"I was shocked," Walsh said of the moment doctors revealed her MRI results. "It was not what I expected."
Walsh was diagnosed with a meningioma: a tumor arising from the lining that surrounds the brain and spinal cord. Within three days, she underwent surgery to have it removed at Cedars-Sinai Medical Center in Los Angeles. After removing the tumor, doctors confirmed that it was benign.
In January 2015, Walsh initially wrote off symptoms as exhaustion, having just executive produced and starred in the NBC series "Bad Judge."
"I could drink five cups of coffee and not wake up," she said.
She found it hard to concentrate. Her balance was off, and she developed shooting headaches. Her Pilates instructor also noticed that she would dip on her right side.
Exercise is "typically when people will see subtle motor changes, because they're really comparing both sides of their body," CNN Chief Medical Correspondent Dr. Sanjay Gupta said.
Her cognitive issues became severe enough that her then-boyfriend urged her to see a neurologist.
"I would reach for words or thoughts, and I just couldn't finish them," Walsh said.
By the time she made it to the doctor in June 2015, he even noticed that the right side of her face was drooping slightly. It was then that doctors found the benign tumor, which was just over 5 centimeters long, pushing on her left frontal lobe.
Doctors put Walsh on a precautionary antiseizure medication before she went under the knife. With a tumor that size pressing on her brain, they told her she was lucky to have had no seizures.
Despite having played Dr. Addison Montgomery on "Grey's Anatomy" and the spinoff "Private Practice," she was blindsided by the diagnosis and her sudden role as the patient. For all the medical jargon she delivered as a TV doctor, she hadn't heard of meningiomas.
"How meta," Walsh said of her transition from onscreen doctor to real-life patient.
Meningiomas, which are more likely to occur in women
, are usually benign and slow-growing. Doctors told Walsh that the tumor may have been there for a while -- but that could mean two years or 10 years, she was told.
Most meningiomas arise close to the skull, but in some cases, a tumor can form as far down as the spinal cord.
These tumors differ greatly from aggressive brain cancers like glioblastoma, which may originate deeper in the brain and can "double in size every couple of weeks," Gupta said. Sen. John McCain was diagnosed with glioblastoma in July.
Meningiomas are the most common tumors that originate in the central nervous system, numbering about 27,000 cases per year, based on 2009-13 data
from the Central Brain Tumor Registry in the United States.
Some are removed surgically without further treatment, as Walsh's was. However, in some cases in which the tumor is very small and causes no symptoms, doctors may simply watch and wait, Gupta said. In less common cases in which malignant cells are found in the tumor or when the tumor is next to delicate structures like the brain stem, doctors may also opt for radiation, Gupta added.
Other celebrities have gone public with their own meningioma diagnoses -- including Sheryl Crow
, Elizabeth Taylor, Mary Tyler Moore and recently Maria Menounos
Immediately after her surgery, Walsh said, she noticed a difference.
"The fog had lifted," she said.
Walsh said that her decision to go public was intended to help raise awareness, especially of a type of tumor that affects mostly women. She joined "Grey's" co-star Patrick Dempsey and other TV doctors in a campaign launched by Cigna that encourages people to get annual checkups.
"I was so relieved to know that it was something" that could be fixed, Walsh said of the tumor. Her advice: "Trust your instincts. Trust your body."