Congenital abnormalities which includes:
1.
Fanconi’s Anaemia: This is an autosomal recessive
preleukaemic condition that often presents as thromobocytopenia with skeletal
or genitourinary abnormalities.
2.
Giant
granular platelets in peripheral blood film are seen in Bernard –soulier
syndrome or may –Hegglin anomaly. Megakaryocytic thrombocytopenia presents with
severe neonatal thrombocytopenia (platelet count < 2 x109/L),
though this often correct itself after the first year of
life.
3.
Wiskolt-aldrich
syndrome:
is an x-linked disorder with a triad of thrombocytopenia, eczema and
immunodeficiency. The platelet count is usually 20-100x109/L and the platelets
are functionally abnormal.
May
Hegglin anomaly and variants of Alport’s syndrome are both characterized by
giant platelets.
Diseases of the platelet storage pool are
deficiencies in either in the α or dense granules causing poor secondary platelet
aggregation.
Acquired Abnormalities
Decrease production of platelets due to
suppression of failure of the bone marrow is the commonest cause of the
thrombocytopenia. In aplastic anaemia, leukaemia and marrow infiltration and after
chemotherapy thrombocytopenia is usually associated with a failure of red and
white cell production but may be an isolated finding secondary to drug toxicity
(pencillamine, costrimozazole, alcohol,
malaria or viral infection (HIV, infectious mononucleosis).
Disorders with
increased platelet consumption
i.
Disorders
with immune mechanism;
a.
Autoimmune-idiopathic thrombocytopenia purpura
b.
Alloimmune-post
transfusion purpura
c.
Neonatal
alloimmune-thrombocytopenia
ii.
Infection
–associated: infectious mononucleosis,
HIV, malaria
iii.
Drug
–induced, heparin, penicillin, quinine, sulphonamide rifampicin
iv.
Thrombotic
thrombocytopenic purpura/haemolytic
urmic syndrome
v.
Hypersplensim
and splenomegaly
vi.
Disseminated
intravascular coagulation
vii.
Massive
transfusion
Acquired
disorders of reduced platelet production
a.
Drug
induced e.g quinine, aspirin (Cheesbrough; 2006)
b.
Leukemia
c.
Metastatic tumor
d.
Aplastic anaemia
e.
Myelodysplasia
f.
Cytotoxic drugs
g.
Radiotherapy
h.
Infections e.g
typhoid and other septicaemias (Cheesbrough; 2006)
i.
Megalobalstic anaemia
j.
Due to bone marrow failure or replacement
Increased
platelet consumption: May be due to immune or non immune mechanisms
a.
Idiopathic thrombocytopenic purpura is a relatively
immune disorder and the most frequent
cause of isolated thrombocytopenia without anaemia or neutropenia. In this
disorder autoantibody produced is usually LgG directed against antigens on the platelet membrane. The platelet lifespan is reduced
to few hours. The platelet count may
vary from < 5 x199/L to near
normal,
b.
Spontaneous skin purpura in
severe immune theombocytopenia
c.
Bone marrow aspirate showing increased magakaryocytes in immune
thrombocytopenia
d.
Neonatal alloimmune thrombocytopenia is
similar to haemolytic disease of the new born infants except that the antigenic stimulus comes from platelet
specific antigens rather than red cell
antigens.
Post
–Transfusion Abnormality:
This is an acquired abnormality. It is a
rare complication of blood transfusion presenting with severe thromobocytopenia
7-10 days after the transfusion. Patients are usually multiparous women who are
negative for the human platelet antigen 1a. Antibodies to this antigen develop
that are somehow responsible for the immune
destruction of the patients own
platelets.
Heparin induced thrombocytopenia occurs during
heparin treatment in up to 5% patients. It may manifest when arterial or venous
thrombosis occurs during a fall in the platelet and this is thought to be due
the formation of antibodies to heparin
that are bound to platelet factor 4, a platelet granule protein. The immune
complexes activate platelets and
endothelial cells, resulting in thrombocytopenia and thrombosis co-exiting.
Causes
of Acquired Platelet Dysfunction;
1.
Aspirin and non-steroidal anti-
inflammatory agents
2.
Penicillins and cephalosporins
3.
Uraemia
4.
Alcohol
5.
Liver disease
6.
Myeloprolifrative disorders
7.
Myeloma
8.
Cardiopulmonary bypass
9.
Fish oils
Thrombocytosis;
a.
Essential (primary) thrombocytosis
b.
Reactive (secondary) thrombocytosis
c.
Infection
d.
Malignant disease
e.
Acute and chronic inflammatory diseases
f.
Pregnancy
g.
After splenectomy
h.
Iron deficiency
i.
Hemorrhage
In essential (primary) thrombocytosis
and reactive (2o) thrombocytosis, the platelet count is raised above
the upper limit of normal.
Abnormalities
of blood stored in anticoagulants include tetany of muscles, decreased packed
cell volume (PCV), increased mean cell volume (MCV), artificial increase in
Platelet count (PC) and decrease white blood cell (WBC) count (Gossens
et al; 1991),(Dacie and Lewis; 1994).