When Nebraska woman Alex Simpson was just two months old, she was diagnosed with a rare congenital brain condition that meant she may not survive to her first birthday.

But despite medical odds, she and her family just celebrated her 20th birthday on November 4 this year.

Simpson's condition, hydranencephaly, means her cerebral hemispheres – the two large lobes that typically make up most of the human brain, responsible for cognitive function, voluntary movement, and sensory processing – are almost nonexistent. While her brainstem and some other brain parts remain, the rest of her cranial cavity is filled instead with cerebrospinal fluid.

Hydraencephaly is incurable, and is managed with intensive supportive care.

"Technically, she has about half the size of my pinky finger of her cerebellum in the back part of her brain, but that's all that's there," Alex's father, Shawn Simpson, told Omaha news station KETV earlier this month.

This means Simpson's vision and hearing are impaired, though her cerebellum maintains some awareness of her surroundings. Her family say they nevertheless have a strong relationship with her.

We don't know what causes hydranencephaly, but some research suggests it may arise when some form of vascular injury, such as a stroke or infection, blocks blood supply to the brain.