Platelet Disorders

Overview

Platelets are the cells responsible for making blood clot. Platelet disorders mean that injured blood vessels bleed more than usual and heal more slowly.

Inherited platelet disorders include Bernard Soulier disease, Glanzmann’s thrombasthenia, Hermansky Pudlak syndrome, Jacobsen syndrome, Lowe syndrome, platelet release and storage pool defects, thrombocytopenia with absent radius (TAR) syndrome and thrombotic thrombocytopenic purpura (TTP).

What is a platelet disorder?

Blood is made up of different types of cells (red blood cells, white blood cells and platelets) all suspended in a straw-coloured liquid called plasma.

• Platelets: Also called thrombocytes, they are formed in the bone marrow from stem cells. Megakaryocytes (large bone marrow cells) are formed, mature and then die, which is when they release platelets.
• Function: When a blood vessel is injured, platelets clump together to block the injury site. They also start off a complicated chemical reaction to form a mesh made of a substance called fibrin. When all of the clotting factors are turned on, the blood forms a clot which stops the injury site bleeding any further.

Over half of the new platelets circulate in the bloodstream and the rest remain in storage in the spleen. Platelets only live for just over a week, then the body destroys them and new ones are released.

Types of platelet disorders

There are lots of different groups of disorders affecting the platelets:

• Thrombocythaemia: where there are too many platelets in circulation
• Thrombocytopenia: where there are too few platelets in circulation
• Dysfunction disorders: where there are the correct number of platelets in circulation but they do not work properly

All of these disorders disrupt the clotting process, leading to abnormal clot formation and bleeding. Generally, symptoms of a platelet disorder are similar, with bruising from minor trauma, bleeding from the mouth, nose or digestive system and excessive bleeding after injury or surgery. They may become apparent soon after birth or later in childhood.

Causes

The majority of platelet disorders are caused by a genetic fault or mutation. In some cases, the gene mutation occurs sporadically (out of the blue), with no family history of clotting disorders.

Autosomal recessive inheritance

In most cases, a mutation in an autosomal gene does not cause problems if the other gene in the pair is normal. However, if someone inherits a faulty gene from both parents, they may be affected.

• A 25% chance that the child will be unaffected and will not carry the mutation
• A 50% chance that the child will not be affected but will carry the mutation
• A 25% chance that the child will be affected

Autosomal dominant inheritance

Some platelet disorders can be caused by a gene mutation being passed on in an autosomal dominant manner. This means a child has to inherit the faulty gene from just one parent to have the condition.

• A 50% chance that the child will be unaffected and will not carry the gene mutation
• A 50% chance that the child will carry the gene mutation and will have the condition

X-linked inheritance

In males, who only have one ‘X’ chromosome, there is not another ‘X’ to provide a functioning gene if one is faulty. This means that only boys are affected by X-linked disorders and the mother is a carrier.

• 25% chance of the child being an unaffected non-carrier girl
• 25% chance of the child being an unaffected carrier girl
• 25% chance of the child being an unaffected boy
• 25% chance of the child being an affected boy

Diagnosis

Doctors will usually start by taking a clinical history of which symptoms are present and when they appeared. They will also look at any other treatments or medications your child is having. A physical examination looks for signs of any bleeding.

Platelet disorders can be diagnosed using a sample of blood for testing in the laboratory. The number and appearance of platelets in the blood will be checked. Clotting tests will also be carried out to record how long it takes a blood sample to form a clot. If an inherited disorder is suspected, genetic testing may help identify the gene mutation.

Specific types of platelet disorders

The most common types are:

• Bernard Soulier disease
• Glanzmann’s thrombasthenia
• Hermansky Pudlak syndrome
• Jacobsen syndrome
• Lowe syndrome
• Platelet release and storage pool defects
• Thrombocytopenia with absent radius (TAR) syndrome
• Thrombotic thrombocytopenic purpura (TTP)

Frequently Asked Questions