OVERVIEW
• Macrocytic red cells (MCV greater than 95fl ) may be associated
with a megaloblastic or normoblastic bone marrow
• Deficiencies of either vitamin B12 or folate lead to defective DNA
synthesis, megaloblastic changes in the bone marrow and many
other cells
• The blood count indices and blood fi lm features of B12 and folate
deficiencies are identical and specific haematinic assays are
required to differentiate between them
• Pernicious anaemia is the commonest cause of B12 deficiency in
the UK
• Folate deficiency occurs in pregnancy, prematurity, chronic
haemolysis and other high cell turnover states
• Vitamin B12 deficiency may lead to progressive neuropathy even in
the absence of anaemia.
Definition:
Macrocytosis is a rise in the mean cell volume (MCV) of red cells
above the normal range (in adults 80–95·fl ). It is detected using a
blood count, in which the MCV and other red cell indices are measured.
The MCV is lower in children than in adults, with a normal
mean of 70·fl at 1 year of age, rising by about 1·fl each year until it
reaches the adult volume at puberty.
The causes of macrocytosis fall into two groups: (i) deficiency of
vitamin B12 (cobalamin) or folate (or rarely abnormalities of their
metabolism), in which the bone marrow is megaloblastic
and (ii) other causes , in which the bone marrow is usually
normoblastic. In this chapter, the two groups are considered separately.
The steps to diagnose the cause of macrocytosis and subsequently
to manage it are then considered.
Megaloblastic bone marrow:
Megaloblastic bone marrow is exemplified by developing red blood
cells that are larger than normal, with nuclei that are more immature
than the cytoplasm. The underlying mechanism is defective DNA
synthesis.
Defects of vitamin B12 metabolism, for example, transcobalamin
II deficiency, nitrous oxide anaesthesia, or of folate metabolism (such
as methotrexate treatment), or rare inherited defects of DNA synthesis,
may all cause megaloblastic anaemia.
Causes of megaloblastic anaemia:
Diet:
• Vitamin B12 deficiency: vegan diet, poor quality diet
• Folate deficiency: poor quality diet, old age, poverty, synthetic diet
without added folic acid, goats’ milk.
Malabsorption:
• Gastric causes of vitamin B12 deficiency: pernicious anaemia,
congenital intrinsic factor deficiency or abnormality, gastrectomy
• Intestinal causes of vitamin B12 deficiency: stagnant loop, congenital
selective malabsorption, ileal resection, Crohn’s disease
• Intestinal causes of folate defi ciency: coeliac disease, tropical
sprue, jejunal resection
Increased cell turnover:
• Folate deficiency: pregnancy, prematurity, chronic haemolytic
anaemia (such as sickle cell anaemia), extensive infl ammatory and
malignant diseases
Renal loss
• Folate deficiency: congestive cardiac failure, dialysis
Drugs
• Folate deficiency: anticonvulsants, sulphasalazine.
Other causes of macrocytosis:
• Alcohol
• Myelodysplasia
• Liver disease
• Cytotoxic drugs
• Hypothyroidism
• Paraproteinaemia (such as myeloma)
• Reticulocytosis
• Pregnancy
• Aplastic anaemia
• Neonatal period
• Red cell aplasia
*These are usually associated with a normoblastic marrow.
Deficiency of vitamin B12 or folate:
Vitamin B12 deficiency:
The body’s requirement for vitamin B12 is about 1·μg daily. This is amply supplied by a normal Western diet (vitamin B12 content 10–
30·μg daily) but not by a strict vegan diet, which excludes all animal
produce (including milk, eggs and cheese). Absorption of vitamin B12
is through the ileum, facilitated by intrinsic factor, which is secreted
by the parietal cells of the stomach. Absorption by this mechanism is
limited to 2–3·μg daily.
