Occasional construction of left Pupil

Hello Vets, My puppy, Blue, an 8 month old Aussie has been having a strange issue arise occasionally. His left pupil will constrict and I cannot pin point the exact cause. Backstory: the first time this happened he was 2-3 months old and was playing in the yard with the kids and hit his head on a trailer. He yelped, I checked him out and he seemed fine, pupils were normal. Fast forward a couple hours he was napping and I woke him up with one pupil constricted so I took him to the ER. By the time we got there his pupils were back to normal. the vet thought maybe he hit a nerve causing the constriction. 2nd time (about 1.5-2 months later) we were camping and he was sitting next to me and I noticed the pupil, but he had not hit his head and it went back to normal a short time later. He hasn’t had another episode until the last couple weeks it’s happened twice. The first of the two I am not sure if had hit is head or not, but the second time he did. Again the left pupil was constricted a short while later but didn’t last long. He hasn’t no symptoms when it happens and it doesn’t seem to bother him. Any ideas as to what is causing this or what we can do to prevent it? He is up to date on all vaccines and intact.

Hello - in my experience, neurological damage is quick to happen and slow to mend; waxing and waning sporadically like this without any fresh insult would be unusual - and if he was hitting his head, it would be odd that the same manifestation of the trauma was seen every time.  I therefore wonder whether any damage may be internal eg a problem to the eye, or whether your dog may be suffering from occasional fits (that could be what caused him to hit his head).  This may be a complicated mystery to solve, but changes affecting pupil size have a way of being serious and I would reccommend seeing your vet and letting them do a neurological examination; they may not, however, solve the mystery in a 10 minute consultation.