such as total gastrectomy, the incidence of complications increases and the mortality rises. For this reason the surgeon now tries to preserve the spleen and maintain the following functions.

 

  Immune function the spleen processes foreign antigen and is a major site of specific immunoglobin M (1gM) production. The nonspecific opsonins tuftin and properdin are synthesised. These antibodies are of both B- and T-cell origin, and react with bacteria and fungi to make them more susceptible to phagocytosis.

Filter function macrophages in the reticulum capture cellular and noncellular material from the blood and plasma; this material includes bacteria, especially pneumococci.

Removal of effete, platelets and red cells this process is called ‘culling’.

Pitting this is the process of removing particulate inclu­sions from red cells and returning the repaired red cell to the circulation. Red blood cell nuclei or malarial parasites can be removed by this process without destroying the cell.

Iron reutilisation the phagocytic reticular cells remove iron from ingested degraded haemoglobin during red cell culling and return the iron to the plasma.

Pooling up to 30—40 per cent of blood platelets are sequestered within the spleen. In splenomegaly up to 80 per cent of the platelet pool can be sequestered in the spleen and this, together with accelerated platelet destruc­tion, can result in thrombocytopenia.

Reservoir function in animals, especially the dog, up to one-third of the blood volume may be sequestered in the

spleen during sleeping and returned to the circulation on waking. This does not occur in humans.

Haematopoiesis this only occurs up to the fifth intra­uterine month and thereafter in certain disease states.

 

Investigation of the spleen

 

Conditions that cause splenomegaly can be diagnosed by his­tory, physical signs and laboratory examination. For instance in haemolytic anaemia a full blood count, reticulocyte count and tests for haemolysis will determine the type of anaemia. Similarly, splenomegaly associated with portal hypertension caused by hepatic cirrhosis is diagnosed by the physical signs of liver dysfunction, abnormal liver function tests and evidence of oesophageal varices. Many conditions that cause splenomegaly also cause lymphadenopathy. The cause of the splenomegaly should then be determined from investigations for diseases known to cause lymphadenopathy and splenomegaly. A lymph node biopsy can be helpful in this respect.

 

Plain radiography

 

This plays little part in investigation now, although calcification in the spleen may suggest an old infarct or hydatid disease. Multiple areas of calcification would suggest splenic tuberculosis.

 

Imaging

 

Ultrasonography of the spleen (Fig. 53.2) is of value in deter­mining its size and consistency, and whether or not cysts are present. It can be used for diagnosis of a ruptured spleen. However, in this case a computerised tomography (CT) scan is more usually undertaken to exclude other intra-abdominal problems. Similarly, a spiral CT scan with contrast enhance­ment will be preferable in the diagnosis of splenomegaly to determine both the extent of the disease and the associated problems such as the extent of lymphadenopathy.

 

Radioisotope scans

Technetium-99m (99mTc)-labelled colloid can provide information about the position and size of the spleen and, with appropriately labelled red cells, the life and place of their destruction can he determined.