If only we had a resident veterinarian to chime in...
I emailed Dr_Blue and gave her a link to this thread. I asked if she would be kind enough to give her opinion on what happened.
She was kind enough to reply. Very informative.
In case anyone is interested, here's what she said:
On rare occasions, otherwise healthy dogs can develop hearing loss after general anesthesia. There are a number of theories as to why this happens. There may be pressure changes in the inner and middle ear due to intubation and gas anesthesia, pressure on the arteries that supply the hearing apparatus (cochlea), low blood pressure during anesthesia, adverse drug reactions, inner/middle ear infection, rupture of the ear drums (they heal!), or simply progression of ongoing hearing loss. Were your dog's ears examined or cleaned under anesthesia? Complications can develop secondary to cleaning, so you should talk to your vet about this if you have questions. Other concerns include unmasking of other disease - including brain tumors. This can happen with otherwise unremarkable anesthetic events, which then give the impression of an acute event. Does your dog have any other clinical signs, such as chronic ear infections, disorientation, circling, head tilt, or different sized pupils?
My recommendations would include thorough otoscopic and neurologic examinations by your vet, complete bloodwork and potentially a referral to a neurologist for advanced imaging and/or a BAER hearing test. I'm not sure if there are neurologists in the St. Louis area, but the Mizzou has some great people.
If there are no other underlying diseases found, it is my impression that unfortunately, these patients do not often regain their hearing. However, a neurologist would likely be better able to answer this question, as this problem is rare, and I do not have direct experience with these guys.
Please keep in mind that dental disease is a serious condition in dogs, and YOU DID THE RIGHT THING by having a dental cleaning performed. I see many dogs who endure needless suffering with dental pain. I don't know your vet, but I'm sure he or she did their very best, and feels very badly that this happened. If your dog's hearing loss is permanent, try to find a silver lining by using this as a training opportunity. You'll have to be extra careful if your dog is off-leash (probably not recommended for a deaf dog anyway), so training with high-value food rewards can be a lot of fun! Talk to your vet about options.
And then she sent another email:
I just found a reference from a neurologist stating that acute hearing loss secondary to anesthesia can take up to 2-4 weeks to return, if it is going to. If there is no other underlying problem, then there really isn't a way to accurately predict which dogs will regain hearing and which ones won't.