SYMPTOM
Third eyelid showing across the eye
Owners notice a pale pink or pigmented membrane creeping in from the inner corner of one or both eyes, sometimes covering a significant portion of the eye's surface, occasionally alongside squinting, discharge, or a generally withdrawn demeanour.
Idiopathic (Haw's Syndrome)
In many cases, both third eyelids elevate without any identifiable eye disease, a pattern most often recognised in young adult cats. It is frequently associated with mild, sometimes self-resolving, gastrointestinal upset.
Gastrointestinal Disturbance
Diarrhoea or other gut upset, including that caused by intestinal parasites, has a recognised association with bilateral third eyelid elevation. This is thought to relate to altered autonomic nervous system signalling rather than any problem within the eye itself.
Ocular Surface Disease
Conjunctivitis, corneal ulceration, or other surface irritation of the eye can cause reflexive protrusion of the third eyelid as a protective response. This is typically confined to the affected eye and often accompanied by discharge or squinting.
Intraocular Inflammation
Inflammation within the eye itself, such as uveitis, can trigger pain-related elevation of the third eyelid. This usually appears alongside a change in pupil size, redness, or cloudiness of the eye.
Horner's Syndrome
Disruption of the sympathetic nerve supply to the eye, which can arise from middle ear disease, trauma, or a lesion along the nerve pathway, produces third eyelid elevation confined to one eye. This is typically alongside a smaller pupil and a slightly sunken appearance.
Systemic Illness or Dehydration
Reduced fat and fluid volume behind the eye, associated with dehydration or significant weight loss from chronic systemic disease, can cause the eye to sit slightly further back in the socket. This allows the third eyelid to passively rise.
Why timing matters
Early observation
A brief appearance that resolves within a few weeks, especially alongside a short bout of loose stool, is consistent with the mild, self-limiting pattern most often seen with the idiopathic bilateral form, which typically takes around four to eight weeks (occasionally longer) to settle fully. An early presentation confined to both eyes equally, without pain or discharge, generally carries a different context to one affecting a single eye from the outset.
Later presentation
Protrusion that persists beyond two to three months, or that is confined to one eye throughout, points away from the typical idiopathic pattern and toward a more localised or ongoing underlying process. The longer a one-sided presentation continues without change, the more it supports a discrete, identifiable cause rather than a generalised, self-resolving one.
Whether the sign affects one eye or both, and whether it is static, intermittent, or gradually worsening, helps separate the generally self-resolving bilateral pattern from a one-sided neurological or ocular process that may need its own characterisation. A shift from bilateral to unilateral, or the emergence of discharge or squinting partway through, would suggest the picture is evolving rather than settling.
Conditions commonly associated
Horner's Syndrome
Disruption of the sympathetic nerve supply to the eye produces one-sided third eyelid elevation, typically alongside a smaller pupil on the affected side.
Conjunctivitis
Inflammation of the ocular surface commonly triggers reflexive third eyelid protrusion, usually confined to the affected eye.
Corneal Ulcers
Pain from a corneal defect can cause protective elevation of the third eyelid over the affected eye, often with squinting or discharge.
Uveitis
Inflammation within the eye can produce pain-related third eyelid elevation alongside changes in pupil size or eye clarity.
Gastroenteritis in Cats
Gastrointestinal upset has a recognised association with bilateral third eyelid elevation, thought to relate to autonomic nervous system changes.
Feline Herpesvirus (FHV-1)
This common cause of feline conjunctivitis can produce ocular surface irritation and secondary third eyelid protrusion.
Giardiasis
This intestinal parasite is among the gastrointestinal causes linked to bilateral third eyelid elevation in affected cats.
When to explore further
Third eyelid protrusion affecting only one eye, particularly alongside a smaller pupil or a slightly sunken appearance on that side, points toward a neurological rather than gastrointestinal cause and may warrant characterisation.
Protrusion accompanied by visible discomfort, discharge, redness, or holding the eye partially closed suggests surface or intraocular involvement rather than the idiopathic bilateral pattern.
Bilateral protrusion occurring alongside diarrhoea or reduced appetite in a young adult cat fits a recognised pattern that often resolves as the gastrointestinal upset settles.
Protrusion appearing gradually alongside weight loss or reduced water intake in an older cat may reflect a systemic or dehydration-related contribution rather than a primary eye problem.
A sudden appearance following any head trauma or recent ear disease is a scenario where the timing adds useful context, given the association between middle ear pathology and one-sided third eyelid elevation.
A photograph taken directly from the front of the face in even lighting, showing both eyes together, is one of the most useful pieces of documentation, as it allows comparison of pupil size and symmetry between the two sides. Noting whether one eye or both are affected, how long the protrusion has persisted, and any accompanying stool changes, discharge, or appetite shift adds further context. Repeating the photograph every few days can help establish whether the pattern is stable, resolving, or progressing.
Last reviewed: 27 August 2026 · Dr Alastair Greenway MRCVS