Holoprosencephaly, apart from being a hard word to spell, is a defect in the brain. It can affect the facial features resulting in closely spaced eyes, small head size and cleft palates and lips. It is the failure of parts of the brain to split fully during development in the womb. This then has an effect on learning development throughout life.
Cases of holoprosencephaly range from the hardly noticeable, to extreme cases of cyclops. There are three main types of holoprosencephaly affecting the main lobes of the brain, with a further Middle Interhemispheric Variant affecting the mid-line division of the brain. It is this later variant that Eva has.
It’s quite hard to find information on it, without it being in depth medical journals, so if you would like to find out more, be prepared to wade through a lot of medical jargon.