Macrocytosis is a rise in the mean cell volume of the red cells
above the normal range (in adults 80-95 fl (femtolitres)).
It is
detected with a blood count, in which the mean cell volume, as
well as other red cell indices, is measured.
The mean cell
volume is lower in children than in adults, with a normal mean
of 70 fl at age 1 year, rising by about 1 fl each year until it
reaches adult volumes at puberty.
The causes of macrocytosis fall into two groups:
(a) deficiency of vitamin B12 (cobalamin) or folate (or rarely
abnormalities of their metabolism) in which the bone marrow
is megaloblastic, and (b) other causes, in which the bone
marrow is usually normoblastic.
The reader is then taken through the
steps to diagnose the cause of macrocytosis, and subsequently
to manage it.
Deficiency of vitamin B12 or folate
Vitamin B12 deficiency
The body’s requirement for vitamin B12 is about 1µg daily.
This is amply supplied by a normal Western diet (vitamin B12
content 10-30µg 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 is limited to 2-3µg 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 age 60; the
condition has a female:male incidence of 1.6:1.0 and is more
common in those with early greying, blue eyes, and blood
group A, and in those with a family history of the disease or of
diseases that may be associated with it for example, vitiligo,
myxoedema, Hashimoto’s disease, Addison’s disease of the
adrenal gland, and hypoparathyroidism.
Other causes of vitamin B12 deficiency are infrequent in
Britain.
Veganism 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 deficiency
The daily requirement for folate is 100-200µg, and a normal
mixed diet contains about 200-300µg.
Natural folates are
largely in the polyglutamate form, and these are absorbed
through the upper small intestine after deconjugation and
conversion to the monoglutamate 5-methyl tetrahydrofolate.
Body stores are sufficient for only about four months.
Folate deficiency may arise because of inadequate dietary intake, malabsorption (especially gluten-induced enteropathy),
or excessive use as proliferating cells 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 deficiencies
Megaloblastic anaemia
Clinical features include pallor and
jaundice.
The onset is gradual, and a severely anaemic patient
may present in 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 precursors).
Biochemically, there is an increase in
plasma of unconjugated bilirubin and serum lactic
dehydrogenase, with, in severe cases, an absence of
haptoglobins and presence in urine of haemosiderin.
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 deficiency 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
hare lip.
No clear relation exists between the incidence of these
defects and 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 methylenetetra
hydrofolate reductase.
Gonadal dysfunction
Deficiency of either vitamin B12 or
folate may cause sterility, which is reversible with appropriate
vitamin supplementation.
Epithelial cell changes
Glossitis 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 Britain 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 mean cell volume to >100 fl, typically without
anaemia or any detectable change in liver function.
The mechanism for the rise in mean cell volume is
uncertain. In liver disease the volume may rise due to excessive
lipid deposition on red cell membranes, and the rise is
particularly pronounced in liver disease caused by alcohol.
A modest rise in mean cell volume is found in severe thyroid
deficiency.
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 presence of a paraprotein (as in
myeloma) may cause a rise in mean cell volume without
macrocytes being present in the blood film.
Physiological
causes of macrocytosis are pregnancy and the neonatal period.
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.
Diagnosis
Biochemical assays
The most widely used screening tests for the deficiencies are
the serum vitamin B12 and folate assays.
A low serum
concentration implies deficiency, but a subnormal serum
concentration may occur in the absence of pronounced body
deficiency for example, in pregnancy (vitamin B12) and with
recent poor dietary intake (folate).
Red cell folate can also be used to screen for folate
deficiency; a low concentration usually implies appreciable
depletion of body folate, but the concentration also falls in
severe vitamin B12 deficiency, so it is more difficult to interpret
the significance of a low red cell than serum folate
concentration in patients with megaloblastic anaemia.
Moreover, if the patient has received a recent blood transfusion
the red cell folate concentration will partly reflect the folate
concentration of the transfused red cells.
Specialist investigations
Assays of serum homocysteine (raised in vitamin B12 or folate
deficiency) or methylmalonic acid (raised in vitamin B12
deficiency) are used in some specialised laboratories.
Serum
homocysteine levels are also raised in renal failure, with certain
drugs, e.g. corticosteroids, and increase with age and smoking.
Autoantibodies
For patients with vitamin B12 or folate deficiency it is important
to establish the underlying cause.
In pernicious anaemia,
intrinsic factor antibodies are present in plasma in 50% of
patients and in parietal cell antibodies in 90%. Antigliadin,
anti-endomysial and antireticulin antibodies are usually positive
in gluten-induced enteropathy.
Other investigations
A bone marrow examination is usually performed to confirm
megaloblastic anaemia.
It is also required for the diagnosis of
myelodysplasia, aplastic anaemia, myeloma, or other marrow
disorders associated with macrocytosis.
Radioactive vitamin B12 absorption studies for example,
Schilling test show impaired absorption of the vitamin in
pernicious anaemia; this can be corrected by giving intrinsic
factor.
In patients with an intestinal lesion, however, absorption
of vitamin B12 cannot be corrected with intrinsic factor. Human
intrinsic factor is no longer licensed for this test because of
concern about transmission of prion disease.
Endoscopy should be performed to confirm atrophic
gastritis and exclude gastric carcinoma or gastric polyps, which are two to three times more common in patients with
pernicious anaemia than in age and sex matched controls.
If folate deficiency is diagnosed, it is important to assess
dietary folate intake and to exclude gluten induced
enteropathy by tests for serum antigliadin and anti-endomysial
antibodies, endoscopy and duodenal biopsy.
The deficiency is
common in patients with diseases of increased cell turnover
who also have a poor diet.
Treatment
Vitamin B12 deficiency is treated initially by giving the patient
six injections of hydroxo-cobalamin 1 mg at intervals of about
three to four 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,
one or two a year may be sufficient, and the patient should be
advised to eat foods to which vitamin B12 has been added, such
as certain fortified breads or other foods.
Folate deficiency is treated with folic acid, usually 5 mg daily
orally for four months, which is continued only if the
underlying cause cannot be corrected.
As prophylaxis against
folate deficiency in patients with a severe haemolytic anaemia
such as sickle cell anaemia 5 mg folic acid once weekly is
probably sufficient.
Vitamin B12 deficiency 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.
