One-sided facial drooping in dogs is caused by damage or disruption to the facial nerve, which controls the muscles of the ears, eyelids, lips, and nose. The most common diagnosis is idiopathic facial nerve paralysis, meaning no underlying cause can be found, accounting for about 30% of cases in one large study. Other causes include ear infections, hypothyroidism, trauma, and less commonly tumors or immune-mediated disease.
What Facial Paralysis Looks Like
The most consistent sign is an inability to blink on the affected side. You may also notice the lip on that side hanging loosely and exposing the inner lining of the mouth, or the ear sitting lower than on the other side. In many dogs, the nose tilts away from the drooping side because the muscles on the working side pull it over.
If the condition has been present for a while and the muscles have started to contract or stiffen, the signs can actually reverse in appearance. The lip on the affected side may sit higher than normal, the nose shifts toward the paralyzed side instead of away from it, and the ear base can ride abnormally high on the head. This can be confusing, so it helps to think about when you first noticed changes and whether the appearance has shifted over time.
Idiopathic Facial Paralysis
This is essentially the dog version of Bell’s palsy in humans. Your vet will diagnose it after ruling out infection, thyroid problems, injury, and tumors. A Sydney-based study covering 15 years of cases found that idiopathic facial nerve paralysis was the single most common diagnosis at 29.5%, and that certain ages, breeds, and sexes were more frequently affected. The onset is typically sudden, and while it can be alarming, many dogs adjust well even if full nerve function doesn’t return.
Ear Infections Are a Major Culprit
The facial nerve runs directly through the middle ear on its way from the brain to the face muscles. This anatomy means that middle or inner ear infections can compress or inflame the nerve and cause drooping on the same side as the infected ear. If your dog has been shaking their head, scratching at one ear, tilting their head, or has a history of chronic ear problems, infection is a strong possibility.
Because other nerves also travel through the middle ear, an infection there can cause additional signs beyond facial drooping. You might see one pupil smaller than the other, a drooping eyelid, or the third eyelid (a pinkish membrane in the inner corner) protruding on the same side. These signs together point strongly toward an ear problem rather than an issue with the nerve itself.
Hypothyroidism and Nerve Damage
Low thyroid function can damage peripheral nerves throughout the body, including the facial nerve. The mechanism involves a few pathways: the thyroid hormone deficit slows cellular energy production in nerve cells, and the high blood fat levels that accompany hypothyroidism can reduce blood flow to the nerve. Deposits of a jelly-like substance can also build up around nerves where they pass through narrow bony channels in the skull, compressing them.
Hypothyroidism tends to cause other noticeable changes as well. Weight gain without increased appetite, a dull or thinning coat, lethargy, and skin infections are common. If your dog has facial drooping alongside any of these signs, thyroid testing through a simple blood draw can confirm or rule out this cause. The good news is that thyroid replacement medication is straightforward, and nerve function can improve once levels normalize.
Trauma and Other Causes
Physical injury to the facial nerve, whether from a car accident, a fall, rough handling, or even prolonged pressure on one side of the face during anesthesia, can cause sudden drooping. Tumors growing near the nerve’s path from the brainstem through the skull are a less common but more serious possibility. Immune-mediated diseases and generalized nerve disorders (where multiple nerves throughout the body are affected) round out the list of rarer causes.
How Your Vet Figures Out the Cause
Your vet will start with a thorough physical and neurological exam, checking whether your dog can blink on the affected side, whether the ear responds normally to touch, and whether other nerves seem involved. They’ll look in the ears for signs of infection and check for the cluster of eye changes that point to middle ear disease. Blood work, particularly thyroid levels, is a standard part of the workup.
If these initial tests don’t reveal a cause, advanced imaging like MRI can visualize the nerve’s path through the skull and identify tumors, deep-seated infections, or inflammation that wouldn’t show up on a basic exam. Electrical testing of the nerve and muscles can help determine how severely the nerve is damaged and whether recovery is likely.
Treatment and Eye Care
Treatment depends entirely on the underlying cause. Ear infections are treated with targeted medications. Hypothyroidism responds to daily thyroid hormone supplementation. Tumors may require surgery, radiation, or management depending on location and type. For idiopathic cases, there’s no specific treatment for the nerve itself.
Regardless of cause, the most important day-to-day concern is protecting the eye on the affected side. Because your dog can’t blink normally, that eye dries out quickly and becomes vulnerable to ulcers and infection. Artificial tears or lubricating eye drops, applied several times daily, keep the cornea moist and protected. Your vet will likely test tear production on the affected side to determine how much supplemental lubrication your dog needs.
Some dogs recover full facial nerve function within weeks to months, particularly when a treatable cause like infection or hypothyroidism is identified and addressed. Dogs with idiopathic paralysis have a less predictable course. Some regain partial or full movement, while others have permanent changes. Most dogs adapt remarkably well, eating, drinking, and living comfortably even with lasting facial asymmetry, as long as the eye stays protected.