Golden Retriever's question about Horner's Syndrome

Question

My almost 9-year-old Golden Retriever, Marley, got attacked 10 days ago by another dog, the tip of the left ear flap was injured, he was treated immediately and luckily healed rather quickly with some antibiotics. Around the same time of the injury his left lower eye lid started to drop just a little. At first it was visible only when he woke up or opened his eyes after having them shut, but 3 days ago, the condition started to rapidly get worse, with his eyelid almost always drooping when the eye is open, the third eyelid bigger and pupil decreasing in size. There is even a slight discharge (which he normally never has). The eye also looks a little sunken and watery. He doesn't try to scratch it and there are no other changes in behaviour. The vet did not find any other signs or symptoms when he saw Marley and suggested we could run tests like MRI or CT scan which he did not think were really necessary given that there were no other signs or symptoms. Could it be that it is the injury that caused all this? Do you think we should run CT scans and all other tests?

Read more at: https://wagwalking.com/condition/horners-syndrome

Horner's Syndrome in Dogs