From the very beginning, doctors wondered if the twins shared sensation; an early video shows one girl being pricked for a blood test as the other starts to cry, her face a perfect mirror image of her sister’s. A pacifier in one mouth seemed to soothe both crying babies.
The above from an article on Krista and Tatiana Hogan in the New York Times magazine. The two conjoined twins are an incredible rarity, perhaps unique in modern medicine. To begin with conjoined twins are rare occuring in approximately 15 of every 1,000,000 births. And cojoined twins joined at the head are even rarer. Known as craniopagus it occurs in just 1 in every 2,000,000 births. There are approximately 135 million births a year on recent years which means less than 140 children are born with craniopagus every year. Such is incredible enough. But the Hogan twins have something that, appears undocumented in history. They share what their neurosurgeon has termed a “thalamic bridge.” A piece of tissue, which anecdotally appears functional, which connects their brains to one another.
As fantastic as it sounds, there is little doubt in [their neurosurgeon Dr.] Cochrane’s mind that the girls share some sensory impressions. When they were 2 years old, he performed a study in which Krista’s eyes were covered and electrodes were glued to her scalp. While a strobe light flashed in Tatiana’s eyes, Krista was emitting a strong electric response from the occipital lobe, which is where images are assembled. The test also worked when the girls switched roles. The results were not published, and some neuroscientists believe that this kind of test, which measures changes in brain activity beneath the skull, is imprecise in determining what region of the brain is at play; but most would agree that any response in the other twin’s brain suggests, at a minimum, connectivity.
Below is an idea of how the seperate brains of most twins with craniopagus share the space. Typically the lobes towards the middle are crowded and underdeveloped. In Krista and Tatiana however there is a bridge which connects their two brains at the middle.
Craniopagus is an incredible condition. The earliest descriptions date from medieval Europe. Below is a lithograph appearing in the 16th century encyclopedia Cosmographia Universalis, which is the first surviving picture depicting craniopagus. The below’s secondary source was Browd and Walker’s great 2004 review article.
Unfortunately, as many conjoined twins, those suffering from craniopagus face unique health problems, sometimes associated with shared circulatory systems and the demands of such. Not to mention their often prematury.
Whatever the unique health issues faced by these childrens the promise of surgical separation remains a risky one. A 1987 case report and a review of the literature showed a mortality rate of much greater than 50% through the whole history of separation from the first in attempt by Cameron in 1928. To be sure imrpoved neuroimaging and surgical techniques have greatly improved outcomes but unique obstacles in cases of craniopagus, including extensive venous sinus sharing, continue to make surgical separation a horrowing undertaking.
In the case of Krista and Tatiana it appears their shared “thalamic bridge” would further complicate any thought of surgical separation. And with their reported health and relatively remarkable development it is no wonder surgery has been forgone.
You can watch more about Krista and Tatiana on the New York Times’ website. Beyond the two journal sources above, the journal Brain also has a good review article on craniopagus which is worth a read.