Under "medical complications" you mean fatal injuries inflicted by demons, right?
Any Dwarf that made it to the hospital who stood a good chance of living but died due to thirst/infection or something like that are classed as those having died under medical complications. Most of those Dwarves will have been those that survived undead attacks but did not make it through hospital, as with demons usually you have Dwarves either fully dead or fully fine. Excluding were-civets, I've only known three Dwarves to be maimed by demons and live.
Most of the syndrome deaths however... Demons. With smidgens of megabeasts.
The Dwarves sparred atop a muddy ditch, any who fell had to endure the fall and slog back up the banks to spar and fall again - until they fell no longer. Originally this ditch was half-filled with water to train Dwarves how to swim, but mothers complained of how hard it was to find babies amidst the water when they inevitably fell in during training.
I had quite the startle when I checked the alert for combat. Usually this just means another Dwarf fell down into the mud or bumped into someone on one of the rampways, but this time there were a dozen combat reports flagged and a forgotten beast was amongst them. All I found was a corridor full of broken bolts, three Dwarves passed out and a great big forgotten beast corpse lying broken in the mud.
A forgotten beast had attacked underbelly of the Fortress, running right into one of the barracks, having found a way in past all the Fort's defences. The comatose Dwarves, notably 3rd Forge Master Edem Beemaster, 15th Forge Master Stukos Anvilshake and Battle Miner Amost Granitetouched, spent weeks resting in the hospital before waking up and getting back to work. I don't actually know what the Forgotten Beast looked like, so I'm going to try and resurrect it. I believe it managed to break through the 1st cavern layer, swim through the artificial lakes and make its way up through one of the arena maintenance tunnels right into the barracks. It has a very potent syndrome that causes this unconsciousness, no doubt if this wasn't the barracks and there weren't so many other Dwarves to finish what the first three started then it would've begun a steady depopulation of the Fortress.
This is one of those exceptions to the usual everyday happy Dwarven life I'm talking about. There are too many bloody beasts roaming the caverns! The Fort was fortunate this time that the syndrome had no longer lasting effects. I may try flushing them out of the caverns somehow...