When a dog suffers a head injury, there will be several telltale indications that the animal has indeed sustained a concussion. One of those signs will be constriction or dilation of the dog's pupils.
According to Allcreatures.com, when a dog sustains a head injury, it will likely result in some downward pressure to the brain, due to swelling. This swelling impairs normal functions, causing the pupils to dilate.
Theveterinarian.com reports that the presence of pupil constriction indicates brain injury severe enough that the prognosis for the animal is quite poor.
Constricted or fixed pupils in a dog with a head injury will not change (become less pinpoint in appearance) when light is shone into the eye. The type of brain damage that is most common when the pupils appear as such is a rostral brainstem injury. The rostral portion of the dog's brainstem is where the ocular nerves are housed.
When a dog has sustained any sort of head trauma, the veterinarian will look for Horner's Syndrome. The syndrome is a group of symptoms that is present when there has been significant damage to the dog's sympathetic nerves. Miotic (constricted) pupils are sometimes a symptom of Horner's Syndrome.
Breeds Susceptible to Horner's Syndrome
According to Peteducation.com, Horner's symptoms commonly presents itself in Golden Retrievers, most often in those dogs that are middle-aged or older.