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I am sorry to hear you are worried about Dillon. My name is Rebecca; I am a veterinarian and will do my best to help.
I would suspect he could have MMM, masticatory myositis. Not being able to open the mouth very far is seen with this condition. It is an auto immune condition that we do not understand completely. The treatment is giving steroids, like prednisone, and it usually will improve with time and treatment. There is a blood test for this condition.
The eye rolling back in the head is not typical with MMM, however. If this is not MMM, it could be a condition affecting the nerves to his face, which could affect both the muscles of his mouth and his eye.
If he were mine, I would probably try a course of steroids, and would do a good neurologic exam and possibly do the blood test to check for MMM. I might try to see if I could open his mouth when he was sedated; if not then this is probably MMM or a neurologic condition. If I suspected a problem with the nerves to his face, I would probably consult a veterinary neurologist.
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