How does Roderick die at the end of "The Fall of the House of Usher?"

In "The Fall of The House of Usher," Poe doesn't specify precisely how Roderick dies. As a master of suspense, he wants to keep us guessing, forcing us to use our imaginations. This is the narrator's account of what happens:

For a moment she remained trembling and reeling to and fro upon the threshold—then, with a low moaning cry, fell heavily inward upon the person of her brother, and in her violent and now final death-agonies, bore him to the floor a corpse, and a victim to the terrors he had anticipated.

The implication here would seem to be that Roderick dies of fright. This explanation is consistent with his behavior prior to Lady Madeline's shock appearance. We know that Roderick is highly-strung, a hypochondriac with what appears to be a severe nervous disorder. Long before Madeline shows up, he is a nervous wreck; one more nasty shock, and he may be done for.
But it is interesting that Madeline also dies at the same time as her petrified brother. Their deaths indicate just how close their relationship had been, a bizarre, almost telepathic connection. The respective fates of brother and sister are inextricably linked to each other, as indeed they are to the fate of the House of Usher itself.