Why is my dog's one eye not dilating to light? What should I do?

Yes, that is definitely something you should be worried about. What you are seeing is called anisocoria, and when it suddenly appears, it is a medical emergency. This can be due to corneal injury, disease or injury to the brain, disease or injury to the nerves running to the eye, or uveitis among other causes. I would take Ace into an ER vet or find a vet on call to see him ASAP to diagnose what is causing the anisocoria so it can be treated.

This can be a sign of an eye problem - the eye can dilate if a toxin has been ingested, or it's possible that the other pupil is actually constricted secondary to pain or a scratch on the eye. I'd recommend seeing your veterinarian for an exam of the eyes. If Nova seems to be painful or disoriented I"d recommend seeing an emergency veterinarian today.

The pupil can constrict with bleeding or inflammation in the anterior chamber of the eye, blunt trauma, or a scratch to the surface of the eye. If you see blood, if your dog is squinting or seems painful, or if there is discharge from the eye I’d recommend traveling to see an emergency vet if at all possible since some conditions could compromise vision if left untreated. Otherwise I think you could monitor her for a day but if the pupil does not return to normal within 24 hours I’d recommend seeing a vet.

Poor pup! Cloudiness and anisocoria (the medical term for unequal pupils) are signs of a significant eye injury. It would be best to have Massimo examined by an emergency vet tonight if possible. If this isn't an option for you, he should be seen by your regular vet first thing in the morning. Prompt assessment and treatment is important in cases like this because the risk of permanent damage or vision loss increases the longer he goes without treatment.

Poor Beau! The eye not dilating is likely a sign of significant head trauma, which also makes sense if he lost a tooth in the collision - it normally takes quite a bit of force to cause this! It sounds like Beau may have a concussion or other serious head injury, and it also sounds like his tooth socket may be infected - especially if part of the tooth root is still present. These things will need veterinary care right away, so I would recommend taking him to the vet today to be checked. Your vet can do a complete neurologic exam to help determine how serious his head injury is, and will also evaluate his mouth to see if he needs surgery to remove any remaining part of the tooth that may be infected and causing pain.

Your pup is experiencing anisocoria. There are many potential causes and I recommend to have your pup examined today: https://vcahospitals.com/know-your-pet/anisocoria-in-dogs It's possible he is in pain (that's why he doesn't want to open the eye). The veterinarian may need to put a dye in his eye to look for a scratch. Other diagnostics may also be recommended pending the physical exam findings. I hope this helps and that he feels better quickly.

Poor Hank! It could be he has a serious eye infection leading to this cloudiness, squinting, and trouble seeing. He could have also injured his eye. It's very concerning this just happened overnight, and any problem with the eye should be considered an emergency. That's because leaving it untreated can lead to vision loss. Please take him into an ER vet clinic; if there isn't one in your area, there will be a vet on call for emergencies. The information to reach that vet is found on a clinic's answering machine or website. Best of luck, I hope this helps!