The most common initial symptom of cavernous sinus thrombosis is a headache.

This usually develops as a sharp pain located behind or around the eyes that steadily gets worse over time.

Symptoms often start within a few days of developing an infection in the face or skull, such as sinusitis or a boil.

It can be several days, or even weeks, before additional symptoms develop after the headache starts.

The eyes

In most cases of cavernous sinus thrombosis, the eyes are affected. You may experience:

• swelling and bulging of the eyes – this usually starts in one eye and spreads to the other eye soon after
• red eyes
• eye pain – which can be severe
• vision problems – such as double vision or blurred vision
• difficulty moving the eyes
• drooping of the eyelids

Other symptoms

Other symptoms of cavernous sinus thrombosis include:

• a high temperature of 38C (100.4F) or above
• vomiting
• seizures (fits)
• changes in mental state, such as feeling very confused

These symptoms usually occur if cavernous sinus thrombosis is left untreated, or if an infection causing the condition spreads throughout the body.

Without treatment, most people with cavernous sinus thrombosis will become increasingly drowsy and eventually fall into a coma.

When to seek medical advice

You should always contact your GP if you have a persistent and severe headache that you've never had before.

While it's highly unlikely to be caused by a cavernous sinus thrombosis, a persistent headache usually requires further investigation.

You should also contact your GP if you develop any of the eye symptoms described above.

Cavernous sinus thrombosis is usually caused by a bacterial infection that spreads from another area of the face or skull.

Many cases are the result of an infection of staphylococcal (staph) bacteria, which can cause:

• sinusitis – an infection of the small cavities behind the cheekbones and forehead
• a boil – a red, painful lump that develops at the site of an infected hair follicle (squeezing a boil can increase the risk of the infection spreading)

Most people have one of these conditions before developing cavernous sinus thrombosis. However, boils and sinusitis are common and it's very rare that they lead to cavernous sinus thrombosis.

Blood clot

In most cases of cavernous sinus thrombosis, a blood clot forms in the cavernous sinuses to try to prevent bacteria spreading further into the body. This is known as thrombosis.

However, the clot usually blocks the flow of blood away from the brain, which increases the pressure in the cavernous sinuses and can damage the brain, eyes and the nerves running between them.

In addition, the blood clot is often unable to prevent the spread of infection. If the condition is left untreated, the infection can spread through the bloodstream, causing blood poisoning (sepsis).

Other causes

Less commonly, a blood clot can develop in the cavernous sinuses, due to:

• a severe head injury
• an infection spreading from the teeth or gums (dental abscess)
• a fungal infection
• a health condition or other underlying factor that makes you more prone to blood clots, the most common being pregnancy
• conditions that cause inflammation to develop inside the body, such as lupus or Behçet's disease
• some types of medication, such as the contraceptive pill, although this is very rare

The cavernous sinuses

The cavernous sinuses are a series of hollow spaces located under the bottom of the brain, behind each eye socket.

Each forms a major vein that is part of a network of sinuses that eventually drain into the jugular veins, which carry blood away from the brain.

Up to 1 in 3 people with cavernous sinus thrombosis may die. Some people who survive it are left with ongoing symptoms such as seizures (fits) and severe headaches.

It can also cause further problems with vision, blood clots and infections.

Vision problems

Problems with vision are a relatively common complication of cavernous sinus thrombosis.

A few people have a degree of permanent visual impairment, although permanent blindness is uncommon.

Blood clots

There's also a risk that another blood clot may develop elsewhere in the body – for example, in the:

• legs – this is known as deep vein thrombosis (DVT)
• lungs – this is known as a pulmonary embolism
  • brain – this triggers a stroke

These conditions are very serious and can be fatal.

Infection

Complications can also occur if the infection spreads beyond the cavernous sinuses. These complications can include:

• meningitis – an infection of the outer protective layer of the brain that can cause symptoms such as a stiff neck, mental confusion and sensitivity to light
• sepsis or blood poisoning – this can cause symptoms such as chills, a fast heartbeat and rapid breathing

Both of these conditions are very serious and can be fatal, particularly if they're not treated promptly.