Pancoast tumors are a rare type of lung cancer that can affect nerves that control the face and eyes, causing a group of symptoms known as Horner syndrome.

The symptoms of Horner syndrome include:

The reason Pancoast tumors can lead to Horner syndrome is due to their location. These tumors grow in the top, or apex, of the lungs, which is close to several important nerves.

These include the brachial plexus, which is a network of nerves that sends signals to the shoulders, arms, and hands. For this reason, Pancoast tumors can also cause pain and weakness in these areas.

The apex of the lungs is also close to sympathetic nerves along the spine and in the neck. These form part of the sympathetic nervous system, which is responsible for the “fight, flight, or freeze” response.

When tumors affect these nerves, it can result in Horner syndrome or another condition known as Harlequin syndrome. This causes increased facial sweating or flushing on one side.

Pancoast tumors affect the function of these nerves by growing nearby and compressing them. Alternatively, they may invade the nerves themselves.

People with these symptoms need to contact a doctor as soon as they can. The sooner a doctor can diagnose the cause and begin treatment, the better the outlook.

However, it is important to note that cancer is not the only potential cause of either Horner or Harlequin syndrome. Around half of all cases of Harlequin syndrome are idiopathic, meaning there is no identifiable cause. Injuries, infections, and other neurological conditions can also cause these syndromes.

A doctor can perform tests to get an accurate diagnosis.