Harvey's Garden

Spleen

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Microscopic anatomy

Red pulp

Red pulp is composed of splenic cords and sinuses (sinusoids).

  • Splenic cords: a reticular meshwork filled with blood (open circulation system) that filters the blood from damaged erythrocytes
  • Spleen sinusoids: long vessels with a fenestrated ring-like (“barrel hoop”) basement membrane that prevent old or malformed RBCs/platelets from reentering venous circulation
    • Open circulation: Blood empties from sheathed capillaries into the splenic cords and then enters the sinusoids through slits in the vessel wall.
    • Closed circulation: Blood empties from sheathed capillaries of the red pulp directly into the sinusoids.
  • Macrophages: found in the cords and around the sinusoids
    • Phagocytosis of damaged RBCs/platelets that do not reenter circulation
    • Capture of viruses and opsonized pathogens that enter the red pulp
  • Blood flow: splenic artery → arterioles → red pulp (cords → sinusoids) → venules → splenic vein → portal circulation

White pulp

  • Periarteriolar lymphatic sheath
    • Surrounds the arterioles
    • Dense lymphoid tissue containing T lymphocytes
  • Splenic follicles
    • Main component of white pulp
    • Close to periarteriolar lymphatic sheath
    • Contain B lymphocytes
  • Splenic marginal zone
    • Located between the red pulp and white pulp
    • Contains antigen-presenting cells (APCs): macrophages and specialized B cells (marginal zone B cells)

Spleen Manifestations in Disease

The spleen’s primary roles are filtering blood and mounting immune responses. Its dysfunction or enlargement is a key sign of various systemic diseases. Spleen manifestations can be broadly categorized into Splenomegaly (enlargement) and Hyposplenism/Asplenia (decreased or absent function).

Splenomegaly (Enlarged Spleen)

  • Etiologies of Splenomegaly
    • Hematologic & Myeloproliferative Disorders
      • Primary Myelofibrosis (PMF): Clonal megakaryocytes (JAK2), marrow fibrosis → massive EMH, dacrocytes (“teardrop” cells), dry tap.
      • Hairy Cell Leukemia: Older males, BRAF V600E → massive splenomegaly + pancytopenia, dry tap, TRAP (+).
      • Chronic Myeloid Leukemia (CML): BCR-ABL1 / t(9;22) → massive splenomegaly, basophilia, ↓ LAP score.
      • Chronic Lymphocytic Leukemia (CLL): Elderly, asymptomatic absolute lymphocytosis, smudge cells, generalized LAD + HSM.
      • Polycythemia Vera (PV): JAK2 mutation, hyperviscosity, aquagenic pruritus, facial plethora.
      • Hodgkin & Non-Hodgkin Lymphoma: Direct tumor infiltration, “B symptoms” (fevers, night sweats, weight loss), painless LAD.
    • Congestive & Vascular
      • Cirrhosis / Portal HTN: Most common overall; red pulp pooling → congestive splenomegaly + secondary hypersplenism (thrombocytopenia).
      • Splenic Vein Thrombosis: Post-pancreatitis or pancreatic CA → isolated gastric varices + splenomegaly (normal liver).
      • Budd-Chiari Syndrome: Hepatic vein/IVC thrombosis → tender hepatomegaly, ascites, secondary splenomegaly.
    • Hemolytic Anemias (RBC Clearance)
      • Hereditary Spherocytosis (HS): Ankyrin/spectrin defect → spherocytes cleared in spleen; triad of anemia + jaundice + splenomegaly; (+) EMA test.
      • Warm AIHA: IgG-mediated splenic RBC clearance; spherocytes, (+) Direct Coombs.
      • Thalassemia Intermedia / Major: Severe ineffective erythropoiesis + chronic hemolysis → EMH, “crew-cut” skull X-ray, HSM.
    • Infections
      • EBV / CMV (Mononucleosis): Reactive lymphoid hyperplasia; high risk of splenic rupture (avoid sports ≥3–4 wks).
      • Infective Endocarditis (IE): Immune clearance or septic emboli → splenic infarction/abscess (fever + LUQ pain).
      • Visceral Leishmaniasis (Kala-azar): Sandfly (Leishmania) → massive splenomegaly + pancytopenia + spiking fevers.
      • Malaria: Intravascular parasite cycling → hyperreactive malarial splenomegaly.
    • Storage, Infiltrative & Autoimmune
      • Gaucher Disease: β-glucocerebrosidase deficiency → massive splenomegaly, severe bone crises/AVN, “wrinkled tissue paper” macrophages.
      • Niemann-Pick Disease: Sphingomyelinase deficiency → neurodegeneration, cherry-red macular spot + prominent HSM.
      • Felty Syndrome: Long-standing Rheumatoid Arthritis (RA) + Neutropenia + Splenomegaly.
      • Systemic Amyloidosis / Sarcoidosis: Splenic parenchyma infiltration by amyloid fibrils (Congo red +) or non-caseating granulomas.
  • High-Yield “Massive Splenomegaly” (>8 cm / Crosses Midline)
    • Primary Myelofibrosis
    • Hairy Cell Leukemia
    • Chronic Myeloid Leukemia (CML)
    • Gaucher Disease
    • Visceral Leishmaniasis (Kala-azar)
  • Conditions WITHOUT Splenomegaly (High-Yield USMLE Traps)
    • Immune Thrombocytopenia (ITP)
      • Isolated severe thrombocytopenia (<30k) + mucosal bleeding.
      • Autoantibody clearance occurs in spleen, but spleen is NOT enlarged (if splenomegaly is present, suspect CLL, SLE, or cirrhosis instead).
    • Thrombotic Thrombocytopenic Purpura (TTP)
      • Pentad: MAHA (schistocytes), Thrombocytopenia, Fever, AKI, Neurologic deficits.
      • Microthrombi consume platelets in systemic microvasculature; normal spleen.
    • Aplastic Anemia
      • Severe pancytopenia + reticulocytopenia from marrow failure/autoimmune stem cell destruction.
      • BM biopsy shows fatty hypocellular marrow; no EMH or splenomegaly (differentiates from PMF/leukemia).
    • Sickle Cell Disease (Adults)
      • Recurrent microvascular sickling and vaso-occlusion during childhood cause autosplenectomy (fibrotic, calcified nub by age ~6). c
      • Spleen is ABSENT in adults (Howell-Jolly bodies on smear; contrast with young children who can have acute splenic sequestration crises).
    • Multiple Myeloma (MM)
      • CRAB criteria (Hypercalcemia, Renal failure, Anemia, Bone lytic lesions) + monoclonal M-spike (IgG/IgA).
      • Plasma cell expansion is restricted to BM; NO splenomegaly or LAD (presence of HSM/LAD points to Waldenström Macroglobulinemia instead).
    • Hemolytic Uremic Syndrome (HUS)
      • MAHA + Thrombocytopenia + AKI following E. coli O157:H7 enteritis.
      • Platelet consumption is localized to renal microthrombi; spleen is normal.
    • Tay-Sachs Disease
      • Hexosaminidase A deficiency → developmental regression, hyperreflexia, cherry-red macular spot.
      • NO hepatosplenomegaly (the single most critical clinical differentiator from Niemann-Pick, which presents identically with HSM).
    • Disseminated Intravascular Coagulation (DIC)
      • Widespread microvascular thrombosis + consumptive coagulopathy (↑ PT, ↑ PTT, ↓ fibrinogen, ↑ D-dimer).
      • Systemic intravascular consumption; spleen is normal.

