In common with lymphoid tissue, the spleen is derived from the mesodermal cells found between the two layers of the dorsal mesogastrium. These cells proliferate in clusters known as splenicules, that later fuse to form the adult spleen. This developmental pattern explains the frequency of accessory spleens (splenicules that failed to fuse) as well as the segmental nature of splenic blood supply, which allows partial resections [1, 2].

While it does not share the architecture of a lymph node, it is the largest secondary lymphoid organ and hosts lymphocyte maturation and antibody production. In addition to its role in the immune system, the spleen is also the chief site of hemocatheresis and serves as a reservoir of neutrophils and platelets. In some species, the spleen possesses a contractile capsule that allows it to release a hemodynamically significant volume of blood, resulting in an autotransfusion. While the spleen serves an important role on in prenatal erythropoiesis, this function is absent in the adult, returning only in pathologic circumstances [3, 4].

The spleen is normally fixed in the left upper quadrant by at least five major ligamentous structures: splenophrenic, gastrosplenic, pancreaticosplenic, splenocolic, and splenorenal ligaments. Wandering spleens are characterized by their lack of peritoneal attachments. They are also termed displaced or ectopic, with the term dislocated reserved for instances in which the spleen is attached in an abnormal location. While commonly asymptomatic, this condition predisposes to splenic torsion. Said torsion may be intermittent, causing pain, but it may also be sustained, leading to infarction. In addition, wandering spleens have been associated with hypersplenism and gastric varices (when the splenic vein is thrombosed). While the traditional approach to the management of wandering spleens was splenectomy, currently splenopexy is preferred. This operation consists of raising peritoneal flaps in the splenic fossa and suturing the splenic capsule to these flaps. Alternatively, synthetic materials have been used to the same end [5–7] (Fig. 19.1).

Accessory spleens are fully vascularized and functional collections of splenic tissue. As mentioned earlier, they represent a failure of fusion of the splenicules. They are the most common splenic anomaly, present in 10–30 % of autopsy cases and 10 % in CT imaging series. Accessory spleens are most often solitary and located in the splenic hilum. Other sites include the omentum, mesentery, lesser sac, and also within the pancreatic tissue. Their importance lies in that they may be mistaken for lymphoid tumors, but more importantly, when unrecognized they may lead to the failure of splenectomy as therapy for hematologic conditions [8].

Congenital asplenia is a rare condition associated with cardiac and pulmonary malformations. It is present in up to 3 % of neonates with cardiac malformations. Cases of isolated asplenia have also been reported. Highlighting the immune function of the spleen, congenitally asplenic patients are vulnerable to overwhelming sepsis by encapsulated bacteria. The diagnosis of asplenia can be suspected by the presence of Howell–Jolly bodies and can be easily confirmed by imaging. These patients should be treated as their surgical asplenic counterparts [9–11].

Splenosis is the result of autotransplantation of viable splenic tissue after trauma to the spleen. In contrast with accessory spleen, splenosis is often multiple. The ectopic tissue lacks independent blood supply and is devoid of its normal immunological function and may be located in remote locations from the spleen. Splenosis has been reported in as many as 65 % of gross splenic rupture. While most commonly asymptomatic, splenosis may also present as a mass or as a source of pain from recurrent hemorrhage and adhesions. The diagnosis can be established with technetium 99 sulfur colloid scan, which is also the test of choice to locate accessory spleens [12].

When compared to the adult, the pediatric spleen presents a thicker capsule and a higher proportion of myoepithelial cells. In addition the arterioles found in the pediatric spleen exhibit more efficient retraction and contraction. These factors determine that the pediatric spleen is more likely to achieve hemostasis after trauma than the adult spleen. The failure rate of nonoperative management for splenic injuries in children is 5 %, compared to 15 % in adults. Due to its thicker capsule, splenorrhaphy is more easily accomplished in children [13, 14].

Please see the discussion for question 5.

Overwhelming postsplenectomy infection is perhaps the most devastating complication of splenectomy. While its lifetime incidence is relatively low (3 %), it carries a morality of 50–70 %. It has been established that its incidence is higher after splenectomy for hematologic or malignant disease as opposed to trauma and in children when compared to adults. Moreover, the majority of cases occur in the first two years following surgery.

The bacteria causing OPSI have a polysaccharide capsule, which helps them evade phagocytosis outside the spleen. The presence of antibodies against the capsule allows bacterial clearance by the Kupffer cells of the liver. Antibodies are not necessary for phagocytosis within the spleen due to its unique sinusoidal vascular pattern that acts as a filter to blood-borne pathogens. This pattern of circulation also makes the spleen vulnerable to septic and metastatic emboli. The bacteria most often involved in OPSI are Streptococcus pneumoniae, Neisseria meningitides, and Haemophilus influenzae.

Clinical features include the presence of a short prodromal phase with fevers, chills, malaise, and GI complaints often without an obvious source of infection. This is followed by a rapid deterioration with hypotension, disseminated intravascular coagulation, respiratory distress, coma, and death within hours of presentation. Survivors often have sequelae from multiple organ failure, as well as peripheral gangrene, bacterial endocarditis, and cardiac valve injuries.

The main line of therapy against OPSI is prevention. Attempts to salvage the spleen should always be made. Additionally, it must be remembered that intact vascular supply. At least 20–30 % of normally vascularized splenic tissue is required to serve the immunological function of this organ. As such, splenic embolization, while physically preserving the spleen, may impair splenic function.

Vaccination against encapsulated pathogens is mandatory. It should be performed 2 weeks before elective splenectomy. In trauma cases, immunization is best given 2 weeks postsplenectomy. In practice immunization is given upon discharge as this allows time for some recovery of the immune system after injury and guarantees 100 % compliance. Boosters must be given every 5 years. In addition, patients should be educated regarding this syndrome. Continuous prophylaxis with penicillin has not proven to be effective. The preferred strategy involves providing patients with antibiotics such as amoxicillin to be taken upon the onset of fevers and rigors, with clear instructions to seek urgent medical care where these symptoms to appear [15, 16].

Due to its role in the destruction of red blood cells, splenectomized patients are at a disadvantage when infected with erythrocyte pathogens such as Plasmodium and Babesia species. The encapsulated bacteria Capnocytophaga canimorsus has been linked to OPSI after dog bites [17, 18].