The Icelandic PM had a swollen foot because of an infection, and he had to go to the hospital in Stockholm to get antibiotics.
Not a fashion statement
The picture here above is from the meeting, where Barack Obama met with the Nordic leaders. One can see that the Icelandic PM is wearing an athletic shoe on his left foot. Johannes Þór Skulason, the PM's assistant, said that the minister was forced to wear an athletic shoe because his left foot was swollen. "He is so swollen because of an infection that this was the only option. This was not a fashion statement. " said Johannes.
It is not known how the PM hurt his leg.
People are already making fun of the incident, see for example here, and the photoshopped picture here below.