The spleen is an organ situated in the upper left abdomen, tucked beneath the rib cage. As part of the lymphatic and hematological systems, it functions primarily as a blood filter, removing old or damaged red blood cells and recycling iron. It also serves as a center for immune response, housing white blood cells and producing antibodies to defend against pathogens.
A splenic lesion is a broad term describing any tissue abnormality or damage within the organ, typically detected through imaging techniques like ultrasound or computed tomography (CT). These findings are often incidental, discovered during scans for unrelated health issues, and are not a diagnosis in themselves. Lesions vary widely in cause and composition, ranging from simple fluid-filled sacs to complex solid masses. While most abnormalities are non-threatening, identifying the specific cause is necessary to rule out serious conditions.
Splenic Cysts (Fluid-Filled Lesions)
Splenic cysts are fluid-filled sacs within the splenic tissue. They are categorized based on the presence or absence of an epithelial lining, which points toward their developmental origin. The three main types include true cysts, pseudocysts, and those caused by parasitic infection.
True cysts, also known as primary or epithelial cysts, are rare developmental lesions lined with epithelial cells. These benign cysts are thought to be congenital and are most often found in young to middle-aged females. They usually grow slowly and may only cause symptoms, such as left upper quadrant discomfort, if they become large enough to compress surrounding organs.
Pseudocysts, or false cysts, are the most frequently encountered non-parasitic splenic cyst and lack a cellular epithelial lining. Their formation typically follows prior abdominal trauma that resulted in a localized collection of blood (a hematoma). Over time, the blood resolves, leaving a fibrous capsule filled with serous fluid, often indicating a past injury the patient may no longer recall.
Parasitic cysts represent a distinct category, with the hydatid cyst being the most common example worldwide. These lesions are caused by the larval stage of the Echinococcus granulosus tapeworm, a zoonotic infection prevalent in certain sheep-rearing regions. Hydatid cysts can grow large and are a concern due to the potential for rupture, which may lead to serious complications including anaphylactic shock or secondary seeding of the abdominal cavity.
Infectious Lesions
Infectious processes can lead to specific splenic lesions, broadly divided into acute, pus-filled pockets and chronic, walled-off reactions. The spleen’s role as a blood filter makes it vulnerable to seeding by circulating pathogens. These lesions can be the first indication of a systemic infection.
Splenic abscesses are localized collections of necrotic tissue and pus, primarily caused by bacterial invasion. They typically arise from hematogenous dissemination, where bacteria are transported through the blood from a distant site of infection, such as septicemia or infective endocarditis. Other pathways include the spread of infection from a nearby organ or the infection of a pre-existing lesion, like an infarct or hematoma.
Abscess formation is relatively rare due to the spleen’s robust immune function, but the risk increases significantly in individuals with underlying conditions such as immunosuppression, diabetes, or hematologic malignancies. The causative organisms are often aerobic bacteria like Streptococcus and Staphylococcus, or anaerobic bacteria like Escherichia coli. These lesions require prompt treatment with antibiotics, often alongside drainage or surgical removal.
Granulomas represent a chronic or resolved inflammatory response to a persistent pathogen. They are small, organized collections of immune cells that the body forms to encapsulate organisms it cannot eliminate, often resulting in calcification. Common causes include fungal infections such as histoplasmosis and mycobacterial diseases like tuberculosis. The presence of splenic granulomas is frequently an asymptomatic, incidental finding that signals past exposure rather than active disease.
Benign Solid Masses and Vascular Events
Solid, non-cancerous masses and blood flow disturbances can create lesions in the spleen. These benign findings are common and often require no treatment, but their appearance on imaging must be carefully differentiated from malignant tumors.
Hemangiomas are the most common type of benign solid splenic lesion, consisting of a tangle of blood vessels. These vascular tumors are often small, asymptomatic, and discovered incidentally during imaging. Larger hemangiomas carry a small risk of spontaneous rupture and hemorrhage, which is the primary concern associated with these lesions. Lymphangiomas are a less frequent type of benign vascular lesion, composed of abnormal lymphatic vessels.
Splenic infarction is a lesion caused by a lack of blood supply, or ischemia, leading to tissue necrosis. This event occurs when a branch of the splenic artery is blocked, often by a clot (thromboembolism) originating from the heart, such as in cases of atrial fibrillation or infective endocarditis. On imaging, an infarct classically appears as a wedge-shaped area of damaged tissue, with the apex pointing toward the spleen’s central blood supply.
Hematologic conditions, most notably sickle cell disease, are frequent causes of splenic infarction, especially in younger patients. The abnormally shaped red blood cells obstruct small vessels, leading to repeated, small infarcts that cause the spleen to shrink and scar over time. Acute hematomas, which are localized collections of blood resulting from blunt abdominal trauma, also create lesions distinct from the resolved pseudocysts they can later form.
Malignant Neoplasms
Malignant lesions in the spleen represent the most concerning category and are divided into cancers that originate in the spleen and those that spread there from other organs. Though primary splenic tumors are rare, their identification is a high priority in diagnostic workup.
The most common primary malignancy of the spleen is lymphoma, a cancer of the lymphatic system that involves the organ’s white pulp. Splenic lymphoma can manifest in several ways, including diffuse enlargement of the entire spleen or the presence of one or multiple distinct solid masses. A much rarer but highly aggressive primary cancer is angiosarcoma, a malignant tumor arising from the blood vessel lining cells within the spleen.
Metastatic disease refers to cancer that has spread to the spleen from a primary tumor elsewhere in the body. While the spleen is a less common site for metastasis compared to organs like the liver or lungs, it occurs, often in the context of advanced, widespread cancer. Cancers most likely to spread to the spleen include those originating from the lung, breast, melanoma, and colorectal areas. The relative rarity of splenic metastasis is often attributed to the organ’s unique anatomical and immunological environment.