Hello,Customer thank you for your recent question. I hope my answer can be helpful to you and give you some ideas as to what might be occuring with your dog.
The symptoms you describe are quite vague, which is one reason I am sure your vet could not find the specific problem with your dog.
Based on these vague symptoms, and the fact there is some trouble with stairs for your dog, and the x-rays not showing much, I would be most suspicious of a back injury.
What you are describing are some classic and suspicious signs of back pain. Back pain in dogs can be due to soft tissue injury (muscle spasms, etc.) or spinal pain, which can range from very mild to severe. Back pain is extremely common in dogs, especially as they age, and can occur even with no history of a specific injury.
The symptoms can range from what you describe--lethargic, moving slowly, trouble on stairs, and can also include things like vocalizing, avoiding or having a hard time jumping up or down, trembling/shaking, reduced appetite, hiding, etc. Equally important is that these dogs typically are not limping, have no other symptoms like vomiting and diarrhea, and are difficult to localize any pain when you feel them, making it challenging to know what is bothering them. the symptoms can come and go too, as you have seen.
Here is some information about one common cause of back pain in older dogs, called disc disease:
Your 5 year old Shih Tzu could have some mild disc disease or as I mentioned, the problem could be more soft tissue/muscular.
The treatment of mild cases of back pain, and I would classify your dog as more mild, is to have the problem confirmed by the vet. An x-ray may be taken to look for obvious signs of disc disease. I don't know if your vet x-rayed the back, but even if so, the x-ray may show nothing obvious in mild cases or cases due to soft tissue problems. The vet then may prescribe strong pain relief medication as well as strict rest to allow the area to heal.
I would recommend resting your dog strictly (no running, jumping or stairs, carry dog if possible up stairs or on/off furniture, leash walk only) for one week to help this heal, and talk to your vet about the possibility of a back injury. Your vet may be willing to prescribe anti-inflammatories to help your dog as well.
thank you, XXXXX XXXXX hope this may help get your dog on the right track. Dr. Marcia