“One day, back in May actually, I found this big mass on my neck,” Williams says in the video. “I was mortified by it. I got tests done, everything you could imagine, everything was negative.”

The mass was diagnosed as a benign branchial cyst. In the caption, Williams wrote that her doctors said she didn’t need to get it removed, “but it kept growing.” Once it got large, her doctors advised her to remove it as soon as possible, since it could leak or get infected.

Here’s what you need to know about the type of cyst Williams had.

What are cysts?

The most common kinds of cysts are epidermoid cysts, which form beneath the skin and are small, harmless bumps most commonly found on the face, neck, or trunk, according to the Mayo Clinic. Epidermoid cysts are caused either by cells that move deeper into the skin rather than shed, or from irritation or injury to the skin or a hair follicle.

They are often slow growing and painless, rarely causing problems or needing treatment; however, cysts are removed when they are bothersome, break open, or become painful or infected.

What is a benign branchial cyst?

The type of cyst Williams had is different from the everyday cysts people usually encounter. A branchial cyst, according to the Cleveland Clinic, is a small fluid-filled sac that may look like a lump under the skin on the side of the neck. It is considered one of the most common neck mass, and while Williams admitted she was “a little scared,” it is largely harmless.

These cysts are considered congenital, meaning they are present at birth. While children are more likely to be diagnosed with them, they can become noticeable later in life if they are infected.

@serena Back in May I found a lump showing on my neck. I immediately went to the doc got a mri and was told I have a brachial cyst. Have you ever heard of that? They said I don’t need to get it removed if I don’t want. So I did not get it but it kept growing. I decided to get more test and 3 test and one biopsy later everything is still negative but doctors advised I get it removed asap because it was the size of a small grapefruit and it could get infected or worse leak. So this is me removing it. I am feelimg so grateful, and fortunate everything worked out, and most of all I’m healthy. I still made it to American doll with Olympia as promised. And yes all is ok. 🙏🏿🙏🏿 #fyp #foryourpage #serenawilliams #mom ♬ original sound – Serenawilliams

What causes a branchial cyst?

The Cleveland Clinic says that branchial cysts can form during fetal development, when branchial arches—which are responsible for developing certain parts of your head and neck—don’t fuse, or grow together. When spaces are left between the arches, certain branchial anomalies like branchial cleft cysts can develop.

How did it get so big?

Branchial cysts can grow if they become infected, usually from an upper respiratory tract infection like the common cold, according to the National Institute of Health. The cyst can also become infected and form an abscess, or a painful, puss-filled pocket.

Once branchial cysts begin growing, doctors will advise immediate removal to prevent further infection. The cyst rarely returns after removal, says the Cleveland Clinic, and the surgery will likely solve the problem entirely. 

For Williams, though the cyst was out, she was exhausted by the process: In the same video explaining her surgery, she included a clip of herself taking her daughter to the American Girl Doll store, pointing out the bandage on her neck, saying she “needed to keep showing up.”

Adds Williams: “I’m gonna go home and pass out.”

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