In Britain, vitamin B12 deficiency is usually due to pernicious
anaemia, which now accounts for up to 80% of all cases of megaloblastic
anaemia. The incidence of the disease is 1:10 000 in northern
Europe and the disease occurs in all races. The underlying mechanism
is an autoimmune gastritis that results in achlorhydria and
the absence of intrinsic factor. The incidence of pernicious anaemia
peaks at 60·years of age; the condition has a female : male incidence
of 1.6 : 1.0 and is more common in those with early greying of hair,
blue eyes, blood group A and in those with a family history of
pernicious anaemia or associated diseases, for example, vitiligo, myxoedema, Hashimoto’s disease, Addison’s disease and hypoparathyroidism.
Other causes of vitamin B12 deficiency are infrequent in the UK. A
vegan lifestyle is an unusual cause of severe deficiency, as most vegetarians
and vegans include some vitamin B12 in their diet. Moreover,
unlike in pernicious anaemia, the enterohepatic circulation for vitamin
B12 is intact in vegans, so vitamin B12 stores are conserved. Gastric
resection and intestinal causes of malabsorption of vitamin B12, for
example, ileal resection or the intestinal stagnant loop syndrome, are
less common now that abdominal tuberculosis is infrequent and H2
antagonists have been introduced for treating peptic ulceration, thus
reducing the need for gastrectomy.
Folate defi ciency
The daily requirement for folate is 100–200·μg and a normal mixed
diet contains about 200–300·μg. Natural folates are largely found
in the polyglutamate form and these are absorbed through the upper
small intestine after deconjugation and conversion to the monoglutamate
5-methyltetrahydrofolate.
Body stores are suffi cient for only about 4 months. Folate defi -
ciency may arise because of inadequate dietary intake, malabsorption
(especially coeliac disease, or excessive use caused by
proliferating cells, which degrade folate. Deficiency in pregnancy may be due partly to inadequate diet, partly to transfer of folate to
the fetus and partly to increased folate degradation.
Consequences of vitamin B12 or folate deficiency:
Megaloblastic anaemia:
Clinical features include pallor and jaundice. The onset is gradual, and
a severely anaemic patient may present with congestive heart failure or
only when an infection supervenes. The blood film shows oval macrocytes
and hypersegmented neutrophil nuclei (with six or more lobes).
In severe cases, the white cell count and platelet count also
fall (pancytopenia). The bone marrow shows characteristic megaloblastic
erythroblasts and giant metamyelocytes (granulocyte precur-sors). Biochemically, there is an increase of unconjugated bilirubin
and serum lactic dehydrogenase in the plasma, with, in severe cases, an
absence of haptoglobins and presence of haemosiderin in the urine.
These changes, including jaundice, are due to increased destruction of
red cell precursors in the marrow (ineffective erythropoiesis).
Vitamin B12 neuropathy
A minority of patients with vitamin B12 defi ciency develop a neuropathy
due to symmetrical damage to the peripheral nerves and
posterior and lateral columns of the spinal cord, the legs being more
affected than the arms. Psychiatric abnormalities and visual disturbance
may also occur. Men are more commonly affected than
women. The neuropathy may occur in the absence of anaemia. Psychiatric
changes and, at most, a mild peripheral neuropathy may be
ascribed to folate deficiency.
Neural tube defects
Folic acid supplements in pregnancy have been shown to reduce the
incidence of neural tube defects (spina bifida, encephalocoele and
anencephaly) in the fetus, and may also reduce the incidence of cleft
palate and harelip. No clear relation exists between the
incidence of these defects and any folate deficiency in the mother,
although the lower the maternal red cell folate (and serum vitamin
B12) concentrations, even within the normal range, the more likely
neural tube defects are to occur in the fetus. An underlying mechanism
in a minority of cases is a genetic defect in folate metabolism, a
mutation in the enzyme 5,10-methylene-tetrahydrofolate reductase.
An autoantibody to folate receptors has been detected in pregnancy
in some women who have babies with neural tube defects.
Gonadal dysfunction
Deficiency of either vitamin B12 or folate may cause sterility, which is
reversible with appropriate vitamin supplementation.
Epithelial cell changes
Glossitis may occur, and other epithelial surfaces may show cytological
abnormalities.
Cardiovascular disease
Raised serum homocysteine concentrations have been associated
with arterial obstruction (myocardial infarct, peripheral vascular
disease or stroke) and venous thrombosis. Trials are under way to
determine whether folic acid supplementation reduces the incidence
of these vascular diseases.
