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 they lead to cavernous sinus thrombosis.

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).

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 medicine, such as the contraceptive pill, although this is very rare
• a complication of coronavirus (COVID-19) infection
• a very rare side effect after having some types of COVID-19 vaccine

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.