Hyposplenism & Asplenia

This refers to decreased or absent splenic function, leading to impaired blood filtering and immune response.

Pathophysiology/Etiology:

  • Splenectomy: Surgical removal (e.g., for trauma, ITP, hereditary spherocytosis).
  • Autosplenectomy: Repetitive infarction and fibrosis, classically seen in Sickle Cell Disease in adults.
  • Congenital Asplenia: Rare developmental failure.
  • Functional Hyposplenism: Spleen is present but dysfunctional (e.g., celiac disease, SLE, inflammatory bowel disease).

Diagnosis (Hallmark Peripheral Smear Findings):

  • Howell-Jolly bodies: Basophilic nuclear remnants in RBCs (normally removed by spleen). Their presence is a key sign of hyposplenism.
  • Target cells (Codocytes): Due to altered RBC membrane.
  • Thrombocytosis: Loss of splenic sequestration of platelets.
  • Acanthocytes (Spur cells): Spiky RBCs.

Complications & Management:

  • Overwhelming Post-Splenectomy Infection (OPSI): High risk of fulminant sepsis from encapsulated organisms due to loss of opsonizing IgM production and phagocytosis. t
    • Key Pathogens: Streptococcus pneumoniae (most common), Haemophilus influenzae type b, Neisseria meningitidis.
    • Mnemonic: SHiN
  • Prevention:
    • Vaccinations: Administer vaccines against pneumococcus, meningococcus, and H. influenzae type b, ideally before elective splenectomy or as soon as possible after.
    • Prophylactic Antibiotics: Often prescribed, especially for children and the immunocompromised (e.g., penicillin).
    • Patient Education: Advise patients to seek immediate medical care for fevers.
DiseaseSpleen FindingMechanism
Sickle Cell DiseaseSplenomegaly (in children) -> Autosplenectomy (in adults)Vaso-occlusion from sickled RBCs causes initial splenic sequestration crisis, followed by recurrent infarctions leading to a shrunken, fibrotic spleen.
Hereditary SpherocytosisSplenomegaly”Work hypertrophy” from clearing spherocytic RBCs. Splenectomy is curative.
Infectious Mononucleosis (EBV)SplenomegalyLymphoid hyperplasia in response to viral infection. High risk of rupture.
Portal HypertensionCongestive SplenomegalyIncreased pressure in the splenic vein obstructs outflow.
MyelofibrosisMassive SplenomegalyExtramedullary hematopoiesis (blood cell production shifts to spleen as bone marrow fails).
Hairy Cell LeukemiaMassive SplenomegalyInfiltration of the red pulp by neoplastic B-cells.
ITPNormal Spleen Size or Mild SplenomegalySpleen is the site of platelet destruction, but does not typically cause significant enlargement.

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