Other causes of macrocytosis
The most common cause of macrocytosis in the UK is alcohol. Fairly
small quantities of alcohol, for example, two gin and tonics or half a
bottle of wine a day, especially in women, may cause a rise of MCV to
100·fl , typically without anaemia or any detectable change in liver
function.
The mechanism for the rise in MCV is uncertain. In liver disease,
the red cell volume may rise as a result of excessive lipid deposition
on red cell membranes, and the rise is particularly pronounced in
liver disease caused by alcohol. A modest rise in MCV is found in
severe thyroid deficiency.
Physiological causes of macrocytosis are pregnancy and the neonatal
period. In other causes of macrocytosis, other haematological
abnormalities are usually present; in myelodysplasia (a frequent
cause of macrocytosis in elderly people), there are usually quantitative
or qualitative changes in the white cells and platelets in the
blood. In aplastic anaemia, pancytopenia is present; pure red cell
aplasia may also cause macrocytosis. Changes in plasma proteins,
for example, presence of a paraprotein (as in myeloma), may cause
a rise in MCV without macrocytes being present in the blood fi lm.
Drugs that affect DNA synthesis, for example, hydroxyurea and azathioprine,
can cause macrocytosis with or without megaloblastic
changes. Finally, a rare, benign familial type of macrocytosis has
been described.
Preventing folate deficiency in pregnancy:
• As prophylaxis against folate deficiency in pregnancy, daily doses
of folic acid 400·μg are usual
• Larger doses are not recommended as they could theoretically
mask megaloblastic anaemia due to vitamin B12 defi ciency and
thus allow B12 neuropathy to develop
• As neural tube defects occur by the 28th day of pregnancy, it is
advisable for a woman’s daily folate intake to be increased by
400··μg/day at the time of conception
• The US Food and Drugs Administration announced in 1996 that
specifi ed grain products (including most enriched breads, fl ours,
cornmeal, rice, noodles and macaroni) will be required to be
fortifi ed with folic acid to levels ranging from 0.43·mg to 1.5·mg
per pound (453·g) of product. Fortifi cation of fl our with folic acid
is currently under discussion in the UK
• For mothers who have already had an infant with a neural tube
defect, larger doses of folic acid, for example, 5·mg daily, are
recommended before and during subsequent pregnancy.
Investigations that may be needed in patients with
macrocytosis
• Serum vitamin B12 assay
• Serum and red cell folate assays
• Liver and thyroid function
• Reticulocyte count
• Serum protein electrophoresis
• For vitamin B12 defi ciency: serum parietal cell and intrinsic factor
antibodies, radioactive vitamin B12 absorption with and without
intrinsic factor (Schilling’s test), possibly serum gastrin concentration
• For folate defi ciency: antiendomysial and antitransglutaminase
antibodies
• Consider bone marrow examination for megaloblastic changes
suggestive of vitamin B12 or folate defi ciency, or alternative diagnoses,
for example, myelodysplasia, aplastic anaemia, myeloma
• Endoscopy: gastric biopsy (vitamin B12 defi ciency); duodenal biopsy
(folate deficiency).
Treatment
Vitamin B12 defi ciency is treated initially by giving the patient six
injections of hydroxocobalamin 1·mg at intervals of about 3–4·days,
followed by four such injections a year for life. For patients undergoing
total gastrectomy or ileal resection, it is sensible to start the
maintenance injections from the time of operation. For vegans, less
frequent injections, for example, 1–2 per year, may be suffi cient, and
the patient should be advised to eat foods to which vitamin B12 has
been added, such as certain fortifi ed breads or other foods.
Folate defi ciency is treated with folic acid, usually 5·mg daily orally
for 4·months, which is continued only if the underlying cause cannot be corrected. As prophylaxis against folate defi ciency in patients
with a severe haemolytic anaemia, such as sickle cell anaemia, 5·mg
folic acid once weekly is probably suffi cient. Vitamin B12 defi ciency
must be excluded in all patients starting folic acid treatment at these
doses, as such treatment may correct the anaemia in vitamin B12
deficiency but allow neurological disease to develop.
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