In antiphospholipid syndrome (APS), the immune system produces abnormal antibodies which make the blood stickier than normal.

This means that a person with APS is more likely to form blood clots in their veins and arteries. This can cause serious, life-threatening health problems, such as deep vein thrombosis (DVT), strokes or heart attacks (see below).

Below are the most common conditions that are caused by APS. It is important to note that most people with APS will not experience all of them.

Deep vein thrombosis

Deep vein thrombosis (DVT) is one of the most common conditions that can occur as a result of APS. It is a blood clot that develops inside one of the body's larger, deeper veins, usually inside the calf or thigh.

Symptoms of DVT include:

• pain, swelling and tenderness in one of your legs (usually only one leg is affected)
• a heavy ache in the affected area
• warm skin in the area of the clot
• redness, particularly at the back of your leg, below the knee

One of the biggest risks is that the blood clot may move out of your leg and travel towards your lungs. A blood clot that reaches the lungs is known as a pulmonary embolism (see below)

Strokes and TIAs

A stroke is a serious condition that is associated with APS. A stroke happens when a blood clot blocks the brain's blood supply.

The most effective way of identifying the symptoms of a stroke is to remember the word FAST, which stands for:

• Face - the face may have fallen on one side, the person may not be able to smile, or their mouth or eye may have drooped.
• Arms - the person may not be able to raise both arms and keep them there due to weakness or numbness.
• Speech - their speech may be slurred.
• Time - it's dial 999 immediately if there are any of these signs or symptoms.

A transient ischaemic attack (TIA), also known as a "mini-stroke", happens when a blood clot causes a temporary blockage of blood to the brain.

TIA symptoms are the same as those of a stroke, but they only last from a few minutes to a few hours before they disappear. However, a TIA should never be ignored because it is a serious warning sign of a problem with the brain's blood supply.

Heart attacks

A heart attack can occur when a blood clot forms in one of your coronary arteries (the blood vessels that lead to the heart).

If the blood clot blocks the blood supply to your heart, it can seriously damage the heart muscles, which can die if left untreated.

Common symptoms of a heart attack are:

• a crushing pain or mild discomfort in the centre of the chest
• shortness of breath
• clammy, sweaty and grey complexion
• dizziness
• nausea and vomiting
• restlessness
• coughing
• a general feeling of being unwell

Dial 999 immediately if you think that you or someone else is having a heart attack.

A heart attack requires immediate emergency treatment so do not wait to get medical assistance.

Pulmonary embolism

A pulmonary embolism is a blockage in one of the blood vessels in the lungs. Most pulmonary embolisms occur when a blood clot that starts in one of the legs travels up into the lungs. Symptoms of a pulmonary embolism include:

• shortness of breath
• severe chest pain
• a persistent cough that may bring up blood-stained phlegm

If it is not treated, a pulmonary embolism can be life-threatening, so contact your GP immediately if you suspect that you have one. If this is not possible, contact your local out-of-hours service or NHS Direct on 0845 46 47.

Pregnancy complications

Women with APS have a much higher risk of developing complications during pregnancy, particularly if it is not treated. Possible complications include:

• recurrent (three or more) early miscarriages, usually during the first 10 weeks of pregnancy
• one or more later miscarriages, usually after week 10 of pregnancy
• premature birth, usually at or before week 34 of pregnancy, which is often caused by pre-eclampsia (where a pregnant woman suddenly experiences an increase in blood pressure)

Livedo reticularis

Livedo reticularis is a skin condition caused by small blood clots that develop inside the blood vessels of the skin.

It causes the skin to take on a blotchy red or blue appearance. Some people also develop ulcers (sores) and nodules (bumps).

Livedo reticularis is often more severe in cold weather.

Superficial thrombophlebitis

Superficial thrombophlebitis is inflammation of the veins just under your skin, usually in your leg. The symptoms are similar to DVT but they are not usually as severe.

The symptoms of superficial thrombophlebitis include swelling, redness and tenderness along the affected vein and a high temperature of 38C (100.4F) or above (although this is less common). The symptoms usually resolve within two to six weeks.