Platelet Disoders
How Platelet Disorders are Inherited
Almost all platelet disorders are caused by autosomal recessive genes. This means that a child has to get the gene for the disorder from both parents. That is why these disorders are so rare. Because the genes are on the autosomes and not on the sex chromosomes, the disorders occur in both males and females.
With a recessive inheritance pattern, if a child gets an unaffected gene from one parent and an affected gene for a platelet disorder from the other parent, the child will not have the disorder. They will be a carrier. This is because the platelet disorder gene is recessive – it is not stronger, or dominant, over the unaffected gene. For the child to have the platelet disorder, he or she must get the affected gene from both parents. The figure below shows the possible gene combinations for these autosomal recessive types of platelet disorders.
If both parents carry a defective recessive gene for a platelet disorder, with each pregnancy there is:
- a 25% (one in four) chance of having a child who is unaffected
- a 25% (one in four) chance of having a child who has the platelet disorder
- a 50% chance of having a child who does not have the platelet disorder but carries the gene
Figure 3-3. An autosomal recessive inheritance pattern – the possible gene combinations with most platelet disorders. The recessive gene is shown with a little r. To have the disorder, a child must inherit a recessive gene from both parents.
How Blood Works with Platelet Disorder
A person with a platelet disorder will have trouble making a platelet plug. Without a platelet plug, blood will continue to leak from an injured blood vessel. In time, the body can make a fibrin clot to stop the bleeding, but it will not be as strong as a clot that had a good platelet plug.
If a person’s platelets have trouble sticking to the wall of an injured blood vessel, they have an adhesion disorder. If the platelets don’t stick well to each other, it is an aggregation disorder. If the platelets don’t release the chemicals that signal other platelets to join in making the plug, it is a secretion disorder. The platelet disorders described in The Handbook are listed below:
Adhesion Disorders
- von Willebrand Disease (VWD)
- Bernard-Soulier Syndrome
Aggregation Disorders
- Glanzmann Thrombasthenia
Secretion Disorders
- Storage Pool Diseases including Alpha Storage Pool Deficiency (Gray Platelet Syndrome), Delta Storage Pool Deficiency, and Alpha/Delta Storage Pool Deficiency
Testing and Diagnosis of Platelet Disorders
Because platelet disorders are rare, it is best to go to a Hemophilia Treatment Center (HTC) for testing. An HTC will also have the special machines necessary to do some of the tests. Even though it is called a Hemophilia Treatment Center, an HTC also gives expert care to people with platelet disorders.
Symptoms of a Platelet Disorder
One of the most important ways of figuring out if someone has a platelet disorder is to look at how she or he has bled in the past. These are some of the most common symptoms:
- Easy bruising – especially bruises that happen when you can’t remember hurting yourself, bruises on several different parts of your body, and large bruises (bigger than 2 inches)
- Frequent nosebleeds – especially ones that are hard to stop, lasting more than 10 minutes or needing packing by your provider
- Gums that bleed easily
For females:
- A monthly period that lasts longer than seven days with a very heavy flow or changing protection every 2 hours
- Bleeding in the stomach or intestine
- A list of drugs and herbs that can affect platelets can be found here.
Most people with a platelet disorder have very mild symptoms. They may not realize that they have a disorder unless they have surgery or are badly injured. Some people, though, will have frequent bleeding that greatly affects their lives.
The doctor or nurse will want to know if other people in your family have had problems with bleeding. They will also ask you about drugs and herbs you are taking that may affect your platelets. A list of drugs and herbs that can affect platelets can be found here.
Tests for Platelet Disorders
To determine if you have a bleeding disorder and what type it might be, the doctor will order blood tests. These are some of the blood tests used to diagnose platelet disorders:
Platelet Count – The number of platelets in a cubic millimeter of blood is counted. The normal amount is between 150,000 and 450,000.
Platelet Aggregation Testing – The platelets from a blood sample are put into tubes. Then different chemicals are added to the tubes to see if they make the platelets stick together (aggregate). A machine is used to detect this. The chemicals used are ones that make normal platelets stick together. These chemicals are called agonists. An important one is an antibiotic called ristocetin.
Bleeding Time – In the past, one of the most common tests for a platelet disorder was the bleeding time. To do this test, a blood pressure cuff is put on your upper arm. It is inflated to a certain pressure. Then a device is put on your forearm that makes a small, usually painless, cut. A stopwatch measures how long it takes for this cut to stop bleeding. Many doctors no longer use this test. It is difficult to standardize. This means that the way one person does the test may be slightly different from the way another person does it. That makes it difficult to compare the test results. Also, the bleeding time is often normal in people even though they have a platelet disorder. It has now been supplanted by the PFA-100®.
PFA-100® – The PFA 100® is a machine that can test how well platelets can stick and clump together. It does this under conditions like those found inside the body when a blood vessel is torn. It measures how long it takes a blood sample to form a platelet plug. The result is called the Closure Time (CT) and is measured in seconds.
Electron Microscopy – By looking at platelets under a powerful electron microscope, a doctor can tell if they have a normal shape. The microscope can also reveal if the storage granules inside the platelet are normal.