Cat Third Eyelid Showing: 0-2 mm Normal, 3 mm Not

A third eyelid that shows 2 mm at rest is doing its job. One that shows 4 mm is a clinical finding with a short differential: dehydration, Horner syndrome, intestinal worms, or loss of the fat pad behind the eye. The membrane moves as a unit with the eyeball, so its position tells you about fluid, nerve supply and orbital fat at the same time.

Measure before you interpret. A cat cornea is roughly 16 mm across, so 3 mm of membrane is nearly a fifth of the visible eye.

0 to 2 mm is the normal window

In an awake, alert cat the membrane should be hidden or show no more than 2 mm at the inner corner. It slides up to 2 mm during sleep, a hard stare or a meal, then retracts within 1 to 2 seconds of the cat being called or standing up.

Symmetry is the cheapest test you own. Photograph the face straight on, weekly, with the cat looking at a light, and compare the 2 corners in one image. A 2 mm difference between eyes that persists for 2 days is worth a call; a 4 mm difference with a small pupil is worth a same-day visit.

Dehydration moves it down and in

Fluid loss shrinks the retrobulbar space, the eye sinks 1 to 2 mm, and the membrane fills the gap. The graded signs run with body weight loss: at 5 percent, gums feel tacky and a skin tent holds 2 seconds; at 7 to 8 percent, the membrane sits 3 to 5 mm out, the eyes look sunken and capillary refill passes 2 seconds; at 10 percent or more the cat is dull, cold at 37 C or below, and in shock.

Translate the percentage into millilitres first. A 4 kg cat at 5 percent is 200 mL short; at 8 percent, 320 mL. Maintenance is 160 to 240 mL a day, or 40 to 60 mL/kg/day, and a cat that is dehydrated and vomiting needs both figures added, not substituted.

Chronic low-grade dehydration is harder: a cat on dry food drinking 100 mL a day can carry a 1 to 2 mm membrane for weeks with normal bloodwork. Wet food at 75 to 80 percent moisture and a fountain add 30 to 60 mL a day.

Horner syndrome: 4 signs, 1 small pupil

Horner syndrome in a cat produces ptosis, miosis, enophthalmos of 1 to 2 mm and a raised membrane, all on one side, with the pupil 1 to 3 mm smaller than its partner. The membrane is often the sign owners see first, because fur hides a 2 mm lid droop and a dark iris hides a 2 mm pupil.

Where the lesion sits decides urgency. Ear disease, a polyp or a middle ear infection causes most feline cases, and deep ear cleaning can do it in minutes. A neck or chest mass produces the same picture, so Horner syndrome without ear disease needs chest imaging within 1 to 2 weeks. The facial nerve branches that lift the membrane also serve the whiskers, described under whisker function. A 1 percent phenylephrine drop helps localize: a postganglionic lesion dilates the pupil within 20 to 30 minutes, while a preganglionic one takes 45 to 90.

Worms, gut disease and the fat pad

Roundworm burden is the classic kitten cause. Toxocara cati is common in kittens from 4 to 8 weeks, and a wormy kitten shows both membranes out 3 to 6 mm, a pot belly, a dull coat and often diarrhea. Deworming at 2, 4, 6 and 8 weeks, then monthly to 6 months, resolves the eye sign in 1 to 3 weeks.

The fat pad is the part owners never think about: a few millilitres of retrobulbar fat hold the eye forward, and it shrinks with age and chronic disease. A 12 year old cat at body condition 3 out of 9 can show 3 to 4 mm of membrane on both sides with perfect hydration and normal nerves, because there is nothing left behind the eye. Chronic enteropathy and intestinal lymphoma in older cats produce the same look plus weight loss above 10 percent, and that combination deserves bloodwork and an ultrasound rather than a bowl of water.

Tear film, gland and the 30 percent

The gland at the base of the membrane supplies 30 to 50 percent of the aqueous tear film. A membrane that stays out therefore threatens the cornea: exposed cornea loses its film in minutes, and an ulcer can form within 2 to 6 hours. Normal Schirmer tear test values in cats run 15 to 25 mm per minute; under 10 mm is suspicious and under 5 mm is keratoconjunctivitis sicca. Reference ranges shift slightly between laboratories and methods, so compare against the range printed with the result.

What each measurement means

Membrane shown	Likely cause	Symmetry	Other signs	Act within
0-2 mm	Normal	Equal	None	Routine
2-3 mm	Mild dehydration, sleep, nausea	Equal	Tacky gums	24 hours
3-5 mm	7-8 percent dehydration, worms	Equal	Sunken eyes, pot belly	12 hours
5-8 mm	Horner syndrome, fat pad loss	One side	Miosis, ptosis	24 hours
8 mm or more	Shock, severe weight loss, mass	Either	Cold, weak, not eating	Now

Flat-faced cats carry shallower orbits and show more membrane for the same body weight, so check the breed quiz before assuming disease in a Persian or a British Shorthair. Read the symmetry column first. Equal membranes point to a body-wide cause: fluid, worms, fat, nausea. Unequal membranes point to a nerve or a local orbital problem, and that is the column where a 24 hour delay changes the outcome.

When to stop watching and go

Five findings end the waiting period: a membrane out on one side only with a pupil difference of 1 mm or more; a membrane covering half the eye or more; any cloudiness, redness or discharge on the cornea; a fever above 39.5 C with lethargy; and a cat that has lost 10 percent of body weight in 3 months. A membrane that stays out after 24 hours of free water access also qualifies.

Never push the membrane back or wipe it with a cloth. The tissue is thin, 1 mm or less, and a wipe can turn a harmless protrusion into a corneal abrasion that needs 5 to 7 days of drops.

Four steps for tonight

1. Photograph both eyes straight on, in the same light, and note the millimetres.
2. Check hydration: gum feel, skin tent time, and weight against last month.
3. Offer wet food and a fountain, and count how much the cat drinks in 24 hours.
4. Book a visit within 24 hours if the membrane is one-sided, and within 12 hours if the cornea looks cloudy.

A visible third eyelid has 4 common answers, and 3 are manageable at home within a day. The fourth, Horner syndrome, shows up on one side only, which makes the symmetry check the most useful 10 seconds of the exam.

Terms mentioned in this article

MM

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