Because there are so many causes, diagnosing the reason why you have developed a rash takes a bit of detective work.
What is purpura?
Purpura just means purple. The term purpura is usually used to refer to a skin rash in which small spots of blood appear on the skin. A purpuric rash is not a disease but it is caused by conditions that result in blood leaking into the skin and other body surfaces.
What are the symptoms of purpura?
The rash looks like little red spots on the skin. It's easy to recognise because - unlike other spotty rashes - the spots don't fade when you press them. The best way to do this is with a drinking glass or other see-through object like a plastic ruler.
There are so many different causes of a purpuric rash it's difficult to list all the symptoms that may occur due to the underlying illness. However, common symptoms you may notice occurring with the rash include:
- Spots inside the mouth.
- Blisters (which may be clear or yellow like little boils).
- Tenderness in the area of the rash.
- A high temperature (fever).
- Feeling under the weather.
- Joint pains.
- Tummy pains.
What causes purpura?
There are many different causes of purpuric rashes. Several of them can be grouped into those caused by lack of platelets and those in which the platelets are present in normal numbers. Conditions in which the platelet numbers are normal are called non-thrombocytopenic. Those in which the platelet numbers are low are called thrombocytopenic.
- Conditions you are born with, such as:
- Osler-Weber-Rendu syndrome.
- Ehlers-Danlos syndrome.
- Pseudoxanthoma elasticum (a condition affecting the elastic tissue of the blood vessels and other parts of the body).
- Infections picked up during pregnancy whilst still in the womb, such as cytomegalovirus and rubella.
- Conditions acquired after you were born, such as:
- Severe infections such as sepsis, infection with one of the germs that cause meningitis (meningococcus).
- Allergy-based conditions such as Henoch-Schönlein purpura.
- Disorders of the connective tissue that connects and binds other bits of the body together, such as systemic lupus erythematosus and rheumatoid arthritis.
- As a side-effect of medicines such as steroids and sulfonamides (antibiotics).
- Other causes, such as ageing of the skin, injury (trauma), lack of vitamin C (scurvy) and poor blood supply, especially to the legs.
- Conditions resulting from problems with platelet production, such as:
- Bone marrow failure - for example:
- Conditions that increase the breakdown of platelets, such as:
- Immune thrombocytopenia.
- Systemic lupus erythematosus.
- Viral infections.
- Conditions affecting the blood clotting (coagulation) system, such as:
- Disseminated intravascular coagulation which causes excessive blood clotting in small blood vessels).
- Haemolytic uraemic syndrome (destruction of blood cells associated with kidney problems).
- Enlarged spleen.
- Conditions causing dilution of the platelets, such as rapid transfusion of large quantities of stored blood.
How is purpura diagnosed?
Because there are so many causes, diagnosing the reason why you have developed a rash takes a bit of detective work. The doctor will need to ask you questions about the rash and your general health (take a history), examine you and do some tests.
What questions will I be asked?
The sort of questions the doctor will ask you may include:
- How long you've had the rash.
- Whether it's changed over time.
- Whether you bruise easily.
- Whether you've been abroad recently.
- Whether you've recently taken any medicines you've bought from a pharmacy.
- If this is not your regular GP:
- Whether you've had any illnesses in the past or have any long-term conditions.
- What prescribed medicines you are taking.
- Whether you have any allergies.
- Questions about your lifestyle (drinking, smoking, etc).
What will the doctor be looking for?
Examination of your rash and general body systems may give a clue as to the cause. The doctor will be looking for:
- The size of the spots, whether they run together, whether there are any blisters (and whether they are filled with clear fluid, blood or pus).
- Tenderness of the spots (this can happen with diseases causing inflammation, such as rheumatoid arthritis).
- Any spots inside your mouth.
- The location of the spots - for example, spots close together in one area are often seen where there has been injury, whereas spots on both lower legs suggest a problem with the circulation in your veins, as in the picture below.
- Swollen organs in your tummy, such as an unusually large liver or spleen.
- Numbness, weakness or other unusual features on examining your nervous system.
Will I need any tests?
There are a huge number of tests which could be arranged, but hopefully by the time the doctor has taken your history and examined you they will have a reasonable idea as to which are the most important. Most tests can be done on blood samples and may include:
- A full blood count to check your platelets, white cells and red cells.
- Inflammatory markers (tests to check for inflammation).
- Tests to check how well your liver is working.
- Tests to check your blood clotting system.
- Tests to check for unusual levels of protein in your blood.
- Tests to check for proteins that attack the body's own cells (autoantibodies).
Other tests may be ordered depending on the suspected cause. For example, you may need a blood culture if your doctor thinks you have an infection, or a lumbar puncture if they think you have a nervous system disorder.
How is a purpura treated?
The treatment will depend on the cause. Leaflets on the specific conditions mentioned in the Causes section will give you more details. If your platelet count is very low, the first treatment you will receive will be a platelet transfusion.
Further reading and references
Coagulation Factors; Lab Tests Online, 2012
Maher GM; Immune thrombocytopenia. S D Med. 2014 Oct67(10):415-7.
Purpura; DermNet NZ
Guidelines on the investigation and management of antiphospholipid syndrome; British Committee for Standards in Haematology (2012)
Thrombophilia due to Thrombin Defect, THPH1; Online Mendelian Inheritance in Man (OMIM)