Haw’s syndrome is a relatively common problem in cats. It is a condition where both third eyelids protrude (or prolapse). Protrusion of the third eyelids can occur for many reasons in cats. When it has a sudden onset, and is associated with diarrhea or other gut conditions, it is called Haw’s syndrome. The protrusion of the third eyelid is not usually painful in cats.

The third eyelid, also known as the nictitating membrane or nictitans, is a structure that sweeps over the surface of the eye in order to spread tears across the cornea and protect the eye. It is usually only possible to see the edge of the third eyelid which is located in the lower corner of the eye near the nose. It is often pale pink in color but can have dark pigment on the surface. Other names for Haw’s syndrome are:

•  Bilateral nictitans protrusion
•  Bilateral third eyelid protrusion.

Protrusion of the third eyelids occurs for many reasons in cats from eye infections to damage to nerves, however it is only called Haw’s syndrome when it occurs suddenly, and if associated with diarrhea or other gut conditions. The cause is unknown (therefore it is called an idiopathic condition). There is thought to be an association with diarrhea caused by a variety of conditions, including tapeworm infestation or virus infection (a toro-like virus has been found in the past, but is not present in every case). Occasionally, other cats in the household will become affected. Sometimes an affected cat will appear dull or depressed and have no appetite, in which case your veterinarian may undertake a more thorough examination. There are other causes of third eyelid protrusion and of diarrhea, so these will be considered by your veterinarian. Haw's syndrome can arise at any age but is much more common in young adult cats.

When you take your cat to the veterinarian, they will perform a thorough eye examination to rule out other conditions that might cause their third eyelid to show. The diagnosis of Haw's syndrome is often made based on several factors:

• The appearance of both third eyelids.
• The absence of other eye problems.
• Possibly a history of recent mild gastrointestinal issues.

The signs may persist for 4 to 6 weeks, but usually the condition is self-limiting, meaning that it resolves without any specific treatment. All cats recover from the condition, so if protrusion of the third eyelids persist for more than four months, the diagnosis will be reconsidered. If a cause can be found for diarrhea, if present, it will be treated specifically. This treatment may include a bland diet, probiotics and if required antibiotics. If the cause is unknown, your veterinarian may choose to treat your cat for diarrhea or administer a dewormer. Treatment of the eyes is not usually necessary. In extreme cases where vision is being obscured by the third eyelids covering much of the eye, 2.5% phenylephrine eye drops can be used to temporarily reverse the third eyelid protrusion. However, this treatment will not shorten the time course of the syndrome.

Contact your veterinarian if you notice:

• Any changes in the appearance of your cat's eyes beyond the visible third eyelid.
• Signs of eye pain or discomfort.
• Discharge from the eyes.
• Redness or swelling around the eyes.
• Worsening of gastrointestinal symptoms.
• The condition persists for more than a few months without improvement.