SYMPTOM
Drooping eyelid with sunken eye
One eyelid appears to droop lower than the other, and that eye can look smaller, sunken, or somehow different in shape, often noticed suddenly as a new facial asymmetry rather than a gradual change.
Idiopathic Horner's Syndrome
In a proportion of dogs, this combination of a drooping eyelid, small pupil, sunken eye and raised third eyelid arises with no identifiable underlying cause and is termed idiopathic; Golden Retrievers appear to be over-represented in reported case series. Idiopathic cases in dogs resolve slowly without treatment: in one series of Golden Retrievers every case resolved within 11 to 20 weeks, and in a larger series of 100 Golden Retrievers the signs had resolved in every dog within 6 months.
Middle Or Inner Ear Disease
The sympathetic nerve fibres responsible for eyelid and pupil tone run close to the middle ear, so infection or inflammation in this region (otitis media or interna) can disrupt the pathway and produce the same eye appearance, sometimes alongside a head tilt or ear discomfort.
Neck Trauma
Injury to the neck, such as from a fall, collision, or rough play, can damage the sympathetic nerve trunk as it runs alongside the jugular vein and carotid artery, producing the same set of eye signs on the affected side.
Thoracic Or Neck Mass
A mass in the chest or neck pressing on the sympathetic chain, such as a nerve sheath tumour of the nerves to the forelimb, is a less common but recognised cause, and tends to be considered when the eye signs persist or other findings accompany them.
Facial Nerve Paralysis
Facial nerve dysfunction produces a different but sometimes visually similar asymmetry, affecting the lip, ear carriage, and blink on the same side, and its mechanism (a motor nerve to facial muscles rather than sympathetic tone to the eye) is distinct from Horner's syndrome.
Vestibular Disease
Disturbance of the balance system can sometimes be accompanied by a droopy-looking eye or altered eyelid position alongside more obvious signs such as a head tilt or loss of balance, and owners occasionally group these together as one presentation.
Painful Eye Disease
Uveitis and glaucoma are painful conditions inside the eye that can produce a similar look, because squinting narrows the eyelid opening and makes the eye seem smaller. Redness, a cloudy or blue surface, light sensitivity or an eye held shut belong to these conditions and are not among the four signs of Horner's syndrome (drooping upper eyelid, small pupil, sunken eye and raised third eyelid). Pupil size, pressure inside the eye and pain cannot be judged reliably at home, and most dogs with primary glaucoma lose vision within hours to days of an acute rise in pressure.
Why timing matters
Early observation
In dogs, a sudden droop and small pupil with no other signs most often proves idiopathic, particularly in Golden Retrievers: in a UK series of 120 dogs that had advanced imaging, a causative lesion was found in 3% of dogs with isolated Horner's syndrome and in 98% of those with additional clinical signs. Whether the signs are truly isolated, and whether the eye itself is comfortable, is established by examination and cannot be settled from appearance at home.
Later presentation
Signs that are joined by additional findings such as a head tilt, facial weakness or forelimb lameness, or that are still present after 6 months, point more towards one of the identifiable structural or nerve-pathway causes.
Idiopathic cases characteristically improve gradually over weeks to months as nerve tone returns, whereas cases driven by an ongoing structural cause, such as a mass, tend to remain static or slowly worsen rather than resolve on their own.
Conditions commonly associated
Horner's Syndrome
This combination of eyelid droop, small pupil, sunken eye and raised third eyelid is the direct clinical picture this presentation describes.
Otitis Media
Infection in the middle ear can disrupt the nearby sympathetic nerve pathway responsible for normal eyelid and pupil tone.
Otitis Interna
Inner ear disease can follow untreated middle ear disease, where the sympathetic pathway is affected, and adds balance-related signs such as a head tilt to the eye changes.
Vestibular Disease
Balance system disturbance can coincide with or be mistaken for the eyelid and eye changes described here.
Facial Nerve Paralysis
A distinct nerve pathway produces a visually similar facial asymmetry that is useful to distinguish from a purely sympathetic cause.
Nerve Sheath Tumour
A nerve sheath tumour of the brachial plexus, the nerves to the forelimb, can involve the first thoracic nerve root and with it the sympathetic supply to the eye on the same side, usually alongside a slowly progressive forelimb lameness and muscle wasting.
Glaucoma in Dogs
Glaucoma is a painful rise in pressure inside the eye. An acute rise shows as a dilated, fixed or sluggish pupil, a red eye and a cloudy cornea, the opposite of the small pupil of Horner's syndrome, although squinting from pain can look like a drooping lid. The pressure can only be measured with a tonometer. In chronic glaucoma, sustained high pressure stretches and enlarges the globe (a change called buphthalmos), so the eye appears larger, not sunken.
Uveitis
Uveitis, inflammation inside the eye, produces a small pupil, low pressure within the eye, redness, light sensitivity and squinting, so it can closely resemble the small pupil and narrowed eyelid opening of Horner's syndrome. It is a painful condition, and its recognised complications include secondary glaucoma, cataract and clouding of the cornea. The two are told apart by examination of the inside of the eye, which cannot be done at home.
When to explore further
A head tilt, loss of balance, or circling alongside the eye change points towards inner ear or vestibular involvement rather than an isolated sympathetic nerve issue.
Drooping of the lip or ear, or a reduced blink, on the same side as the eye change suggests facial nerve involvement in addition to or instead of Horner's syndrome.
A palpable lump in the neck, or altered breathing sounds, may warrant characterisation of a chest or neck mass as a contributing factor.
Signs that follow a recent fall, road traffic incident, or rough interaction with another animal fit a trauma-related pattern.
Idiopathic cases in dogs take months to clear, so slow change alone does not separate one cause from another; eye changes still present after 6 months fall outside the published idiopathic pattern.
Squinting, redness, a cloudy or blue surface to the eye, light sensitivity, or a pupil in the drooping eye that is larger than the pupil of the other eye fits painful disease inside the eye such as uveitis or glaucoma, not Horner's syndrome, and in glaucoma sight can be lost within hours to days.
Photographing both eyes together in the same lighting from the first day gives a record of pupil size, eyelid position and third eyelid visibility that can be compared over time. Photographs record appearance only: they do not show pain, the pressure inside the eye or the cause. A short video capturing gait and head position can document any head tilt or unsteadiness. Noting whether the ear on the same side seems itchy, painful, or has been shaken more than usual rounds out the picture.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS