Sideroblastic anemias are a heterogeneous group of disorders with two common features: ring sideroblasts and affected heme biosynthesis. In this article, the most important forms of sideroblastic anemia are presented as well as their etiology, diagnosis and therapy.
Ring Sideroblast smear

Image: "Ring sideroblasts: abnormal deposition of iron in the mitochondria of red cell precursors forming a ring around the nucleus." by Paulo Henrique Orlandi Mourao. License: CC BY-SA 3.0

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Definition of Sideroblastic Anemia

A microcytic anemia

Sideroblastic anemia is defined as a microcytic anemia in which the bone marrow produces sideroblasts, which are ring shaped blood cells versus normal round shaped red blood cells. This is due to the inability of the body to place iron properly into hemoglobin. The shape can be described as abnormal nucleated erythroblasts, with iron saturating the mitochondria. This forms a ring around the nucleus giving the classic shape.

Epidemiology of Sideroblastic Anemia

A study in the USA, in which 25 children with anemia underwent a bone marrow biopsie, showed a prevalence of sideroblasts of 8 %.

In the United Kingdom, 29 % of healthy men and 19 % of healthy woman showed siderotic granules (not ring sideroblasts) present in their bone marrow.

Congenital X-linked sideroblastic anemia usually manifests in childhood, but can remain even undiagnosed for a long time and then only present late in the 4th – 8th decade of life. The primary acquired sideroblastic anemia occurs with a median age of 75 years.

Etiology of Sideroblastic Anemia

Sideroblastic anemia due to congenital defects or acquired causes

Sideroblastic anemia can be congenital or acquired as shown in table 1. Congenital defects are due to defective porphyrin synthesis due to abnormal Aminolevulinic acid synthetase (ALAS). Most commonly seen X-linked in individuals with ALAS abnormalities. This enzyme is responsible for the conversion of aminolevulinic acid to produce the vital end product, heme, as explained in the following:

ALAS ⇒ converts succinyl CoA with Vit B6 → Aminolevulinic acid (ALA) ⇒ Aminolevulinic acid dehydrogenase converts ALA → porphobilinogen → protoporphyrin → attaches to iron → heme

Acquired causes include alcoholism, drug induced and vitamin deficiencies. Alcohol tends to be the most common and detrimental to the mitochondrial process. Alcoholics are also generally afflicted with liver disease, folate deficiency, blood loss, hemolysis, and hypersplenism leading to RBC loss and sequestering.

Lead poisoning inhibits the conversion of protoporphyrins necessary for heme synthesis. Pyridoxine (vit B6) deficiency which is a necessary cofactor for protoporphyrin synthesis is another cause. This deficiency is most commonly seen in patients treated with isoniazid and chloramphenicol.

Copper deficiency is another cause seen in those after prolonged ingestion of zinc. Patients who received bariatric surgery or those who are malnutritioned are also at risk for copper deficiency induced sideroblastic anemia.

Lastly, myelodysplastic syndrome may result in sideroblastic anemia. The bone marrow shows abnormal erythroid hyperplasia with poorly hemoglobinized cytoplasm also found in folate deficiency.

Acquired causes Congenital causes
Lead poisoning
Myelodysplastic syndrome
Medications (isoniazid for TB)
Copper deficiency
Zinc poisoning
Mitochondrial disorders

Table 1.

Symptoms of Sideroblastic Anemia

Signs of sideroblastic anemia

Symptoms of anemia

Image 1: “Main symptoms that may appear in anemia.” by Mikael Häggström. License: Public Domain

A variety of signs, symptoms and laboratory findings represent the diagnostic features pointing to sideroblastic anemia. Signs and symptoms manifest similar to those with general anemia and vary from growth retardation in children, to adults with hypothermia, dental lead lines, photosensitivity, ataxia, fatigue and muscle weakness (Image 1). Those afflicted due to Pyridoxine (vit B6) deficiency commonly complain of peripheral neuropathy. Whereas those with lead poisoning often present with abdominal pain, peripheral neuropathy, encephalopathy, mental and growth retardation especially in children.

Diagnosis of Sideroblastic Anemia

Iron overload in sideroblastic anemia

Diagnosis can be made with the fulfillment of the following criteria. Laboratory findings most commonly show excessive iron overload. A complete blood count is first done, usually showing the presence of microcytosis with an MCV < 80. Variations in RBC size and shape is reflected with an increased RDW (Red blood cell distribution width). Hemoglobin levels are usually below 7 g/dl. Iron studies show a serum iron level of high, elevated ferritin levels, normal or decreased tibc (total iron binding capacity), and elevated transferrin saturation. Laboratory prussian blue staining show hypochromic RBC with basophilic granules staining positive for iron, giving the classic ring sideroblast shape (Image 2). It can also be seen in peripheral blood with those having severe anemia.


Image 2: “Sideroblast, as seen in sideroblastic anemia.” by Mikael Häggström,Original by Paulo Henrique Orlandi Mourao. License: CC BY-SA 3.0

MRI can also show iron overload in organs such as the heart, liver or bone marrow. Bone marrow exams offer diagnostic imagery. The marrow shows erythroid hyperplasia with poor hemoglobin concentration with the presence of ring sideroblasts. The amount of iron seen in the bone marrow is heightened due to ineffective intramedullary hemolysis.

Therapy and Management of Sideroblastic Anemia

How to treat sideroblastic anemia

Management differs on the level of severity and syndromic forms of sideroblastic anemia. Therapy aims to prevent organ damage from subsequent iron overload as well as control and prevent symptoms of anemia. If anemia is severe, transfusion may be required. In the case of iron overload, phlebotomy, given no contraindications such as CHF exist, is used to prevent iron accumulation. Those who cannot undergo phlebotomy may be given trials of iron chelation therapy.

Vitamin supplementation with Vitamin B6, folic acid and thiamine is also given to reduce neuropathic effects, aid in normal erythropoiesis, as well as correct drug induced sideroblastic anemia seen in patients and alcoholics. Alcohol avoidance should also be maintained. Most drug and alcohol induced sideroblastic anemias are corrected upon removal of offending agent. Splenectomy is not recommended for these patients due thrombotic risks and complications.

Review Questions

The answers are below the references.

1. A 22 year old patient comes into the clinic complaining of fatigue, inability to concentrate and pale skin. Patient is a strict vegan and was diagnosed with a “congenital” blood disorder she is unsure of. Lab work shoes MCV of 70 and hemoglobin of 6. Which of the following tests would correctly identify the anemia she is having?

  1. Biopsy of the bone marrow
  2. Physical exam
  3. CT of the head
  4. CBC

2. A 43 y/o male presents for a follow up after complaining of poor vision. Lab results show iron buildup in his eyes and liver. CBC shows anemia and blood smear shows which of the following?

  1. Irregular shaped and unequal sized red blood cells
  2. Erythroid hyperplasia with poor hemoglobin concentration
  3. Mostly small red blood cells
  4. Normal red blood cells
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