There's no additional rule either in Basic set or in Martial Arts. As B391 suggests, you can defend against any attack from a known enemy at -2, except for situations when the defending character doesn't know from where (and when? This is not so clear) the attack is coming.
A character with the Peripheral Vision advantage can defend at -2 even if approached from behind. What such a person sees is just a shade somewhere in the extreme periphery of sight, leading to a rough idea that someone behind is attacking. It is similar when someone jumps behind me: if I can't turn, I can just do a "wild parry" (which means -2 to the roll).
There is another interresting problem: will the fighter with an enemy in his back (assuming he doesn't want to turn, perhaps due to another enemy in front of him) be able to defend in subsequent turns? The rules are not clear on this.
I would assume the answer is no: the defender knows that someone is behind him, but he can hardly expect when the strike is coming. Anyone who ever fought with a sword (including wooden/ plastic LARP swords) would agree that timing is crucial while parrying. So my quick and easy answer is that you can only defend against the first attack. The longer answer (how I solve it in my game) is as follows, but it is rather a house rule than an official rule:
The defender can try to dodge at -4 (-2 for not seeing the enemy and extra -2 for guessing when the attack is coming; Peripheral vision buys off the first penalty and Danger sense the second if the defender wins PER vs. higher of attackers Stealth and combat skill in the turn), moving either constantly or from time to time (all-out defence or defensive attack). Parrying or blocking is trickier. The defender can choose to swing his blade wildly behind his back, allowing him to defend at -2 - but this is for whole turn, which means this is possible only when the defender chooses all-out defence (Double Defense with one hand with a weapon/ shield committed to this enemy and useless against any other attack). Or he can defend at -4 (extra -2 for not knowing the timing). If attacker chooses "wait until there's no defence", no defence should be possible unless all-out defense was chosen by the defender (the wait maneuver would waste the attack in this situation).