Unfortunately, lung cancer can be difficult to diagnose since symptoms tend to appear only in the disease’s advanced stages. “Many patients with lung cancer report delays in diagnosing their disease. This may contribute to advanced stage at diagnosis and poor long term survival,” says one study published in the Journal of Thoracic Disease. For this reason, it’s essential to know the signs so that when they do present themselves, you can get the help you need. One symptom in particular, which you may notice in your eyes, may tip you off to a rare but serious type of lung cancer that’s considered especially aggressive. Read on to find out which strange ocular symptom to look out for, and what to do if you suspect a problem. RELATED: If These 2 Body Parts Hurt You, It Could Be a Sign of Cancer, Study Says. According to the American Cancer Society (ACS), experiencing drooping or weakness in one eyelid can tip you off to a rare form of lung cancer, called a Pancoast tumor. This type of tumor is located at the top of one side of the lungs—an area known as the apical segment—and often spreads to the nerves, thoracic ribs, blood vessels, lymph nodes, and upper vertebrae as it grows. In particular, Pancoast tumors tend to affect the nerve pathway that connects the brain to the eyes and face, triggering a bundle of symptoms known as Horner Syndrome. Besides drooping in one eyelid, patients often have a smaller pupil in the affected eye, as well as minimal or no sweating on the affected side of the face. The Mayo Clinic adds that some people experience other ocular symptoms, including “sunken appearance to the eye,” an inability for the pupils to fully dilate in dim light, and “upside-down ptosis”—a slightly elevated lower eyelid on one side. RELATED: If You Notice This on Your Skin, You Could Be at Risk for 13 Cancers. The Mayo Clinic explains that these symptoms arise because the tumor or other underlying cause can damage the sympathetic nervous system. This important system is usually responsible for regulating the body’s responses to environmental changes—for instance, sweating to lower the body’s temperature in hot weather. Pancoast tumor patients may find their body less able to respond to these environmental changes, which may translate into abnormal blood pressure, heart rate, or perspiration. Noting any changes in these functions may help you reach a diagnosis sooner. For more health news sent directly to your inbox, sign up for our daily newsletter. Experts say that if you notice symptoms of Horner Syndrome you should have your lungs examined for any abnormalities. However, in addition to Pancoast tumors, Horner Syndrome can also be the result of a stroke or spinal cord injury, according to the Mayo Clinic. In children, Horner Syndrome can occur due to underlying neuroblastoma, a type of cancer that affects the adrenal glands, spinal cord, neck, or chest.ae0fcc31ae342fd3a1346ebb1f342fcb While there is no treatment available for Horner Syndrome, you may be able to restore normal nerve function by treating the tumor or other underlying cause. The location of a Pancoast tumor can trigger another specific set of symptoms, which may help signal a diagnosis. “Because the cancer is at the top of the lungs, it might put pressure on or damage a group of nerves that runs from the upper chest into your neck and arms. The group of nerves is called the brachial plexus,” explains Cancer Research U.K. Their experts add that pressure on the brachial plexus is known to trigger severe shoulder pain, pain in the arm of the affected side, and weakness in the hand on that same side. Regardless of your other symptoms, be sure to call a doctor for immediate medical assistance if signs of Horner Syndrome are accompanied by vision loss, dizziness, lack of muscle control, or sudden pain in the head or neck. RELATED: If You’re Sweating at Night, It Could Be a Sign of These Kinds of Cancer.