Bruno Fernandes, who is one of St-Pierre's jiu-jitsu coaches as well as a doctor of ophthalmology in Montreal, told MMA Fighting that St-Pierre's vision was drastically compromised by the injury, which reduced his capability of seeing out of his left eye to about 10 percent.
"We don't really measure vision in terms of percentage but he lost field of vision, clarity and depth perception," Fernandes said. "When you have hyphema, it is like looking through a really dirty window. Your vision is significantly impaired, and it is amazing he was even able to fight with such a degree of impairment."
In the two weeks since the fight, the blood in his eye has been reabsorbed and he's back to normal, but extreme cases of hyphema can result in permanent visual impairment.
Fernandes, who is a full-time professor at McGill University in Montreal, did not examine St-Pierre on the night of the fight but helped arrange his immediate aftercare, corresponded with the attending doctor and saw the champion upon his return to Montreal.
But in watching the fight as it happened, Fernandes, who has been working with St-Pierre since 2006, knew something was wrong.
"Trying to hit a moving target with one eye only is incredibly hard, "said Fernandes, who is a black belt under Carlos Gracie, Jr., and runs Gracie Barra Montreal. "Trying to punch someone moving all the time is very hard. It was really challenging for him to perform under these circumstances.
"Everything from the left, he couldn't see coming, and that became clear when you saw any attacks from the left were landing," he continued. "That's not common to see when he fights."
Some wondered why St-Pierre didn't make more of an effort to put the fight on the ground, where he would have been less likely to be hit, but Fernandes said that when St-Pierre was robbed of his depth perception, it made shooting in for takedowns a much riskier proposition. His timing likely would have been off due to misjudging distance.
Asked how he would have responded to the problem had he been in St-Pierre's corner, Fernandes admits the decision would have left him torn. On one hand, he understands the time and sacrifice St-Pierre puts into training, and would have wanted him to continue. On the other, he would have been concerned with his safety and long-term vision.
But Fernandes says there's a huge difficulty in that split-second moment when doctors have to decide whether or not a fighter can continue, and that comes from the limited instruments they have with which to make a diagnosis at cageside.
"By using instruments, if I had an idea how bad it was, if I knew there was no retina detachment, no open wound, and if it's not 100 percent but he can see fingers, using the tools available at that moment it would be OK to let fight continue," he said. "But keep in mind it's up to the fighter to know how well he's seeing with that amount of vision to continue. Would other fighters stop? Maybe. He showed a lot of heart. He really wanted to continue."