I had a young apple whose bark was half stripped by rabbits, I think because the bark was not completly ringed there was enough left to support the tree, being young it seems to have regrown the bark.
I bought a longer rabbit guard.

Are you convinced its slugs?
If yours is not completly barked all round it may stand a chance if you can prevent any more damage

Thank you for your interest. The lower stems are completely stripped of the outer layers right down to the wood. Yes, I am pretty sure that it is slug damage. the strange thing is that I noticed this 2 days ago and yet the young stems and leaves just a little higher up are still lush and it has been pretty warm here in the UK. That is why I wondered whether hydrangeas had a different stem structure to other bushes and trees such as apple trees.

Hi Ray, I'm sorry I would seriously doubt that slugs could strip the bark from your hydrangea they eat soft vegetable material not bark. Suggest it is more likely to be rabbits or even deer. If it has been completely ring barked then the hydrangea will die but it isn't an instant process.

Hmmm, that is sad but the plant is very young. Perhaps I should use the shoots for cuttings?

Take some cuttings now before the top growth does wilt.

And do not be in too big a hurry to write off the plant. H. paniculata will grow from below ground level buds, sometimes.

Exactly. The top may hang on, despite the raspings of slug teeth, but I would expect it to struggle. Hydrangea paniculata is a toughie, and the stems are all pith so may withstand the attack. The soft bark is attractive to slugs and snails, especially when the hot weather makes things difficult for them. The good thing is, next year it'll be stronger still because you'll be pruning it down so that it comes up from lower, and it'll be thicker barked

It could also be voles - I lost a 12 foot cotoneaster this way. Try a good sprinkling chilli pepper round the base as this should put off whatever it is.

Thanks to all for your advice, I think Owdboggy and Worthy1 nailed it. Sadly, today there are signs of wilting. I have taken 5 cuttings. I am pretty sure it was slugs as an earlier plant went the same way over winter/spring but I thought it was frost damage. I don't intend to give up though. I aim to purchase a bigger plant and up the pest protection to high security levels. :-)