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PA19.4 | Splenomegaly — Causes & Differentiation — Part 1
CLINICAL SCENARIO
A 34-year-old man from Bihar presents with a massively enlarged spleen reaching the right iliac fossa, mild fever for six months, and profound anaemia. His blood film shows no malaria parasites. His colleague — also from Bihar — had an identical presentation two years ago and responded to sodium stibogluconate.
Massive splenomegaly demands a short, specific differential. By the end of this session you will not only name that list but explain why each disease produces a spleen big enough to fill half the abdomen.
WHY THIS MATTERS
PA19.4 is tested in every university written paper as a structured long-answer question: "Classify the causes of splenomegaly with examples." It also anchors the clinical chapters on portal hypertension, haematological malignancies, and tropical infections. Surgeons and physicians both ask about hypersplenism and splenectomy consequences in post-graduate entrance examinations. There is no shortcut — the mechanistic framework is the shortcut.
RECALL
Before proceeding, activate what you already know:
- Cluster H5 — thalassaemia major and hereditary spherocytosis both cause haemolytic anaemia. Where does the excess haemolysis occur?
- Cluster H7 — CML is the prototypic myeloproliferative neoplasm. What is the key chromosomal abnormality?
- Cluster H8 — lymphomas infiltrate lymphoid tissue. Can they infiltrate the spleen?
- From Year-1 Anatomy: the spleen lies in the left hypochondrium, between ribs 9–11. A spleen you can palpate has at least doubled in size.
Hold these in mind — they will slot into the framework as you build it.
Normal Spleen: Structure and Functions
The adult spleen weighs 150–200 g. Its parenchyma has two compartments:
Red pulp (~75%) — sinuses lined by littoral cells (specialised macrophages) and cords of Billroth. Functions:
• Filtration/culling: removes aged, rigid, or abnormal RBCs that cannot deform through 3-µm slit pores between sinusoids and cords.
• Pitting: excises intra-erythrocytic inclusions (Howell-Jolly bodies, Heinz bodies, parasites) while returning the cell to circulation.
• Blood reservoir: stores ~30–40 mL RBCs and a platelet reservoir (~1/3 of total platelets).
White pulp (~25%) — lymphoid tissue arranged around the periarteriolar lymphoid sheath (PALS) of T cells, with follicles (B cells) forming germinal centres on stimulation.
• Immune surveillance: IgM production (especially against encapsulated bacteria); opsonisation.
Haematopoietic function: primary haematopoietic organ in the fetus (from week 5 to ~7 months gestation). In adults, extramedullary haematopoiesis resumes in the spleen only when marrow is replaced or severely damaged (e.g., myelofibrosis).
Normal Spleen Histology and Splenomegaly Classification
Splenomegaly and Massive Splenomegaly — Definitions
Splenomegaly is defined clinically as a palpable spleen (implying weight >400 g or extension below the left costal margin) and pathologically as spleen weight >250 g.
Massive splenomegaly — no universally agreed threshold, but conventionally: spleen extending to or beyond the umbilicus, or weight >1 kg. In practice, a spleen reaching the right iliac fossa weighs 3–5 kg.
Why size matters diagnostically: the degree of enlargement correlates loosely with the mechanism:
| Degree | Examples |
|---|---|
| Mild (250–500 g) | Acute infections, congestive heart failure, early portal hypertension |
| Moderate (500–1,000 g) | Lymphomas, chronic hepatic disease, haemolytic anaemia |
| Massive (>1,000 g) | CML, myelofibrosis, kala-azar, malaria, Gaucher, thalassaemia major |
Mechanistic Framework: Five Causes of Splenomegaly
Think of every cause of splenomegaly as an exaggeration of one of the spleen's normal functions. Five mechanisms cover the syllabus:
- Congestive — raised splenic venous pressure → passive congestion → sinusoidal dilatation.
- Infective/Inflammatory — antigenic stimulation → white pulp hyperplasia; macrophage activation → red pulp expansion.
- Haematological — haemolytic stress (work hypertrophy of filtration) OR infiltration by malignant haematopoietic cells.
- Infiltrative/Storage — accumulation of metabolic substrates or protein deposits distends macrophages.
- Immune — autoimmune activation (complement-tagged cells) → macrophage hyperplasia.
Five Mechanistic Categories of Splenomegaly