14.3 Spleen

Anatomy, Size, and Echotexture

The spleen is an intraperitoneal organ in the left upper quadrant, protected by the 9th through 11th ribs. Its convex diaphragmatic surface abuts the left hemidiaphragm, while its visceral surface bears the hilum, where the splenic artery and vein enter and where the pancreatic tail lies immediately adjacent — a relationship that matters when searching for splenic vein thrombosis or accessory spleens. The splenic artery follows a characteristically tortuous course, while the splenic vein runs a straighter path from the hilum toward the portal confluence behind the pancreatic body. Normal splenic length, measured in a coronal plane through the hilum, is up to about 11-12 cm (some references allow 13 cm), with a width near 7 cm and an anteroposterior thickness of 4-5 cm. Splenic size correlates with body habitus, so interpret borderline measurements in clinical context.

Splenic parenchyma is homogeneous and equal to or slightly more echogenic than the liver, and distinctly more echogenic than the renal cortex. Comparing the spleen with the left kidney is a quick internal check for diffuse disease of either organ.

Scan Technique

The spleen is best imaged intercostally with the patient supine or in right lateral decubitus, using coronal and oblique planes during quiet respiration or a held deep inspiration, which moves the spleen below the costal margin. Document the maximum length at the hilum and compare echotexture with the left kidney. Unlike the gallbladder examination, prolonged fasting is not required, though fasting does reduce gastric gas that can obscure the splenic hilum and pancreatic tail.

Normal Variants

An accessory spleen (splenunculus) is a congenital rest of splenic tissue found in roughly 10-30% of the population, most often near the splenic hilum or the pancreatic tail. It appears as a small, round, well-marginated mass with echotexture identical to the main spleen. Its importance is twofold: it can mimic a pancreatic tail or adrenal mass, and after splenectomy accessory spleens can hypertrophy and cause recurrent disease (for example, recurrent thrombocytopenia).

Situs anomalies change the picture entirely. Asplenia syndrome (right isomerism, Ivemark syndrome) pairs an absent spleen with a midline liver and severe congenital heart disease. Polysplenia syndrome (left isomerism) shows multiple small splenules and is characteristically associated with azygos continuation of the inferior vena cava and intestinal malrotation. Before declaring true asplenia, remember that a wandering spleen on an elongated pedicle may lie ectopically in the mid-abdomen or pelvis and is at risk for torsion.

Splenomegaly

Splenomegaly is generally a splenic length exceeding roughly 12-13 cm. Rather than relying on a single number, combine size with shape: an enlarged spleen loses its crescentic taper, and its inferior pole extends toward the left kidney and iliac fossa. Causes are organized by mechanism:

The spleen is central to the portal hypertension assessment. Look for a dilated splenic vein (normally under about 10 mm) with sluggish or hepatofugal Doppler flow, varices at the splenic hilum, splenorenal collaterals, a recanalized paraumbilical vein, and ascites. Splenomegaly accompanied by these findings is congestive until proven otherwise. Note that an acutely enlarged spleen from mononucleosis is friable, and patients are counseled to avoid contact sports because of the risk of rupture.

Trauma

The spleen is the most commonly injured organ in blunt abdominal trauma. Findings include a subcapsular hematoma (a crescent-shaped collection that flattens the underlying parenchyma), parenchymal lacerations, and hemoperitoneum — check Morison's pouch and the pelvis for free fluid, the basis of the FAST (Focused Assessment with Sonography for Trauma) examination. Acute hemorrhage can be echogenic and nearly isoechoic with parenchyma, so a normal-appearing spleen does not exclude injury, and delayed splenic rupture may occur days after the initial event.

Infarction, Cysts, and Masses

• Splenic infarct — the classic appearance is a peripheral, wedge-shaped, hypoechoic defect with its base on the capsule and its apex pointing toward the hilum; causes include emboli (endocarditis, atrial fibrillation) and sickle cell disease. There is no internal Doppler flow. In sickle cell disease, repeated infarction over years eventually produces a small, shrunken, echogenic spleen — so-called autosplenectomy.
• Cysts — true congenital cysts have an epithelial lining; acquired pseudocysts (usually post-traumatic, and the most common splenic cyst) do not. Both are anechoic with posterior enhancement. In endemic regions a hydatid (echinococcal) cyst may contain daughter cysts.
• Masses — cavernous hemangioma is the most common benign primary neoplasm (echogenic, similar in appearance to liver hemangiomas); lymphoma is the most common malignancy to involve the spleen, typically producing multiple hypoechoic target lesions with splenomegaly. Splenic metastases are rare.