Your dog's eyes may sink in for several reasons ranging from localized eye problems to systemic disorders requiring immediate veterinary attention. The medical term for sunken eyes is enophthalmos and only one eye or both eyes can be affected. Because many causes of sinking eyes in dogs are quite serious, it's important to have your dog seen by your veterinarian for proper diagnosis and treatment.
Painful Eye Disorders
Unlike humans, a dog's eyes are equipped with retractor bulbi, special muscles that allow dogs to retract their eyes back into their sockets. When a dog presents with a sunken eye, this can be indicative of severe eye pain as these muscles tend to spasm as a reaction to painful conditions affecting the surface of the eye as seen in corneal ulcers. A corneal ulcer can be detected by staining the eye's surface with a fluorescent-green dye.
Severe Eye Problems
Sometimes, severe and destructive eye diseases and traumatic injuries cause the eyes to undergo irreversible damage, which leads to eye shrinkage, also known as phthisis bulbi, along with sinking into the socket. This can occur because of complications arising from intraocular eye surgery, glaucoma, ocular trauma and intraocular inflammation. A thorough eye examination conducted by a veterinarian or veterinary opthamologist can determine the presence of these eye conditions.
Presence of Masses
In some cases, a dog's eyeball may recede because of the presence of masses, which can be cancerous. Because of limited space found within the orbit, masses or swellings located in the frontal part of the orbit cause the affected eye to sink in since the eyeball is pushed back. Along with sunken eyes, masses in the eye may also cause loss of ocular motility, blindness, strabismus, protrusion of the third eyelid and possibly pain. X-rays of the skull can help to determine the exact location of any masses.
Sign of Dehydration
In severely dehydrated and emaciated dogs, both eyes may sink deep into the orbit because of loss of substance in the fat pads located just behind the dog's eyes. In such advanced cases, dogs risk becoming victims of shock and collapse. Treatment consists of immediate veterinary intervention to correct the dehydration through intravenous fluid therapy and prevent further loss of fluids.
Possible Horner's Syndrome
When the nerves that lead to the eye are affected, dogs may develop a condition known as Horner's syndrome. Middle ear diseases, trauma to the neck and tumors may be culprits, but sometimes a precise cause cannot be found. Idiopathic Horner's Syndrome means its cause remains unknown. The syndrome is characterized by sunken eyes, third eyelid protrusion, constricted pupils and droopy eyelids. Horner's often resolves spontaneously; however, it's important to see your vet to determine if there's an underlying cause so it can be treated accordingly.
Other Systemic Disorders
Several other disorders affecting multiple parts of the dog's body may include sunken eyes as a symptom. Tetanus is known for causing bilateral sunken eyes because of spasms of the retractor muscles causing the eyeballs to recede along with the protrusion of the third eyelids. Strychnine poisoning and chronic masticatory myositis are other culprits, while tick-borne diseases are also known for causing sunken eyes along with many other symptoms.