This is a refractory anaemia defined by the presence of many pathological ring sideroblasts in the bone marrow (Fig. 3.14). These are abnormal erythroblasts containing numerous iron granules arranged in a ring or collar around the nucleus instead of the few randomly distributed iron granules seen when normal erythroblasts are stained for iron. There is also usually erythroid hyperplasia with ineffective erythropoiesis. Sideroblastic anaemia is diagnosed when 15% or more of marrow erythroblasts are ring sideroblasts. They can be found at lower numbers in a variety of haematological conditions.
Sideroblastic anaemia is classified into different types (Table 3.8) and the common link is a defect in haem synthesis. In the hereditary forms the anaemia is usually characterized by a markedly hypochromic and microcytic blood picture. The most common mutations are in the ALA‐S gene which is on the X chromosome. Pyridoxal‐6‐phosphate is a coenzyme for ALA‐S. Other rare types include an X‐linked disease with spinocerebellar degeneration and ataxia, mitochondrial defects (e.g. Pearson’s syndrome when there is also pancreatic insufficiency), thiamine‐responsive and other autosomal defects. The much more common form is refractory anaemia with ring sideroblasts, which is a subtype of myelodysplasia (see Chapter 16). Acquired reversible forms may be due to alcohol, lead and drugs, e.g. isoniazid.