Anyone who thinks their sibling messes with their head may be more right than they know. An Indiana woman had a tumor removed from her brain that turned out to be the remains of a twin who never developed. The phenomenon is unusual, but not as rare as you might expect, or hope.

Yamini Karanam had no idea she could have had a twin sister. However, while studying for her PhD at Indiana University, she started noticing cognitive lapses that hadn't occurred before: “Problems with reading comprehension, listening comprehension. If a couple people were talking in a room, I wouldn't understand what was happening," Karanam told NBC.

Specialists disagreed on the source of the problem and whether something could be done. Karanam's investigations led her to Dr. Hrayr Shahinian of the Skull Base Institute in Los Angeles. Shahinian pioneers minimally invasive techniques for removing brain tumors. Karanam thought that when the tumor is located deep inside the brain, minimally invasive sounded like a good option.

“Unlike traditional brain surgery where you open the skull and use metal retractors and you bring a microscope to see in the depths of the brain, what we're doing is keyhole surgery," Shahinian explained. Although this type of brain surgery has been performed thousands of times, what Shahinian found when he removed Karanam's tumor gave him the opportunity to bring it to prominence.

Karanam's tumor was a teratoma, which she calls “the evil twin that has been torturing me for 26 years.” Teratomas form from germ layers that fail to develop properly. Instead of a fertilized egg splitting with both parts becoming twins, one lodges inside the other. Most teratomas are made up of unstructured tissues from specific organs. Some contain simple bodily elements such as hair, teeth or bone. In much rarer cases, whole organs or limbs develop without a body to attach to.

Teratomas can be malignant, but most, including Karanam's, are benign. Consequently, she is expected to make a full recovery. Teratomas can also appear in many parts of the body, although the ovaries, testes and coccyx are particularly common. While the case of Gavin Hyatt, whose 2009 abdominal teratoma was considered a case for warrant officer Ripley, may sound horrific, having one in your brain is probably a fair bit worse.

H/T: Huffington Post