Karlsson said Åkesson had been steadily drained over the years by the physical and mental pressures of leading the party, which is treated as a pariah by Sweden's political establishment.
Åkesson's issues were no secret to the party, Karlsson said.
"I've been aware that he has been on the verge of breaking-point psychologically for some time. I don't want to go into details but I've seen the warning signs for at least four years," Karlsson told the newspaper.
Karlsson said that Åkesson even had problems recalling what he had did during a party leaders debate in parliament just before he went on indefinite sick leave.
"He had a faint recollection that he had been there, but he did not remember what he did there," Karlsson said of a telephone conversation with Akesson.
Karlsson told Swedish Radio News that the party was behind Åkesson and hope to have back leading the party in the near future.