Splenomegaly is an enlargement of the spleen. The spleen usually lies in the left upper quadrant (LUQ) of the human abdomen. Splenomegaly is one of the four cardinal signs of hypersplenism which include: some reduction in number of circulating blood cells affecting granulocytes, erythrocytes or platelets in any combination; a compensatory proliferative response in the bone marrow; and the potential for correction of these abnormalities by splenectomy. Splenomegaly is usually associated with increased workload (such as in hemolytic anemias), which suggests that it is a response to hyperfunction. It is therefore not surprising that splenomegaly is associated with any disease process that involves abnormal red blood cells being destroyed in the spleen. Other common causes include congestion due to portal hypertension and infiltration by leukemias and lymphomas. Thus, the finding of an enlarged spleen, along with caput medusae, is an important sign of portal hypertension. Definition
The standard system for classifying splenomegaly on radiography is:
Normal (not splenomegaly): the largest dimension is less than 11 cm
Moderate splenomegaly: the largest dimension is between 11 and 20 cm
Severe splenomegaly: the largest dimension is greater than 20 cmAlso, a cutoff of a craniocaudal height of 13 cm is also used to define splenomegaly.
In patients where the likelihood of splenomegaly is high, the physical exam is not sufficiently sensitive to detect it; abdominal imaging is indicated in such patients.In cases of infectious mononucleosis splenomegaly is a common symptom and health care providers may consider using abdominal ultrasonography to get insight into a persons condition. However, because spleen size varies greatly, ultrasonography is not a valid technique for assessing spleen enlargement and should not be used in typical circumstances or to make routine decisions about fitness for playing sports. Causes
The most common causes of splenomegaly in developed countries are infectious mononucleosis, splenic infiltration with cancer cells from a hematological malignancy and portal hypertension (most commonly secondary to liver disease, and sarcoidosis). Splenomegaly may also come from bacterial infections, such as syphilis or an infection of the hearts inner lining (endocarditis). Splenomegaly also occurs in mammals parasitized by Cuterebra fontinella.The possible causes of moderate splenomegaly (spleen <1000 g) are many, and include:

The causes of massive splenomegaly (spleen >1000 g) are

chronic myelogenous leukemia
myelofibrosis
malaria
splenic marginal zone lymphoma

Pathophysiology
Splenomegaly can be classified based on its pathophysiologic mechanism:

Congestive, by pooled blood (e.g., portal hypertension)
Infiltrative, by invasion by cells foreign to the splenic environment (e.g., metastases, myeloid neoplasms, lipid storage diseases)
Immune, by an increase in immunologic activity and subsequent hyperplasia (e.g., endocarditis, sarcoidosis, rheumatoid arthritis)
Neoplastic, when resident immune cells originate a neoplasm (e.g., lymphoma). Diagnosis
Abdominal CT is the most accurate and spleen need to be larger 2-3 times to be palpable below the costal margin in physical examination that can help in diagnosis.