About PNH

PNH is a rare blood condition where blood cells are prone to be attacked by the body’s immune system. The process where the red blood cells are destroyed is called “haemolysis” and it is responsible for many of the symptoms of the disease. “Haemolytic PNH” affects about 5 people in a million of the general population. Another 10 people per million have a small PNH clone, but few PNH-type symptoms.

Not everyone diagnosed with PNH will be affected in the same way. Whilst some patients may have no symptoms, others can experience lots of different complications.

“Paroxysmal” refers to symptoms occurring intermittently, “haemoglobinuria” to haemoglobin seen in urine (discolouring it dark red or black urine) and “nocturnal” to it occurring at night (or usually first thing in the morning).

In reality the condition is present all the time and not everyone may experience all the symptoms, eg dark urine.


It is not fully understood why people develop PNH. However, it is thought that PNH occurs due to a change (or mutation) in a gene in one of the stem cells in the bone marrow. This mutation happens in a gene called PIG-A and the cells made by this stem cell lack certain proteins on their surface making them ‘weaker’ than normal blood cells.

The mutation is “acquired” meaning that it develops in the bone marrow stem cell whilst it is multiplying, but is not present from birth. There is evidence that almost all people in the population will develop a PNH mutation at some time but as the resulting PNH cells are weak they rarely grow to a level that they can be routinely detected.

Normal blood cells have proteins on the surface that protect the cells from part of the immune system. PNH cells don’t have these protective proteins, which is what makes them weaker, leading to them be either destroyed (red cell haemolysis) or activated, or stimulated (white blood cells and platelets).

Who does PNH affect?

PNH is a rare condition and is thought to affect around 16 people per million in the United Kingdom. It is not an inherited condition, meaning it cannot be passed on from parents to children. Equally, it is not contagious - you cannot catch PNH from someone who has it. It can hit people of any age or gender.

Connection between Aplastic Anaemia and PNH

Having aplastic anaemia is the only known risk factor for developing PNH. More than 10% of people with aplastic anaemia will develop PNH. In addition, some people with PNH will develop aplastic anaemia. People with PNH can share symptoms with aplastic anaemia patients, such as low blood cell counts.

On average, 2% of patients with PNH go on to develop myelodysplastic syndrome (MDS).

How can you get help?

There is a PNH national service – an NHS commissioned service to provide specialist support to patients and their families with PNH. It is based at St James’s Hospital in Leeds and King’s College Hospital, London.

There is also a PNH Support Group UK, where you can get access to additional information and support.