Sideroblastic anemia

1. Overview & Pathophysiology

Sideroblastic anemia is a heterogeneous group of disorders in which the bone marrow cannot incorporate iron into heme. The result is a triad of: (1) defective heme synthesis inside erythroblasts, (2) mitochondrial iron accumulation encircling the nucleus → ringed sideroblasts, and (3) ineffective erythropoiesis — precursors die in the marrow, so anemia develops despite abundant iron.

• Anemia type: usually microcytic, hypochromic (can be normocytic or dimorphic, especially with alcohol/MDS).
• Paradox: iron utilization fails, but absorption continues → progressive systemic iron overload (hemochromatosis-like picture).
• Hallmark labs: ↑ serum iron, ↑ ferritin, ↑ transferrin saturation, ↓ TIBC — the mirror image of iron deficiency.
• Diagnostic finding: ringed sideroblasts on bone marrow Prussian blue stain.

The heme synthesis pathway

Every cause of sideroblastic anemia maps onto one step of heme synthesis:

1. Glycine + Succinyl-CoA → δ-ALA by ALA synthase (mitochondria; rate-limiting; needs vitamin B6 as cofactor). Impaired by B6 deficiency, isoniazid, and the X-linked ALAS2 defect.
2. δ-ALA → Porphobilinogen by ALA dehydratase (cytoplasm). Inhibited by lead.
3. Several cytoplasmic steps → Protoporphyrin IX.
4. Back in mitochondria: Protoporphyrin IX + Fe²⁺ → Heme by ferrochelatase. Inhibited by lead.

When any step fails, iron is still shuttled into mitochondria but cannot be loaded onto protoporphyrin → it piles up around the nucleus (the ringed sideroblast). Defective precursors undergo intramedullary death (ineffective erythropoiesis), hepcidin is suppressed, and intestinal iron absorption rises — driving the iron overload.

2. Etiology & Clinical Presentation

Group the causes by the "Rule of 3": hereditary, acquired–reversible (drugs/toxins/nutrition), and acquired–clonal (bone marrow disease). For exams, anchor on the high-yield quartet: Alcohol, Lead, Isoniazid, and MDS — alcohol is the commonest acquired cause overall, while MDS dominates in the elderly.

The isoniazid mechanism is a recurring exam target: INH inhibits pyridoxine phosphokinase, blocking conversion of B6 to its active cofactor needed by ALA synthase.

Clinical presentation

• General anemia: fatigue, weakness, pallor, exertional dyspnea, palpitations, dizziness.
• Iron overload (chronic cases — looks like secondary hemochromatosis): bronze skin pigmentation, hepatomegaly/liver dysfunction, diabetes mellitus, hypogonadism, cardiomyopathy and arrhythmias.

Clues to the specific cause

• Lead: abdominal colic, constipation, peripheral neuropathy (wrist/foot drop in adults), encephalopathy and developmental delay in children, gingival lead line (Burton lines); smear shows basophilic stippling.
• Alcohol: coexisting macrocytosis (dimorphic smear) and signs of liver disease.
• Isoniazid: patient on anti-TB therapy without B6 supplementation.
• MDS: elderly patient, frequently with cytopenias/pancytopenia.
• Hereditary (X-linked): young male, family history, possible splenomegaly.

The lead smear finding is highly testable:

3. Diagnostic Approach

Lead with the iron-study signature — it is the single fastest discriminator and the most heavily tested fact:

• Serum iron: ↑ high
• Ferritin: ↑ high
• TIBC / transferrin: ↓ low or normal
• Transferrin saturation: ↑↑ markedly high (often > 50%)
• Reticulocytes: inappropriately low (ineffective erythropoiesis)

The four microcytic anemias side-by-side

Any microcytic anemia (MCV < 80 fL) is worked through the TICS differential, and iron studies separate them at a glance:

See the iron studies in microcytic anemia comparison for the full ferritin / TIBC / saturation breakdown, and the IDA vs α-thalassemia vs β-thalassemia minor table for the RBC count and electrophoresis features that separate the normal-iron microcytic anemias.

Smear and bone marrow findings

• Peripheral smear: hypochromic, microcytic RBCs, often a dimorphic population; basophilic stippling (lead); Pappenheimer bodies (iron-containing granules).
• Bone marrow (Prussian blue) — diagnostic: ringed sideroblasts ≥ 15%, i.e., ≥ 5 iron granules encircling ≥ one-third of the nuclear circumference.
• If lead suspected: ↑ blood lead level, ↑ zinc protoporphyrin, ↑ urinary δ-ALA.

The marrow hallmark — iron studded around the nucleus on Prussian blue stain:

On the peripheral smear, the corresponding iron inclusions are

4. Management & Complications

Management has three pillars: treat the cause, manage iron overload, and supportive care — with complications driven mainly by chronic iron loading.

1. Treat the underlying cause

• Hereditary (X-linked, ALAS2): pyridoxine (vitamin B6) 50–200 mg PO daily, often lifelong; many respond partially.
• Alcohol-induced: stop alcohol → anemia usually reverses over weeks.
• Isoniazid-induced: add/increase pyridoxine 25–50 mg PO daily; always co-prescribe B6 with INH to prevent it.
• Lead poisoning: remove exposure + chelation — DMSA (succimer) PO for moderate levels; CaNa₂-EDTA ± dimercaprol (BAL) for severe disease/encephalopathy.
• Copper deficiency: copper replacement and stop excess zinc.
• MDS-RS: supportive transfusions; luspatercept, erythropoiesis-stimulating agents, or hypomethylating agents (azacitidine); allogeneic stem cell transplant in selected younger patients.

For the marrow features and classification of the clonal form, refer to the clinical features of myelodysplastic syndrome, which covers MDS-RS and its SF3B1 association.

2. Manage iron overload

• Iron chelation when ferritin is markedly elevated or after repeated transfusions: deferasirox 20 mg/kg PO daily, deferiprone, or deferoxamine (SC/IV infusion).
• Never give iron supplements — the defect is iron utilization, not iron lack; supplementation only accelerates overload.

3. Supportive care

• Transfuse for symptomatic anemia, but sparingly to limit iron loading.
• Folic acid supplementation (high marrow turnover).
• Monitor end-organ damage; treat infections and vaccinate.

Complications

• Secondary iron overload (hemosiderosis / hemochromatosis-like): liver — hepatomegaly, cirrhosis, hepatocellular carcinoma; heart — cardiomyopathy, heart failure, arrhythmias; endocrine — diabetes mellitus, hypogonadism, hypothyroidism; skin — bronze pigmentation.
• Transfusion-related: alloimmunization, infection, additional iron load.
• Progression to AML in MDS-associated disease.
• Lead-specific: encephalopathy, peripheral neuropathy, nephropathy, and developmental delay in children.

5. High-Yield Exam Summary

The must-know facts, distilled for rapid review: